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Near-lossless / lossy FLAC

This is an (unoriginal) idea / work in progress. I make no claims for it, but it might be interesting or useful for someone. It is not competitive with wavpack lossy. It is not "finished" either! As far as I know, it is 100% compatible with existing recent lossless FLAC implementations.


The idea is simple: lossless codecs use a lot of bits coding the difference between their prediction, and the actual signal. The more complex (hence, unpredictable) the signal, the more bits this takes up. However, the more complex the signal, the more "noise like" it often is. It's seems silly spending all these bits carefully coding noise / randomness.

So, why not find the noise floor, and dump everything below it?

This isn't about psychoacoustics. What you can or can't hear doesn't come into it. Instead, you perform a spectrum analysis of the signal, note what the lowest spectrum level is, and throw away everything below it. (If this seems a little harsh, you can throw in an offset to this calculation, e.g. -6dB to make it more careful, or +6dB to make it more aggressive!).


How is this applied to FLAC? FLAC has a nice featured called "wasted_bits". If it finds all bits below a certain bit are consistently zero, it simply stores: "the bottom 3 bits are all zeros" and then takes no more effort in encoding them. It checks this once per frame. In FLAC frames can be variable length, but current encoders use a fixed 4096 sample length.

This means if you have a 24-bit file, but it only contains 16-bit audio data (i.e. the bottom 8 bits are zero throughout) then FLAC encodes it just as efficiently as a 16-bit file. The only overhead is a few bits every 4096 samples saying "wasted_bits=8".

It also means that if, say, you have a normal 16bit CD and you find the noise floor during a certain 4096 samples never falls below the 12th bit, you can set bits 13-16 to zero, then feed the result to FLAC, and it will automatically use a lower bitrate for that frame than if you fed it all 16 bits.

Hence "lossy FLAC" is a wav pre-processor for regular lossless FLAC. The interim stage is a "lossy" wav file with 0s in some least significant bits. The final output is a 100% compliant FLAC, which faithfully reproduces this "lossy" wav file. The lossy stage is therefore the pre-processor, and the processed "lossy" wav file, when encoded to FLAC, results in a lower bitrate than the original wav file when encoded to FLAC.


Potentially the quality is very near to what you started with, and more than good enough for many applications. In most places where mp3 doesn't work, I believe that lossy FLAC will.


On music which FLAC already compresses very well, lossy FLAC gives little advantage. Often it does exactly nothing (full 16 bits preserved), or nearly nothing (the last bit or two dropped occasionally). On music which causes the FLAC bitrate to go comparatively high, lossy FLAC usually brings a significant gain. I've seen bitrates fall by 20%-50%. Still, it's not low bitrate encoding, and it's pure VBR.


Problem samples? I don't know - I'm hoping some HA regulars can lend their ears and detective skills here. Standard lossy codec problem samples are probably not that relevant. Wavpack lossy problem samples are more relevant, but lossy FLAC does seem to spot some of these and either quantises less aggressively or not at all (i.e. encoding is pure lossless).


So what can people download? Well, sadly, I'm not a C programmer. I'm attaching a MATLAB script that works as a lossy FLAC pre-processor. You run a .wav file through this, and then encode it to FLAC as normal.

If you haven't got MATLAB, but have an idea for a useful sample to test, upload it to HA (maximum 30 seconds; shorter=better because MATLAB is slow and the code isn't optimised at all!) and I'll upload a lossy FLAC version when I get a chance.


I'll post more about the algorithm later.

Cheers,
David.

P.S. the attachment should be "lossyFLAC.m" but HA won't allow me to upload .m, so I've changed it to .txt.

Near-lossless / lossy FLAC

Reply #1 –
For those who don't want to read MATLAB code but want to know what's happening...

The algorithm is quite simple. Pick two FFT sizes - one long one, useful for catching tonal signals, one short one, useful for catching transients. Find out where the quantisation noise due to truncating at each bit will fall in these sized FFTs. Store this data in a look-up table.

Now go through the audio file. For each 4096-sample block, look at the long and short FFTs across that block separately, and find the lowest value in each, look up the implied number of wasted bits for each, and then use the lowest value of wasted bits to round the audio in that block to that many bits.

Job done.

However, there are some "bodges" in there.

