WHAT EXACTLY IS 8-VSB ANYWAY? by David Sparano
The USA invented it, the ATTC tested it, the FCC accepted it, everyone is talking about it, soon we will all get it in our homes - but what is 8-VSB anyway? Simply put, 8-VSB is the RF modulation format utilized by the DTV (ATSC) digital television standard to transmit digital bits over the airwaves to the home consumer. Since any terrestrial TV system must overcome numerous channel impairments such as ghosts, noi bursts, signal fades, and interference in order to reach the home viewer, the lection of the right RF modulation format is critical. The 8-VSB format is the cornerstone upon which the DTV standard is bad; developing a basic understanding of 8-VSB is imperative for tho who will be working around DTV in the future.
In the alphabet soup world of digital communications, there are two big names to remember when thinking about the complete DTV system: 8-VSB and MPEG-II. 8-VSB is the RF modulation format and MPEG-II is the video compression/packetization format. To convert high definition studio video into a form suitable for over-the-air broadcast, according to DTV
standards, two stages of processing are needed: MPEG-II encoding and 8-VSB modulation. Accordingly, two major pieces of equipment are required: an MPEG-II encoder and an 8-VSB exciter.
The MPEG-II encoder takes baband digital video and performs bit rate compression using the techniques of discrete cosine transform, run length coding, and bi-directional motion prediction - all of which are beyond the scope of this article. The MPEG-II encoder then multiplexes this compresd video information together with pre-coded Dolby AC-3 audio and any ancillary data that will be transmitted. The result is a stream of highly compresd MPEG-II data packets with a data frequency of only 19.39 Mbit/Sec. This is by no means a trivial task since the high resolution digital video (or multiple standard resolution video) input to the MPEG-II encoder could easily have a data rate of 1 Gbit/c or more. This 19.39 Mbit/c data stream is known as the DTV Transport Layer. It is output from the MPEG-II encoder and input to the 8-VSB exciter in rial form, according to the SMPTE-310 interface standard.
烤鱿鱼Although MPEG-II compression techniques can achieve stunning bit-rate reduction results, still more tricks must be employed to squeeze the 19.39 Mbit/c DTV Transport Layer signal into a slender 6 MHz RF channel and transmit it (hopefully without errors) to the eager consumer waiting at home in front of the TV t. This is the job of the 8-VSB exciter.
Figure 1 is a block diagram of a typical 8-VSB exciter. In this article, we will walk through the major process that occur in the 8-VSB exciter - identifying the major components of the 8-VSB signal and explaining how this signal is generated.
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FIGURE 1: BLOCK DIAGRAM, 8-VSB EXCITER
DATA SYNCHRONIZATION 旋钮式换挡>烧公鸡的做法
The first thing that the 8-VSB exciter does upon receiving the MPEG-II data packets is to synchronize its own internal circuits to the incoming signal. Before any signal processing can occur, the 8-VSB exciter must correctly identify the start and end points of each MPEG-II data packet. This is accomplished using the MPEG-II sync byte. MPEG-II packets are 188 bytes in length with the first byte in each packet always being the sync byte. The MPEG-II sync byte is then discarded; it will ultimately be replaced by the ATSC gment sync in a later stage of processing.
另一位研究者
DATA RANDOMIZER
大山的图片With the exception of the gment and field syncs (to be discusd later), the 8-VSB bit stream must have a completely random, noi-like nature. This is becau our transmitted signal frequency respon must have a flat noi-like spectrum in order to us
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e the allotted channel space with maximum efficiency. If our data contained repetitious patterns, the recurring rhythm of the patterns would cau the RF energy content of our transmitted signal to "lump" together at certain discrete points in our frequency spectrum, thereby leaving holes at other frequencies. This implies that certain parts of our 6 MHz channel would be overud, while other parts would be underud. Plus, the large concentrations of RF energy at certain modulating frequencies would be more likely to create discernible beat patterns in an NTSC television t, if DTV-to-NTSC interference were experienced.
In the data randomizer, each byte value is changed according to known pattern of pudo-random number generation. This process is reverd in the receiver in order to recover the proper data values.
REED-SOLOMON ENCODING
Reed Solomon encoding is a Forward Error Correction (FEC) scheme applied to the incoming data stream. Forward error correction is a general term ud to describe a varie
ty of techniques that can be ud to correct bit errors that occur during transmission. Atmospheric noi, multipath propagation, signal fades, and transmitter non-linearities may all create received bit errors. Forward error correction can detect and correct the errors, up to a reasonable limit.
The Reed-Solomon encoder takes all 187 bytes of an incoming MPEG-II data packet (the packet sync byte has been removed) and mathematically manipulates them as a block to create a sort of "digital thumbnail sketch" of the block contents. This "sketch" occupies 20 additional bytes which are then tacked onto the tail end of the original 187 byte packet. The 20 bytes are known as Reed-Solomon parity bytes
