[Media_api] portsoundfile?

[Roger Dannenberg]

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