VIDEO SIGNAL PROCESSING AND RECORDING SYSTEM

Abstract: The present invention includes a system for processing and recording digitized component television signals onto analog video tape. Such a system includes a digital color transformation matrix for generating a digital luminance signal, a first digital chrominance signal, and a second digital chrominance signal. The signals are then sample rate reduced, interleaved, and stored in a storage subsystem (72), which may include a disk array. The storage subsystem allows a constant rate, uninterrupted data stream to be produced. To record the signals onto video tape, the signal data is read from the storage subsystem and separated. A series of digital signal processing elements, coupled to the digital luminance signal (read from the storage subsystem), are used to generate a digital filtered and frequency modulated luminance signal. A second series of digital signal processing elements, coupled to the first and second digital chrominance signals (also read from the storage subsystem), are used to generate a digital low pass filtered and amplitude modulated composite chrominance signal. These signals are combined, then converted into an analog composite recording signal compatible with a predetermined VCR standard (74). The system may also include a controlled video tape mechanism (78) which allows frame accurate recording so that a single, long program may be recorded as a series of concatenated shorter length clips.

Description: VIDEO SIGNAL PROCESSING AND RECORDING SYSTEM Background of the Invention The present invention relates generally to the processing and recording of video signals, and more particularly to the conversion, temporary storage, and recording of digitized baseband component video signals such as red, green, and blue (RGB) or any of its linear transformations (e. g., YUV or YIQ). Specialized combinations of computer hardware and software, such as QuickTime@ by Apple Computer, Inc., allow users to create and edit video movies using a combination of video, graphics, and sound data. Each frame of a movie exists in a digitized baseband component format, which allows the images to be stored and manipulated by a computer. In the case of QuickTime@ presentations, the movies are stored with a resolution of 640 pixels per line, 480 lines per frame and 30 frames per second. While the digitized format is ideal for creating and editing movie frames on a computer, this format is not compatible with video cassette recorders (VCRs) which require a composite television signal (usually NTSC or PAL) input. Because of memory and disk limitations, it is frequently difficult to store and record continuous movies of relatively long durations. As is well known to those skilled in the art, NTSC and PAL composite television signals are recorded using frequency modulation of the luminance (Y) component, which is then used as the AC bias signal for the downconverted QAM encoded chrominance components. Quadrature amplitude modulation encoding, or QAM, is the suppressed carrier amplitude modulation of two subcarriers in quadrature (I and Q for NTSC, or U and V for PAL). The signal parameters of these and other composite television standards can be found in"Report 624-4, Characteristics of Television Systems,"Reports of the International Radio Consultative Committee (CCIR), 1990, pp. 1-33. Prior practice for converting and recording digitized component video signals onto analog video tape has been to convert the digitized components into RGB analog components, encode them to form a composite television signal, and then apply this composite signal as input to a video cassette recorder (VCR). Common consumer VCR standards are VHS, S-VHS, Video-8, and Hi8. The parameters of VHS are defined in Helical-scan Video Tape Cassette System Using 12.65 mm (0.5 in) Magnetic Tape on Type VHS. IEC Standard 774, First Edition, 1983. The parameters of S-VHS (a proprietary system of JVC) are defined in a paper by Y. Nagaoka, M. Tsurata, and H. Fujiwara entitled"High Performance VTR Based on the S-VHS Format,"IEEE Trans. Consumer Electronics, 34 (3), Aug. 1988, pp. 560-565. The parameters of Video-8 are defined in Helical-scan Video tape Cassette System Using 8 mm Magnetic Tape-Video 8, IEC Standard 843, first edition, 1987. The parameters of Hi8 (a proprietary system of Sony Corporation) are defined in a paper by K. Tsuneki, T. Ezaki, and Y. Kubota entitled"Development of the High-Band 8mm Video System", IEEE Trans. Consumer Electronics, 35 (3), Aug. 1989, pp. 436-441. All four VCR systems described above require a composite television signal input which is processed for recording by separating the luminance and encoded chrominance components. These signals are then further encoded for video recording. The luminance signal is applied to an FM modulator, and the encoded chrominance is down-converted to a lower subcarrier frequency. All of the video recording systems use a frequency division multiplex approach where encoded luminance and chrominance occupy separate well defined bands, with strict requirements as to modulated signal bandwidth. Artifacts commonly associated with composite television signal encoding are cross-chroma and cross-luma (together generically called cross-color) and smear. Cross-color is caused by crosstalk between luminance and chrominance signals. This problem is often quite severe on synthetic imagery, such as graphics. Smear is caused by excessive delay between different channels, and is corrected through the use of matched analog filters and delay lines. Each stage of the process described above (analog component generation, composite television signal encoding, and video recording) is typically accomplished with analog circuitry. This analog signal processing adds unavoidable noise and distortion to the recorded video signal. In addition to the signal quality problems discussed above, the recording of computer generated movies onto video tape requires the production of an uninterrupted stream of video signal data. Currently, fast computer systems are used to minimize latency losses of data transfer from mass storage devices. For example, a random access memory (RAM) may be used as a disk cache. A microprocessor may control data transfer from the disk to the RAM, and from the RAM to an output device. The data transfers may also be executed by direct memory access (DMA) devices, in which case the microprocessor sets up each DMA before each data transfer, and keeps track of the RAM contents. In either case, a fast and therefore expensive microprocessor is required. Finally, the ability to record frame accurate material is essential to the creation of movies of arbitrary length because of the inherent limitations of most digital data storage devices. Currently, only frame accurate positioning systems allow the concatenation of a series of shorter length"clips"without noticeable processing artifacts. Accordingly, an object of the present invention is to convert and record digitized component video signals onto analog video tape without first generating the associated composite television signals. Another object of the present invention is to convert and record digitized component video signals onto analog video tape with fewer processing stages to suppress noise and distortion. Still another object of the present invention is to convert and record digitized component video signals onto analog video tape using digital processing up to the final analog video recording stage to further suppress or eliminate noise and distortion. Yet another object of the present invention is to convert and record digitized component video signals onto analog video tape avoiding cross-color artifacts associated with composite television signal encoding. Still another object of the present invention is to convert and record digitized component video signals onto analog video tape while reducing analog related"smear"by digital processing. Yet another object of the present invention is to convert and record digitized component video signals onto analog video tape while eliminating the drift, alignment, and calibration problems of a conventional analog implementation. Another object of the present invention is to convert and record digitized component video signals onto analog tape to create movies of arbitrary length. Still another object of the present invention is to relatively inexpensively render movies onto analog tape with fully synchronized audio and video. