Abstract:
A method and apparatus for splicing a first information stream into a second information stream to produce an output information stream containing compressed digital data. The first and second information streams are synchronized using a common timing reference, thereby avoiding temporal discontinuity errors in an information stream decoder.

Description:
The U.S. Government has certain rights in this invention pursuant to Contract No. 70NANB5H1174. 
    
    
     The invention relates to communication systems in general, and more particularly, the invention relates to a method and apparatus for synchronizing a plurality of compressed data streams to facilitate stream selection and other operations. 
     BACKGROUND OF THE DISCLOSURE 
     There are several major television networks providing national news and other programming to local or regional affiliate stations. These affiliate stations receive a National Television Systems Commission (NTSC) network feed via, e.g., a satellite link and synchronize the NTSC feed to a local NTSC synchronizing, or “genlock” signal. The term “genlocking” refers to the process of synchronizing one or more signals, or the equipment producing the signals, to a studio timing reference. The NTSC television signal includes synchronizing pulses which delineate horizontal lines of video and, therefore, allow for simple calculations of video field and frame locations within the NTSC signal. Thus, the affiliate is able to align the video and audio information on, e.g., a line by line, field by field, or frame by frame basis. The affiliate station may then easily insert (“splice in”) advertising or local programming into the received signal. The resultant spliced signal, which includes network programming and locally inserted material, is then transmitted via, e.g., terrestrial broadcast. 
     In several communications systems, the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172. The second, known as MPEG-2, refers to ISO/IEC standards 13818. 
     The proposed use of such compressed data by a network and its affiliate poses several technical challenges since MPEG-based video and audio synchronization is not as straightforward as NTSC-based synchronization. Moreover, the affiliate station must be able to subject an MPEG data stream to operations like genlock or splicing (e.g., adding commercials to programs) prior to re-transmission by the affiliate station. 
     There is considerable expense involved in performing the above synchronization, splicing and other operations on a compressed (e.g., MPEG) data stream. This is because the compressed network feed or other stream must be decoded, processed and re-encoded prior to transmission to a local user. In addition to the expense, this approach also degrades the signal quality of the network fed video and audio because the steps of decoding and encoding are not entirely lossless (i.e., portions of the data representing an image may be ignored or truncated during at least the encoding process). 
     Therefore, a need exists in the art for a cost-efficient method and apparatus for applying splicing and other processing techniques to information contained in a compressed data stream. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for processing a compressed information signal includes a timing source for providing a plurality of synchronized timing information signals to one or more compressed information processors. The information processors operate on respective compressed information signals in a synchronous manner such that it is not necessary to compare baseband or elementary information signals when performing signal splicing or insertion operations. 
     More specifically, an HDTV pass-through station is disclosed in which a synchronizing source provides a plurality of timing signals derived from a common frequency source. The timing signals are used to synchronize an MPEG-compliant network feed signal to MPEG-compliant live feed signals and stored feed signals such that switching or splicing amongst the three feed signals may be performed at a transport level, thereby avoiding baseband or elementary stream manipulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a block diagram of a high definition television (HDTV) pass-through station according to the invention; and 
     FIG. 2 shows a block diagram of a studio timing source suitable for use in the pass-through station of FIG.  1 . 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described within the context of a high definition television (HDTV) pass-through station (e.g., a network affiliate station) which is operative upon MPEG-compliant information streams, including video and audio information streams. The invention provides a cost effective solution for designers of such pass-through stations. However, it must be noted that the embodiment of the invention described herein may be modified for use in other compressed data systems where, e.g., it is desirable for a plurality of information processors to operate on respective compressed information signals in a synchronous manner such that it is not necessary to compare baseband or elementary information signals when performing signal switching, splicing or insertion operations. 
     The MPEG standards address the timing and synchronization issues for decoders of MPEG data streams (e.g., video, audio, data, and the like) as follows. A sample of a 27 MHz referenced clock is transmitted in a program clock reference (PCR) field of a transport stream packet. The PCR indicates a time when the transport decoder is expected to have completed reading the PCR field. This information may be used by a far-end decoder, such as a high definition television receiver, to synchronize a system clock within the receiver. That is, the phase of the local clock running at the far-end decoder is compared to the PCR value in the bit stream at the instant at which the PCR is obtained to determine whether the decoding process is synchronized to ensure accurate decoding and decompression of the data stream. In general, the PCR from the transport stream does not directly change the phase of the system clock of the decoder, but only serves as an input to adjust the clock rate. 
