On 14/06/2010, at 6:54 PM, Jan Stary wrote:

quoted text
> On Jun 14 11:37:52, Paul M wrote:
>> 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.
>
> It is good practice to leave a little headroom (say, 6dB)
> for further processing. You might want to do some noise
> reduction, compressing, whatever, and the effects will clip
> if the signal already is saturated.

I'll leave enough headroom to allow for the highest peaks, but
I'm not planning on doing any additional processing during the
initial conversion. I can attenuate the signal later if I add
any post processing.


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> Also, "the input to the adc" is not all there is to it;
> there are other mixer settings that affect the signal
> that will eventually end in your file.wav
>
>> Ideally, I'll get the workstation hardware
>> set to certin defaults, then adjust the incomming audio as
>> required.
>
> There are no "defaults". Your analog inputs can
> (and most probably will) vary greatly.

I dont really want to futz with the audio hardware once it's set
up, so these 'defaults' would be such that a certain input signal
level will produce a clean digital file with no clipping and
good dynamic range. I'll then use a preamp with decent VU meters
to ensure the signal sent to the computer is optimal.


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>> This leads to a couple of questions:
>> Are there (typicaly) any variable gain stages in the analog input
>> path in the computer.
>
> Yes. 'mixerctl -a' will shouw you how the azalia 'widgets' are
> interconnected on your codec.
>
>> 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. However there is also
>> 'record.volume', though I would assume this operates on the mixed
>> digital signals at the end of the chain.
>
> record.volume
> 	Amplifier gain control for widgets listed in record.volume.slaves.

Thanks, I wondered about the slaves.


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>> 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?
>
> What's a "0 gain setting"?

Unity gain, or 0dB gain. Signal level out = signal level in (for
that stage).


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> I believe Jacob Meuser has work going on to make the numbers
> on the azalia knobs correspond to actual decibels,
> but I don't know if it's current yet.
>
>> bios0: ASUSTeK Computer INC. P5QPL-AM
>> azalia0 at pci0 dev 27 function 0 "Intel 82801GB HD Audio" rev 0x01:
>> apic 2 int 21 (irq 5)
>> azalia0: codecs: Realtek/0x0887
>> audio0 at azalia0
>
> It would be my guess that this is the audio chip that's integrated
> with the Asus P5QPL-AM motherboard.  If you are really after "best
> transfer quality", you might want to use something else in the
> first place.

Good point, thanks for the reality check.
Most of what I have to do is not the best quality anyway, but that
doesn't mean I'm happy to introduce unnecessary generation loss by
being sloppy. There is some though that is very good, so that may well
need something better.


paulm