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims. Summary of the Invention The present invention includes a system for processing and recording digitized component television signals onto analog video tape. Such a system includes a digital color transformation matrix for generating a digital luminance signal, a first digital chrominance signal, and a second digital chrominance signal. The signals are then sample rate reduced, interleaved, and stored in a storage subsystem, which may include a disk array. The storage subsystem allows a constant rate, uninterrupted data stream to be produced. To record the signals onto video tape, the signal data is read from the storage subsystem and separated. A series of digital signal processing elements, coupled to the digital luminance signal (read from the storage subsystem), are used to generate a digital filtered and frequency modulated luminance signal. A second series of digital signal processing elements, coupled to the first and second digital chrominance signals (also read from the storage subsystem), are used to generate a digital low pass filtered and amplitude modulated composite chrominance signal. These signals are combined, then converted into an analog composite recording signal compatible with a predetermined VCR standard. The system may also include a controlled video tape mechanism which allows frame accurate recording so that a single, long program may be recorded as a series of concatenated shorter length"clips"without requiring a frame accurate positioning system. Brief Description of the Drawings The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the invention and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. FIG. 1 is a block diagram of a Video Signal Processing and Recording System according to the present invention. FIG. 2 is a block diagram of a prior art system for converting and recording digitized component signals onto analog video tape. FIG. 3 is a block diagram showing the signal processing operations performed by the host computer system of FIG. 1. FIG. 4 is a block diagram showing the digital storage subsystem of FIG. 1. FIG. 5 is a block diagram of the VCR waveform generator of FIG. 1. FIG. 6 is a block diagram of an alternative embodiment of the VCR waveform generator of FIG. 1. FIG. 7 is a block diagram of the analog signal processing subsystem of FIG. 1. FIG. 8 is a block diagram of the video recording mechanism of FIG. 1. FIG. 9 is a schematic diagram of a formatted, helical scan video tape. Detailed Description of the Drawings The present invention will be described in terms of the preferred embodiment. The present invention is a Video Signal Processing and Recording System (VSPRS) for the conversion, temporary storage, and recording of digitized component video signals onto analog video tape. Such a system is shown in FIG. 1. The VSPRS is referred to by general reference numeral 20. For purposes of this discussion, the use of VSPRS 20 with digitized RGB baseband component signals and NTSC standard composite television signals will be explained. However, it will be apparent to those skilled in the art that VSPRS 20 can be easily modified for use with any linear transformation of digitized RGB baseband component signals. It will also be apparent to those skilled in the art that VSPRS 20 can be easily modified for use with the PAL composite television standard. Or with further modifications, VSPRS 20 can be modified for use with other composite television standards. Also, common signal processing requirements well known in the art and disclosed in the various standards (e. g. synchronization pulses, color burst generation, signal delays, composite to component separation and demodulation, etc.), will not be discussed to avoid obscuring the present invention. To better explain a preferred embodiment of the present invention, a selected prior art system for converting and recording digitized component signals onto analog video tape will be described below. Such a prior art system is shown in FIG. 2. The prior art system shown in FIG. 2 includes three main components: a video source22 which provides digitized RGB component data representative of the frame or frames in a video movie; a composite television encoder (contained within dotted line 24); and a VCR (contained within dotted line 26). Composite television encoder 24 includes a Digital to Analog (D/A) converter 28, which receives digitized RGB component data from video source22. D/A converter 28 converts digitized RGB component data into analog RGB component data, which is used as an input by NTSC color transformation matrix 30. As is well known in the art, color transformation matrix 30 transforms analog RGB signals into YIQ color space signals, where Y, I, and Q are all functions of time and space (e. g., Y (x, y, t), and so forth), and: Y = 0.299R + 0.587G + 0.114B; I =-0.27 (B-Y) + 0.74 (R-Y); and Q = 0.41 (B-Y) + 0.48 (R-Y) Next, the I and Q signals are used to modulate a subcarrier frequency fs C, generated by an oscillator 32. In the NTSC standard, fs is approximately 3.58 MHz. I modulator 34 receives the reference signal fsc from oscillator 32, and the analog I signal from color transformation matrix 30. Q modulator 36 receives the reference signal fs (phase shifted 90 behind that received by I modulator 34) from oscillator 32 and 90 phase shifter 38. Q modulator 36 also receives as an input the analog Q signal from color transformation matrix 30. The modulated I and Q signals are generated by I modulator 34 and Q modulator 36, then added together in adder 40. Adder 40 generates a chrominance signal C, which is the phasor sum of the modulated I and Q signals. Mathematically, the modulated chrominance signal C equals: I (t) cos (2FtfSCt) + Q (t) sin (2nfset) which also equals: EMI7.1 where e is the base of the natural system of logarithms, and j is the imaginary basis. This method of modulating and combining the I and Q signals to generate a C signal is called Quadrature Amplitude Modulation (QAM). Signals Y and C are then added together in a video summer 42, which generates a total composite video signal T. It is this signal T that is then applied to VCR 26. In VCR 26, luminance filter 44 is used to extract luminance signal Y from composite video signal T, and chrominance filter 46 is used to extract chrominance signal C from video signal T. Next, luminance signal Y is modified for recording by a single-zero high pass pre-emphasis filter 48, then by frequency modulator 50, and finally by high pass filter 52, all of which are well known in the art. Chrominance signal C is downconverted (typically to a frequency of about 700 kHz, referred to as fsc') by frequency converter 54, and then filtered by band pass filter 56, as is also well known in the art. The modulated and frequency converted Y and C signals are then added together in a video summer 58, which generates a frequency division multiplexed output that drives a video recording head 60. If a video movie including a series of concatenated"clips"is to be created without fading or picture interruptions, video head 60 must be precisely turned on and off. This method of concatenating single frames requires a frame accurate positioning system. As was mentioned above, the encoding, decoding, and extensive analog signal processing used in the prior art technique adds unavoidable cross-color, smear, noise, and distortion to the recorded signal. Referring now to FIG. 1, a block diagram of VSPRS 20 according to the present invention is shown. VSPRS 20 includes six major subsystems: a host computer 70, a storage subsystem 72, a VCR waveform generator 74, and an analog signal processing subsystem 76, a video recording mechanismE78, and a system controllerE80. System controller 80 is used to monitor the status of and control the interaction between each of the subsystems within VSPRS 20. Referring now to FIG. 3, a more detailed diagram of host computer 70 is shown. In a preferred embodiment, host computer 70 processes data"off line" ; that is, the processing is not done in real time. Therefore, the time it takes host computer 70 to perform a computation has no effect on the quality of the signal generated by host computer 70. Host computer 70 receives digitized, progressive scan RGB component data signals from a video source 22. These digitized RGB data signals are then converted into digital YUV color space by digital color transformation matrix 82, according to the formulas Y = 0.299R + 0.587G + 0.114B; U = 0.493 (B-Y); and V = 0.877 (R-Y). Digital color transformation matrix 82 may be in the form of a look-up table which generates the single luminance and two chrominance signals. In an alternative embodiment, video source 22 may itself generate YUV color space signals, eliminating the need for digital color transformation matrix 82. In applications complying with the NTSC standard, the resulting Y, U, and V data stream may be a (24 bits/pixel) X (240 pixels/line) X (486 lines/frame) X (29.97 frames/Second) movie, regardless of the original movie format. This assumption greatly simplifies the timing and format choices with which VSPRS 20 must be compatible. In a preferred embodiment, the digitized RGB signals, as well as the digital YUV signals generated by color transformation matrix 82, are all sampled at the same rate. According to CCIR Recommendation 601-2, "Encoding Parameters of Digital Television for Studios" (1982,1986,1990), digitized