[mythtv-users] OT: ATSC/QAM Encode to distribute HD over longer runs.

Chris Ribe chrisribe at gmail.com
Sat May 10 00:17:54 UTC 2008


On Fri, May 9, 2008 at 7:33 PM, Dan Ritter <dsr-myth at tao.merseine.nu> wrote:

> On Fri, May 09, 2008 at 07:16:51PM -0400, Chris Ribe wrote:
> >
> > This is true, but it would be much more difficult to provide multiple
> > streams from a single backend if you were feeding ATSC streams.  Without
> the
> > ability to have independent frontends, such a system would be of limited
> > appeal.  Also, consider that very few TVs have multiple ATSC tuners, so
> you
> > would need to work around the problem of still allowing those TVs to
> watch
> > off ait TV.
>
> It's just a transport for video and audio; the frontend still
> needs to exist somewhere.


Yes, so unless you have a very high monitor-to-frontend ratio, it isn't
going to be cost competitive with a simple set top box that plugs into the
network and outputs video.



>
> But as to your second point -- an ATSC channel is the same 6 MHz
> as an NTSC channel; once you've produced it, probably at a
> standard 'channel 3' setting, can't you use a $50 modulator
> (http://www.smarthome.com/7764.html is the first to hand) to
> assign it to a dead channel that you create with a notch filter? Add
> another frontend, add another modulator. You should be able to
> handle four or five channels that way.
>

No, that won't work.  The modulation schemes are substantially
incompatible.  A 6Mhz NTSC band contains separate signals for luma, chroma,
and audio.  If you fed an RF modulator like the one you linked with a
SMPTE-310 (baseband ATSC, roughly speaking) signal on the video input, it
would output an undecodable mangled signal.

This isn't to say that an ATSC compatible RF modulator need be any more
expensive than an NTSC model, just that currently available NTSC models
wouldn't work.

-chris

-- 
Chris Ribe
TV/IT Engineer
WCJB-TV/DT Gainesville, FL
(352) 416 0648
cribe at wcjb.com
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