[LAU] [solved] Focusrite 18i20 balanced -> unbalanced

Ralf Mardorf ralf.mardorf at alice-dsl.net
Fri Jan 20 07:20:53 UTC 2017


I don't know what solved the issue, but actually the output sound is
ok. It's still not in the same class as RME, but from good hifi quality.

For the cable plugged into channel 2 of the Focusrite only tip
and sleeve are soldered.
For the cable plugged into channel 1 of the Focusrite only tip
and sleeve are soldered, too, but by a crocodile clip cable I connected
and disconnected ring with sleeve [1].
There's neither a difference in volume, nor in tone, with ring and
sleeve short circuit or not.
A big surprise was, that comparing the output of the Focusrite, with
the output of a CD player, everything was ok yesterday, when the
testing was done. I couldn't reproduce the bad sound I experienced
before. The output level of the Focusrite was ok as well. No other
analog audio cables were connected to the other channels. Perhaps some
of the other audio cables, connected to other outputs caused problems by
the tests, I made a while ago.
However, this morning I checked channels 3, 5, 7 and 9 against channel
2, again with a short circuit and without it and after that I compared
channels 4, 6, 8 and 10 with channel 9, but only without a short
circuit. And finally I repeated to compare channel 1 and 2 with a CD
player. Everything sounded very well.

With linux 4.9.0-rt1 it's possible to go down to 32 frames at 48 KHz,
getting lots of inaudible xruns, at 16 frames as well as 64 frames the
interface is completely unusable. At 128 frames it's the same as for a
vanilla kernel with threadirqs, just scrolling with the USB mouse wheel
in roxterm usually does cause issues, very seldom a random xrun could
happen, when switching to another window. At 256 frames it seems to be
absolutely stable, however, it seems to be usable at 128 frames, with
linux-rt as well as a vanilla linux with threadirqs. FWIW using another
USB cable at 128 frames doesn't make a difference. Actually it even
seems to be usable at 32 frames, the xruns were inaudible. I don't know
why it's completely broken at 64 frames. 16 and 64 frames cause long
interrupted audio signals.


[1] http://picpaste.com/pics/IMG_2250_unbalanced.1484895640.jpg

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