R, G, and B signals that are destined for television service are all to be sampled at a studio standard rate of 13.5 million samples per second (megasamples/Second). This 13.5 megasamples/Second rate was chosen by the CCIR to provide compatibility between digital implementations of the PAL and NTSC television systems. The digital Y, U, and V signals generated by digital color transformation matrix 82 (or video source 22, in an alternative embodiment) are also sampled at a rate of 13.5 megasamples/Second each. This sampling rate structure, where the Y, U, and V signals are all sampled at 13.5 megasamples/Second, is usually expressed as the ratio 4: 4: 4. In an alternative embodiment, the digital Y, U, and V signals need not be sampled at the same rate, and sampling rates greater or less than 13.5 megasamples/Second may be used. For compatibility between digital implementations of the NTSC and PAL television systems, the sampling period of each signal should be a common divisor of the NTSC and PAL line durations of 63.5555 and 64.0 microseconds. Many digital implementations of broadcast television systems perform baseband chrominance bandwidth reduction to take advantage of the eye's reduced chrominance spatial bandwidth capability. Above-mentioned CCIR Standard 601 defines a basic studio standard YUV signal with co-sited samples in the ratio 4: 2: 2. That is, the Y signal is sampled at 13.5 megasamples/Second, the U signal is sampled at 6.75 megasamples/Second, and the V signal is also sampled at 6.75 megasamples/Second. The present invention is adaptable to much lower sampling rate structures. The actual sampling rate and sampling rate structure chosen is mostly a function of cost, convention, and the nature of the chrominance signals themselves. No specific sampling rate structure is required for VSPRS 20 to work for its intended purpose. The YUV sampling rate structure is modified from signals according to above-mentioned CCIR Standard 601 at a base sampling frequency of 13.5 megasamples/Second, to signals according to above-mentioned CCIR Standard 601 at a base sampling frequency of 18 megasamples/Second. In other words, the sampling frequencies for the Y, U, and V signals are changed from 13.5 megasamples/Second to 25 and 2.25 megasamples/Second, respectively. There are several methods of decimation, or sample rate reduction, known to those skilled in the art. Examples of sample rate reduction techniques can be found in Digital Television, C. P. Sandbank, Ed., J. Wiley & Sons, 1990, pp. 585592. Most such methods require low pass filtering before the signal is sample rate reduced. Host computer 70 incorporates a digital time domain low pass filter 84 to filter the Y data signal. In a preferred embodiment, digital time domain low pass filter 84 has two dimensional symmetric and preferably isotropic response for isotropic smear. Next, the filtered Y signal is sample rate reduced by sample rate reduction circuit 86 to a rate of approximately 4.5 megasamples/Second. Preferably, sample rate reduction circuit 86 performs a two dimensional sample rate reduction to approximate an isotropic response. In an alternative embodiment, the functions of time domain low pass filter 84 and sample rate reduction circuit 86 may be combined in a single time domain low pass filter and sample rate reduction circuit (not shown). Preferably, the digital Y signal is temporarily stored in Y buffer 88, which may store a frame, a field, or a line of Y signal data. The Y signal data stored in Y buffer 88 is used as an input by multiplexer 90. The 13.5 megasamples/Second U signal generated by digital color transformation matrix 82 is filtered by digital time domain low pass filter 92, which preferably has two dimensional symmetric and preferably isotropic response for isotropic smear. Next, the filtered U signal is sample rate reduced by sample rate reduction circuit 94 to a rate of approximately 2.25 megasamples/Second. Preferably, sample rate reduction circuit 94 performs a two dimensional sample rate reduction to approximate an isotropic response. In an alternative embodiment, the functions of time domain low pass filter 92 and sample rate reduction circuit 94 may be combined in a single time domain low pass filter and sample rate reduction circuit (not shown). The filtered and sample rate reduced U signal generated by sample rate reduction circuit 94 is then (preferably) temporarily stored in U buffer 96, which may also store a frame, a field, or a line of U signal data. U signal data stored in U buffer 96 is used as an input by multiplexer 90. The digital V signal generated by digital color transformation matrix 82 is processed in the same manner as the digital U signal. First, the 13.5 megasamples/Second V signal is filtered by digital time domain low pass filter 98, which preferably has two dimensional symmetric and preferably isotropic response for isotropic smear. The filtered V signal is then sample rate reduced by sample rate reduction circuit 100 to a sample rate of approximately 2.25 megasamples/Second. Preferably, sample rate reduction circuit 100 performs a two dimensional sample rate reduction to approximate an isotropic response. In an alternative embodiment, the functions of time domain low pass filter 98 and sample rate reduction circuit 100 may be combined in a single time domain low pass filter and sample rate reduction circuit (not shown). The filtered and sample rate reduced V signal generated by sample rate reduction circuit 100 is then (preferably) temporarily stored in V buffer 102, which may also store a frame, a field, or a line of V signal data. V signal data stored in V buffer 102 is used as an input by multiplexer 90. In an alternative embodiment, video source 22 may generate digital YUV signals having the proper sampling rate structure needed for data storage. In this case, time domain low pass filters 84,92 and 98, as well as sample rate reduction circuits 86,94 and 100, would not be needed. Next, the sample rate reduced baseband luminance and chrominance signals are multiplexed so they may be temporarily stored in storage device 104 (See FIG. 4). Multiplexer 90 selects when each input data from Y buffer 88, U buffer 96, or V buffer 102) will be output to form time division multiplexed signal 103. It will be apparent to those skilled in the art that any convenient multiplexing scheme may be used to interleave the Y, U and V data to be stored in storage device 104. A preferred technique for the generation of time division multiplexed signal 103, as well as the inclusion of synchronized audio information in time division multiplexed signal 103, is described more fully in U. S. Patent Application serial number 08/219,950, entitled"SYSTEM AND METHOD FOR SYNCHRONIZED PRESENTATION OF VIDEO AND AUDIO SIGNALS", by Lawrence F. Heyl and Henry N. Kannapell, filed March 30,1994, (PCT/US95/03807) and assigned to the assignee of the present invention. A copy is included herein as Appendix A, pages 20-31 below (without claims). Three appendix drawing pages are included with the present drawings. Referring now to FIG. 4, a block diagram of digital storage subsystem 72 is shown. As was mentioned above, time division multiplexed signal 103 is used as an input by storage device 104. In a preferred embodiment, storage device 104 is a mass storage device capable of a sustained data rate of about 9 megabytes/Second. By way of example, storage device 104 may be a two disk RAID (redundant array of inexpensive disks) 3 system. This is a simple striping system designed to increase the sustained transfer rate of the disks. The total size and capacity of the array used depends on the minimum length of a video segment to be recorded. Preferably, data is stored and retrieved from the disk in a scatter/gather format, which is well known to those skilled in the art. For VSPRS 20 to generate a high quality video recording, digital storage subsystem 72 must provide VCR waveform generator 74 with an uninterrupted data stream 110. The uninterrupted data stream should last at least several seconds, and more preferably tens of seconds. Discontinuities in data stream 110 due to the seek characteristics of storage device 104 are removed through the use of communicating direct memory access (DMA) controllers 106 and 107, and RAM 108. Communicating DMA controllers 106 and 107 use a small amount of auxiliary logic circuitry (not shown) to coordinate their access to RAM 108. RAM 108 acts as a buffer to temporarily hold data retrieved from storage device 104. This allows the creation of an uninterrupted data stream 110 suitable for use by VCR waveform generator 74, despite the access delays (i. e., seek time) when data is transferred from storage device 104 and RAM 108. The capacity required of RAM 108 is proportional to the seek time of storage device 104. For example, if storage device 104 has a 15 ms seek time, RAM 108 must be able to store (9 megabytes/Second) x (15 ms) = 135 kilobytes of data. Because