Firstly, a frequency range is specified. FFT bins outside this frequency range won't be checked. Otherwise, a sharp 20kHz low pass filter in the original would force "wasted_bits" to zero simply to maintain a -96dB noise floor above 20kHz.

Secondly, the FFT's are "spread" before finding the lowest value. This isn't some clever psychoacoustic ear/masking spreading function - it's just a simple average. The reason is quite simple: in almost any windowed FFT, you'll get some bins into which almost no energy falls. This really isn't significant, but if we didn't ignore these bins, they'd force us to keep all the bits all the time. As it is, I've averaged over 4 bins using a rectangular spreading function before finding the lowest. If this gives you cause for concern, this should allay your fears: there are still enough low bins that 8-bit dither, pasted into a 16-bit file, is still encoded with 10-bit accuracy! In other words, when encoding pure noise, there's still a 2-bit "safety margin". Whether this works for all signals is one reason I'd like to people listen.

Thirdly, it's trivial to shift the thresholds, so I've put that feature in, though set it to 0 by default.


There are issues which remain to be solved.

It seems to work OK with clipped files, which is a surprise, because a positive clipped integer sample (e.g. 16bit audio) is all ones, hence wasted_bits=0. I need to look into this. Converting to 24-bits and dropping the audio by 6dB would be a solution (already implemented) if this was a problem.

There is no checking of the mid or side channels yet. Ideally, the algorithm should check mid and side in the same way as left and right, and pick the global noise floor. One caveat is that any channel which is digital silence (or "near" digital silence - there's a can of worms) needs to be ignored.

You can run many many generations with lossy FLAC before problems arise. I've gone to 50 generations with trivial processing and dithering at each generation. The quantisation noise was 1-2 bits higher in the 50th generation than in the first lossy FLAC generation. If this is a problem (I couldn't hear it) I assume you could set a -12dB noise threshold offset to solve it, though the efficiency would decrease dramatically.

Finally, this will lead to FLAC files that look like they're lossless (because FLAC is normally lossless) but are in fact lossy. Never fear! A simple utility (someone else can write one) to check the value of "wasted_bits" will soon tell you what you have. Real FLAC files almost almost never have non-zero "wasted_bits". lossy FLAC files will have load.


To answer the obvious question about bitrates, here is a screen grab from foobar2k showing the bitrates of some wavpack lossy problem samples in lossy FLAC.
[attachment=3286:attachment]


Here is an unrelated file containing a random mixture of music samples from a recent listening test.
This is the waveform (top view) and lossy FLAC quantisation noise (bottom view)
[attachment=3288:attachment]

This is a graph of the number of bits removed (i.e. the quantisation / rounding level) in each FLAC frame/block:
[attachment=3287:attachment]

Obviously the quantisation noise and number of bits removed are correlated (perfectly)..

Cheers,
David.

Near-lossless / lossy FLAC

Reply #2 –
Here are some examples - only a couple, because I'm on dial up.

The originals are elsewhere on HA - do a search if you want to grab them to compare.


Penultimate comment: I have no plans for a lossy+correction=lossless version. It would be possible to do it crudely with two FLAC files (one lossy, one residual) and adding them; or smartly by integrating this within FLAC itself. Not my job. Not sure it's worth it.

Finally (for now), as discussed in a recent thread, if you can't hear above 16kHz, then you can often reduce FLAC bitrates by resampling to 32kHz. Combining that with lossy FLAC pre-processing brings the bitrate down still further. I'm almost tempted to use it.


Let the hunt for problem samples begin!

Cheers,
David.

Near-lossless / lossy FLAC

Reply #3 –
Interesting approach.

I did something similar in the very early days (1997) of TAK. Well, i haven't used your FFT approach but something more simple but nevertheless efficient.

I remember that the frame size was very important. 4096 samples is definitely too much! The signal amplitude will often change considerably in those 93 ms. You will have to keep too many bits to avoid distortions in the frame parts with low amplitude.

I don't know if i still had golden ears in 1997, but for me my bit reduction approach was transparent at about 440 kbps. Well, should be considerably less wth TAK's later compression improvements...

