[mythtv-users] Help, comments, suggestions, thoughts on tuning Myth

Michael J. Lynch mlynch at gcom.com
Mon Sep 27 09:51:14 EDT 2004

I've been running Myth for about 3 months now and have been happy with 
every aspect except one.  When a recording quality problem occurs, it is 
very difficult to figure out which of the frontend and/or backend 
controls needs to be adjusted in order to correct the problem.  At the 
moment my recordings are very good if the video content has a high 
dynamic range, however, when video content has low dynamic range quality 
suffers and I've been trying to figure out what needs to be adjusted to 
correct the problem.  Playing around with different frontend and backend 
controls is not only time consuming but may not result in the best 
possible settings for some of the controls.

To solve this problem I've been mulling over the idea of approaching it 
the way a radio receiver tech approaches repair of a radio receiver.  
The idea here is to use a signal generator to inject known good signals 
into the circuit starting at the speaker and working back to the RF 
section.  When the known good signal is not heard though the speaker as 
a "good" signal, the source of the problem has been determined and 
repair can procede.

Using this philosophy with myth, there are really only two points at 
which injection of a known good signal can occur.  The first point would 
be at the interface between the frontend and backend.  E.G.  Where  the 
backend stores it's recordings.  The second injection point is at the 
tuner card.

The following couple of paragraphs are thoughts and questions about how 
to go about signal injection at the backend/frontend interface point for 
purposes of "tuning" the frontend settings.  Any comments and/or 
suggestions would be appreciated.

So the first thing I figure I need to do is get a known "good" signal.  
I think it would be safe to assume that a VOB file from a DVD RIP of a 
high quality DVD would work as a known "good" signal.

The next thing that would need to be done would be the convert the VOB 
file, without modifying the audio and video data, to an NUV file that 
myth understands.  I know that myth uses a modified version of the NUV 
format and that modified versions of MPLAYER are able to understand and 
display those modified NUV format files.  I also know that MENCODER can 
convert NUV to any number of formats but I do not know if it can convert 
VOB to modified NUV.  Does anyone know if MENCODER can convert VOB to 
modified NUV without modifying the video and audio data?  If so, can 
someone suggest what the command line might look like to do this?  Now 
that I think about it, does Myth have it's own playback or does it 
simply use MPLAYER to playback recordings?  I think the answer is it has 
its own playback.

Once the VOB has been converted to NUV, the final step would be to 
convince Myth that the file is a recording that can be played back.  I 
could be wrong but I don't think I can simply copy the file to the 
"recordings" directory and expect to be able to play it back.  Would I 
need to format the name of the file the same way that Myth does when it 
creates a recording?  Also, how would I make an entry into the database 
for this "known good" recording so it can be played back?  Could I just 
record something, then replace the NUV file generated by that recording 
with my known good NUV and it would just work?

Once the frontend has been "tuned" then tuning the backend just becomes 
a matter of injecting a "reference standard" signal at the tuner and 
adjusting the backend settings for the best possible quality for the 
"tuned" frontend.

What does anyone think about this idea and would a procedure of this 
type be useful to anyone?

Thanks in advance for any comments and/or suggestions.

Michael J. Lynch

What if the hokey pokey IS what it's all about -- author unknown

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