of error processing and other system requirements, the actual capacity of RAM 108 should be higher. The generation of an uninterrupted data stream 110 of the type described above with respect to FIG. 4 is described more fully in PCT Application < serial number not yet assigned, based on U. S. Patent Application serial number 08/219,987 > , entitled"STREAMING STORAGE SYSTEM AND STREAMING DATA BUFFER", by Henry N. Kannapell, filed March 27,1995, and assigned to the assignee of the present invention. The entire contents of the above-referenced patent application are incorporated herein by reference. Referring now to FIG. 5, a block diagram of VCR waveform generator 74 is shown. Data stream 110 generated by storage subsystem 72 is used as an input by demultiplexer 112. Demultiplexer 112 generates as its outputs the sample rate reduced 4.5 megasamples/Second Y signal, and 2.25 megasamples/Second U and V signals. The Y signal is used as an input by interpolation (sample rate increase) circuit 114. The Y signal is preferably interpolated to a sample rate of between about 10 and 30 and more preferably about 18 or 27 megasamples/Second. This relatively high sample rate is chosen to minimize sampling artifacts in D/A conversion and reconstruction filtering. Similarly, the U and V signals are sample rate increased by interpolation circuits 116 and 118, respectively. The U and V signals are also preferably interpolated to a sample rate of between about 10 and 30 megasamples/Second to minimize sampling artifacts in D/A conversion and reconstruction filtering. However, the sample rate structure chosen is dependent upon the quality desired of the recorded signal, and the VCR format chosen. Preferably, interpolation circuits 114,116, and 118 perform a three dimensional (two in space and one in time) sample rate increase to approximate an isotropic response. A method for sampling a television signal appropriate for VHS video tape is described in PCT Application < serial number not yet assigned, based on U. S. Patent Application serial number 08/220,085 > , entitled"VIDEO IMAGE SAMPLING LATTICE FOR VHS FORMAT MAGNETIC TAPE APPLICATIONS", by Henry N. Kannapell, filed March 27,1995, and assigned to the assignee of the present invention. The entire contents of the abovereferenced patent application are incorporated herein by reference. Since the Y, U, and V signals have, up to this point, been processed in a progressive scan format, the interpolated Y, U, and V signals are used as an input by progressive to interlaced scan converter 120, which generates as its output interlaced Y, U, and V signals. The operation of scan converter 120 is well known to those skilled in the art. Next, the signal processing of the luminance (Y) signal will be discussed. Digital pre-emphasis filter 122 receives as an input the luminance signal Y generated by scan converter 120. The single zero high pass filter required by all VCR formats may be implemented as a time domain digital filter. The filtered luminance signal generated by pre-emphasis filter 122 is next used as an input by digital frequency modulator 124, which is used to generate an FM encoded luminance signal as required by the various VCR standards. To generate the FM encoded lumi 1991, pages 1142-1144. High pass filter 126 may also have two dimensional isotropic response. The high-pass filtered luminance signal is then used as one input by digital video summer 128. Next, the signal processing of the chrominance signals will be discussed. The prior art QAM technique described above applies U and V baseband video to a modulator along with a subcarrier to generate a signal C, which equals: U (t) cos (2nfSCt) + V (t) sin (2nfsct) which can also be represented in polar form as: EMI15.1 where fsc is the subcarrier frequency used in an NTSC (or other composite) television signal. Since VSPRS 20 directly computes the recording waveform, there is no need to amplitude modulate the subcarrier fs and down convert to fSC'Instead, the VCR standard subcarrier frequency fSc'is modulated with the U and V signals. This can be done digitally, as described below. First, U (t) and V (t) can be transformed from rectangular to polar coordinates by way of a CORDIC rotation. The CORDIC rotation was first described by J. Volder in"The CORDIC Trigonometric Computing Technique," IRE Trans. on Electronic Computers, Sept. 1959, pp. 330-334. The CORDIC rotation is a digital technique used in solving the trigonometric relationships involved in conversion from rectangular to polar coordinates. The phasor sum of U and V in polar coordinates would have two components: a length r (U, V), which equals: EMI15.2 and an angle 6 (U, V) which equals: arctan U/V. A CORDIC rotation device 130 receives as input the digital U and V signals generated by scan converter 120. The outputs r (U, V) and O (U, V) of CORDIC rotation device 130 are then further processed to generate the frequency converted C signal. The 0 signal acts as a phase offset for a conventional phase integrator frequency synthesizer 132. Phase integrator frequency synthesizer 132 generates an output signal, C (foc') which is representative of the phase and frequency of subcarrier fs c'. (D (fs c') and O (U, V) are added together in summer 134, which generates a composite phase signal. The composite phase signal and the r (U, V) signal are applied to a look-up table 136 that generates a weighted phasor output, which is the frequency converted C signal. This scheme is a reduced complexity alternative to direct implementation of an analog QAM encoder. Next, the frequency converted C signal is low pass filtered and interpolated by a digital low pass filter 138. Digital video summer 128 then receives as an input the filtered C signal generated by low pass filter 138. Video summer 128 adds the Y and C signals together, generating a digital recording waveform 140. Referring now to FIG. 6, a block diagram of an alternative waveform generator 74A is shown. Instead of using the CORDIC rotation signal processing technique described above with respect to FIG. 5, the alternative embodiment shown in FIG. 6 uses a modified digital version of the standard QAM encoding technique described above with respect to FIG. 2. Digital U modulator 142 receives the VCR standard subcarrier signal fsc' from an oscillator 144, and the digital U signal from scan converter 120. Digital V modulator 146 receives the signal fs c' (phase shifted 90 behind that received by digital U modulator 142) from oscillator 144 and 90 phase shifter 148. Digital V modulator 146 also receives as an input the digital V signal from scan converter 120. The modulated U and V signals generated by digital U modulator 142 and digital V modulator 146 are then added together in a summer 150. Summer 150 generates a digital chrominance signal C, which is the phasor sum of the modulated U and V signals. Digital chrominance signal C is then filtered by digital low pass filter 138. All other signal processing performed in the embodiment shown in FIG. 6 is identical to that described above with respect to FIG. 5. Referring now to FIG. 7, a block diagram of analog signal processing subsystem 76 is shown. Digital to analog (D/A) converter 152 receives digital recording waveform 140 from VCR waveform generator 74. D/A converter 152 generates as its output an analog recording waveform 154, which is applied as an input to analog reconstruction filter 156. Filter 156 generates as its output a filtered analog recording waveform 158. Referring now to FIG. 8, a block diagram of video recording mechanism 78 is shown. Recording mechanism 78 includes a tape drive 160, which interacts with system controller 80. Tape drive 160 may have frame accurate positioning capability. However, in a preferred embodiment, tape drive 160 is a consumer-grade drive which does not have frame accurate positioning capability. Tape drive 160 receives as one of its inputs the filtered analog recording waveform 158 generated by analog signal processing subsystem 76. System controller 80 communicates with tape drive 160 via a SYNC signal 162, a COMMAND signal 166, a RECORD ENABLE signal 168, a TIME CODE signal 170, and a STATUS signal 171. STATUS signal 171 is used to inform system controller 80 of the status of tape drive 160. COMMAND signal 166 is used to place tape drive 160 in one of the following modes: STOP, RECORD, INSERT EDIT, FAST FORWARD and REWIND. In the INSERT EDIT mode, waveform 158 may be recorded at a precise location on a video tape. Additional modes such as PAUSE may be controlled by COMMAND signal 166. With the exception of INSERT EDIT, these modes do not differ from those offered in consumer grade VCRs. Referring now to FIG. 9, system controller 80 and tape drive 160 may be used to generate a formatted video tape such as HiFi VHS videotape 172, which includes a prerecorded control track 174, a prerecorded time code track 176, unrecorded video tracks 178, and unrecorded audio tracks 180. Control track 