Originally Posted by rebkell
I'm pretty sure the compression talk is about national HD channels, USA, TNT, SciFi, TLC, etc... I think the locals are still getting passed through. We do need some way to figure it out though. I can record in VMC w/clear QAM. we need a similar recording via OTA so we can compare bitrates.
Each QAM channel has a bandwidth of ~36-38Mb/s... so if you look at the SiliconDust lineup... no more than two full Stations that broadcast HD are on a single QAM channel. A good example is QAM channel 77... which houses WTCIDT 1&2, and WTVCDT 1&2. Since WTCIDT is a 1080i station, and WTVCDT is 720p... the full bitrate of both can be passed in a single QAM channel.
Typical bit rates are:
(Note - The above are full bandwidth bitrates. A station doesn't have to use all of it, or it's split up between the subchannels)
Now, a bad example is QAM channel 79, which houses WRCBDT 1&2 and WDEFDT. Since they are both 1080i stations... there has to be some compression to fit both maximum 19MB/s streams into a 36MB/s QAM channel.
Now... here's the difference between OTA ATSC broadcasts, and Cable's approach. In OTA ATSC... each station has the ability to on the fly adjust up or down the bitrate used on each subchannel as needed. Some stations that don't have a subchannel (like WDEF, I believe) tend to use the full "pipe" that the 19Mb/s allows. Other stations (like WTCI) will have to divy up the bandwidth between the subchannels to fit within the 19Mb/s "pipe". So even though WTCI is showing an HD program on the first subchannel... it's bit-starved to allow the other 3 subchannels to have their piece of the pie. Though, typically... I've seen WTCI lower the bitrate on the lesser subchannels dramatically to leave enough headroom for the main channel.
Cable stuffs 2 or 3 HD sub channels per each ~36-38Mb/s 256-QAM channel but often uses bandwidth sharing (aka statistical multiplex or "mux") to lower average bit rate. So the same broadcast that is using 19Mb/s on WDEF's OTA broadcast can be paired down to 12Mb/s (or lower) using on the fly re-encoding techniques that lower the bitrate... but don't lower the detail. Because even with OTA... not many times does the HD broadcast need the full 19MB/s that ATSC provides.
A good example of programming that doesn't need the full 19MB/s is something like the Tonight Show. Most of the screen is static... and in the MPEG-2 world you don't have to have much of any data for a static block of screen. So when Conan is sitting there talking to someone for several seconds, there isn't anywhere near 19Mb/s worth of data.
A bad example of programming that is bit-starved... is typically on CBS when they show football games, and the camera pans around the stadium. You have tons of people jumping up and down and waving around... you usually end up with macro-blocking because the data rate is so high that 19Mb/s just isn't enough.
You have to remember, the TV stations get the full uncompressed video... which doesn't have any bit-starved issues or macro blocking. When they have to compress it down to 19Mb/s (or lower if they have subchannels) you can get artifacts.
The result? OTA broadcasts use more data to *potentially* have more visual detail. Thus... you have larger DVR recordings when using OTA.
Proponents say that Cable's muxing helps because lots of time HD signals don't need the full 19Mb/s of bandwidth. Critics agree that muxing can help but they also agree that no matter how good the statistical multiplexing is, there will times when an increased number of digital artifacts will occur.
As in my CBS football example above, on the crowd-pan scene, it will need to get up to 19Mb/s for reasonable quality... and I'm sure you've all seen it when even that's not enough. The result is that if the CBS channel on cable gets the required bandwidth then the other two channels in the mux will have to split the remaining bandwidth between them resulting in poor HD quality on the other two stations in the mux.
I don't think Comcast has 3 HD channels per QAM channel... yet. At least they don't for the local affiliate feeds. Now, the other HD feeds (like the HGTV-HDs and MTV-HDs, etc) might be muxed 3 per QAM channel.
I hope this long-winded post helps.