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

[Holger Marzen]

Hi all,

since I only need one microphone when recording I am happy with my
Focusrite Scarlett 2i4. I use the 2i4 for recording and direct
monitoring with the line outputs feeding a headphones amp. I use the
builtin headphones amp as well, both when recording and for mixing.

So far, so good.

Then I wanted 2 additional outputs to feed my active speakers (Neumann
KH120A plus KH805 sub). Because of my room's damping of the high
frequencies (at least compared to my headphones AKG K812) I need an EQ
before them. So I use a Scarlett 2i2 for this purpose, with CALF
Jackhost and a Calf EQ feeding this outputs.

Now I have 2 audio interfaces that can't be synchronized with a
wordclock cable. So I have to drive the 2i2 for the speakers in an async
way. Since I use this output only for mixing/mastering I can live with
some additional latency.

I tried several ways to use the 2nd interface:
- alsa_out
- zita_j2a
- audioadapter (jack_load -i "-dhw:2 -q0 -n2 -p128 -r48000" 2i2 audioadapter)

The audioadapter is the most robust solution and doesn't crash when I
resize jackd's bufsize on the fly, so I'd like to stay with it.

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?

Regards
Holger