174 and time code track 176 allow identification of individual frames on the video tape. SYNC signal 162 is output by tape drive 160 during the INSERT EDIT mode, and is used to synchronize system controller 80 with prerecorded control track 174. During INSERT EDIT mode, TIME CODE signal 170 is read by tape drive 160 from prerecorded time code track 176, and supplied to system controller 80. This allows system controller 80 to unambiguously identify each frame recorded on the video tape. RECORD ENABLE signal 168 is generated by system controller 80 and read by tape drive 160. RECORD ENABLE signal 168 is used during the INSERT EDIT mode to enable and disable recording with frame accuracy while servos (not shown) within tape drive 160 are in continuous motion. All of the signals exchanged between system controller 80 and tape drive 160 (with the exception of RECORD ENABLE signal 168) are available in midto high-end consumer grade VCR tape drives. Those skilled in the art will know how to adapt the firmware and electronics of such tape drives to provide the RECORD ENABLE capability. The RECORD ENABLE signal 168 generation and the response of the electronics (not shown) within tape drive 160 must be precise enough to begin or end recording between two predetermined adjacent frames. In the operation of video recording mechanism 78, a video tape is first formatted with control track 174 and time code track 176. Alternatively, a preformatted video tape may be used. The video tape is then rewound to a point before a predetermined frame. The servos of tape drive 160 are then activated (by COMMAND signal 166), and the time code track 176 is monitored (by TIME CODE signal 170) so that the predetermined frame can be identified. When the predetermined frame is located, RECORD ENABLE signal 168 is asserted by system controller 80, allowing analog recording waveform 158 to be recorded onto the video tape. By using this method, arbitrary length, continuous video recordings can be made by concatenating a series of shorter length video clips. The apparatus and method performed by video recording mechanism 78 is more fully described in PCT Application < serial number not yet assigned > , based on U. S. Patent Application serial number 08/219,955, entitled"METHOD FOR RECORDING FRAME ACCURATE TELEVISION SIGNALS ON MAGNETIC TAPE WITHOUT FRAME ACCURATE POSITIONING", by Henry N. Kannapell, filed March 29,1995, and assigned to the assignee of the present invention. The entire contents of the above-referenced patent application are incorporated herein by reference. In summary, VSPRS 20 allows the precise recording and editing of computer generated video information onto consumer grade video tape so that a single, long program may be recorded as a series of concatenated shorter length clips. The frequency division multiplexed signal of a VCR is directly implemented, which avoids the artifacts of composite television signal encoding. An additional benefit of digital processing is the ability to realize video filters with two dimensional isotropic response, which allows for smear compensation. This system does not require an expensive computer system for most of the digital signal processing. The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims. SYSTEM AND METHOD FOR SYNCHRONIZED PRESENTATION OF VIDEO AND AUDIO SIGNALS BACKGROUND OF THE INVENTION The present invention relates generally to multimedia computer applications, and more particularly to the synchronized presentation of signals such as video and audio signals. An increasing fraction of video presentations are computer-generated motion pictures and synchronized audio such as those produced with the QuickTime system of Apple Computer, Inc. (QuickTime is a registered trademark of Apple Computer, Inc.). Such multimedia presentations may be used for promotional, instructional or entertainment purposes. Often times, a multimedia presentation made with a comparatively expensive computer system is desired in a more ubiquitous format such as VHS consumer video tape. The present invention is an arrangement of computer hardware and software that allows video and audio information resident on the computer to be transformed to video and audio signals that may be viewed directly or recorded on video tape. For the purpose of computer processing, synchronized video and audio data are currently kept in separate files. Time codes, which are binary sequences inserted into these files, are used to indicate the timing of the video and audio data, which are synchronized by being locked to a common time code reference. Examples of time codes that can be used are Society of Motion Picture and Television Engineers (SMPTE) and European Broadcasting Union (EBU) time codes. Computer-stored video and audio presentations have heretofore been reproduced using independent video and audio interfaces as input-output (I/O) devices. The video and audio data are read from a mass storage device into random access memory (RAM) buffers in the main memory using separate I/O processes and held in the main memory at the same time. Separate I/O processes are then used to transfer the data from the main memory to the video and audio interfaces. The system must be fast enough to provide the data at rates corresponding to the sampling rates of the audio and video signals. While this approach allows the most general sort of image and sound manipulations, it is also expensive to implement due to the high processing and I/O speeds required. Accordingly, an object of the present invention is to provide a system for transferring data to video and audio interfaces using a single I/O device. Another object of the present invention is to provide a method for storing and presenting synchronized video and audio signals using and reading a single file. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims. SUMMARY OF THE INVENTION The present invention is directed to a system for providing synchronized signals. The system comprises a demultiplexer having an input and a plurality of outputs. When a composite digital data stream containing information characterizing the signals to be provided is supplied to the input, each output outputs demultiplexed digital information characterizing one of the signals. Data processors for converting the demultiplexed digital information to synchronized signals are connected to the outputs of the demultiplexer. The method of the present invention is a method for generating synchronized signals. A file containing composite digital data containing information characterizing the synchronized signals is provided. The data of the file is then supplied as a data stream to the input of a system according to the present invention such as, for example, the one described above. The data may be an interleaved sequence of samples of the synchronized signals. Because the video and audio digital data are precisely treated and composited while digital signals, a very high precision is maintained in their synchronization. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the invention and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. Figure l is a schematic block diagram of the processing of video and audio data held in separate files to obtain a composite file. Figure 2 is a diagram of the format of the composite file. Figure 3 is a schematic block diagram of a system for producing real time video and audio signals using data from a composite file. Figure 4 is a schematic block diagram of the demultiplexer of the system of Figure 3. Figure 5 is a timing diagram of the audio data outputs of the demultiplexer of the system of Figure 3. