Abstract:
A time-based, digital FM demodulator circuit receives a stream of digital samples corresponding to an analog FM waveform. The samples are provided to a zero crossing detector, which allows a counter to determine a number of clock cycles between zero crossings. The resolution of this coarse period determination is further refined by an intercept calculation, which further localizes the zero crossing of the FM waveform based on interpolation between samples on either side of the zero crossing. Accuracy of the period determination may be further enhanced by use of a sinusoidal correction filter, which minimizes error caused by the linear interpolation performed on the sinusoidal waveform. Although the FM demodulator circuit is particularly suitable for demodulation of the chroma component of a SECAM video signal, it may advantageously be applied in a wide variety of FM demodulation applications.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention: 
   The invention generally relates to video decoders and more specifically to composite video decoders and FM demodulators for use in decoding the color information of a SECAM (Sequential Couleur Avec Mémoire) composite video signal. 
   2. Description of Related Art 
   There is a large surge in the use of digital video devices today. Examples include: digital televisions, LCD (Liquid Crystal Display) TVs (televisions) and monitors, DVD (digital versatile disk) recorders, personal video recorders, PC (personal computer) video cards, video capture and streaming applications, and video conferencing. In many cases, these units need to receive an analog video signal, which may be one of the composite signals, such as NTSC (National Television Standards Committee), PAL (Phase Alternating Line) or SECAM; S-video; component video or RGB (Red-Green-Blue). It is then desirable to produce the proper digital output, such as eight or ten bit ITU-R (International Telecommunications Union-Radio Communication) BT (Broadcast Television) 656. It is preferred that all the video decoding be done in a single chip for all of these formats. The decoder not only has to handle composite signals, which means it must be able to determine the chroma and luma values, but it also must handle vertical blanking interval (VBI) data and handle VCRs (video cassette recorders), which frequently have unstable timing signals. 
   Although a number of such systems have been developed, it is always desirable to improve the output and capabilities of the particular video decoder. For example, it is desirable to support as many video formats as possible while maintaining a certain level of circuit simplicity, both for cost and reliability purposes. Unlike NTSC and PAL composite video, SECAM video includes color information that is frequency modulated. Thus to be able to decode SECAM video, a video decoder must include an FM demodulator. Historically, phase locked loops (PLLs) have been used for FM demodulation. However, these components are susceptible to various forms of electrical noise, and further add to the cost and complexity of the video decoder circuits. 
   Therefore a need exists in the art for a means of performing time-based FM demodulation to eliminate the need for the PLL. It would be further advantageous to perform the demodulation in the digital domain, so as to reduce the susceptibility of the circuitry to various forms of noise and error. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a time-based, digital FM demodulator circuit. The FM demodulator receives a stream of digital samples corresponding to an analog FM waveform. These samples are provided to a zero crossing detector, which allows a counter to determine a number of clock cycles between zero crossings. The resolution of this coarse period determination is further refined by an intercept calculation, which further localizes the zero crossing of the FM waveform based on interpolation between samples on either side of the zero crossing. Accuracy of the period determination may be further enhanced by use of a sinusoidal correction filter, which minimizes error caused by the linear interpolation performed on the sinusoidal waveform. Although the FM demodulator circuit is particularly suitable for demodulation of the chroma component of a SECAM video signal, it may advantageously be applied in a wide variety of FM demodulation applications. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  displays a block diagram of an exemplary personal video recorder using an analog video decoder according to the present invention. 
       FIG. 2  is a block diagram of an analog video decoder according to the present invention. 
       FIG. 3  is a block diagram depicting a composite decoder portion of an analog video decoder according to the present invention. 
       FIG. 4  illustrates a prior art analog time-based FM demodulator. 
       FIG. 5  illustrates a digital time-based FM demodulator according to the present invention. 