     FIG. 1 shows a block diagram of a HDTV pass-through station  100  according to the invention. The pass-through station  100  receives a compressed information signal from a television network (i.e., a network feed) and inserts or “splices in” advertising or local programming into the received signal. The resultant spliced signal, which includes network programming and locally inserted material, is then transmitted via terrestrial broadcast to an end user, e.g., an Advanced Television Systems Committee (ATSC) receiver or HDTV receiver. 
     Specifically, a studio timing source  200  provides a 27 MHz system clock SYSCLOCK, an 8.07 MHz transport data clock TDATCLOCK and three video synchronizing signals; a horizontal H synchronization signal, a vertical V synchronization signal and composite CSYNC synchronization signal. The studio timing source  200  may be used to provide other clock and synchronizing signals, e.g., the 19.93 MHz or 2.4 MHz clocks favored by some equipment manufacturers. It is important to note that all of these clock and synchronization signals are locked to a common reference frequency source (e.g., 27 MHz) within timing source  200 , as will be described below with respect to FIG.  2 . 
     A network feed signal S 1 , including an MPEG-compliant information stream, is transmitted by a satellite  123 , received by an antenna  125  and demodulated by a satellite signal demodulator  130 . The demodulated network feed signal S 3  comprises, e.g., an MPEG-compliant transport stream that has been encoded at the network transmitter (not shown) using one or more known error correction or other encoding schemes (e.g., randomization, Reed-Solomon, Trellis encoding and the like). The demodulated network feed signal S 3  and the clock signal TDATCLOCK are coupled to a receiver interface  135 . Receiver interface  135  utilizes the transport data clock TDATCLOCK to extract an MPEG transport stream S 4  from the demodulated network feed signal S 3 . The receiver interface  135  may comprise, e.g., a small first-in, first-out (FIFO) memory synchronized to a stable studio timing reference, illustratively, the TDATCLOCK. This arrangement allows the satellite feed to be synchronized to the studio reference. 
     The extracted MPEG transport stream S 4  comprises the programming stream that is to be retransmitted (along with any locally-inserted program material or commercials) by the pass-through station. The extracted MPEG transport stream S 4  includes a program clock reference (PCR) that was ultimately derived from the common reference frequency source in the timing source  200 . Thus, the extracted MPEG transport stream S 4  representing the network feed is synchronized at the transport level to the studio timing source  200 . 
     A camera  105  receives “live” image information (e.g., a live news program) via a lens  105 L. The camera also receives the horizontal H and vertical V synchronization signals, which are used to format the image information to produce a synchronized image signal S 5 . A video encoder  110  receives the synchronized image signal S 5  and the clock signal SYSCLOCK and produces a video elementary stream S 6 L. A tape machine  107  is used to play tape containing, e.g., recorded images produced by camera  105 . The tape machine  107  receives composite synchronization signal CSYNC to synchronize an output elementary stream S 6 T. A transport encoder  125  receives the two video elementary streams S 6 L, S 6 T and the clock signal SYSCLOCK, selects one of video elementary streams S 6 L, S 6 T for encoding, and produces an MPEG-compliant “live feed” transport stream S 7 . The live feed stream S 7  may be in any video format (e.g., 1125 line 30 Hz interlaced, other HDTV, NTSC and the like). Since the transport encoder  115  utilizes the clock signal SYSCLOCK, the timing information (i.e., PCR) of the resultant “live feed” transport stream S 7  is synchronized at the transport level to the studio timing source  200 . 
     A play-to-air server  140  (e.g., a video disk, tape machine, or other storage device) stores advertisement and/or local programming. The advertisement and/or local programming information is stored as elementary of packetized elementary video and audio information. The play-to-air server  140  receives the clock signal TDATCLOCK and in response to a control signal PLAY from a controller  125 , produces an MPEG-compliant “stored feed” transport stream S 8 . Since the play-to-air server  140  utilizes the clock signal TDATCLOCK, the timing information (i.e., PCR) of the resultant “stored feed” transport stream S 8  is synchronized at the transport level to the studio timing source  200 . 
