[mythtv-users] ATSC & QAM
beww at beww.org
Thu Sep 14 19:49:15 UTC 2006
On Sep 14, 2006, at 1:11 PM, Jafa wrote:
> Aaron Howard wrote:
>> My cable provider does not carry all of my local OTA HD broadcasts
>> to contract disputes. Therefore, some local HD channels (as well as
>> some non-local HD like ESPNHD, TNTHD, etc.) are available via QAM-256
>> from the cable system and some are only available via ATSC OTA.
>> Two questions:
>> 1. Is it possible/feasible/easy to combine two sources into one (the
>> reverse of a splitter?) to get an OTA antenna and the cable signal
>> onto one RG-6 input to a tuner card?
> Short answer - no. The OTA channels and Cable channels are on top of
> each other.
Longer answer, perhaps, under certain circumstances.
If the Cable system is using standard broadcast frequency assignments
(not HRC or IRC), and also has no signal on the channels you want to
insert from an antenna you could theoretically mix them with a combiner.
The off-air signals would interfere with an upper adjacent channel if
the cable system had one, in other words if you combined an off-air
channel 7 into the stream it would likely cause beats in channel 8.
This is because broadcast signals have the aural carrier 10db. below
the visual, and cable systems run the aural 15-17db. down so as not
to cause upper adjacent beats. You will never see two adjacent off-
air channels assigned by the FCC in the same market (4 and 5 are not
really adjacent, nor are 6 and 7, even though their numbers are
sequential, so you do see those combinations in some markets).
You could also use a filter to remove one or more channels from
either the cable or OA signal, but good filters of this type are
expensive and cheap ones don't work well. It would cost several
hundred dollars for a good quality filter to do this job, more than
another tuner card.
You would also have to match the levels of the signals from both
sources pretty closely, and since both cable and off-air signals can
vary significantly this would be hard to do without processors
equipped with AGC, which are also much more expensive than tuner cards.
So it *is* possible but not practical, and the only reasonable answer
is a second tuner. You could also use a set-top-box that had baseband
video and audio outputs and go into the baseband inputs of the tuner
card, but this is really just using another tuner card, it just
happens to be located in the STB instead of your Myth machine.
But any splitter is also a combiner if connected the other way,
unless specifically designed to *not* do that, which would greatly
increase the cost, thus most splitters in common use are combiners as
well. if that is what you were actually asking.
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