[Linrad] Re: Delta 44 ALSA gain setting

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Michel and All,

> In general:
> -Some years ago, OSS (4Front) allowed 2 settings for the Delta 44 gain.
> (-10 and +4dB)
> This has never been possible with ALSA.
4Front has three settings: -10, CONSUMER and +4dB. They work as
one would expect and saturate at 3.6V, 7.1V and 14.1V.

> -So now we're left in ALSA with the amixer or alsamixer (resp. text or
> GUI) switch and value settings for soundcard gain control. These are
> linear settings from 0-100%.
Not really. There are several settings that may affect the gain. It 
depends on how many channels you have selected. I tried one channel and 
then must set A/D to 100, click H/W Multi= capture and set it to 100. 
One must also set IPGA to 0. Saturation will then happen for 14.1 V with
good dynamic range. The output is shifted by 2 bits however so one
should set the first FFT gain to 4000.

With two channels it is different. The H/W Multi has no effect.
A/D level and IPGA affect each other and only the one latest touched
has any effect. Make the last setting A/D=100 (which forces IPGA to
zero) Then saturation is at 14.1V with a noise floor that is one
dB better compared to OSS (at about 15 kHz.)

Four channels work the same way as two channels. I suggest setting
IPGA (input analog amplifier gain) to zero and not rely on the
history of which control was last set.....

It seems like OSS uses a little analog gain and compensates for that
on the digital side. That would explain the 1 dB higher noise floor.


73

Leif / SM5BSZ


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