Re: [Jack-Devel] query jack scaling factor?

On Mon, May 30, 2011 at 08:50:21PM +0300, Dan Muresan wrote:

> So, for now (October 2008) we use 2^(N-1)-1 as the scaling factor.
> 
> Which is part 1 of my point -- unless Jack promises *never* to change
> this scaling strategy, it should be available for those apps that need
> bit-exactness.

AFAIK, the only point about bit-exactness is for legal purposes,
when you have to prove that some data is 'original' by comparing
its cryptographic digest to a certified one. Which means you
can't modify it in any way, and there's no point sending it
throuhg Jack.

Apart from that, there is nothing magical about the bit patterns
produced by an AD converter. Change the analog gain by 0.1 dB and
you get completely different samples and roundoff errors, for the
same input signal. They are neither better nor worse than the
original ones.

> Part 2 of my point was: can I find out whether Jack is reading 16-bit,
> 24-bit, or something else from ALSA? There's no point in wasting disk
> / memory space if I want to convert back to PCM in my app.

There are good reasons for converting to float for computation.
Disk and memory space are cheap these days, and most apps writing
sound files will offer you the choice of 16, 24 or FP. If you
modify the samples in any way, the conversion to 16 or 24 bit
should be the last step, not an intermediate one, and Jack will
take care of it.

If the format matters you have to read the specs of your soundcard.
Jack will report the sample *format* when starting up, but that
doesn't need to be the same as the actual number of bits from the
converter. Many 24-bit cards use a 32-bit format for example.

Ciao,

-- 
FA

1306780162.20976_0.ltw:2,a <20110530182901.GD27205 at linuxaudio dot org>