On Tue, Apr 15, 2008 at 03:43:30PM +0100, James Courtier-Dutton wrote:
John Rigg wrote:
On Monday 14 April 2008 23:07:22 James Courtier-Dutton wrote:
The aim of the dB values in the ALSA mixer is this.
For Playback: Set all the mixer controls to 0dB. Send a sample digital signal from the CPU to the sound card. The measured analog signal on the line-out should be the same for ALL sound cards.
The dB setting in a mixer refers to gain, not level. Expecting the same absolute level from pro and consumer sound cards at the same gain setting is a little ridiculous. If I set a control at 0dB I want just that: 0dB gain.
John
But surely it is a bit more than that. If I work with Pro gear and I capture some sound via line-in. I then send the digital sound file produced to another person who also has pro gear. When they output that digital sound file to their line-out, it should be identical, level wise, to the captured line-in was so long as both people have the same dB gain levels set. In this case, we would need some way to convert Volts from the line-in to specific digital samples in the sound file. I have not been able to find out what this conversion should be. E.g. a 16bit signed digital sample with value +5000 equals X Volts output from line-out. What is that X value?
In pro gear, levels are referenced to digital full scale in the A-D and D-A converters. The actual analogue signal voltage level that corresponds to this varies between interfaces. In most pro converters, full scale corresponds to about +20dBu (about 11 volts peak) but there is some variation (+/- 2 or 3dB) between manufacturers. I'd expect the line in and line out levels on a consumer sound card to be much lower than this, so obviously the same standard can not be applied to both.
John