     A play-to-air switcher (i.e., splicer)  120  receives the MPEG transport streams representing the studio feed S 4 , the live feed S 7  and the stored feed S 8 . As previously discussed, these three streams S 4 , S 7 , S 8  have been transport-encoded using clock signals which are derived from a common frequency source. The play-to-air switcher is responsive to a control signal CONT from controller  125  to select one S 9  of the streams S 4 , S 7 , S 8  for subsequent storage and broadcast. It is important to note that the selection of a transport stream in this manner is only possible because the available transport streams S 4 , S 7 , S 8  are already synchronized, or “genlocked,” to the studio reference clock. This means that the PCR time stamps in the respective streams S 4 , S 7 , S 8  are the temporally aligned (i.e., respective PCR reference packets arriving simultaneously at switcher  120  will be identical). 
     An off-air recording server  150  receives the selected stream S 9  and the clock signal TDATCLOCK. The record server  150  includes a transport stream decoder (not shown) that utilizes the clock signal TDATCLOCK to decode the selected stream S 9  to a packetized elementary stream level for storage on a digital storage medium. A vestigial sideband (VSB) transmitter  155  also receives the selected stream S 9  and the clock signal TDATCLOCK. The VSB transmitter  155  encodes the selected stream S 9  using, e.g., data randomization, Reed-Solomon encoding, interleaving and Trellis encoding. The encoded signal is then modulated using known VSB modulation techniques to produce a VSB modulated signal S 10  for broadcast by an HDTV transmitter  160 . Since the off-air recording server  150  and VSB transmitter  155  utilizes the clock signal TDATCLOCK, the timing information included in the recorded and broadcast signals will be synchronized. 
     The pass-through station  100  also includes an HDTV to NTSC converter  165  that scan converts the selected stream S 9  to an NTSC signal S 11 . An NTSC transmitter modulates the NTSC signal S 11  using known techniques to produce an NTSC modulated signal S 12  for broadcast by an NTSC transmitter  175 . The NTSC modulated signal S 12  and VSB modulated signal S 10  are broadcast simultaneously. The simultaneous broadcast is only required during an NTSC to HDTV transitional period mandated by the Federal Communications Commission (FCC), the U.S. government regulatory agency responsible for radio-spectrum usage. 
     In the pass-through station  100  of FIG. 1, the play-to-air switcher  120  selects one of the three signals available streams S 4 , S 7 , S 8  as stream S 9  for subsequent storage and broadcast. For example, the network feed stream S 4  may include a 20 minute television program which is transmitted in several predetermined portions over a 30 minute time period. The non-program portions (i.e., the 10 remaining minutes) are the portions of the 30 minute time period during which the local affiliate may insert commercials or other information. During the program portions of the 30 minute time period, the network feed S 4  is coupled to the transmitters as selected stream S 9 . During the non-program portions of the 30 minute time period, the stored feed S 8  or the live feed S 7  may be coupled to the transmitters as selected stream S 9 . The controller  125  receives the system clock signal SYSCLOCK, determines the appropriate time to switch and, via control signal CONT, causes the switcher  120  to select the desired stream. 
     The controller  125  optionally receives a monitor signal MON which allows the controller  125  to monitor the data streams received by switcher  120 . For example, the controller may be used to determine the format of the network feed signal S 4  and responsively provide a signal FORMAT indicative of the determined format. The format indicative signal FORMAT may be used by, e.g., a synchronization generator device which will be described in more detail with respect to FIG.  2 . 
     It is important to note that the selected stream S 9  is, in effect, a multiplexed transport stream comprising portions of, e.g., the network feed stream S 4  and the stored feed stream S 8 . The multiplexed stream S 9  will not be decoded at the receiver properly if the time stamps of the network S 4  and stored S 8  streams are not synchronized. The improper decoding produces a timing discontinuity at the switching points in the stream that may cause the decoder to improperly present several frames as timing circuitry within the decoder slews from, e.g., PCR data contained in the network feed S 4  to PCR data contained in the stored feed S 8 . Some decoders sense this timing error and simply produce a “blank” output signal until the decoder timing circuitry adjusts. This condition is not desirable. 
