On Wed, 18 Apr 2018 17:03:09 +0200, Ricard Wanderlof wrote:
The gain algorithm used in softvol can handle gain factors of up to 32767 which is slightly more than 90 dB, so allow a max_dB of 90 dB. This doesn't affect existing asound.conf files, but does allow a max_dB of up to 90 dB when needed.
Tested using Audacity that there is no undue distorsion or other artefacts when 90 dB of gain is applied to a suitable signal (i.e. a signal quiet enough not be clipped whan applying 90 dB of gain).
Signed-off-by: Ricard Wanderlof ricardw@axis.com
I discussed this with Takashi and Jaroslav back in January, but the discussion petered out and didn't reach a conclusion on which maximum gain would be acceptable, so I thought I'd send an actual patch and we can take it from here.
Ah, this ended up to null just because I went to my vacation...
We have a couple of usecases where we are using digital microphones, which have 24 bit resolution, and with an acoustic overload point of about 120 dB SPL, the output level can be quite low when the sound source is a fair distance form the microphone. The current softvol limit of 50 dB is therefore a bit restrictive.
When studying the code, it becomes apparent that the limit of the actual gain function is a gain factor of 32767, which corresponds to 90.3 dB. So it makes sense to set the MAX_DB_UPPER_LIMIT (whose only function is to cause _snd_pcm_softvol_open() to bail out and emit an error message if the gain is set higher than this limit) to 90 dB as it corresponds to the technical limitations of the gain algorithm. Granted, 90 dB is probably more than most people need, but there's no real harm in allowing a large gain; the user gain limit in a given application is still set by the max_dB parameter in asound.conf . The worst that can happen is that the signal becomes clipped as would happen with any signal if the applied gain drives it over 0 dB. If in fact a signal is close to 0 dB to start with, then the resulting clipping when increased by 90 dB is not much worse than when increased by 50 dB.
Honestly speaking, I still don't like to change something just because the number can reach. But this is a minor issue and if any damage happens, it can be said to be self-responsibility, so I applied as is.
thanks,
Takashi