On Mon, Jun 14, 2010 at 11:37:52AM +1200, 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. 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. > 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. > 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). > 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 > 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 ... > 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. > 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. -- jake...@sdf.lonestar.org SDF Public Access UNIX System - http://sdf.lonestar.org