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will be described in terms of the preferred embodiment. The preferred embodiment is an apparatus and method for synchronized digital-toanalog conversion of video and audio signals. As shown in Figures 1 and 3, in the preferred embodiment of the present invention, image or video data 12 is read and composited with synchronization and blanking signals appropriate for National Television System Committee (NTSC) or Phase Alternation Line (PAL) television systems. These component video signals are then combined, as will be described below, with a serial digital audio stream derived from the sound or audio file data 14. The result is stored in a mass storage composite file 16 including video, video-sync and audio. Composite file 16 is then input to a decoder and signal processing circuit 18 (Figure 3) which separates the video data for digital to analog reconstruction and television encoding, and the audio data for digital to analog reconstruction. The analog video and audio signals produced may be displayed on standard video and audio equipment. Video data 12 and audio data 14 are band limited and held in digital form in separate files on a host computer. The data files are assumed to contain a synchronized presentation; that is, if the video and audio files are read out for use at their respective sampling rates starting at the same time, they will end at the same time. As is well known, video images are represented electronically using a temporal succession of frames. Each frame consists of a plurality of continuous horizontal lines. A video image may be sampled temporally and vertically. Such sampling is necessary in order to represent an image having continuous extent in three dimensions (horizontal, vertical and time), using signals that are continuous in only one dimension (time). For each point on a horizontal line, three signal values are necessary to render a color image. The brightness of the point may be rendered using one of the signals, termed luminance, and the color information by two signals termed chrominance. In the NTSC standard, the video signal runs at 29.97 frames per second (fps). In other countries, a frequency of 25 frames per second is used, which does not preclude the use of the present invention. The input video data file 12 is assumed to represent an NTSC (nom. 30 fps) or PAL (25 fps). motion video sequence of Luminance (Y), In Phase Chrominance (U), and Quadrature Chrominance (V) samples represented in an 8 bit linear pulse-code modulation (PCM) format sited in accord with International Radio Consultative Committee (CCIR) Recommendation 601-2 (1982-1986-1990) "Encoding Parameters of Digital Television for Studios." It should be noted that the present invention may be practiced using other color spaces and chrominance signals such as YIQ. Because the output is intended for recording on standard video ecruipment, the video data is converted to a 4: 2: 2 sampling rate hierarchy for YUV, as indicated by block 20 in Figure i, resulting in sampling rates of 4.5 MHz (Y) and 2.25 MHz (U, V). This allows an adequate bandwidth for luminance and exceptional bandwidth for chrominance for signals destined for VHS. The image data is read line by line, normalized, and composited with samples generated in software representing NTSC or PAL synchronization and blanking as indicated by block 22. The result is an aggregate 9 Megabyte/Sec data stream consisting of 8 bit YUV quantities, referenced in Figure 1 as 26,28 and 30, respectively. This quantization will achieve a nominal 49 decibel (dB) signal to noise ratio (SNR). As chrominance recording with an SNR:SNR: dB is acceptable, it is practical to discard the video information 36 and 38, held by the two least significant bits (LSBs) of the U and V chrominance channels, respectively. These bits can then be used to carry audio data. This use of the LSBs of the U and V chrominance channels reduces the composited video from linear PCM represented as 8: 8: 8 (bits/sample, YUV) to 8: 7: 7 (bits/sample, YUV). The scheme used to achieve the synchronization trades a small amount of chrominance SNR in order to insert a digital audio bit stream into the U and V chrominance channels. It can be seen that composite file 16 contains an interleaved sequence of samples for the signals to be played back in a synchronized fashion. In the preferred embodiment, audio processing hardware 78 (Figure 3) requires a serial audio data signal 80 and a serial audio synchronization signal 81 used to distinguish between data for the right audio channel and data for the left audio channel. Because these extra"serial channels"are sampled, as discussed above, at the chrominance rate of 2.25 MHz, and two 16 bit sound samples (the first for the stereo Left channel, the second for the stereo Right channel) are output alternately, the digital sound information may be sampled at 2.25 MHz/ (2 x 16) = 70.3125 kHz. This rate is not an obvious multiple of any of the accepted sound sampling rates of 48 kHz, 44.1 kHz or 32 kHz. The conversion of signals to the 70.3125 kHz sampling rate is achieved by successive applications of digital sample rate changing techniques including finite impulse response (FIR) half-band interpolation and decimation, and linear interpolation indicated generally by block 24 of Figure 1. These techniques are well known to those skilled in the art. The linear interpolation is used to precisely compute the interpolated output samples from the oversampled band limited input sequence. The precision required of the linear interpolator is a function of the synchronization precision desired. Maintaining at worst one frame time in 30 minutes, or, (for NTSC) 1/ (30 fps x 60 Sec/min x 30 min) = 1/54,000, can be easily seen to be near the limit of 16 bit arithmetic (1/65536). In the preferred embodiment, 32 bit arithmetic is used to dispense with this problem. After the sample rate conversion is achieved on the 16 bit PCM sound samples, the samples 32 (Figure 1) are read in bit serial form and composited in place of the LSBs of the U chrominance samples 28. An audio sync signal 34 that indicates whether the bits of signal 32 are for the stereo Left channel or the stereo Right channel is also generated and composited in place of the LSBs of the V chrominance samples 30. The Y, U/audio-data and V/audio-sync signals are then multiplexed as indicated by block 40 of Figure 1 and stored as a composite video/video-sync/audio-data/audiosync file 16. This file is now in a form where it can be"played back"at a 9 Megabyte/Sec rate to realize the desired video and audio. Figure 2 illustrates a possible format for the composite file. Alternating U and V bytes are separated by Y bytes, resulting in twice as many Y bytes as there are U and V bytes. Only the seven most significant bits of the U and V bytes are used to hold video information. The LSB of the U bytes carries audio-data signal information. The LSB of the V bytes carries the audiosync signal that indicates whether the LSB of the corresponding U byte has right audio channel or left audio channel information. Sixteen consecutive U LSBs are used for each audio sample, and the 16 bit sequences alternate between the left and the right audio channels. A timing diagram for three such 16 bit sequences is shown in Figure 5. Audio data signal 80 is a one bit serial signal clocked at 2.25 MHz. It is a succession of alternating 16-bit words 102 and 104 each being a sample for the stereo Left channel and the stereo Right channel, respectively. Audio sync signal 81 alternates between two values and changes every 16 bits of the audio data signal. The data processing described above, which concludes with the creation of composite file 16, does not need to occur in real time. It may be performed using a wide variety of combinations of hardware and software. In particular, it may be performed using a general purpose personal computer. In order to produce the synchronized standard video and audio signals, the composite file is presented to the output hardware 18 of Figure 3 as a 9 Megabyte/Sec stream. System 18 demultiplexes the video and audio, performs digital to analog reconstruction, and provides video encoding. The digital data stream is demultiplexed by demultiplexer 42 to provide 8 bit Luminance and 7 bit U and V chrominance, and the 2 bit serial audio stream. As noted above, the 4.5 MHz sample rate of the Y luminance signal is twice the 2.25 MHz sample rate for the U and V chrominance signals. As shown in Figure 4, demultiplexer 42 receives 32 bits of input 98 on every cycle of a 2.25 MHz clock and latches them using latches 100. The 7 bit U and V signals, and the 1 bit audio data and audio sync signals are output directly from their corresponding latches at a rate of 2.25 MHz. The two bytes of Y data are multiplexed using multiplexer 102 and output at a rate of 4.5 MHz one 8 bit sample at a time. The data output by demultiplexer 42 is delayed (not shown) in order to minimize Half Amplitude Delay (HAD) on the eventual analog outputs. Data from the several outputs is processed in parallel by separate devices. The three video data streams are applied to a digital-to-analog (D/A) converter 44 which generates a zero order hold representation of the signal. Digital to analog converter 44 may be, for example, model CXD1178Q produced by Sony corporation. The three Y, U and V analog signals 46,48 and 50, respectively, are then smoothed by low pass filters (LPF) 52,54 and 56 respectively. These filters are 5th order with a Transitional Butterworth-Thompson response. This transfer function was chosen to give an acceptable step response for the video as well as to provide good discrete time signal image attenuation in order to suppress cross-color artifacts. The U and V channel filters have a lower cut-off frequency than the Y channel, and in consequence a greater group delay; this is compensated for, as mentioned, by delaying the luminance data so the HAD between the three channels is minimal. The output of these filters are YUV analog baseband signals 58,60 and 62. The synthesis of such filters is taught, for example, in Anatol Zverev, Handbook of Filter Synthesis, John Wiley and Sons, 1967. YUV analog baseband signals 58,60 and 62 are then applied to a composite encoder