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|reply to Clearpix |
Re: Picture quality question Comcast vs DirecTV
said by Clearpix:It doesn't work this way. said by ComcastTech:
Standard def: Comcast is better
High Definition: Direct TV is a hair better.
Comcast HD: 12Mbps (3 HDs per QAM)
DTV and FiOS: 19.4 Mbps (2 HDs per QAM)
Hmm bad math...that would be 39% better. I think the Comcast tech is not only a bit biased, but thinks we are really stupid.
1. PQ isn't determined by bit rate. While the two are related, one number divided by the other is not the percentage of quality. D*, E*, and Uverse transmit ESPN HD at 4 mbps: Some times it looks superior, some times it looks inferior, but at no time is it consistantly "20% as good as FiOS".
2. Comcast does not allocate 12 mbps per HD channel. The "3:1 HD compression" is not "3:1", it is variable bit rate of 3 HD streams across 38.8 mbps. At any one time, a stream could be 22 mbps and another could be 5 mbps. FoxNews and CNN don't need much more, for example... not much is changing on the screen within the micro blocks. Is it "39% better PQ" if a weather map with half a woman's arm is using 19.4 mbps of bandwidth instead of 5 mbps required to deliver 1920 x 1080 pixels of clear picture by only updating those blocks that changed from the last frame?
DTV, as in over the air "Digital Television", allocates 18.3 mbps for video and audio, the rest is taken up with FEC, PSIP, and other overhead. Of that 18.3, FoX does not deliver the full spectrum to it's affiliates, never did, and tops at around 15-16 mbps. ABC and CBS may use the full spectrum, depending on affiliate, and how many secondary streams they have. If you have an accuweather or traffic/weather sub-station along with the HD, it is using less than 17.3 for the HD stream. Almost all NBC affiliates have a sub-station, with many having 2, dropping the HD stream to sub 16 bit rates.
FiOS uses "two in one QAM" for most, not all, of their HD streams. Comcast uses "two in one QAM" for a selection of it's HD streams and does not use "3:1" for all. FiOS does have more channels using a higher bit-rate, but due to it's technological ONT specifications to match "cable coax", even if an ESPN 720p non 5.1 audio stream only uses 9 mbps of bandwidth, FiOS broadcasts the stream at 19.4 mbps split into "half QAM" for compatibility, same as cable, that tuners demodulating the signal need. Each 6mhz QAM "slot" must be "filled" for the tuner to lock, either with audio+video or null zeros. Cable, same as FiOS, must fill the QAM. Comcast deployed a variable bit rate technology that "packages" channels into a mux. When it first rolled out through HITS to our local head-ends, a few of the channels "packaged" were bad choices and the hardware doing the encoding was not up to the task, many of these oversights have been fixed in the last 2 years and "Monday Night RAW" on USA has been watchable for a long time now. Choosing 3 channels that together won't supercede 38.8 mbps en masse of bandwidth, allows the three to fill the QAM instead of 2 streams + nulls and zeroes. This does not affect picture quality on MPEG2 as the micro can't be compressed further beyond the macro as it can in MPEG4, resulting in an oil slick looking digital fuzz of 8 colors for background when the foreground can't be compressed any further. It can cause buffer overrun, which results in micro and macro blocking (little white and black squares on the video) and audio feedback (tinny, fuzzy, audio errors). Video errors, not necessarily "picture quality" under the most used definition meaning how it "looks", results from these buffer overruns.
Are there channels on FiOS that result in better PQ than cable due to fixed bit rate from less macro compression? Yes. Is it all of them? No. Is it visible on any product made by Vizio, Sanyo, Pioneer, Westinghouse, Insignia, etc? No. Is MPEG4 superior to MPEG2 in "picture quality"? NO. MPEG4 is superior to MPEG2 in providing "suitable picture quality" at a lower bitrate, as defined by ITU-T Study Group 16 (VCEG) and ISO/IEC JTC 1/SC 29/WG 11 (MPEG), it is not superior in quality, it is superior in compression.
I can haz competition?