[mythtv-users] Getting 1080p from comcast cable boxes (Was VUDU)

[Brad Templeton]

>> Note that when providing movies, 1080p is no particular technical
>> challenge.   Movies are shot at 24fps, and doing them at 60 fields/second
>> interlaced is actually a terrible shame, though that is what is done
>> on a the broadcast and many cable/satellite channels.
>>
>> I was quite pleased to learn that some of the cable channels,
>> such as AMC-HD, provide their movies at 1080 p, 30fps.   This is
>> good because it means a smaller file and no deinterlacing required
>> to watch or transcode.
>>
>>     
>
> Well, it's actually 1080*i* at 60 fields per second/30 frames per
> second.  Obviously no way for me to show that your local headend isn't
> doing deinterlace themselves, but AMC HD, at least, broadcasts in
> 1080i.
>
>   
>> The comcast cable box spits out mp2 over the firewire.  Does anybody
>> know if Comcast actually sends mp2 over thier QAM channels, or
>> do they send mp4 and then have the cable-box up-transcode it to mp2
>> to fit the spec (and then I re-transcode it down to mp4 again if I
>> will not be watching it for a while, sigh
>>     
>
> The entire system is in MPEG-2.
>
> Robert
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>   
The AMC-HD recordings I get out of firewire are definitely progressive
(or at least identified as such by mythtv) and at 30 fps.

I am curious as to how you know that AMC-HD broadcasts in 1080i?   I
don't doubt it could be true, but it seems rather odd since the
alternative makes a lot of sense.  AMC-HD does nothing but movies which
are 24fps.   The cable and satellite companies, if they are able to
receive it, would be well served to get 1080p 24fps from the source
channels, since that is the easiest thing for them to mogrify into
whatever format they want to use, even for them to do the telecine to
get 1080i.   It's the easiest thing to transcode, to downres etc. and it
makes a smaller bitstream than a telecine'd video.   However, since
24fps is not a standard rate, I think, for many devices, I can see them
doing that at 30fps, which does not make a much bigger mpeg2 since the
dup frames take up very little space.

You suggest AMC-HD sends out 1080i to Comcast, and they inverse telecine
it and send it out over QAM as 1080p, and then have their cable boxes
convert it to 1080i and 720p to drive HDTVs?    Could be true but makes
a lot less sense than the other.

Of course, we myth users would love it if they just gave us 24fps files
of movies.  mpeg4s even better.   I am told the satellite companies use
mpeg4 natively.


I have notice a fair bit of difference in the bitrates used by various
stations.  OTA ATSC stations tend to run 11 to 13 megabits of mpeg2.
My local PBS, sadly, does 1080i at 9 megabits, which is way too low, and
it looks terrible.  (They try to air 3 other SD channels on the same
multiplex.  Greedy.)

AMC-HD takes about 9 megabits and looks good (because it is 30fps
movies).   SciFi HD which I record a lot seems to run as high as 16
megabits.

I have also seen 1080p 30fps on Universal HD.   FXHD seems to air in
720p.  I guess we should make a chart of these.