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No per-individual line-level adjust: bug or missing feature?

 

 

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f-myth-users at media

Jan 14, 2006, 7:45 PM

Post #1 of 2 (1350 views)
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No per-individual line-level adjust: bug or missing feature?

Should I open a ticket on this [*], or will it be closed as "feature
request without attached patch"? If I hear nothing (or encouragement),
I'll do the former; if I hear the latter, I'll add it to the wishlist
at the new wiki.

In short, it seems buggy to me that Myth assumes that all capture
cards will agree on their audio level when capturing. Apparently
Hauppauge cards can differ by at least 9dB across revisions and
models, and it'd be asking a lot to expect differing brands to agree
with each other, too, not to mention analog vs digital capture.

Not only is such a dramatic level difference irritating (and you never
know what it's going to be, since it depends on which tuner Myth picked),
in my case it was preventing me from getting unity gain on my 250's---
to get unity for cable->250->myth->tv vs cable->tv, I had to max out
the capture levels on the 250's and on the relevant ALSA mixers. But
such a high capture level causes my 350 to clip badly if -it- happened
to get picked as the tuner, so as a workaround, I'm doing a disgusting
whack-a-mole approach of simply checking the relevant 350 register at
1 Hz and, if necessary, smashing it with ivtvctl so Myth can't
override it, as it tries to do so at the start of every capture. [1]
(My 350 is ~9dB hotter than the 250's, even though both were made in
the same month; Hans reports dramatic differences across new & old
350's, etc.)

My suggestion is to put a per-card slider into mythtv-setup's screen
for each capture card (where this is supported by the card; some might
not). This slider is used in addition to the master capture slider set
in whichever recording profile is active. [And see [+] for a further
suggestion.] The value should get stored in some appropriate table,
consulted when a capture commences, and be multiplied together with
the master capture mixer when initing the card. (A more user-friendly
approach would be to be able to adjust this in real time, instead of
via repeated trips to mythtv-setup, but that seems like a lot more work.)
Presumably the slider should be an attenuator, like the rest of them
in the UI, so 100% is unity & everything else is quieter, and should
default to 100%.

[I asked about this on mythtv-users and ivtv-users; see [1] and [2]
for relevant threads. No one claimed this was already implemented,
and I don't see anything about it in any archives.]

[1] http://www.gossamer-threads.com/lists/mythtv/users/173374
[2] http://ivtvdriver.org/pipermail/ivtv-users/2006-January/001062.html
[.There are also reports in ivtv's Trac about these problems;
one reports something that sounds worse than 12dB mismatch!]

[*] ...which I might actually code and close, but it won't be until
after .19 is released if so, because I need to be more stable than
current SVN and don't have enough spare hardware to run a parallel
SVN machine. I'm hoping this is a really quick hack for someone
already running SVN and familiar w/the code.

[+] Another hack, along the "while we're in here" approach: Also
implement per-input and per-channel sliders. This would probably be
really quick to write if the approach is "look in the database for
values, but don't bother to add to the visible UI for it" and would
presumably mean that the final setting sent to the card is a simple
multiplication of master x card x input x channel, with all defaults
being unity and all missing values assumed default. The rationale
here is that not all set-top boxes allow setting their output line
levels, or there may be other reasons not to fiddle with it (e.g.,
split setups where the STB drives something else as well); and also
that some individual channels run -very- hot---my own cable feed has
mismatches of ~9dB across certain channels I watch, and this mismatch
has been consistent across -years-. If I could stop constantly
fiddling with the TV volume every time I change channels, I'd love it;
this applies in spades when using a DVR, since I might not even know
what channel something aired on to begin with---until it starts
playing and blasts me out of my seat because the volume is way too
high... (An unfortunate choice of tuner & channel damn near killed
my speakers recently because of this combination of issues...)
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danielk at cuymedia

Jan 15, 2006, 7:16 AM

Post #2 of 2 (1294 views)
Permalink
Re: No per-individual line-level adjust: bug or missing feature? [In reply to]

On Sat, 2006-01-14 at 22:45 -0500, f-myth-users [at] media wrote:
> Should I open a ticket on this [*], or will it be closed as "feature
> request without attached patch"?
It will be closed...

> If I hear nothing (or encouragement),
> I'll do the former; if I hear the latter, I'll add it to the wishlist
> at the new wiki.
Right place.

> In short, it seems buggy to me that Myth assumes that all capture
> cards will agree on their audio level when capturing. Apparently
This doesn't seem like a big problem on my machines. There is a slight
difference between the PVR-500 & PVR-250, but at a few dB it is hardly
noticeable.

> My suggestion is to put a per-card slider into mythtv-setup's screen
> for each capture card (where this is supported by the card; some might
> not). This slider is used in addition to the master capture slider set
> in whichever recording profile is active. [And see [+] for a further
> suggestion.] The value should get stored in some appropriate table,
> consulted when a capture commences, and be multiplied together with
> the master capture mixer when initing the card. (A more user-friendly
> approach would be to be able to adjust this in real time, instead of
> via repeated trips to mythtv-setup, but that seems like a lot more work.)
> Presumably the slider should be an attenuator, like the rest of them
> in the UI, so 100% is unity & everything else is quieter, and should
> default to 100%.
Sounds like a good idea, it could be adjusted while recording just like
the recording picture attributes 'G'. I don't think 100% should be the
default though, you tend to get clipping if you go above 70% on any amp.

> split setups where the STB drives something else as well); and also
> that some individual channels run -very- hot---my own cable feed has
> mismatches of ~9dB across certain channels I watch, and this mismatch
> has been consistent across -years-. If I could stop constantly
They are probably just using range compression on the loud station, this
is a technique used by pop bands and advertisers to sound louder without
actually increasing the peak volume (what you set with the volume knob.)
I'd love to have an audio filter that detected restricted range in the
audio and undid the damage as best it could (and reduced the loudness).
But using the volume control on these loud stations would work...

-- Daniel


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