[mythtv-users] ATSC Unwatchable

Devin Heitmueller dheitmueller at kernellabs.com
Thu Sep 6 17:33:18 UTC 2012

On Thu, Sep 6, 2012 at 12:58 PM, Keith Wait <kwait at alumni.rice.edu> wrote:
>> You should double check this.  The SNR field shown in azap is in 0.1
>> dB increments, so 0xff is 25.5 dB (which is a marginal signal).  In
>> other words, you'll get lots of errors.  Try a better antenna, or
>> temporarily move the computer to a location where it can get better
>> signal conditions.
>> At this juncture all data points to "crappy signal conditions" rather
>> than anything wrong with the device or the software stack.
>> Devin
>> --
>> Devin J. Heitmueller - Kernel Labs
>> http://www.kernellabs.com
> I did some more testing and you are correct; it is possible for me to
> get signal values above 0xff.  I've gotten at most up to 1e0.  So yes,
> I believe that what the linux PC is being fed is a lousy signal.

I cannot imagine how that would be possible.  0x1e0 would translate to
48.0 dB, which is above the maximum the demodulator is capable of

> However, I don't agree with you that 'At this juncture all data points
> to "crappy signal conditions".'  If that were the case, why (as
> originally noted) does plugging the antenna directly into the TV next
> to the PC (TV, PC, antenna all unmoved) give me clear picture?  And
> why does the card in a windows PC located in roughly the same place in
> the house work just as well as the TV?

There indeed could be a driver problem, or the problem could be with
the application.  One thing worth trying would be to run "azap -r
CHANNELNAME" and then "cat /dev/dvb/adapter0/dvr0 > foo.mpg".  Then
play the resulting foo.mpg in mplayer or VLC.  If it really was a
signal problem, then the video would be corrupted there as well.  If
it plays fine, then you've definitely got a problem with Myth.

> Is there anything other than azap I can use to get a second opinion on
> signal strength?  Is it possible the frequency table isn't the same as
> is used in my two non-linux PC cases?  Are there any modprobe options
> I can use to try and mitigate this problem?  Are there other questions
> I should be asking but am not?  Sorry for the hysterics, but the SO is
> out of patience.

Azap is just reporting the signal stats provided by the driver.  There
are probably modprobe options you can provide to the tda18271 driver
to see what the target frequency is, which would allow you to compare
(I would probably start with something like "modprobe tda18271


Devin J. Heitmueller - Kernel Labs

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