       FIG. 6  illustrates a frequency modulated waveform and a digitally sampled version of the FM waveform to better understand certain teachings of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an exemplary personal video recorder (PVR)  100  is shown. PVR  100  is an exemplary use of analog video decoder  102 , and it is understood that the analog video decoder can be used in multiple applications including digital televisions, LCD TVs, DVD recorders, video capture situations, and the like. A radio frequency (RF) or broadcast signal is provided to a tuner  104 . The tuner  104  provides both video and audio outputs. The video output from the tuner  104  or a video signal from an external connection is provided to analog video decoder  102 . The audio signal from the tuner  104  or an external audio signal is provided to an audio decoder  106 . The output in the analog video decoder  102  is preferably an ITU-R BT 656 format digital signal, which may be either an eight or ten bit signal. This digital signal from analog video decoder  102  is provided to an MPEG (Motion Picture Experts Group) codec (coder-decoder)  108  to perform video compression in the digital domain. Similarly, the audio decoder  102  provides a PCM (Pulse Code Modulated) signal to the MPEG codec  108  to allow MPEG codec  108  to perform compression of the digital audio signal. The MPEG codec  108  in output mode provides an ITU-R BT 656 digital stream to an analog video encoder  110 , which in turn produces an analog video signal output. Similarly, the MPEG codec  108  provides a PCM digital signal stream to an audio encoder  112 , which provides an analog audio signal output. 
   The MPEG codec  108  is connected to a host bus  114  of a host CPU (Central Processing Unit)  116 . The host CPU  116  performs processing operations and controls the various devices located in the PVR  100 . The host CPU  116  is connected to flash memory  118  to hold its program and RAM (Random Access Memory)  120  for data storage. The host CPU  116  also interfaces with a front panel  122 . A hard drive interface  124  is also connected to the host bus  114 , with a hard drive  126  connected to the hard drive interface. The various decoders  102  and  106  and encoders  110  and  112  are also connected to the host bus  114  to allow control and setup by the host CPU  116 . 
   In operation, audio and video signals are provided to the analog video decoder  102  and the audio decoder  106 , which then provide their digital streams to the MPEG codec  108 . The host CPU  116  programs the MPEG codec  108  to transfer data to the hard drive interface  124 , and thus to the hard drive  126 , for storage. The host CPU  116  could at a later time direct data to be transferred from the hard drive  126  to the MPEG codec  108  for playback. Thus an analog video decoder  102  is an important part of such analog-to-digital video devices. 
   A block diagram of an exemplary analog video decoder  202  is shown in  FIG. 2 . The video signal is provided to an external capacitor  202 , and is then provided to a clamp, buffer, automatic gain control (AGC) and sample and hold (S/H) block  204 . Block  204  provides clamping of the video signal to ensure that the video signal does not exceed limits, impedance buffering and line driving, and automatic gain control and sample and hold. The output is then utilized by an analog-to-digital converter (ADC)  206  which does the actual analog-to-digital conversion of the video rate signals. The ADC  206  is preferably operated on a sample clock, which is a free running sample clock and is not locked to the source video in the preferred embodiment. In alternate embodiments, a source locked clock signal could be used. The output of the ADC  206  is provided to an anti-aliasing/decimation filter  208  because preferably, the ADC  206  oversamples the video signal for increased accuracy. The anti-aliasing portion is a low pass filter used to remove sampling alias effects. The decimation filter then reduces the effective sample rate down to the desired rate, such as 27 MHz. The output of the anti-aliasing/decimation filter  208  is provided to a composite decoder  210  in the case of a composite video signal such as NTSC, PAL or SECAM. The composite decoder  210  separates the luma and chroma signals and provides these signals to a digital output formatter  212 , which produces a 4:2:2, eight or ten bit signal according to the ITU-R BT 656 standard. 
   The output of the analog-to-digital converter  206  is also provided to a low pass filter  214  which removes any of the video content, leaving the sync signals. The output of the filter  214  is then provided to a sync detector  216 , having outputs that are horizontal and vertical sync signals. The low pass filter  214  output is also connected to a clock generator  218 , which is effectively a PLL and produces a source locked clock used by other devices, if appropriate. 
