Low input sensitivity

robiwan · Post by **robiwan** » Wed Aug 30, 2017 9:36 pm

Hi Matt, I have a solution now which streams audio, but I notice that full output from a USB audio device couple to the input of the I2S card does not even remotely reach +0db peak level, more like -20 dB peak. Is there a way to set the PGA of the WM8731 for higher sensitivity ?

flatmax · Post by **flatmax** » Wed Sep 06, 2017 8:07 am

Hi there,

Have you bumped the gain up in alsamixer ? Did that have any effect ?
Matt

robiwan · Post by **robiwan** » Wed Sep 06, 2017 4:40 pm

Yup, I have everything set to max, still having this issue. I will try to do a measurement of the exact levels, analog and digital.

robiwan · Post by **robiwan** » Mon Nov 20, 2017 12:22 am

Ok, I've done a loopback test where I playback (with aplay) a 0 dB fullscale WAV file, and the result is recorded with arecord. I would have expected that the result would be approx. 0 dB peak, but it is far from it. The result is at least 6 dB lower than fullscale. See attached files.

Could it be a driver issue ? I.e. that the PGAs of WM8731 aren't setup correctly ?

Regards
/Robert

robiwan · Post by **robiwan** » Mon Nov 20, 2017 12:34 am

Furthermore, here is an Oscilloscope screenshot of the output of the soundcard. The datasheet states 1.0Vrms*(AVdd/3.3) and as the AVdd is 3.3V, the output should be peak +/- 1.4142V, which the scope screenshot verifies. So the problem is on the input side.

moggyd · Post by **moggyd** » Mon Nov 20, 2017 8:51 am

See this thread http://forum.audioinjector.net/viewtopic.php?f=5&t=3129

robiwan · Post by **robiwan** » Mon Nov 20, 2017 9:12 am

Ok, I'm not sure what this means. It is ambiguous. Is there a 6 dB attenuation on the input or not ? But not on the zero card ? The boost mentioned seems to imply soft volume, which means digital boost of 6 dB. That is unacceptable. The datasheet of WM8731 says 1Vrms*AVdd/3.3 at 0 dB FS. That is what I am after.

moggyd · Post by **moggyd** » Mon Nov 20, 2017 9:23 am

There is a 6dB limit on input of the full size card but not the zero version. I have no idea why.
Using softvol is far from ideal for me but does make the card usable

flatmax · Post by **flatmax** » Fri Nov 24, 2017 8:13 am

The full sized card has a resistor divider on the input side. This causes the 6 dB attenuation.
To bypass this attenuation, please short resistors R6 and R7.

As mentioned, the zero sized card doesn't have this attenuation on the input.

thanks
Matt

robiwan · Post by **robiwan** » Fri Nov 24, 2017 7:18 pm

Yay! I suspected something of the sort

Thanks!

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