On Mon, Jun 14, 2010 at 11:37:52AM +1200, Paul M wrote:
quoted text
> I have a large amount of analog audio I need to digitize and
> naturaly want to ensure best transfer quality. So I need to set
> the analog level at the input to the adc as high as possible
> without clipping. Ideally, I'll get the workstation hardware
> set to certin defaults, then adjust the incomming audio as
> required.
> This leads to a couple of questions:
> Are there (typicaly) any variable gain stages in the analog input
> path in the computer.

varies depending on hardware, but often there is a gain
at the input and a gain at the ADC.

quoted text
> Mixerctl -av (full output below) shows a
> node called 'record.adc'. It seems reasonable that this might
> opperate on the analog input to the adc

generically speaking, yes.

quoted text
> However there is also
> 'record.volume', though I would assume this operates on the mixed
> digital signals at the end of the chain.

no.  record.volume is essenially an alias.  on your hardware with the
configuration you've posted, it's a shortcut for setting both
record.adc and record.adc2.  this is explained in azalia(4) (though
maybe that info didn't make it into 4.6, the info in -current azalia(4)
is mostly relevant even for 4.6).

quoted text
> Also: a lot of the gain stages have defaults of 120.120. Would
> it be reasonable to assume that this is the 0 gain setting?

no.  unfortunately, the mixer interface, like a lot of audio(4)
related stuff, is designed for "consumer usage".  so, we just have
a range that is essentially 0-100% - it has no relevance to anything
except the knob.  truly the  worst kind of knobs are those that
have no outside meaning, but apparently people like this.  *shrug*

http://marc.info/?l=openbsd-tech&m=123101323408867&w=2

quoted text
> Any thoughts appreciated.

if you really want to know how to do this right, your best bet is to
find the datasheet for your codec.

now, your codec is a Realtek, which is common for azalia(4), and
I happen to know them pretty well ...

quoted text
> inputs.line=85,85
this is a 0 (0), 10 (85), 20 (170), 30 (255) dB gain on the line-in
jack.  values in () are the corresponding mixerctl values.

quoted text
> record.adc=248,248
this is the ADC input gain.  0 dB should be around '88'.
0..255 here represents the hardware's -16.5 to 30 dB
in 1.5 dB steps.

these are the only gains on the recording path of your device.

-- 
jakemsr@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org