   One portion of video decoder  102  is composite decoder  210 . One advantageous aspect of a composite decoder is the ability to demodulate SECAM signals. SECAM (Sequential Couleur Avec Mémoire) is a composite video standard generally used in France, some French-speaking countries, as well as Russia and some former Soviet republics. Details of SECAM and other composite video systems are generally known to those skilled in the art, and thus aspects not critical in the context of the present invention are not repeated here. 
   SECAM is similar to the PAL standard used in most countries other than the United States and Japan. In SECAM video, the line, field, and frame timing is identical to the PAL standard. However, unlike PAL, SECAM chroma (color) information is frequency modulated (FM) rather than phase modulated (PM). An FM modulated signal is simply a signal that will change in frequency by the rate of the modulating signal. 
   All color video signals comprise three color components: red, green, and blue. These colors are the three primary colors of light, and any color can be made by some combination of these three colors. SECAM brightness (luma) signal Y is simply a weighted sum of the red, green, and blue components. The luma signal alone can be displayed as a monochrome image, i.e., black and white television. SECAM color information comprises two color difference signals Dr and Db. The Dr signal is a scaled difference between the red signal and the luma signal. The Db signal is a scaled difference between the blue signal and the luma signal. Thus the original red, green, and blue components can be readily derived from the luma signal Y and the color difference signals Dr and Db. 
   The color difference signals Dr and Db are FM modulated on a 4.43 MHz center carrier ±approximately 200 kHz. Thus to provide for demodulation of SECAM signals, an FM demodulator is required. Furthermore, it is advantageous to provide the capability to perform FM demodulation without the addition of substantial additional circuitry and/or logic to a video decoder. 
   One technique for accomplishing this objective is to use the time-based FM demodulation apparatus and technique of the present invention. In the case of SECAM video signals, the bandwidth of the Dr and Db signals is limited to approximately 1.2 MHz. This relatively limited bandwidth means that adequate samples of the carrier are available to compute the zero crossing change of the carrier (and hence FM demodulate). This advantageously allows FM demodulation to be performed using the time base correction module that is already present in a video decoder. Although described in terms of demodulating SECAM video signals, the present invention is not so limited, and may advantageously be used to demodulate any FM signal. 
   The SECAM composite video signal is composed of luma information “Y” (brightness) and chroma information “C” (color). With reference to  FIG. 3 , a sampled (digitized) version of the SECAM composite video signal  301  is input into composite decoder  210 . Specifically, the signal is input into YC separator  302 , which subtracts the luma information from the composite signal. This luma signal Y is one output of composite decoder  210 . The color signal C, which remains after the luma signal is separated, is passed into chroma demodulator  303 . Chroma demodulator  303  will separate the chroma signal C into its two components Dr and Db. Alternating lines of the SECAM signal will include either the Dr or Db signal. Thus a first line will be encoded, for example, with the Dr color information, and a next line will be encoded with the Db color information. 
   Chroma demodulator  303  is essentially an FM demodulator, with a commutation switch so that the demodulated color difference signal can be output to the correct Dr or Db line depending on which color difference signal is present for the current line. A typical prior art time-based FM demodulator used in many DSP applications is illustrated in  FIG. 4 . An FM modulated signal  401  is input into a high gain amplifier  402 . The output of high gain amplifier  402  saturates from virtually any input, creating a square wave  403  at its output, with the square wave having a period, i.e., time between zero crossings, identical to the FM modulated signal  401 . 
   This square wave  403  serves as the input to counter  404 . Counter  404  also has as an input high frequency clock  410 . Typically high frequency clock  410  is about 128 times faster than period of the square wave. The number of counts output from counter  404  are used to determine the period of the square wave by period calculator  405 . This calculated period is inverted by period inverter  406  to determine a frequency. The determined frequency is input into summing module  407 , which subtracts the carrier frequency f s . The determined frequency minus the carrier frequency is the demodulated signal  409 . 
