[flac-dev] C API: How to get a seektable for very long files?

[Martijn van Beurden]

Op zo 13 okt 2024 om 22:33 schreef Stefan Oltmanns <stefan-oltmanns at gmx.net>:
>
> Is the seektable written at the beginning of the file in the metadata
> block or can there also be a second metadata block at the end?
>

Only at the start of the file.

>
> If it's at the beginning, would it possible to reserve space for N seek
> points and during encoding remember a seek point after X samples,
> resulting in M seek points when encoding is finished. If M <= N all seek
> points are written, otherwise only every 2nd, 3rd etc.
> Is it possible to do that? The functions all expect a a total_samples
> argument, which is not known at the beginning.
>

No, you can only provide a seek table template with specific sample
numbers. There is no way to ask the encoder to add a seek point every
so many samples.

>
> The signal is the FM-modulated video signal of video tapes (like VHS).
> The idea is to capture the signal directly from the video head amplifier
> in the VCR and later demodulate/decode it in software, providing higher
> quality than traditional capture of analog video. See this project:
> https://github.com/oyvindln/vhs-decode/
> I started to design a capture device, as there is no 40 MHz continuous
> sampling hardware available at consumer prices:
> https://github.com/Stefan-Olt/MISRC
>

I've seen similar uses before. Maybe this one can serve as some
inspiration: https://www.youtube.com/watch?v=ZrEFU22C8l8 According to
that guy, he used cheap hardware.

Op ma 14 okt 2024 om 00:09 schreef Stefan Oltmanns <stefan-oltmanns at gmx.net>:
>
> I think there is another major issue for me: In
> METADATA_BLOCK_STREAMINFO the field for the length is only 36 bit,
> that's not even half an hour at 40 MHz sample rate, resulting in that
> the encoder sets it to 0 for longer captures. In the seekpoint the
> sample number is 64 bit, which is more than enough.
> But how does the decoder handle the seektable when the total number of
> samples is unknown? Or does the seektable override the info from
> METADATA_BLOCK_STREAMINFO?

When a suitable seektable is found, it overrides the information from
streaminfo, yes.

>
> I used this functions now to add seekpoints, but all remain placeholders
> according to metaflac:
>
> FLAC__metadata_object_new
> FLAC__metadata_object_seektable_template_append_placeholders
> FLAC__stream_encoder_set_metadata
> (encoder init & loop)
> FLAC__metadata_object_seektable_template_sort
>

Yes, that is correct, because you asked for placeholder points. You
should ask for spaced points. I just tested what happens if you make a
seek table template with a total_samples that is bigger than the
eventual total_samples that is encoded, and found a bug in the
encoder. It works, but the resulting seek table isn't valid. You could
try to use that approach anyway in the meantime, perhaps it works just
fine?

I'd say, have your implementation prepare a seek table template for 5
hours of recording (I assume that is above the upper bound of such
captures?), the stream encoder will fill in those seek points when it
reaches them, and leaves the unused ones unfilled. I will work on a
fix, so the stream encoder converts those unused points to placeholder
points in the future.

The stream encoder works pretty simple: you give it metadata to add to
the start of the stream, it adds those verbatim. After encoding is
finished, it will update streaminfo and seektable. There is no
metadata at the end of the stream, and the metadata blocks should not
be changed during encoding.

I see the difficulty here now by the way: metaflac also refuses to
write a seektable when the streaminfo metadata block specifies 0 total
samples, which is unavoidable in your case.