  Thomas

Near-lossless / lossy FLAC

Reply #4 –
So ... basically you are applying a variable or 'gliding' noise gate if I understood correctly?
The name was Plex The Ripper, not Jack The Ripper

Near-lossless / lossy FLAC

Reply #5 –
that was my hunch too, that for noisy samples you might get better results with shorter blocks.

clever idea.  in practice I think the file should also be tagged with the preprocessing parameters so it could be identified without analyzing all the frames.

Near-lossless / lossy FLAC

Reply #6 –
Quote from: jcoalson on
that was my hunch too, that for noisy samples you might get better results with shorter blocks.

clever idea.  in practice I think the file should also be tagged with the preprocessing parameters so it could be identified without analyzing all the frames.


You would have better control of the noise floor with shorter (or variable) blocks, but my guess is there would be some kind of trade-off as shorter blocks would often make FLAC itself less efficient. How efficient is FLAC with, say, 1024-sample blocks?

I agree it would be sensible to "tag" the files as lossy in some way, but it should be a way which isn't easily removed by careless use of a tag editor. This implies something at the frame level.


These are both things which can only be done from within the FLAC encoder. I am not skilled enough to start playing around in there myself!

Cheers,
David.


Quote from: JeanLuc on
So ... basically you are applying a variable or 'gliding' noise gate if I understood correctly?


Kind of. Technically it doesn't remove noise, since by definition any change to the signal is unwanted, and hence "noise". So it actually adds more noise, at/below the existing noise floor.

The only exception is if you force the threshold up (i.e. make it much more aggressive), and then it's just possible that it could quantise a noise floor (in isolation) out of existence - i.e. if the signal consists of white noise at -90B, it could quantise it to all zeros. I must stress that this isn't default behaviour - you'd have to raise the thresholds by 12dB or more to make this happen. By default, it will preserve all noise, but often add a little more noise several dB below the existing noise - a change which I suspect is both inaudible and almost always irrelevant.

Cheers,
David.

Near-lossless / lossy FLAC

Reply #7 –
I've attached some lossy and lossless files for ABXing if anyone is interested / willing.

These were grabbed from various threads about wavpack lossy problem samples, since these are the most likely to cause problems with lossy FLAC.

If anyone has any other potential problem samples, please let me know / upload them.

Cheers,
David.


I forgot to mention...

If anyone thinks this is useful enough to code into a real programming language, please do!

If anyone wants to adapt this idea for other lossless codecs, feel free.

If anyone wants to argue that I should have included dither, please don't. Used properly, the signals are self dithering at the chosen quantisation level (so it's largely unnecessary) and dither adds an extra bit of noise which reduces the efficiency of the whole process (i.e. you have to keep ~one more bit of data than you would otherwise just to counteract the dither).

Most importantly, I'm asking if people can listen, ABX, and find problem samples. I've never been sensitive to background noise, so I don't know if this approach works fine as it is, needs tweaking, or is useless.

Cheers,
David.

Near-lossless / lossy FLAC

Reply #8 –
I am sensitive to this noise with wavpack and dualstream at 250 k. A casual abx: all is good thus far. Avg bitrate is 550k. I don't know how to compare though as wavpack and dualstream are fine at 350 k even on most hard stuff.
wavpack 4.8 -b256hx6c

Near-lossless / lossy FLAC

Reply #9 –
Thanks. If you know of anything which wavpack and dualstream can't handle at 350k, that might be an interesting test.


The lossy FLAC bitrate will never be competitive in this incarnation for two reasons:

1) it's just a preprocessor, so it has to work within the limits of the host format (in this case, FLAC).
2) it doesn't use any noise shaping.

It would be interesting to see the lossy FLAC method of setting the noise floor integrated into something like wavpack lossy, with or without wavpack lossy's noise shaping.


btw, the most interesting problem sample I found was "short block test 2". Lossy FLAC does absolutely nothing to it, judging the noise floor to be at or below the 16th bit. Hence it gets encoded losslessly at 137kbps.

Cheers,
David.

Near-lossless / lossy FLAC

Reply #10 –
I am opposed to calling any lossy implementation of FLAC still FLAC. FLAC has positioned itself as a "Free Lossless Audio Codec" (it's name) and changing the name or what it means now would be detrimental and confusing to users I believe. FLAC has also stood for LOSSLESS -- that's why so many people use and like it (no loss of audio quality/data).

