A basic question I'm afraid! Wikipedia https://en.wikipedia.org/wiki/FLAC (https://en.wikipedia.org/wiki/FLAC) says
FLAC supports only integer samples, not floating-point (signed/unsigned?). It can handle any PCM bit resolution from 4 to 32 bits per sample, any sampling rate from 1 Hz to 65,535 Hz in 1 Hz increments or from 10 Hz to 655,350 Hz in 10 Hz increments, and any number of channels from 1 to 8.
It doesn't say whether these integers are signed or unsigned though, and I can't find the information anywhere else. Is there a choice, if not which is it?
Flac samples are unsigned, FWIW. The significance of being unsigned is that a bits per sample of n represents a sample in the range zero to 2^(n-1). That almost misses the point of how Flac samples are actually represented, since they are a sequence of bits independent of any particular hardware representation of integers. They are stored as 4-32 bit (in practice only up to 24 bit in any current implementation) unsigned integer "words" stored in big-endian format. An encoder can of course represent Flac samples any way it chooses internally, including as signed integers of a sufficient size.
A basic question I'm afraid! Wikipedia https://en.wikipedia.org/wiki/FLAC (https://en.wikipedia.org/wiki/FLAC) says
FLAC supports only integer samples, not floating-point (signed/unsigned?). It can handle any PCM bit resolution from 4 to 32 bits per sample, any sampling rate from 1 Hz to 65,535 Hz in 1 Hz increments or from 10 Hz to 655,350 Hz in 10 Hz increments, and any number of channels from 1 to 8.
It doesn't say whether these integers are signed or unsigned though, and I can't find the information anywhere else. Is there a choice, if not which is it?
This question doesn't make sense. What are you actually trying to do?
The output is signed or unsigned depending on whether the input was signed or unsigned. In other words the output is identical to the input, whatever that was.
I've concluded that so long as they are both integers (they are), it doesn't matter, as it's just a number so could be converted without loss from one to the other.
FLAC compressed 16-bit integers losslessly, so it doesn't matter whether they are signed or unsigned, you'll get the same values out that you put in (which has been said). However, the quality of the compression (i.e., the ratio) will suffer greatly is the wrong interpretation is used.
In other words, 16-bit audio in a WAV file is signed, and FLAC knows this and compresses as though the values 0 and -1 are very close together (and they are very close together in an analog signal). However, if you converted that to unsigned by adding 0x8000 then those two values become 0x8000 and 0x7FFF which are the furthest two values apart (opposite rails). The lossless compression ratio is going to be terrible. And it wouldn't sound so good either, but it would still be lossless.
I'm interested in this question as well as I'm now computing checksums for my ALAC files and want to know out of curiosity how the FLAC's internal checksums are computed.
Looking at the source code of flac, one can see what unsigned and/or big endian data are preprocessed into signed and little endian data before the actual encoding. This is true for 8,16,24 bit data https://github.com/xiph/flac/blob/27c615706cedd252a206dd77e3910dfa395dcc49/src/flac/encode.c#L2398