Air Versus Cable Part 1
I want to take a few minutes to talk about invisible signals in the air, and in part 2, what QAM looks like. It’s impossible to “see” RF signals with the naked eye, so we rely on test equipment to show us what is going on in the high speed RF spectrum. You may know by now TV stations in the United States use a system called 8-VSB to deliver digital TV to you homes. Each channel can use around 19.4 million bits per second to deliver digital TV to your home. I have some screen captures from my Sencore 1850 MPEG analyzer that shows you what the signal looks like.
This is a representation of the 8-VSB signal. The yellow dots on the black square in the upper right hand corner shows the carriers lined up in 8 columns. Each one of those dots should line up fairly straight, in order for your receiver to decode the signals contained there. The squiggles in the bottom right corner displays the mathematical relationships the carriers have to each other. If anything gets out of line, the receiver can’t lock onto the signal enough to decode whats contained in it. When the signal bounces off a building, tree, or other object, it will have an affect on this signal, either adding or subtracting to the main signal. The receiver can get confused if any of these carriers get sloppy, and aren’t in a close line with the others. Remember, there’s a lot more data contained in the signal than just video and audio. There’s many tables and error correction bits sent to ensure the receiver has the best opportunity to decode the signal.
Above is an overall view of the tables that make up an over the air DTV signal.
Here you can see a chart of the actual bitrates on the air. The top bar on the graph is for null packets, These are basically empty packets of data to fill in the full bandwidth allotted. As you can see, I am not wasting very many bits on these things, and have most of my resources allocated to the main streams. The second bar shows how much the data tables take up.
Our HD stream is running at 14.35 million bits per second and is taking up almost 74% of my bandwidth. Universal Sports is running at 3.1 million bits per second, and Weather Plus is only slightly over 1 million since it is mainly still graphics with little motion.
Now this is our reference, or non Olympic bitrates. (You should have known I was going to mention the Olympics eventually)
In years past, we have shut down the Weather Sub Channel and throttled down the second channel to the bare minimum.
This time, Universal Sports will be carrying many live events, so I have to crank up the bitrate for it as high as I can. We just installed a new receiver for Universal Sports, so we able to capture the full bandwidth they are sending on the satellite, which is just under 5 million bits per second. However, to get 2 more megabits, I have to steal it from somewhere else, or one of my other channels. I can get about half a million from the Weather Plus channel, and pull another 1.5 million from the main HD channel.
Naturally, I don’t want to reduce the HD bandwidth any more than I have to. The less bits you have, the more compression you have to do. In a high speed sporting event, that can be less than desirable.
The system uses statistical multiplexing, which means the streams can be shared in a pool, and bits are assigned to the stream that needs it the most. That way, when one channel has a scene with little motion, the extra bits can be used on the higher motion stream. In other words, the HD will always have top priority, and get all the spare bits required to deliver the best picture from the other channels which may not have a high demand for bits. The other channels get what is left over, which will still render a good picture.
I will be changing the bitrates very close to the Olympics. The Opening Ceremonies are amazing to see and use every amount of detail the cameras can capture. If you’ve never seen the Opening Ceremonies in HD before, you are in for a real treat!
If you enjoyed this post, please consider leaving a comment or subscribe to the feed and get future articles delivered to your feed reader or email.
Subscribe to RSS
Very interesting and informative. Thanks!