[opendtv] Re: Transition confusion
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 31 Oct 2007 14:39:08 -0400
Cliff Benham wrote:
> Gary Hughes wrote:
>>
>> 256QAM itself does not require the bitrate be reduced.
>
> No, it doesn't, but the cable companies do it to increase the
> number of programs they can provide /regardless of how they
> end up looking./
>
> The cable company I am most familiar with [I used to work
> there] 'packs' 6 or 7 standard definition programs into one
> 256 QAM 'channel' after dumming each one down to around 240
> lines of dull and blurry looking resolution. I can only recall
> them encoding two HD programs per 256 QAM channel.
Still, Gary said it right. Using 256-QAM allows for more bit rate in
each 6 MHz channel, and certainly does not degrade the image quality by
itself.
8-VSB when used over the air, i.e. 8T-VSB, gives you 19+ Mb/s in a 6 MHz
channel.
256-QAM or 16-VSB over cable, i.e. with more bits per symbol and also
much less or no convolutional FEC, gives you 38 Mb/s in that same 6 MHz
channel.
So cable systems can double the amount of DTV they transmit in 6 MHz of
spectrum, with NO loss in quality (assuming the same codec), if they so
choose.
So, two 60p HDTV programs in a single 6 MHz 256-QAM channel should look
at least as good as a single 60p stream OTA. I say "at least" because
you can use some stat mux to get more headroom in the cable example.
Typically, both HD streams not be at peak bit rate at exactly the same
time.
Similarly, 6 or 7 SD streams over 256-QAM should be at least as good as
3-4 SD streams OTA.
If they had to dumb those down, it must have been when MPEG-2 encoders
were still rather primitive. Because 3 SD programs in a single OTA 6 MHz
channel can look decent, judging from what I'm seeing these days.
Bert
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