circuit 64. This circuit consists of a subcarrier oscillator and a quadrature amplitude modulation (QAM) circuitry used to generate NTSC or PAL encoded chrominance. The subcarrier may be generated in a manner that is locked to the video raster if desired. This step is seen as optional as many VCRs are not phase coherent, i. e., the subcarrier-horizontal (SCH) phase is not well defined. A possible encoder is model CXA1219 available from Sony corporation. The luminance output 66 from encoder 64 is then buffered using buffers 68 and 70, and made available to drive an S-video output 76, and also for separate use such as locking a video editor. As is known, an S-video output is comprised of a separate wire for luminance with synchronization pulses and a separate wire for the QAM encoded chrominance, to reduce cross-color distortion. The chrominance output 72 is also buffered using a buffer 74 and drives S-video output 76. Optionally, the hardware may be configured to supply a composite NTSC or PAL output. The serial digital audio stream 80 and audio sync signal 81 are applied to a stereo digital-to-analog converter 78 that generates left and right analog signals 82 and 84, respectively. The digital-to-analog converter may be, for example, model TDA1543 produced by Philips BV, Eindhoven, The Netherlands. The baseband signals 86 and 88 are obtained using low-pass filters 90 and 92, and buffered using buffers 94 and 96, respectively. Buffers 94 and 96 may be used to drive an audio amplifier. The above techniques for recovering analog sound are similar to those used for Compact Disk (CD) digital audio applications. The performance of hardware 18 is strongly influenced by the layout of its printed circuit board. The actual D/A conversion hardware may be advantageously located in a small volume such as a part of a computer peripheral card. Such a card would include a digital interface comprising demultiplexer 42, video D/A comprising converter 44, audio D/A comprising converter 78, analog reconstruction filters 52,54,56,90 and 92, analog video processing 64, and regulated power to the analog components. To avoid annoying impairments due to crosstalk between the digital, video, or audio sections, rigorous ground isolation should be provided. As is well known, such ground isolation may be accomplished by using a star ground system with different ground islands. Separate voltage regulators may be used to isolate the power supplies used for the various circuit functions. By way of example, four ground islands may be provided. The digital logic 42 and the digital portion of the video D/A converters 44 may use one such island. A second island may be provided for the analog portion of the video D/A 44 and reconstruction filters 52,54 and 56. If the reconstruction filters use emitter followers, these should be positioned over the ground plane for optimum stability. The capacitors determining the response of analog video reconstruction filters 52, 54 and 56 should be positioned within a well wherein the ground plane has been removed, in order to minimize the parasitic capacitance around the capacitors. Parasitic capacitance, in particular, will change the effective capacitance in the filters, and cause the filter response to be distorted or cause the filter corner frequency to drop. The composite video encoder 64 and output drivers 68,70 and 74 may be positioned on a third ground island to decouple them from the video D/A 44 and filters 52,54 and 56. This eliminates ground return paths that could cause high frequency oscillations or ground side noise coupling. The ground planes also reduce parasitic impedances that could adversely affect the stability of the output drivers. The audio section comprising D/A converter 78 and lowpass filters 90 and 92 may be positioned on a fourth ground island, separated from the video section except at the star point, and located as close as possible to the audio output connectors. In summary, an apparatus and method for synchronized digital-to-analog conversion of video and audio signals has been described. The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims.

Claims: CLAIMS: 1. A video signal processing system for recording computer generated movies onto analog video tape, comprising: means for generating a digital luminance signal, a first digital chrominance signal, and a second digital chrominance signal, said digital luminance signal, said first digital chrominance signal, and said second digital chrominance signal compatible with a predetermined composite television signal standard, and representative of a portion, or clip, of a computer generated movie; multiplexer means, coupled to said digital luminance signal, said first digital chrominance signal, and said second digital chrominance signal, for generating therefrom a digital time division multiplexed video storage signal; digital storage means, coupled to said multiplexer means, for temporarily storing said digital time division multiplexed video storage signal; buffer means for reading said time division multiplexed video storage signal from said digital storage means, and for generating. therefrom an uninterrupted time division multiplexed data stream of at least several seconds duration; demultiplexer means, coupled to said data stream, for separating said data stream into said digital luminance signal, said first digital chrominance signal, and said second digital chrominance signal; digital signal processing means, coupled to said demultiplexed digital luminance signal, for generating therefrom a digital filtered and frequency modulated luminance signal; second digital signal processing means, coupled to said demultiplexed first and said demultiplexed second digital chrominance signals, for generating therefrom a digital low pass filtered and amplitude modulated composite chrominance signal; means for combining said digital low pass filtered and amplitude modulated composite chrominance signal with said digital filtered and frequency modulated luminance signal, and for generating therefrom a digital recording signal; digital to analog converter means for converting said digital recording signal into an analog recording signal compatible with a predetermined VCR standard; means for recording said analog recording signal onto said video tape; and means for controlling said recording means, to allow frame accurate recording. 2. The system of Claim 1 wherein said digital signal processing means includes: digital pre-emphasis filter means, coupled to said digital luminance signal, for generating a digital high-pass filtered luminance signal; digital frequency modulator means, coupled to said digital high pass filtered luminance signal, for generating a digital frequency modulated luminance signal; and high pass filter means, coupled to said digital frequency modulated luminance signal, for generating said digital filtered and frequency modulated luminance signal. 3. The system of Claim 1 wherein said means for generating a digital luminance signal, a first digital chrominance signal, and a second digital chrominance signal includes a digital color transformation matrix. 4. The system of Claim 1 wherein said second digital signal processing means includes: CORDIC rotation means, coupled to said first and said second digital chrominance signals, for computing a phasor sum of said first and said second digital chrominance signals, and for generating a first output signal, representative of the magnitude of said phasor sum, and a second output signal, representative of the phase angle of said phasor sum; phase integrator means for generating a composite phase output signal which is a combination of the phase and frequency of a predetermined subcarrier and said second output of said CORDIC rotation means; look up table means, coupled to said composite phase output signal and said first output signal of said CORDIC rotation means, for generating a digital amplitude modulated composite chrominance signal; and digital low pass filter means, coupled to said digital amplitude modulated composite chrominance signal, for generating said digital low pass filtered and amplitude modulated composite chrominance signal. 5. The system of Claim 1 wherein said second digital signal processing means includes: digital oscillator means for generating a predetermined subcarrier signal and a 90 phase shifted subcarrier signal; first amplitude modulator means, coupled to said predetermined subcarrier signal and said first digital chrominance signal, for generating a first digital amplitude modulated chrominance signal; second amplitude modulator means, coupled to said 90 phase shifted subcarrier signal and said second digital chrominance signal, for generating a second digital amplitude modulated chrominance signal; means for combining said first digital amplitude modulated chrominance signal and said second digital amplitude modulated chrominance signal and for generating therefrom a digital amplitude modulated composite chrominance signal; and digital low pass filter means, coupled to said digital amplitude modulated composite chrominance signal, for generating said digital low pass filtered and amplitude modulated composite chrominance signal. 