mythtv and digital tv

Derek Atkins warlord at MIT.EDU
Fri Jul 25 12:20:49 EDT 2008


Quoting Ben Scott <dragonhawk at gmail.com>:

>> The goal is to write mpeg2 files to the server for playback like the pvr-250
>> does via its encoder.  I'd like to believe that the term "digital streams"
>> means mpg2/aac ...
>
>  ATSC is MPEG-2 video and AC-3 audio.  I've read digital cable is
> basically ATSC with different modulation and a few extra tweaks.

Correct.  Cable uses QAM modulation (instead of VSB-8, which is what
gets used for over-the-air ATSC transmission).  The data stream is
effectively the same as you'd get for over-the-air digital, MPEG-2
transport stream.

The biggest difference is that cable companies can choose to
encrypt their QAM, which means you either need a cablebox or a
device with a cablecard.  As far as I know no device usable by
mythtv directly can use a cablecard without having yet another
Digital-Analog-Digital conversion.

So for example, for SD you could have mythtv control a cablebox and
then read it in via a PVR-250.   For HD you could effectively do
the same but use an HD-PVR box.   But if your cable company is
really nice (RCN no longer falls under this category) then they
don't encrypt your QAM and you could just plug an HD-Homerun
directly into your cable.   *sighs*

> -- Ben

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available



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