Please call your variant LFLAC or something other than FLAC please to avoid confusion with older decoders/hardware implementations which likely won't support any lossy variant. Thanks!

Near-lossless / lossy FLAC

Reply #11 –
Support for the LFLAC name (or possible Lossy Free Audio Codec - LFAC?).....

I love the idea of LFLAC - I recently set my PC transcoding individual track FLAC > whole album OGG and it sat for 8 hours or so. The reason I picked OGG is due to the predisposed good opinion of it on these forums and the fact that GSPlayer plays it on my iPAQ.

However, I would be interested in LFLAC as a portable variant of my FLAC collection.......
lossyWAV -q X -a 4 -s h -A --feedback 2 --limit 15848| FLAC -5 -e -p -b 512 -P=4096 -S-

Near-lossless / lossy FLAC

Reply #12 –
Quote from: goodnews on
Please call your variant LFLAC or something other than FLAC please to avoid confusion with older decoders/hardware implementations which likely won't support any lossy variant. Thanks!


As currently implemented, it is a pre-process to a standard FLAC encoder.

As such, it is 100% compatible with all FLAC compliant decoders, requires no change to the format, and the final file will be a standard .flac file from a standard FLAC encoder.

Given your concerns, this should scare you far more than the name (which can be anything - well, anything sensible).


However, I've already addressed this concern earlier in the thread: if users can't be trusted to tag (or not to untag) lossy FLAC files properly, the only way to recognise them is from something at the FLAC frame level (the "wasted_bits" data already tells you what is happening), or by spotting rows of 0s in the LSBs of the decoded audio data.


If an incompatible "LFLAC" format can do the job better (i.e. more efficiently; same performance in fewer bits) than standard FLAC with the lossy FLAC pre-processor, then it'll probably be created, and you'll have nothing to worry about.

However, the beauty of lossy FLAC (as a pre-processor) is that it's compatible with all the FLAC implementations out there. Unless "LFLAC" brings big advantages, making an intentionally incompatible "LFLAC" format just to hold lossy FLAC data won't stop the problem you envisage: On day 1, nothing will play it back, but it could easly be transcoded losslessly (i.e. maintaining the same losses!) into standard FLAC, maintaining the bitrate advantage and playing back correctly on everything that supports FLAC. So if I or someone else were to force a different lossy FLAC / LFLAC format onto the world, people would transcode it to standard FLAC to get it to play on various devices. Then there would be exactly the same "lossy FLAC" files that I've provided above.


Look at it this way: at least with lossy FLAC there's an easy way to check that it's lossy. However, if someone transcodes a traditional high bitrate lossy file without a lowpass to FLAC and gives it to you, the only way of knowing is by listening.


It's ironic that you're facing this problem because FLAC is open source. If it was closed source, I'd never have been able to do this.

Don't panic though. This is still at the "proof of concept" stage. It might not work. If it does work, I'm sure someone will implement it properly, and they might not base that implementation on FLAC at all.

Cheers,
David.

Near-lossless / lossy FLAC

Reply #13 –
This sounds like something that could be achieved through collaboration with FLAC's developer - add a command to FLAC.EXE to select a "quality" which might equate to how aggressive the algorithm is and output to a FLAC file.

The issue of "is it a lossless FLAC file or a lossy FLAC file" would remain for those who do not create their own FLAC files - however a simple checksum of the resultant WAV file (and an Accurate Rip style database) would instantly indicate whether the file was lossy / lossless.

Good luck with implementation........
lossyWAV -q X -a 4 -s h -A --feedback 2 --limit 15848| FLAC -5 -e -p -b 512 -P=4096 -S-

Near-lossless / lossy FLAC

Reply #14 –
David,

I understand more about what you are attempting, but I still don't like FLAC being "forked" like this. Not that you can't do it legally (i.e. open source). But Josh has said before that FLAC hasn't been "forked" in all the years that it has been out, and I believe that "forking" it now would damage FLAC's reputation unless the name and file extension were changed.

When I see a FLAC file, I know it's lossless. FLAC is synonymous with lossless. Changing to to a "forked" lossy version where now a FLAC file could be lossless or could be lossy would confuse many people and IMO detract from the format's name and reputation among users that FLAC has built up all these years.

