Abstract:
A method and apparatus for improving the reception of digitally modulated signals. A main signal and a supplemental signal are provided in the transmitter. The signals may be substantially identical except that the supplemental signal is advanced in time with respect to the main signal. The main and supplemental signals are sent from the transmitter to the receiver modulated on a signal. At the receiver, the supplemental signal is stored in a buffer. If the main signal is undesirably changed during transmission, corresponding portions of the supplement signal are substituted for the undesired portions of the main signal.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application 60/(PU 010153) filed Jul. 19, 2001. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a system for improving the reception of the signal used in digital television. More particularly, the present invention is useful in mobile digital television receivers.  
         [0004]     2. Discussion of Related Art  
         [0005]     Any terrestrial TV system must overcome a number of problems in transmitting signals to a receiver. For example, the United States has adopted eight-level vestigial side band (8-VSB) modulation, as proposed by the Advanced Television Systems Committee (ATSC), as its terrestrial digital television system modulation standard. The VSB system, being a single carrier modulation system, is susceptible to fading caused by multipath and signal attenuation. Any of the signal fading that is frequency selective may be corrected by equalization techniques. However this can result in degraded performance when fading occurs. If the fade is deep, wide and long enough in duration, however, the signal will be lost and the demodulator system in the TV receiver will lose synchronization. Such fading is particularly severe in mobile reception of the signal used in digital television.  
         [0006]     The present invention seeks to overcome these problems by utilizing two sets of program material from a source in a transmitter. One of the sets is delayed in time with respect to the other. Thus, if the delayed set is used for reception and fading occurs, the set that is advanced in time can be substituted for the faded or missing portion of the signal.  
         [0007]     While the detailed description of the current invention below focuses on the details of the 8-VSB system, it must be recognized that the solution of the current invention is equally applicable to any digital broadcast transmission system that is subject to a fading channel environment.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with principles of the present invention a method and apparatus for improving the reception of digitally modulated signals operates as follows. A main signal and a supplemental signal are provided in the transmitter. The signals may be substantially identical except that the supplemental signal is advanced in time with respect to the main signal. The main and supplemental signals are sent from the transmitter to the receiver modulated on a signal. At the receiver, the supplemental signal is stored in a buffer. If the main signal is undesirably changed during transmission, corresponding portions of the supplement signal are substituted for the undesired portions of the main signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic diagram of a VSB transmitter incorporating the principles of the present invention.  FIG. 1  includes sub  FIG. 1A  having an MPEG Encoder and  FIG. 1B  having a hierarchical source encoder;  
         [0010]      FIG. 2  is a schematic diagram of a VSB receiver incorporating the principles of the present invention; and  
         [0011]      FIG. 3  is an illustration of groups of video packets received by the receiver wherein a fade has occurred during transmission. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring to the drawings and more particularly to  FIG. 1A , a schematic diagram of a transmitter incorporating the principles of the present invention is shown. The transmitter operates in accordance with the provisions of the Advanced Television Standards Committee (ATSC) Digital Television Standard dated Sep. 16, 1995, which is incorporated herein by reference. The digital television system includes three sections namely a source encoding and compression section a transport multiplexing section and an RF/transmission section.  
         [0013]     The source material is applied on an input conductor  10  to an MPEG encoder  20  which provides the source encoding and compression, typically in accordance with MPEG standards, e.g. MPEG-2. The source material can include video and audio signals, for example, which are encoded in the encoder  20  into a digital data stream. The encoding can utilize known bit rate reduction methods and compression techniques which are appropriate for the particular signals involved. The compressed data stream provided from the encoder  20  is divided into packets of information, each packet including data identifying that packet.  
         [0014]     Also in accordance with the principles of the present invention, a second encoder  30  is provided for the source material  10 . In the encoder  30  the source material is encoded into a digital packet data stream in the same manner as in the encoder  20 . However the output from the encoder  30  is applied on a conductor  31  to a packet buffer  32  which delays the data stream from the encoder  30  in time with respect to the output signal from the encoder  20 . The output signal from the encoder  20  is identified as the supplemental signal while the output of the encoder  30  is identified as the main signal.  
