Re: [ecasound] W64 format conversion correctness

From: Kai Vehmanen <kvehmanen@email-addr-hidden>
Date: Fri Jun 10 2005 - 00:44:49 EEST


On Thu, 2 Jun 2005, Free Lunch wrote:
>> Before processing, ecasound converts all input data to its internal format
>> which is 32bit float atm.
> Just to clarify, is that true for cases when recording from alsa with
> ecasound? Does data coming in at 24 bits get converted to 32b and
> back to 24b before being written to disk?

yes, it applies to all ecasound use-scenarios (data flow inside ecasound
is based on passing around buffers of float samples).

Adding special-case handling for the "fixed-A -> float -> fixed-B" case,
where A and B are the same, has been considered in the past, but so far
not actually implemented. The needed change is not a major one, but
one needs to be _very_ careful so that other uses of ecasound are not

>> Neither ecasound or libsndfile supports dithering (although it is on
>> libsndfile's todo-file and will be utilized by ecasound also when ready),
> While dithering would be an improvement, it is a complicated subject
> and there are so Many ways to do dithering..

Yes, and that's why I'd prefer to utilize common implementations: JACK
implements various dithering algorithms for audio device i/o, libsndfile
will hopefully do the same for file i/o. Those are the only places where
we must convert from/to fixed-integer sample presentation.

> For my audio recording purposes, any conversion is bad. The streams
> must be bit-accurate so that the integrity of the audio chain can be
> verified.

Ugh, then you'd need the above optimization (or just use arecord or some
other recording app -> qarecord might be a good canditate, it offers
native ALSA support plus multithreaded disk i/o subsystem for recording).
All JACK apps use float sample presentation internally, so you have the
same problem of fixed<->float conversions...

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Received on Fri Jun 10 04:15:05 2005

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