Cable bit rates for HD

[bdlucas]

I'm in the process of switching from DirecTV to cable (CableVision), so I'm paying a little attention to picture quality. Subjectively it seems fairly comparable, although I would say that while DirecTV's main failing is a sometimes soft picture, the cable picture's main failing is a sometimes noisy picture. Some of this seems to be analog noise presumably present prior to compression, while some of it is digital artifacting.

I was a little surprised to see how low a bit rate is being used for HD. Typical seems to be around 10-15 Mb/s, with 12 Mb/s being fairly common. Is this typical of cable companies? Would be interested to hear about other cable companies and other CableVision customers. And who does the compression - is it the cable head-end, or is the cable system just re-transmitting an already-compressed stream that they receive from the content provider?

I'll come back to this thread later today and post some numbers for some specific channels and programs. Would be interested in seeing some numbers from others for comparison.

[bkdtv]

Rate shaping (re-compression) dynamically re-allocates bandwidth on the fly based on channel bitrate requirements at a given instant. All providers use rate shaping on SD channels, and a number of cable providers now use it on HD channels, much like DirecTV, but with comparatively less compression.

DirecTV gets ~31.6Mbps from each DBS transponder, which they spread across three HD channels with rate shaping, for an average bitrate of 10.5Mbps each. To attain such a low bitrate, DirecTV downconverts most 1920x1080i HD channels to 1280x1080i. With SD, DirecTV spreads 12-13 channels across each transponder, for an average of 2.4-2.6Mbps each; to attain such a low bitrate, they downconvert SD channels to 480x480.

Cable companies have 38.8Mbps available per 6MHz channel slot. A number of cable providers now use rate shaping to fit three 1080i HD channels per slot for an average bit rate of ~13Mbps each. Most cable providers fit 12-14 SD channels per slot for an average bit rate of 2.8-3.2Mbps each, with resolution of 528x480.

Examples of cable providers that now use rate shaping with HD include Cablevision, Charter, Cox, and Time Warner. For the longest time, Comcast avoided rate shaping with HD, but they are now using it on some capacity-constrained 750MHz systems. As far as I know, 860MHz (and better) Comcast systems still do not use rate shaping with HD. Note rate shaping is used with every implementation of SDV.

Verizon FiOS does not use any rate shaping with HD. With Verizon FiOS, channels are passed as-is from the content provider without extra processing or compression, typically at bitrates of 17-19Mbps. Channels like Discovery Theater, Hdnet, Hdnet Movies, MTV-HD, NGC-HD, and TNT-HD are all 17.5-19Mbps on FiOS. Verizon FiOS fits 9-10 SD channels per slot with an average bit rate of 3.9-4.3Mbps each; a few channels are 704x480 while most are 528x480 or 544x480.

[slimoli]

Unless you get a very bad cable , HD will look like a real HD and not the Directv "HD Lite". SD digital will look better but many cable companies still have some analog channels. Overall, I would say nothing can look worse than Directv. I have plans to go back to D* if they really improve the picture and launch the promised 100 national HD channels but I have a feeling that they will once again maximize the transponders and continue to give us the HD-Lite.

[atmuscarella]

This subject seems to pop up on many forums now and then best I can tell there a allot of factors that determine how "good" (a very subjective term) of a picture someone gets.


The quantity of data being transmitted. More data doesn't always mean a better picture.
The quality of the data - all compression equipment/software is not equal.
The TV & cables including:

Actual overall quality of the TV & cables
The size of the TV
How well the TV has been calibrated.

The distance a person is from the TV
The eyes of the person watching the TV.


I find the bit rate info very interesting but when it comes to actually maximizing the "user experience" you have allot more control over the TV/audio/video equipment and your physical setup. Just my 2 cents.

Thanks,

[jmoak]

Comparison of DirecTV 1280x1080i to 1920x1080i (http://www.widemovies.com/directv-resolution.html)
The Big Squeeze - Are you getting all the resolution you're paying for on your satellite and cable high-def channels? (http://www.soundandvisionmag.com/article.asp?section_id=82&article_id=2061&page_number=1&print_page=y)
Not So High on High Definition - Squeezed Signals Mean Some HDTV Customers Aren't Happy With What They See (http://www.multichannel.com/article/CA6400524.html)

and just fyi, (it's data is 2 years old):

Dallas/Ft. Worth HDTV Bitrate Monitor (http://www.widemovies.com/dfwbitrate.html)

The numbers you mention are common for what I have seen.

My local PBS DT runs multiple channels at 2 to 6 Mb/s with the highest at ~12. Our local NBC is at about the same rate. We have a local wb dt that runs a single channel at ~17 Mb/s constant. Directv and Dish (reported) run from 8 to 12 with the canadian expressvu service being the highest I've ever seen broadcast at 24 to 27 Mb/s during the last superbowl and the occasional ufc fight. (11 to 17 normally)

[vman41]

For some reason, my cable company isn't encrypting UniversalHD. I was surprised to see it in the 11-13 mbit/sec range. The local digital channels it carries, also in the clear, appear to be a straight pass through with no rate shaping.

[PRMan]

I think Time Warner in my area is now worse than D*.

I just went down to the local cable office and looked at all the HD channels on their HDTV. It was easy to see that they were very pixellated and a little blurry.

[bdlucas]

Thanks for all the information. I've put together a little google spreadsheet (http://spreadsheets.google.com/pub?key=pU-T5iA3Gy7Yf5tNvhnHddQ) with some bitrates of recent programs on Cablevision.

Subjectively, I'm not sure I'd say that Cablevision quality is particularly better than DirecTV. I think Cablevision in general has a little more digital artifacting. Cablevision may be sharper, but with my oldish HDTV (vintage 2000) I can't see that, at least on HD. On SD my impression is that Cablevision is generally noisier (some digital artifacts, some analog noise) but sharper.

[cableguy763]

In Austin (TWC) the HD for non-switched channels is not compressed at all. Currently TNT HD is running at 19mbps, Showtime HD is 17 mbps, HBO HD is 15 mbps. I can go on but depending on what is being broadcast the bitrate raises or lowers. We broadcast what we receive.