         [0015]     The output from the encoder  20  is applied on a conductor  21  to a first input of a transport multiplexer  40  and the output from the packet buffer  32  is applied to a second input of the transport multiplexer  40 . Additional data signals (not shown) could also be applied to the multiplexer  40 , for example, control data to be utilized in the DTV receiver. The data streams supplied to the transport multiplexer  40  are multiplexed into a single data stream by the transport multiplexer  40 .  
         [0016]     The output of the multiplexer  40  is channel coded and modulated by the channel coding section  50 , the symbol mapping section  60 , and the mixer  70  utilizing the carrier oscillator  80 . These circuits also insert the various “helper” signals that will aid the 8-VSB receiver in accurately locating and demodulating the transmitted RF signal. These include the ATSC pilot tone, segment sync, and frame sync components.  
         [0017]     The main signal, as it is transmitted, is shown in  FIG. 3  as  310  and runs from “A” to “Z”. The alphabetic sequence represents the time ordered sequence of video packets. The supplemental signal, as it is transmitted, is shown in  FIG. 3  as  300  and runs from “a” to “jj”. In the embodiment illustrated in  FIG. 3 , the supplemental sequence is advanced in time by more than  6  packet times, and more specifically, is illustrated in  FIG. 3  as being advanced by 10 packet times.  
         [0018]     In accordance with the principles of the present invention, the method of transmitting two separate substantially identical signals, shifted in time is identified as “staggercasting”. Thus,  FIG. 3  represents a staggercasted transmitted signal.  
         [0019]     The main stream  310  of information and the supplemental stream  300  of information can be identical except for information in each packet to identify them. However in order to conserve channel bandwidth, the main stream could contain data representing video and/or audio at “full resolution” while the supplemental stream would contain reduced resolution data.  
         [0020]     Instead of using the encoders  20  and  30  as shown in  FIG. 1A  it is possible to also use a hierarchical coding method to supply the main and supplemental channels, as illustrated in  FIG. 1B . The main channel  310  would be supplied with all the components but the supplemental channel  300  would have only the high priority components.  
         [0021]      FIG. 1B  shows the source material being applied via the terminal  10 ′ to the hierarchical source encoder  20 ′. The output on the conductor  21 ′ is the supplemental, time-advanced, stream  300  while the output on the conductor  31 ′ is the main stream  310 . Note that the main stream  310  is delayed in the packet buffer  32 ′. In this embodiment, the supplemental channel would have only the high priority information on conductor  21 ′ while the main stream would include both the high priority information from conductor  21 ′ and the low priority information from conductor  31 ′ as combined in the multiplexer  33 . The supplemental output from the hierarchical source encoder  20 ′ is applied to a first input of the transport multiplexer  40  while the output from the buffer  32 ′ would be applied to the second input of the transport multiplexer  40 , as shown in  FIG. 1A . Otherwise the transmitter functions are identical.  
         [0022]     The use of hierarchical source coding permits the high priority data to appear in both the main and supplemental channels while all the low priority data is also available only in the main channel. Images transmitted by such a system could be displayed on mobile devices such as personal digital assistants equipped with VSB demodulators.  
         [0023]     Referring now to  FIG. 2 a  schematic diagram for a VSB receiver incorporating the principles of the present invention is illustrated. In the 8-VSB transmitted signal, the digital information is transmitted exclusively in the amplitude of the RF envelope and not in the phase. The eight levels of the transmitted signal are recovered by sampling only the I-channel or in-phase information.  
         [0024]     In the receiver shown in  FIG. 2 , the transmitted signal is demodulated by applying the reverse principles that were applied in the transmitter. That is the incoming VSB signal is received, downconverted, filtered and then detected. The segment and frame syncs are recovered. This is accomplished by the mixer  100 , the local oscillator  101 , the lowpass filter  102 , the analog to digital converter  103 , the mixer  104  and the carrier recovery circuit  106  as well as the interpolator  107  and the symbol timing recovery circuit  108 , all in a known manner.  
