8-VSB revisited…
When the question is asked…”what is 8-VSB” most engineers (with a deer in the headlight look) say it’s that rack mounted unit that is part of DTV transmission system.
Actually it’s that rack mounted unit that converts our nice stream of 0’s and 1’s into a signal that can modulate the transmitter. When you stop and look at it…the transmitter is an AM transmitter like the ones we have been using since Guglielmo Marconi invented AM radio in the late 1800’s. Simply put an AM transmitter varies the RF output level corresponding to the rate and level of a program signal. First we have to convert our flow of digital 0’s and 1’s into something that can modulate an AM transmitter.
The process of conversion started earlier in the stream as the high definition video is compressed by a MPEG-II encoder. We had created very nice high quality video, but unfortunately, we only have a pipe of 6 MHz to send our signal through to Aunt Bessie’s television set.
After compression the MPEG-II encoder then multiplexes this compressed video information together with pre-coded Dolby AC-3 audio and any ancillary data that will be transmitted.
That created data stream (now call the Transport Stream) has a data rate of 19.39 Mbit/Sec. Quite a reduction from the over 1 Gbit/Sec of the original video and audio. As the stream begins the travel from the MPEG encoder to the 8-VSB exciter it will take on a serial form and use the protocol named SMPTE-310 interface standard.
When the stream reaches the 8-VSB exciter it under goes a great deal of rearranging. This process which includes the sections in diagram below is designed to add forward error correction which will help overcome interference and errors introduced in the signal along the path from the station antenna to Aunt Bessie’s TV set.
Because of all the information added for error correction in the above blocks, our original data package of 19.39 Mbits/sec has increased to 32.28 Mbit/sec at the output of the trellis coder.
The segment sync and field sync pulses along with the pilot allows the receiver to “lock” onto the incoming signal and begin decoding, even in the presence of heavy ghosting and high noise levels.
In the NTSC signal we had a video carrier for the receiver to lock onto, plus horizontal and vertical sync plus to aid in proper video lock. In ATSC the pilot replaces the video carrier, segment sync is somewhat like NTSC horizontal sync and field sync is roughly analogous to the NTSC vertical interval. Note, however, that unlike NTSC, the ATSC syncs do not play any role in the framing of the displayed image on the receiver picture tube. This information is encoded digitally as part of the MPEG packet address information.
The Trellis encoder also has the job of configuring the data stream into a format that can modulate the RF carrier. This is accomplished by dividing the bits in the stream into 3 bit words. Each word contains 2 bits of the payload and 1 bit of error correction.
If you recall your binary theory, a 3 bit word can represent 8 different combinations of 0’s and 1’s. 2 raised to the power of 3 = 8. These 8 combinations can represent 8 different voltage levels which then can modulate the RF carrier.
Our eight level baseband DTV signal, with syncs and DC pilot shift added, is then amplitude modulated onto an intermediate frequency (IF) carrier. This creates a large, double sideband IF spectrum about our carrier frequency, as is shown in Figure 3.
This is way too much information to transmit in 6 MHz. Since information in the upper and lower sidebands is redundant we can eliminate all but a small portion of the lower sideband. This is called Vestigial Side Band. The word vestigial means forming a very small remnant of something that was once much larger or more noticeable
Now let’s review some numbers….the gross data rate coming out of the Trellis encoder is 32.28 Mbit/sec, which includes the 19.36 payload with the remainder used for error correction overhead.
Since each of the 8 levels of modulation (now called symbols) can handle 3 bits, the resulting
symbol rate is 32Mb / 3 = 10.76 Million symbols/sec.
After some clever shaping and filtering using the Nyquist Theory the final RF carrier appears as in figure 4.
That my friend is a simplified explanation of how the 8-VSB signal is put together. Of course there are a lot of things that go on behind the scene in the various stages of creating the signal. You can find a lot of detail information including all the mathematical equations that are used it putting this together on the internet.
Check out the article “What exactly is 8-VSB anyway?” by David Sparano.
But be careful, some of it (as one engineer put it) can turn your mind to mulch!
A BIG thanks to Josh Bohn with Bohn Broadcast Services for becoming a supporting sponsor of the ABA Engineering Academy.
Visit their web site www.bohnbroadcast.com
