On Aug 2, 2021, at 08:40, Federico Miyara <fmiy...@fceia.unr.edu.ar> wrote:
> 1) Is there a way to get the audio size (number of samples) and other
> information, such as number of channels, from a flac file without fully
> decoding it?
Yes. There is a specific header with this information. Look for the
documentation of the FLAC format for reference, and then look through the
library for calls that might return this information.
I usually start from https://xiph.org/flac/documentation_format_overview.html
for the official specification of the file format.
Note that FLAC is mostly a streaming format, where metadata is not generally
used. But the file does have this information early on. See
https://xiph.org/flac/format.html#def_STREAMINFO where it says that the
STREAMINFO packet must be the first packet in the file.
> I've found that the WAV header is replicated after the "riffRIFF" keyword,
> but I don't seem to be able to predict where it is located or whether it is
> safe or not to asume that the first time such keyword appears is the correct
> one, and if there is an upper bound for its location; for instance, some text
> such as the name of a song or some comment could contain that keyword, even
> if it is unlikely. The information preceding the "reference libFLAC 1.3.3"
> encoder version seems to be non-text information.
You should not use the RIFF information. It is not part of the FLAC
specification. It is an optional enhancement to store information that is only
about the input WAV files. It will strictly be missing from FLAC files that are
recorded live, converted from AIFF, and will even be missing when WAV is the
source if the option is not added.
> 2) I decode using the option --skip=0 --until=1. I would expect to get a wav
> file with only 1 sample, but I get 3 samples. Stictly no sample should be
> decoded since 0 samples should be skipped and the help says:
>
> --until={#|[+|-]mm:ss.ss} Stop at the given sample number for each input
> file. The given sample number is not included in the decoded output.
>
> so sample #1 should not be included, unless the first sample is sample #0, in
> which case only 1 sample, and not 3 samples, should be included in the output
> file.
Are you seeing 3 bytes for 1 sample? ... or are you seeing 3 samples?
Also, I recall that the FLAC library returns 32-bit numbers, so you have to
convert these to 16-bit or 24-bit samples.
Brian
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