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

[Alistair Buxton]

Another SDR user here. It was me who reported the bug where total samples
wraps around on overflow.

FLAC performs extremely well on SDR samples, both speed and compression
ratio. In my testing it outperforms any other free lossless codec by a
large margin, being 20% smaller and 10% faster than the next best (which
was ffv1). The problem is the metadata, and not just total samples. We also
can't put true values in the sample rate field because it doesn't have
enough bits (I have files with 35468950MHz nominal sample rate for
example), and there is no way to record that samples have been padded eg
from 10 bits to 16 bits, which seems to be very common in SDR applications.
These are just two examples off the top of my head - there are probably
more.

The problems around total samples and seek table allocation could be
alleviated by using multiple files as mentioned previously, but that
introduces a new problem: how do you know if you have the full set of
files? How do you know which file contains the nth sample? This would still
require extra metadata somewhere.

I would like to see this kind of thing put into a secondary metadata block
aimed specifically at SDR. This could be completely ignored by regular
audio players - these files are not meant to be listened to anyway. I could
probably figure out how to implement that, I even started looking into it
once, but I realised that 1. nobody would adopt it if it is just me behind
it and 2. I don't know enough to make it suitable for all use cases. So I
cannot and should not do this alone, as it would just be yet another
half-baked adhoc "standard" that causes more problems than it solves. For
example I had not even considered the idea of using multiple files.

On the topic of seeking, it is also a problem for my specific use case. I
am interested in digital signals (teletext) hidden in analog video (CVBS
samples), and they just aren't sequential in the same way that audio and
video usually are. I need to seek to an arbitrary video line as fast as
possible, preferably in constant time. A line is usually 2048 samples but
can vary between 1000 and 3000 samples long (always fixed size within a
given file, but can vary depending on the hardware used to record). A
typical recording will have 32 lines per frame (because the picture has
already been discarded), 25 frames per second, and be up to 12 hours long =
35 million lines and 70 billion samples. That would result in a ~700MB seek
table. And making the block size = 1 line also has a negative effect on
compression ratio. Finally since the files are huge I am storing them on
cheap USB HDDs which have terrible seek and read times to begin with. Note
though that very high resolution seeking is not really necessary for most
SDR uses and is mainly a quirk of the fact that teletext was one of the
earliest attempts at digital data transmission and has very small and fixed
size data packets that map directly to video lines. As such this would
probably be best handled with a custom, app-specific seek table in a
separate file, built on-demand and possibly stored on a faster disk.

-- 
Alistair Buxton
a.j.buxton at gmail.com
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