[Jack-Devel] audioadapter: quality setting important when not changing sampling frequency?

Holger Marzen holger at marzen.de
Fri Sep 28 20:51:09 CEST 2018

On Mon, 24 Sep 2018, Holger Marzen wrote:

> On Mon, 24 Sep 2018, David Kastrup wrote:
> > Holger Marzen <holger at marzen.de> writes:
> > 
> The output of the first adapter is used for
> - direct monitoring
> - headphone monitoring and mixing/mastering, but without EQ
> > > As I played around with the quality setting (-q0 ... -q4) I noticed an
> > > enormous CPU load with -q4 (sinc) but couldn't hear any differences in
> > > audio quality. -q0 sounds as good as -q4.
> > >
> > > Now my question:
> > > When the audioadapter has to resample to the same frequency (from 48000
> > > to 48000), do higher -q settings improve the audio quality? As I
> > > understand there is no interpolation needed at all so -q0 would be
> > > enough to have a perfect audio quality.
> > >
> > > Any comments/ideas to this?
> > 
> > Interpolation will be needed when the clocks drift.
> Or you can simply wait til the "right" moment und use the original
> values. But that would not use more CPU depending on the quality
> setting. So there seems to bee an interpolation happening.
> It looks as if I have to do a looptest (play through the 2nd interface
> and record with the 1st) to get an impression how's the signal distorted
> at the different quality settings.

I used the interface that's connected via audioadapter for playback and
the "main" interface that's directly controlled by jackd for recording.
As a signal source I used Audacity with generated sine waves of 5000 and
10000 Hz. I played this signal and recorded it (audio cable between
interfaces). The result showed that the signal gets indeed resampled and
that the quality setting matters.

q0 (SRC_LINEAR) and q1 (SRC_ZERO_ORDER_HOLD) showed a non-flat envelope
of the recorded signal - bad.

q2 (SRC_SINC_FASTEST) showed the desired flat envelope and seems to be
the best compromise between CPU load and quality.

-> That's the setting I'll use from now on.

I cannot to further examination because the spectrum analyzer in
Audacity can't show the harmonics accurately. I expect a single peak in
a sine wave's spectrum but I got a softer curve.

SRC seems to bee an abbreviation for Secret Rabbit Code

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