david.henningsson at canonical.com
Tue Dec 20 01:33:04 UTC 2011
(Please don't respond in private if anyone can benefit from the answer)
On 12/20/2011 02:01 AM, Len Ovens wrote:
> On Mon, December 19, 2011 3:56 pm, David Henningsson wrote:
>> Why is this bad? If you tell PulseAudio to achieve +30 dB, whether
>> PulseAudio chooses to set "Boost=+30 dB, Capture=0 dB" or "Boost=+12 dB,
>> Capture=+18 dB" should lead to equivalent results. If anything, having
>> boost high and capture down should theoretically lead to better quality
>> as the signal level would be higher between the "boost" (which comes
>> first) and "capture" volume controls.
> Huh??? No. Attenuation off first. Boost is an amplifier. Amplifiers are
> always noisier than less attenuation. The capture level is a level of
> attenuation. No attenuation is high level and lots of attenuation is a low
> level. Boost is an extra amplifier that takes whatever level the
> attenuator passes and amplifies it. In my opinion very much a pulse audio
> problem. Talk to some one who does sound engineering. Try it both ways,
> listen, compare. This is why on a sound board the input pre level is set
> with the fader (attenuator) set high... for least noise.
You did not give me your alsa info, so I'm assuming it looks like mine.
The signal chain looks like this:
Physical Mic Jack => Mic Boost control => Capture control => ADC
So far I've seen only amplification on "Mic Boost", and most "Capture"
controls being able to do both amplification and attenuation. On my
computer, my Mic Boost goes from 0 dB to +30 dB (in 10 dB steps), and
Capture goes from -16.5 dB to +30 dB.
If you're familiar with mixer boards, you can think of "Mic boost" being
the small "Gain" knob you have first on every mic input.
For mic inputs, you almost always want more than 0 dB in total. There is
no attenuation. PulseAudio first tries to adjust "Mic Boost" to get at
close as possible, then does the fine tuning in "Capture".
David Henningsson, Canonical Ltd.
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