[Audacity-devel] Project rate

[Juhana S. <ko...@ni...>]

Hello. I'm puzzling on the following.

I think the project rate should not be the play and record rate.
The play and record rate should be the rates optimized by the
manufacturer for their audio card. Even if a card supports low
rates, the circuitry may be optimized only for highest rate
(say, 44100 Hz). That means that people recording at 8000 Hz would
do better by recording at 44100 Hz and downconverting to 8000 Hz
later (done automatically by Audacity with project rate set to
8000 Hz). The similar argument stands for playing 8000 Hz output.

So, what about the following functional structure in the following
example? The example project uses two audio cards just for showing
the idea better.

  card 1 rate: 48000 Hz (in preferences, in menu)
  card 2 rate: 44100 Hz (in preferences, in menu)

  card 1, A/D (48000) --> SRC (rec rate 32000 or off) --> disk
  card 2, A/D (44100) --> SRC (rec rate 32000 or off) --> disk

  disk --> SRC (project rate 8000) --> SRC (48000) --> card 1, D/A (48000)
  disk --> SRC (project rate 8000) --> SRC (44100) --> card 2, D/A (44100)
  disk --> SRC (project rate 8000) --> disk

That is, we may record with the best hardware rates and store
that audio directly to disk (SRC = off). Or, if the disk space
or the archive space is an issue, user may select a suitable rec
rate (32000 Hz). While the audio is stored to disk at the rec
rate, the recording still happens at the best hardware rate.
Further, if the sample rate conversion (SRC) is the bottleneck,
user may set SRC off and change the hardware rates.

As seen in the last part of the example, the project will be played
at the hardware rates and can be exported to disk with the project
rate.

With the same notation, the current situation would look following,
please compare.

  card 1 rate: 8000 Hz (only one rate possible, right?)
  card 2 rate: 8000 Hz

  card 1, A/D (8000) --> disk
  card 2, A/D (8000) --> disk

  disk --> SRC (project rate 8000) --> card 1, D/A (8000)
  disk --> SRC (project rate 8000) --> card 2, D/A (8000)
  disk --> SRC (project rate 8000) --> disk

Of course, one is able to use a higher project rate and change it
to 8000 Hz only at the final stage.

So, should we separate the hardware rates, recording rate,
and the project rate as I have suggested? The advantage for me is
that when I permanently set the hardware rate to best possible and
the rec rate converter to off, then the settings always stay fixed
and I don't accidentally record anything with a lower quality if
I change the project rate.

Best regards,

Juhana