[opendtv] Re: Analog v Digital TV
- From: "John Willkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 15 Jan 2007 21:43:22 -0800
Barry;
To rule out intermodulation/image issues, you will need to do the
appropriate engineering analysis. This can be a significant issue. I've
never done it with digital, but with analog (and there are still analog
stations around), the separation between an undesired station 7 channels
below a desired one is almost the same distance as between first adjacent
channels. That's because the IF frequency for analog tv sets is 45 mhz,
which is between 7 and 8 channels and is almost as significant as first
adjacent channels.
The antenna preamp might also be interacting with a slightly out of
tolerance component in the Dish receiver.
If you've got a friend with the same receiver, it might be informative to
trade receivers for a few days. If that receiver has the same issue, you
might try to put successive 3dB pads in the antenna line and see what
happens.
You could also be getting more VSWR than is desirable due to mismatches
between the impedance on the preamp output, the receiver input, the
connectors on the cable, and the cable itself. You could take the receiver
in question up into the attic along with a receiver, and hook it up to the
antenna with and without the preamp.
Lots of luck on this one.
John Willkie
_____
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] On
Behalf Of Barry Brown
Sent: Monday, January 15, 2007 8:18 PM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: Analog v Digital TV
I appreciate all the comments, however, front end overload doesn't seem to
be a condition at my location. I'm about 35 miles from the closest
transmitter. There are no stations in the area that are adjacent to the
problem channel. Intermodulation, I guess, could be a possibility if it's a
condition created by the problem transmitter. Also, I'm not the only one in
this area experiencing the indicated random signal loss of this channel. The
fact that I have to take the signal down 21 dB in a grade B area is somewhat
a mystery. BTW, I have a preamp on my attic antenna, but that is required to
receive my weaker channels.
On Jan 15, 2007, at 6:36 PM, Allen Le Roy Limberg wrote:
Front end overload, possibly by nearby channels. This is owing to the AGC
design of the receiver, not something to do with the ATSC modulation scheme.
Al
----- Original Message -----
From: <mailto:barrysb@xxxxxxxxxxx> Barry Brown
To: <mailto:opendtv@xxxxxxxxxxxxx> opendtv@xxxxxxxxxxxxx
Sent: Monday, January 15, 2007 11:53 AM
Subject: [opendtv] Re: Analog v Digital TV
On Jan 15, 2007, at 8:57 AM, Craig Birkmaier wrote:
Anything downstream in an ATSC transmission is error protection with the
sole purpose of trying to get the data in the files I described above to the
decoder in the receiver with the lowest number of errors possible. IF you
can receive the ATSC transmitted bits perfectly, you can reconstruct the
MPEG video stream at the same level of accuracy, as a decoder that is
connected to the output of the encoder ( i.e. NO CHANNEL ERRORS).
What ATSC signal conditions, other than multi-path errors, would cause some
receivers to give an intermittent "Loss of Signal" indication (might be loss
of sync) where inserting 21 dB of attenuation in the antenna connection
corrects the problem? One channel in my area is such a case. It is not the
strongest or weakest channel nor is there any indication of multi-path
errors on analog channels (not adjacent) either side of the problem child .
In fact one of the analogs is transmitted from the same tower as the DT
channel.
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