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It affect your measurent if you are using the mic on your usb audio if

the

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automatic gain control is switch on

Automatic gain control is now switched off.

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Background noise affect the result if you are not using line in and line out with a loopback cable

I am using loopback, noise should not be an issue.

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How many steps need to reduce amplitude of the recorded sine wave by half when you play a sine wav through your usb audio and record it back using audacity?

Using recorded sine wav provided by 'speaker-test -t sin' amplitude of recorded sine wave reduced by half at step 77

output of 'amixer -c 2 set Headphone 77': Simple mixer control 'Headphone',0 Capabilities: pvolume pswitch pswitch-joined Playback channels: Front Left - Front Right Limits: Playback 0 - 151 Mono: Front Left: Playback 77 [51%] [-13.94dB] [on] Front Right: Playback 77 [51%] [-13.94dB] [on]

Does this number of steps remain the same when your starting volume are not at 151?

Your value 77 seem quite different from alsamixertest

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You many need to change the name of the volume control to headphone playback volume in alsamixertest

Done. alsamixertest output:http://ix.io/ezd

You may need David to analyse the result

Do those measured value match with those in datasheet?

Seem the measured dB range is larger than 27 dB

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/log/sound/usb?q...