[linux-audio-dev] ALSA features.

[James Courtier-Dutton]

Jens M Andreasen wrote:
>>
>>Any other suggestions?
> 
> Nothing special except for the obvious limiter upfront before you start
> sampling.
>  

Jens,

What do you mean by "limiter upfront". I am not an audio mixer studio 
expert. Is this something for the detection of clipping in the audio 
path? If so, I am afraid that I don't know of any sound card hardware 
that has that feature.

Too me, a limiter would be something that limits the max peaks of a 
signal. For playback, one could use the application to do the limiting 
and set the sound card for 0dB so that no extra clipping happened in the 
output path, but for capture it is more difficult. The limiter should 
really be in the pre-amp prior to the ADC. Once the signal is digital, 
one can set all the sound cards digital mixers to 0dB and then just pass 
it to the application knowing that no further clipping or quality loss 
has happened.

With a better understanding of the capture path on the sound card.
e.g. Pre-amp(gain controlled) -> ADC -> Digital mixer -> PCM.
One could implement some basic limiting by letting the application 
detect any digital sample getting close to the limit and controlling the 
Pre-amp gain control as a result. In this example, one would want the 
"Digital mixer" to be set to 0dB so that the samples reaching the PCM 
were exactly the same as the samples leaving the ADC.
So, one step to achieve this would be to clearly identify all the 
alsamixer controls that control the analog "Pre-amp" parts before the ADC.

In any case, the first step to achieving all this is to get alsamixer to 
show dB gain instead of %

James