Am 14.10.24 um 09:11 schrieb Martijn van Beurden:
Op zo 13 okt 2024 om 22:33 schreef Stefan Oltmanns <stefan-oltma...@gmx.net>:
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.
That's indeed an interesting device.
Op ma 14 okt 2024 om 00:09 schreef Stefan Oltmanns <stefan-oltma...@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.
Unfortunately that doesn't seem to be the case. I just made a capture
that is > 30 Minutes with total samples set to 0 and a seek table: All
players I tried cannot seek in the file and cannot determine it's
length: VLC, Celluloid and DeaDBeef
I wondered why I can seek files I manually compressed with flac from
raw: The total samples field is just wrong, the seek table is correct,
having sample_numer values a lot higher than total samples. The players
display the wrong length. Seems to be a bug in flac.
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 fixed that manually after calling
FLAC__metadata_object_seektable_template_sort, according to metaflac
everything is correct:
for(int i = seektable->data.seek_table.num_points-1; i>=0; i--) {
if (seektable->data.seek_table.points[i].stream_offset != 0) break;
seektable->data.seek_table.points[i].sample_number =
0xFFFFFFFFFFFFFFFF;
}
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.
I have done that now, 2^18 seekpoints for a maximum length of 2^41
samples, that sould be enough.
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.
Just an idea: Of course the size of the total samples field cannot be
changed, and a seektable currently doesn't seem to work with total
samples = 0.
Is it possible to extend the specification with full backward
compatibility in a way like this?
If total samples is 0, the last seek point in the seek table that is not
a place holder can indicate the total number of samples:
In case frame_samples is 0, stream_offset is pointing to the last frame
and sample_number is the total number of samples.
FLAC__stream_decoder_get_total_samples would check that and return the
correct value.
Not sure if that breaks any old versions / 3rd party decoder. Another
option would be to use the first place holder, set stream_offset to the
total samples and some magic value in frame_samples. That should
definitly be backward compatible as these values are undefined for
placeholders.
Another option would of course be to add a new type of
metadata, like METADATA_BLOCK_STREAMINFO2, but I don't know how
backwards compatible that is.
Best regards
Stefan
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