[alsa-devel] wrong decibel data?

[Raymond Yau]

2010/6/8 Colin Guthrie <gmane at colin.guthr.ie>

> 'Twas brillig, and Raymond Yau at 08/06/10 01:47 did gyre and gimble:
> >> Here is the function in PA's pulse/volume.c:
> >>
> >>  pa_volume_t pa_sw_volume_from_linear(double v) {
> >>
> >>    if (v <= 0.0)
> >>        return PA_VOLUME_MUTED;
> >>
> >>    /*
> >>     * We use a cubic mapping here, as suggested and discussed here:
> >>     *
> >>     * http://www.robotplanet.dk/audio/audio_gui_design/
> >>     *
> >>
> >>
> http://lists.linuxaudio.org/pipermail/linux-audio-dev/2009-May/thread.html#23151
> >>     *
> >>     * We make sure that the conversion to linear and back yields the
> >>     * same volume value! That's why we need the lround() below!
> >>     */
> >>
> >>    return (pa_volume_t) lround(cbrt(v) * PA_VOLUME_NORM);
> >>  }
> >>
> >>
> > can you provide the forumla for the "Master" volume control of pulse
> device
> > ? ctl.pulse
> >
> > there are 65536 steps ,
> > where are  0dB  , -6dB  and -inf dB on this cubic mapping ?
>
> Well 0 dB and -inf dB are obvious: 100% and 0% respectively.
>
> -6dB is about 80%:
>
> 1. Convert to Linear:
>        10 ^ (-6 / 20.0) = 10 ^ -0.3 = 0.501187
> 2. Cube Root:
>         0.7943
>
> Here is the code from volume.c that does this (combined with the
> function already pasted above). It's probably easier just to follow the
> code rather than have me describe it (no doubt poorly!):
>
> static double dB_to_linear(double v) {
>    return pow(10.0, v / 20.0);
> }
>
> pa_volume_t pa_sw_volume_from_dB(double dB) {
>    if (isinf(dB) < 0 || dB <= PA_DECIBEL_MININFTY)
>        return PA_VOLUME_MUTED;
>
>    return pa_sw_volume_from_linear(dB_to_linear(dB));
> }
>
>
>
> The final stage in pa_sw_volume_from_linear() (the * by PA_VOLUME_NORM
> and the lround()) is just converting to PA's own internal representation.
>
> HTHs
>
> Just for clarity, the reverse of for my approximate cut off at 16% would
> be:
>  16% = 0.16
>  0.16 ^ 3 = 0.004096
>  20 * log(0.004096) = -47.75dB
>
>
Can you explain how PA handle the volume controls of ac97 codec ?

PCM -34.5dB to +*12* dB
Master -46.5dB to 0dB

The total dB range (PCM + MASTER) is -81dB to +*12*dB


Most user concern about recording without distrotion. (i.e. best result when
Capture Volume at 0dB , PCM and Master Volume at 0dB ) and they need where
are 0dB points