   A time-based FM demodulator according to the present invention is illustrated in  FIG. 5 . The digital samples of the FM signal  501  are input into a zero crossing detector  502 . Zero crossing detector  502  creates a signal that will change state when the digital sampled data passes through zero (i.e., changes sign). The zero crossing “trigger” signal is input to counter  506 , which is driven by clock  507 . For purposes of the video demodulation application described herein, the clock is driven at 27 MHz; however, the clock may be driven at any appropriate frequency for other applications. In any case, the clock signal and zero crossing pulses input into counter  506  allow the counter to determine the number of clock cycles between zero crossings, and thus coarsely determine the frequency. The frequency is only coarsely determined because the resolution is limited to the resolution of clock  507 . However, the present invention further provides for additional resolution enhancement of the frequency determination. 
   The digital samples of FM signal  501  are also input along path  503  to intercept calculator  504 . The zero crossing trigger signal generated by zero crossing detector  502  is also input into intercept calculator  504 , causing it to store the sample value prior to the zero crossing and the sample value post zero crossing. This operation may be further understood with reference to  FIG. 6 . The analog FM modulated sine wave  601  is represented as a series of digital samples, indicated by waveform  602 . The sampled values are input into the zero crossing detector  502 . When zero crossing detector  502  detects the sign change between sample  603  (a positive value) and sample  604  (a negative value), the output signal of the zero crossing detector  502  causes the intercept calculator to retain the sample values  603  and  604  for further processing. 
   Intercept calculator  504  performs an interpolation between the samples  603  and  604  to approximate the actual intercept  605  of the FM waveform. The calculated intercept indicates what fraction of a cycle of the demodulation clock  507  should be subtracted from the count determined by counter  506  to further refine the demodulated frequency. By calculating the intercept, and reducing the integer clock count by this fractional value, the effective resolution to signal period is significantly enhanced. Furthermore, this enhanced resolution is achieved without the need to increase the sample frequency as typically done in prior art time-based FM demodulators. 
   In a preferred embodiment, the calculated intercept determined by interpolation by intercept calculator  504  may be further refined by sin(x) corrector  505 . As noted above, intercept calculator  504  determines the location of intercept  605  by linear interpolation between samples  603  and  604 . However, the actual analog signal is a sine wave, not a linear function. The deviation between the linear function used in the interpolation and the actual value of the intercept of the sine wave may impose additional error on the order of several percent, which introduces noise into the signal. Therefore sin(x) corrector  505  takes advantage of the edge rates and pre-emphasis of SECAM signaling to reduce the error of intercept calculation and reduce the noise in the signal. In a preferred embodiment, this correction is performed by a lookup table arrangement, although other techniques will be apparent to those skilled in the art. 
   The total period (and thus frequency) of the FM waveform is then determined at  508  by taking the number of clock pulses counted by counter  506  and subtracting the fraction of a clock pulse indicated by the intercept calculation and optional sin(x) refinement. Because video data is continuous, it is necessary to determine the moving period of the FM signal, which is performed by operational block  509 . At this point, the FM demodulation is substantially complete. However, additional post processing based on known characteristics of the SECAM video signal may be applied to further reduce noise. For example, limiter  510  may be applied to limit the data ranges and reject spurious frequencies by checking the computed period based on expected signal profiles of SECAM signaling. The limiter will reject unexpected values (based on SECAM signal profiles) further reducing noise and decode errors. Additionally, infinite impulse response (IIR) low pass filter  511  may be used to de-emphasize certain parts of the signal that are emphasized as part of standard SECAM signaling. This filter  511  may also be used to integrate the FM signal, which further reduces noise in the system. 
   Thus by providing a mechanism for digital time-based demodulation of the chroma component of a SECAM composite video signal, the error/noise performance of a video decoder may be substantially enhanced while reducing the cost and complexity of the video decoder. While illustrative embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.