6. The system of Claim 1 wherein said predetermined VCR standard is VHS. 7. The system of Claim 1 wherein said predetermined VCR standard is S-VHS. 8. The system of Claim 1 wherein said predetermined VCR standard is Video 8. 9. The system of Claim 1 wherein said predetermined VCR standard is Hi8. 10. The system of Claims 6,7,8, or 9 wherein said predetermined composite television standard is NTSC. 11. The system of Claims 6,7,8, or 9 wherein said predetermined composite television standard is PAL. 12. The system of Claim 2 wherein said pre-emphasis filter means is a single zero high pass filter. 13. The system of Claim 12 wherein said high pass filter means is a finite impulse response digital filter whose tap weights are a single power of two shifts and two adds. 14. The system of Claim 13 wherein said pre-emphasis filter means and said high pass filter means have two dimensional isotropic response. 15. The system of Claim 4 wherein said low pass filter means has two dimensional isotropic response. 16. The system of Claim 5 wherein said low pass filter means has two dimensional isotropic response. 17. The system of Claim 1 wherein said digital storage means includes an array of hard disks. 18. The system of Claim 1 wherein said buffer means includes a RAM and two communicating DMA controllers. 19. The system of Claim 1 wherein said means for recording is a VCR tape drive. 20. The system of Claim 19 wherein said video tape includes a prerecorded control track and a prerecorded time control track. 21. The system of Claim 20 wherein said controller means includes means for monitoring said prerecorded control track and said prerecorded time code track. 22. A video signal processing system for recording computer generated movies onto analog video tape, comprising: means for generating a digital luminance signal having a first sampling rate and a corresponding first sampling period, a first digital chrominance signal having a second sampling rate and a corresponding second sampling period, and a second digital chrominance signal having a third sampling rate and a corresponding third sampling period, said digital luminance signal, said first digital chrominance signal, and said second digital chrominance signal compatible with a predetermined composite television signal standard, and representative of a portion, or clip, of a computer generated movie; decimation means, coupled to said digital luminance signal, said first digital chrominance signal and said second digital chrominance signal, for generating therefrom a decimated digital luminance signal, a first decimated digital chrominance signal, and a second decimated digital chrominance signal, said decimated digital luminance signal having a fourth sampling rate and a corresponding fourth sampling period, said first decimated digital chrominance signal having a fifth sampling rate and a corresponding fifth sampling period, and said second decimated digital chrominance signal having a sixth sampling rate and a corresponding sixth sampling period; multiplexer means, coupled to said decimated digital luminance signal, said first decimated digital chrominance signal, and said second decimated digital chrominance signal, for generating therefrom a digital time division multiplexed video storage signal; digital storage means, coupled to said multiplexer means, for temporarily storing said digital time division multiplexed video storage signal; buffer means for reading said time division multiplexed video storage signal from said digital storage means, and for generating therefrom an uninterrupted time division multiplexed data stream of at least several seconds duration; demultiplexer means, coupled to said data stream, for separating said data stream into said decimated digital luminance signal, said first decimated digital chrominance signal, and said second decimated digital chrominance signal; interpolation means, coupled to said demultiplexed decimated digital luminance signal, said demultiplexed first decimated digital chrominance signal, and said demultiplexed second decimated digital chrominance signal, for generating therefrom an interpolated digital luminance signal having a seventh sampling rate, a first interpolated digital chrominance signal having an eighth sampling rate, and a second interpolated digital chrominance signal having a ninth sampling rate; digital signal processing means, coupled to said interpolated digital luminance signal, for generating therefrom a digital filtered and frequency modulated luminance signal; second digital signal processing means, coupled to said first interpolated and said second interpolated digital chrominance signals, for generating therefrom a digital low pass filtered and amplitude modulated composite chrominance signal; means for combining said digital low pass filtered and amplitude modulated composite chrominance signal with said digital filtered and frequency modulated luminance signal, and for generating therefrom a digital recording signal; digital to analog converter means for converting said digital recording signal into an analog recording signal compatible with a predetermined VCR standard; means for recording said analog recording signal onto said video tape; and means for controlling said recording means, to allow frame accurate recording. 23. The system of Claim 22 wherein said first, said second, and said third sampling rates are 13.5 megasamples/Second. 24. The system of Claim 22 wherein said fourth sampling rate is 4.5 megasamples/Second, and said fifth and said sixth sampling rates are 2.25 megasamples/Second. 25. The system of Claim 22 wherein said seventh, said eighth, and said ninth sampling rates are equal. 26. The system of Claim 25 wherein said seventh, said eighth, and said ninth sampling rates are between 10 and 20 megasamples/Second. 27. A method for recording computer generated movies onto analog video tape comprising the steps of : generating a digital luminance signal, a first digital chrominance signal, and a second digital chrominance signal, said digital luminance signal, said first digital chrominance signal, and said second digital chrominance signal compatible with a predetermined composite television signal standard, and representative of a portion, or clip, of a computer generated movie; generating a decimated digital luminance signal from said digital luminance signal, a first decimated digital chrominance signal from said first digital chrominance signal, and a second decimated digital chrominance signal from said second digital chrominance signal; multiplexing said decimated digital luminance signal, said first decimated digital chrominance signal, and said second decimated digital chrominance signal, to generate therefrom a digital time division multiplexed video storage signal; temporarily storing said digital time division multiplexed storage signal in a storage device; reading said time division multiplexed video storage signal from said storage device, and generating therefrom an uninterrupted time division multiplexed data stream of at least several seconds duration; demultiplexing said data stream into said decimated digital luminance signal, said first decimated digital chrominance signal, and said second decimated digital chrominance signal; interpolating said decimated digital luminance signal, said first decimated digital chrominance signal, and said second decimated digital chrominance signal, to generate therefrom an interpolated digital luminance signal, a first interpolated digital chrominance signal, and a second interpolated digital chrominance signal; generating a digital filtered and frequency modulated luminance signal from said interpolated digital luminance signal; generating a digital amplitude modulated composite chrominance signal from said first interpolated digital chrominance signal and said second interpolated digital chrominance signal; low pass filtering said digital amplitude modulated composite chrominance signal, to generate a digital low pass filtered and amplitude modulated composite chrominance signal; combining said digital low pass filtered and amplitude modulated composite chrominance signal with said digital filtered and frequency modulated luminance signal, to generate a digital recording signal; converting said digital recording signal into an analog recording signal; and recording said analog recording signal on a video tape having a prerecorded time code track and a prerecorded control track.

Inventors: HEYL LAWRENCE F, KANNAPELL HENRY N, STEVENSON DAVID K, HEYL, LAWRENCE, F., KANNAPELL, HENRY, N., STEVENSON, DAVID, K.
Applicants: APPLE COMPUTER, APPLE COMPUTER, INC.
Application year: 1995
Main classification: H04N9
IPC codes: G11B27, H04N5, H04N9