[mythtv-users] ATSC & QAM

Brian Wood 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:
>> Background:
>> My cable provider does not carry all of my local OTA HD broadcasts  
>> due
>> 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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