I suggest you use a different extension .LFL or .LFLAC instead of .FLAC to avoid any chance of confusion. Look at how Apple uses .M4A for lossy and now Apple lossless. You just can't always easily tell in 3rd part apps if you are playing a lossy or lossless file (other than perhaps by the file size). Many apps will choke on an Apple Lossless .M4A file as they think it is a MPEG 4 (AAC audio) file.

I'd hate to see the FLAC name and file extension "bastardized" to mean "it could be lossless or it could be lossy, your guess?" My vote is for FLAC to remain FLAC (LOSSLESS) and please choose some other file extension for a lossy implementation of FLAC, if you so desire to make one.

Near-lossless / lossy FLAC

Reply #15 –
goodnews,

I have no desire to damage FLAC. I would be quite happy to call "lossy FLAC" LFAC and have a .lfac extension.

The immediate problem I have with this is that I have to rename the .lfac files to .flac in order to get foobar2k (or anything else!) to play them. Everyone else in the world will face the same problem.


To be honest, if David (Bryant, wavpack developer) is interested, I think the method I'm using would sit better within his encoder.

Also Josh is free to put this in FLAC in a compatible but identifiable way.

We'll see. Let's figure out if it works first.

Cheers,
David.

Near-lossless / lossy FLAC

Reply #16 –
FLAC as such remains lossless of course.
You can never prevent people from doing pre-processing for whatever reason so you never know when getting a FLAC file whether it was preprocessed before encoding or not.
I have some (few) oldies tracks in my ape archive that are important to me and with which I did some preprocessing (denoising/declicking/bringing artificial brilliance to them cause they sounded pretty dull).
Sure these ape files are not identical with the original source (but I enjoy them a lot more).

Whenever you get a file from somebody else you're always in an unsecure position. The most probable issue isn't preprocessing but potential mediocre quality of the original source used (the FLAC file may be the Non-DRM version of a 128 kbps DRM-WMA track for instance). But you can decide by listening whether you like it or not.

David's idea is great to me just because it's a pre-processor machinery leaving the FLAC world as it is.
Moreover he has found a mechanism which may be valuable for other encoder developers. Maybe David Bryant can use the idea for bringing a quality control to wavPack lossy if he likes to.
lame3995o -Q1.7
opus --bitrate 140

Near-lossless / lossy FLAC

Reply #17 –
I don't see this as "damaging" FLAC at all - as halb27 said, you never know if the WAV input to FLAC has been processed in any way before encoding. As David said, the files are fully FLAC compliant and therefore are FLAC files - the fact that the input WAV was pre-processed is neither here nor there.

Now, if a Foobar based transcoder could be implemented, I could drop OGG and use the LFAC pre-processor to shrink my FLAC-for-iPAQ files..... (I just found the GSPlayer gspflac.dll file  )
lossyWAV -q X -a 4 -s h -A --feedback 2 --limit 15848| FLAC -5 -e -p -b 512 -P=4096 -S-

Near-lossless / lossy FLAC

Reply #18 –
Quote from: 2Bdecided on
The idea is simple: lossless codecs use a lot of bits coding the difference between their prediction, and the actual signal. The more complex (hence, unpredictable) the signal, the more bits this takes up. However, the more complex the signal, the more "noise like" it often is. It's seems silly spending all these bits carefully coding noise / randomness.

So, why not find the noise floor, and dump everything below it?


This is sort of what speech codecs do, actually.

your signal --[LPC analysis filter H(z)]--> pretty white noisy residual --[lossy coding]--> pretty white noisy residual + white noisy errors --[LPC synthesis filter 1/H(z)]--> your approximation with q-noise "hidden behind" your signal.

If you want a similar preprocessing for FLAC or WavPack you'd do something like this:
- estimate LPC filter coeffs (H(z)) and temporarily filter the block to get the residual
- check the residual's power and select "wasted_bits" accordingly
- quantize original (unfiltered) samples so that the "wasted_bits" least sigcificant bits are zero
- use 1/H(z) as noise shaping filter.

If you further check what psychoacoustic models usually do you'll notice that they allocate more bits to lower frequencies than to higher frequencies (higher SNR for lower freqs) most of the time. You then can tweak the noise shaping filter to W(z)/H(z) where W(z) is some fixed weighting so that you have a higher SNR for lower freqs.

