[mythtv-users] Possibly a "dumb" question

[Devin Heitmueller]

On Fri, Feb 5, 2010 at 10:49 AM, Brian Wood <beww at beww.org> wrote:
> On Friday 05 February 2010 08:40:52 am Jim Morton wrote:
>> Can someone explain this to me? I just don't get it...
>>
>> Myth records shows as mpeg or whatever is being broadcast with no
>> decoding or other changes effectively capturing it unaltered.
>> During playback, Myth has a myriad of ways to process and render
>> video/audio to then deliver to the TV.
>> Why can't it just stream the exact file to the TV and let the TV tuner
>> receive/decode/render/interlace/deinterlace it. In other words hand it
>> to the TV exactly as it were broadcast.
>>
>> I believe cable boxes do just that. Provide the broadcast stream on
>> channel 3 or 4 and let the TV deal with it.
>
> If you mean a cable STV outputs ATSC, just like it would be received off the
> air? I don't think so. Those I have seen output baseband video as composite,
> S-Video or component, or RF modulated as NTSC.
>
> ATSC modulators are still expensive, much more than the entire STB, and cable
> operators are cheap.
>
>>
>> The TV should not know any difference between live OTA, live cable, and
>> a playback of a recording from Myth...
>
> Most of us use the DVI, HDMI or VGA connector to connect the frontend to the
> TV set, and nobody I am aware of outputs ATSC from their frontend.
>
> No TV set I am aware can accept ATSC as anything other than RF.

Brian is correct - HD cable boxes do not output an HD signal on the RF
output.  They will output standard definition NTSC only.  The only way
to get an HD signal out of a cable box is via the component video or
HDMI outputs (or via firewire for those boxes where it actually
works).

ATSC doesn't work like NTSC - the cable box cannot output an ATSC
signal "on channel 3" like it used to be able to for analog.

It's also worth noting that cable boxes still do quite a bit of work
transforming the signal (assuming you have digital cable).  They take
the MPEG stream, decrypt it, decode it, deinterlace it (if necessary),
rescale it, and send it out to the TV.  It is not a straight
passthrough.

Devin


-- 
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com