         [0025]     The output of the interpolator  107  is applied to the equalizer  110 . The segment sync signal aids in the receiver clock recovery while the field sync signal is used to train the adaptive equalizer  110 . The output of the equalizer  110  is applied to a forward error correction circuit (FEC)  120 . The error corrected signals provided by the forward error correction circuit  120  are applied to and utilized in the transport demultiplexer  130 . The output from the transport demultiplexer  130  includes both the supplemental stream signals on conductor  131  and the main stream signals on conductor  132 . Under normal circumstances, the main stream signals are applied directly to the stream select circuit  140  while the supplemental signals are applied to a packet buffer delay circuit  150  which has a delay that matches the time period by which the supplemental signal is advanced in the transmitter. Accordingly the two streams applied to the stream select circuit  140  are now aligned in time.  
         [0026]     The stream select circuit  140  normally is conditioned to pass the main stream signals to the MPEG decoder  160 . If, however, a fading event occurs in the received VSB signal signal, then the main stream signals will be degraded, possibly to the point of being unusable. If the main stream signals become unusable, then the stream select circuit  140  will be conditioned to pass the buffered supplemental stream signals to the MPEG decoder  160 . This is determined by the error detection circuit  121  connected to the outputs of the forward error correction circuit  120  and the transport demultiplexer  130 .  
         [0027]     The occurrence of a fading event can be detected by a number of possible measures in the physical layer. For example, a signal-to-noise ratio detector (SNR) may be used. This would be detected as a change in amplitude of the processed main signal. As another example, it is possible to use a bit-error rate detector. In yet another example, it is possible to use the undecodable error flag indication from the forward error correction system. When the circuit  121  determines that the main signal is corrupt it instructs the stream select circuit  140  to utilize the supplemental channel data.  
         [0028]     The use of the supplemental data will continue until either the data in the buffer  150  is exhausted, or the receiver recovers and the main channel is restored to a predetermined quality threshold. It is evident that to be prepared for another fade in the main stream signal, once the receiver recovers it must stay recovered long enough to permit the supplemental packet buffer  150  to refill. The delay introduced into the main signal must be long enough to cover the expected time duration of fading events while not taking a long time period to recover from such fading events. In a preferred embodiment, the time delay introduced to the main signal by the packet buffer  32  or  32 ′ in the transmitter and the packet buffer delay  150  in the receiver may be selected to be between around 500 ms and a few seconds.  
         [0029]     Also shown in  FIG. 2  is a block representing a display processor and display device  180  which receives the output of the MPEG decoder  160  and develops decoded image data for an onscreen display image to be displayed on the display device, and decoded sound data to be reproduced on a speaker, in a conventional manner.  
         [0030]     Referring now to  FIG. 3 , an illustration is provided of the staggercasting principles in a packet stream.  FIG. 3  is a time diagram with the groups of video and/or audio packets representing the supplemental sequence ( 300 ) being advanced in time with respect to the main sequence ( 310 ) and, as noted above, running from “a” to “jj”. It can be seen that the supplemental channel  300  illustrated in the upper portion of the diagram is advanced in time by a time period “T adv ” of roughly ten packets in this example.  
         [0031]     The main channel  310  is represented by the packets “A” to “Z” in the lower portion of the diagram where packet A in the main channel  310  corresponds to packet a in the supplemental channel, packet B in the main channel corresponds to packet b in the supplemental channel, and so forth. In  FIG. 3  the first ten packets in the main channel  310  are indicated as zero since this is the time period by which the main channel  310  is delayed in the transmitter. This is the time period during which packets “a” to “j” are loaded into the buffer  150  in the receiver prior to the reception of the first corresponding packet “A” in the main stream  310 . One skilled in the art will understand, however, that the main stream  310  may contain main packets corresponding to preceding packets in the supplemental channel.  
         [0032]      FIG. 3  shows an example of a complete fade of the VSB signal in its transmission from the transmitter to the VSB receiver. The fade begins at time t 1 , and ends at time t 2 . After the fade, however, the circuitry in the receiver requires recovery time to resynchronize its clock to the received signal and reacquire error correction lock. This recovery time begins at time t 2 , after the fade ends, and continues until time t 3 . The illustrated fade in the packet sequences, thus, causes the loss of six packets from both the main  310  and supplemental  300  channels. That is, in the main channel, packets H-M are lost: packets H, I, J are lost due to the fade and packets K, L, M are lost due to the demodulator and FEC recovery; and in the supplemental channel, packets r-w are lost for the same reasons.  
