Axel pointed out the sf_command interface and SFC_SET_CLIPPING parameter
in libsndfile. You're right! It's strange to change the scale factor
when you turn on clipping, but that's what happens. I still think it's
asking for trouble to scale differently on input and output by default,
but at least there's a simple workaround.
However, I don't think ogg vorbis is supported -- not only did Eric just
tell me a couple people started but didn't finish an implementation, I
just downloaded libsndfile-1.0.17 and ogg_open() in ogg.c returns
SFE_UNIMPLEMENTED. Is there really an Ogg Vorbis implementation?
Michael asked about distortion. Here's my analysis: reading 16-bit pcm
samples scales them by 1/32768. Writing scales by 32767, so the net
scale factor (with libsndfile default behavior) is 32767/32768. This
does not change values below 16K (the rounding bumps the samples back to
their original values) but subtracts one for values above 16K (let's
keep it simple by just looking at positive samples). So if you look at
the input-to-output mapping, it's equivalent to passing the signal
through a function that's linear except for a least-significant-bit
glitch at 16K. This is admittedly a small distortion, but with people
already questioning whether 16 bits is adequate for audio, wiring a
non-linearity into audio I/O seems unwise. If you iterate this map, as
in reading and writing a file repeatedly, each iteration subtracts 1
from all samples over 16K, making the nonlinearity grow. Eventually,
everything above 16K is simply clipped. A similar thing happens with
rounding toward zero (as in libaudiofile), except the non-linearity is
at 0 and if you iteratively apply rounding, the problem does not get
worse. Eric (libsndfile) argues that the error is small and if you are
iteratively reading and writing integer samples, you are probably doing
something wrong. I agree, and my my main concern is just that having two
different mappings between integers and floats is bad practice. If I
pipe a file through a software mixer with gain set to unity, shouldn't
the samples come back out without modification? The SFC_SET_CLIPPING
option appears to fix this behavior.
-Roger
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