     In the pass-through station  100  of FIG. 1, the above-described discontinuity error is avoided by “genlocking” the three streams S 4 , S 7 , S 8  to a common frequency source prior to switching. This “genlocking,” or synchronization, means that the selected stream S 9  may be decoded properly (i.e., without discontinuities) in a transport decoder even after a switching operation that combines multiple streams produced by different sources. Moreover, the switching operation is greatly simplified since the main criterion for switching is simply the “wall clock” time for inserting a commercial. 
     The arrangement of FIG. 1 allows a satellite feed to be synchronized to a studio reference. Prior art arrangements required that a satellite feed be decoded to an elementary stream or baseband signal, time-base corrected and coupled to a switcher along with a number of other time-base corrected baseband signals. The selected signal would then be re-encoded and transmitted. This process was cumbersome, expensive and tended to degrade the quality of the selected video or audio signal. The invention avoids this problem. 
     FIG. 2 shows a block diagram of a studio timing source  200  suitable for use in the pass-through station of FIG.  1 . Timing source  200  includes a stable reference frequency source  210  (e.g., a 27 MHz source) having an output coupled to a first buffered splitter  220 . The buffered splitter receives a 27 MHz signal from the source and provides a plurality of buffered 27 MHz signals at respective outputs. Several buffered 27 MHz output signals (illustratively, two) are coupled to timing source outputs as SYSCLOCK signals. The 27 MHz clock frequency is the standard system clock frequency for an MPEG system. Of course, in non-MPEG systems, the reference clock frequency may be other than 27 MHz. 
     One of the buffered 27 MHz output signals is coupled to a frequency converter  230 , illustratively a phase locked loop (PLL) type, which produces an 8.07 MHz output signal. The 8.07 MHz output signal is buffered by a second buffered splitter  240 . Several buffered 8.07 MHz output signals (illustratively, two) are coupled to the timing source outputs as TDATCLOCK signals. The 8.07 MHz clock frequency is the standard clock frequency used for transferring transport packets from one device to another (e.g., from one server to another or from a transport encoder to a splicer). 
     One of the buffered 27 MHz output signals is coupled to a frequency converter  250 , illustratively a phase locked loop (PLL) type, which produces an 74.25 MHz output signal. The 74.25 MHz output signal is buffered by a third buffered splitter  260 . A buffered 74.25 MHz signal is coupled to a synchronization generator  270  that generates HDTV horizontal H, vertical V and composite synchronization CSYNC signals and couples these signals to the timing source outputs. The synchronizing signals H, V, CSYNC are generated by counting down the buffered 74.25 MHz signal (i.e., pixel clock signal). The synchronization signals H, V, CSYNC are suitable for use by, e.g., an HDTV camera or tape machine. 
     It should be noted that by using a 74.25 MHz pixel clock, the synchronization generator  270  of timing source  200  produces synchronization signals H, V, CSYNC that are appropriate for use in the 1125 lines, 30 Hz interlaced HDTV format. Synchronization signals for other HDTV formats or conventional television formats may be generated by changing the output frequency of the frequency converter  250 . This change may be made in response to a signal FORMAT which causes the frequency converter  250  to change its output signal frequency. This signal may be provided by, e.g., the controller  225  in the pass-through station  100  of FIG.  1 . 
     The invention has been described within the context of a high definition television (HDTV) pass-through station (e.g., a network affiliate station) that is operative upon MPEG-compliant information streams, including video and audio information streams. The invention provides a cost effective solution for designers of such pass-through stations. For example, a common frequency source is used to derive all the timing signals required by such a station. Moreover, the use of this common frequency source allows the “genlocking” or synchronizing of a network feed carrying MPEG-compliant programming, a server storing MPEG-compliant commercials and a commercial inserter for inserting the commercials into the network feed for subsequent broadcast. In this manner, timing discontinuities are avoided and the station, therefore, does not need to retime the combined signal prior to broadcast. Also, by locking all the output clocks together, the above-described genlock arrangement advantageously prevents an accumulation of jitter in a signal being coupled from one piece of equipment to another (e.g., from a server to a splicer to a VSB modulator). Furthermore, the genlock arrangement allows for control and synchronization of an HDTV camera or tape machine and the insertion of the live or taped information into the network feed. 
     However, it must be noted that the embodiment of the invention described herein may be modified for use in other compressed data systems where, e.g., it is desirable for a plurality of information processors to operate on respective compressed information signals in a synchronous manner such that it is not necessary to compare baseband or elementary information signals when performing signal splicing or insertion operations. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.