[mythtv-users] Possibly a "dumb" question

[Brian Wood]

On Friday 05 February 2010 09:34:33 am Robert McNamara wrote:
> On Fri, Feb 5, 2010 at 8:24 AM, Brian Wood <beww at beww.org> wrote:
> > On Friday 05 February 2010 09:14:58 am Eric Sharkey wrote:
> >> On Fri, Feb 5, 2010 at 10:55 AM, Robert McNamara
> >>
> >> <robert.mcnamara at gmail.com> wrote:
> >> >> 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.
> >> >
> >> > That would require an QAM modulator
> >
> > Actually an ATSC modulator, though the two are very similar, and the same
> > device could probably do both, perhaps with a firmware change.
> 
> Actually, if you're attempting to pass through a cable signal
> unmolested, you would use a like-type modulation.  Since ATSC and QAM
> have different bitrate ceilings, remodulating a signal received via
> QAM as ATSC (with its lower bitrate limit) would necessarily require a
> transcode in at least some cases.  So yeah, passing through a cable
> signal to your TV would require a QAM modulator.

A cable system could theoretically just heterodyne an off-air ATSC signal to 
another frequency and put it on the wire, then use an STB to simply mix it 
down to channel 2 or 3, all the information would be untouched, and the TV set 
wouldn't know the difference.

But you would not be able to have things like OSDs, or even volume control in 
the STB.

But off-air channels are in the minority on cable systems, and they really 
don't care about them since they can't insert commercials, and sometimes even 
have to pay for the retransmission rights.

Cable companies hate the off-air channels, for a lot of reasons.

Heterodyne processors were the standard way of handling off-air NTSC channels 
for decades, they worked quite well, but didn't generate revenue.