         [0033]     However, it may be seen that, supplemental packets h-m, corresponding to main packets H-M, were received from time t 4  to time t 5 , before the fade began and, therefore, are stored in the packet buffer  150 . Because the supplemental packet sequence  300  has been advanced by more than 6 packets, which is the duration of the exemplary fade and reacquisition, the supplemental sequence h-m can be read from the packet buffer  150  when the main sequence H-M is lost due to the fading event.  
         [0034]     The system is vulnerable to fades until the supplemental buffer  150  is repleted. This is because both the main and supplemental streams (and any others in the transport stream) were lost in the fade. More specifically, from time t 6  to t 7 , the receiver receives main packets R-W. However,. as described above, the corresponding supplemental packets r-w were lost during the fade. Thus, there are no supplemental packets stored in the packet buffer  150 , and no protection for fades is available, for this time period. Full protection is available again after time t 7 . Additional supplemental streams, advanced by different time periods, could be used to ride out multiple close successive fades at the expense of consuming more bandwidth.  
         [0035]     Also shown in  FIG. 3  are shadings, which help to identify the processing of respective packets in the main and supplemental streams. The packets shaded as illustrated by shading  301  are the packets decoded by the MPEG decoder  160  at the receiver. The packets shaded as illustrated by the shading  302  are packets that are lost due to the loss of signal in transmission. The packets shaded as illustrated by the shading  303  are packets that are lost due to receiver re-acquisition while the unshaded boxes (shading  304 ) are packets that are available in either the main or the supplemental channels, but not decoded by the MPEG decoder  160 .  
         [0036]     The concept of using a supplemental signal to contain information to be processed during a fade event provides the same quality or a graceful degradation of the image. A lower quality supplemental signal requires lower throughput and less bandwidth to transmit than the full resolution main signals, but the lower quality image from the supplemental signal is slightly degraded from the full resolution image of the main signal. It is also conceivable to use a signal staggered in time of the same quality and even with a different compression format.  
         [0037]     It is clear that the method and apparatus incorporating the principles of the present invention as described above helps to correct some of the weaknesses in the VSB system or any other modulation system that is susceptible to fading in a transmission channel. The VSB system is a single carrier modulation system and accordingly is susceptible to fading caused by multipath and signal attenuation. The use of the equalizer corrects many frequency selective fades but this is at the expense of increasing noise in the bands when actual fading occurs. If the fade is deep, wide and long enough in duration the modulator system can lose synchronization and the signal will be lost.  
         [0038]     In accordance with the principles of the present invention, by having an advance copy of the program material in memory, it is possible to continue demodulating by switching to the advanced (supplemental) transport system. Thus the demodulator will continue to try to recover and If the fade is of modest duration the main stream will come back on line before the stored advance stream is exhausted. When the main program packets are available, the decoder will resume demodulating the mainstream and begin buffering the advanced packets of the supplemental stream awaiting the next disruption in the received signal.  
         [0039]     The described method and apparatus are particularly useful for mobile reception of the VSB signal. It is evident that mobile receivers are prone to severe fading as the receiver is moved through different areas. This can cause interruption of the received signal. As noted above, the apparatus and method according to the principles of the present invention provide a means of graceful degradation of this received program under temporary loss of signal due to fading.  
         [0040]     This approach utilizes the transmission of a synchronously encoded, optionally reduced resolution, advanced set of program material from the same source, called the supplemental signal. The technique is applicable to any streaming data but is directly useful for video and audio since lower resolution material could be used to conserve bandwidth. As also noted above, this system could be particularly useful to users of wireless personal digital assistants and entertainment digital assistants.  
         [0041]     While the present invention has been described with respect to a particular embodiment and a particular illustrative example it is evident that the principles of the present invention may be embodied in other arrangements without departing from the scope of the present invention as defined by the following claims.