Although I think the utility for this is limited (how often do any of us work with 4 bit samples?) i think it could still be useful for the other aspects of a FLAC container for audio besides compression, like a stored mD5 or other metadata that wav does not offer.

The problem is that at some point (which isn't really clear-cut) storing lower bit depths cannot be compressed by FLAC. The residual coding stage cannot in any way use less than 1 bit per sample, so FLAC is never able to compress 1-bit streams (like DSD) with the methods currently implemented. Similarly, performance for 2-bit and 3-bit streams will probably be very poor.

The 4-bit boundary is somewhat arbitrary.

The following is kinda a moot discussion - who has ever seen a 3-bit file?

Quote from: ktf on 2023-10-08 13:41:03

The 4-bit boundary is somewhat arbitrary.

In the sense that nothing in the code forbids 2 or 3 "except an IF condition that has a 4 because that is what the specification says", right?
IDK whether 1 could lead to weird things.

sox input.flac output.flac dither -p 2

I take note of your explanations. Thank you very much for answering my questions (even if the last one was perhaps not clear, (for which I apologize), you have answered it)!

FLAC compresses this using wasted bits of course. On the rarewares test samples set, I get a file size of 76% of the uncompressed 2-bit data (if you could store it in uncompressed form without padding). The audio sounds like an AM international broadcast: just barely intelligible, almost completely drowned in noise.

]$ sox "Dan Worral - I Won The Loudness War.flac" -n stats
             Overall     Left      Right
Pk lev dB     -24.08    -24.08    -24.08
RMS lev dB    -24.25    -24.25    -24.25
...
Bit-depth       1/5       1/5       1/5

]$ ls -l "Dan Worral - I Won The Loudness War."*
...   862311 ... 'Dan Worral - I Won The Loudness War.flac'
... 15209036 ... 'Dan Worral - I Won The Loudness War.wav'

The version in the attachment uses only 12th bit in 16-bit container (and a digital silence between the fragments):

$ flac -ac Dan\ Worral\ -\ I\ Won\ The\ Loudness\ War.flac | grep wasted | less | awk '{print $2}' | sort | uniq -c

flac 1.4.2
Copyright (C) 2000-2009  Josh Coalson, 2011-2022  Xiph.Org Foundation
flac comes with ABSOLUTELY NO WARRANTY.  This is free software, and you are
welcome to redistribute it under certain conditions.  Type `flac' for details.

Dan Worral - I Won The Loudness War.flac: done
    332 wasted_bits=0
    958 wasted_bits=11
    568 wasted_bits=12

So, there are 332 frames with no wasted bits (these are frames with only 0 or 1 samples)

958 frames with 11 wasted bits

and 568 frames with 12 wasted bits

]$ flac -ac "Dan Worral - I Won The Loudness War.flac" 2>/dev/null | grep -B1 'subframe=0.*wasted_bits=12' | grep assignment | cut -f7 | sort -u
channel_assignment=RIGHT_SIDE

]$ flac -ac "Dan Worral - I Won The Loudness War.flac" 2>/dev/null | grep -B5 'subframe=1.*wasted_bits=12' | grep assignment | cut -f7 | sort -u
channel_assignment=LEFT_SIDE
channel_assignment=MID_SIDE

Here is a DSD-like (1-bit 705.6kHz) wav file which can be encoded to flac by using --lax.

There's always "I Won The Loudness War" by Dan Worrall

That's one mofo of a file.
[...]I wonder if the original lossless PCM (that hasn't been through Opus for youtubing) is just as wild compression-wise.

% of WAVE, that is, lower is better. It makes a fool out of any OptimFROG except the lightest mode