On 2011-05-13 18:20, Tanu Kaskinen wrote:
On Fri, 2011-05-13 at 17:29 +0200, David Henningsson wrote:
In short; if we e g have Mic Boost levels at (0dB, 20dB, 40dB and 60dB)
and the user wants 30 dB, better have 20dB in hardware and +10dB in
software than 40dB in hardware and -10dB in software, as the latter one
is more likely to have digital distortion when the signal passes through
the ADC.
There is a longer email on this list a while back, explaining more of
the philosophy behind, but I can't seem to find it right now.
Ok, I think I understand why rounding down makes sense when recording.
With playback, however, rounding towards 0 dB is definetely broken. My
ability to think clearly happens to be currently a bit weakened, so I
have to ask you: what's so magic about 0 dB?
Here's how I wrote it a while ago, quoting a thread named "Mic Input
Volume Controls":
"
We want to maximise quality while avoiding digital distortion, that's
basically the problem in a nutshell. We're assuming (sometimes
incorrectly; but that's our best guess) that this golden spot will be
achieved with all sliders at 0 dB.
I think my approach makes sense, unless I'm missing something: If we're
aiming for something above 0 dB, let's round down to make sure we avoid
distortion, and if we're below 0 dB, let's round up to make sure we get
maximum quality.
And then we always start with the control that's closest to the physical
hardware and work our way in.
"
Rethinking that, I don't think I've ever seen any playback volume
control that goes above 0 dB. All HDA's I've seen go up to 0 dB only,
whereas for recording there is both above and below 0 dB. Therefore the
scenario you state has not crossed my mind. So maybe you're right about
always rounding up with playback. If there actually was a say +6 dB
switch and you wanted +4 dB, then we should do +6 dB in hw and -2 dB in
sw as +4 dB in software would potentially cause digital distortion. Right?
> With playback rounding should always be done up -
> would it make sense to round always down with capture?
Maybe, if we give up the thought of 0 dB being magic.
--
David Henningsson
http://launchpad.net/~diwic
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