This is actually what I did when I experimented with "high data rate steganography for audio carriers" and it worked pretty well. The only difference was that instead of zeroing LSBs i "simulated" data to be carried by randomly filling those LSBs which is like subtractive dithering.

BTW: FLAC's default blocksize is 4608, isn't it? The encoder's blocksize should match the preprocessor's blocksize so no "wasted bits" are coded. Also, for noisy transients it's good to be able to quickly change "wasted_bits" which suggests merging preprocessor + encoder into one program that uses variable length blocks. As a matter of fact I recently (couple of weeks ago) read the FLAC file format spec again to check wheter I should give it a try ...


Cheers!
SG

Near-lossless / lossy FLAC

Reply #19 –
Nice. This is the same way MPEG-4 SLS becomes lossy, there have been some good results reported with that.

Near-lossless / lossy FLAC

Reply #20 –
SebG,

Thanks for your response, but I'm confused. Are you saying what I've done is equivalent to what you describe? Or better/worse? Or didn't you look at what I'd done? As far as I can tell (and I know almost nothing about LPC analysis!) what I'm doing is more accurate, and gives a better "guarantee" of transparency.

As for skewing the noise or noise calculation towards lower frequencies - I intentionally don't want to put any psychoacoustics in there, other than some very simple assumptions which are required to make it work at all.

The FLAC block size is supposedly 4096, which is what I've used. It would make sense to use/try something smaller, but that's out of my hands.

Cheers,
David.
 

Near-lossless / lossy FLAC

Reply #21 –
Quote from: Nick.C on
This sounds like something that could be achieved through collaboration with FLAC's developer - add a command to FLAC.EXE to select a "quality" which might equate to how aggressive the algorithm is and output to a FLAC file.
right now I'm thinking it should remain outside any "flac"-named encoder since flac has always meant lossless.  if it turned out to be really useful then we could probably figure out a way to make it into a proper tool that also wouldn't cause confusion.

Quote from: SebastianG on
BTW: FLAC's default blocksize is 4608, isn't it? The encoder's blocksize should match the preprocessor's blocksize so no "wasted bits" are coded.
yes, you're right, they should definitely match.  the default blocksize switched to 4096 samples in 1.1.4

Quote from: SebastianG on
Also, for noisy transients it's good to be able to quickly change "wasted_bits" which suggests merging preprocessor + encoder into one program that uses variable length blocks. As a matter of fact I recently (couple of weeks ago) read the FLAC file format spec again to check wheter I should give it a try ...
I've been working on supporting variable blocksize properly; currently thespec is ambiguous in some cases... stay tuned.

Josh

Near-lossless / lossy FLAC

Reply #22 –
Not suggesting that you compromise the excellent reputation that FLAC has - it seems to be becoming a more mainstream codec with support in Volvo cars (of all things, but great start!).

I just like the idea of only using one codec for all my encoding / transcoding - and one that allows lossy coding in a container that will work exactly the same as the lossless parent version.
lossyWAV -q X -a 4 -s h -A --feedback 2 --limit 15848| FLAC -5 -e -p -b 512 -P=4096 -S-

Near-lossless / lossy FLAC

Reply #23 –
Quote from: Nick.C on
Now, if a Foobar based transcoder could be implemented, I could drop OGG and use the LFAC pre-processor to shrink my FLAC-for-iPAQ files..... (I just found the GSPlayer gspflac.dll file  )
How big is your iPaq's memory? Even with LFAC I don't think you'll fit more than 1 album's worth.

Slightly offtopic: Where'd you get the gspflac.dll??? I wanna!11!!! 

Near-lossless / lossy FLAC

Reply #24 –
Hi, David and Josh!

Quote from: 2Bdecided on
Are you saying what I've done is equivalent to what you describe? Or better/worse? Or didn't you look at what I'd done?

No, it's not equivalent to what you've done. It's just my interpretation of the text I quoted (noise floor not necessarily flat) and sort of a suggestion because I believe it to be a clever thing. But if you don't like the idea of shaping the noise at all your approach (= only introducing white noise below the threshold of hearing) is already as good as it can get, I suppose.

However, I'd like to note that by properly colouring the noise you can theoretically set more LSBs to zero (=> lower bitrate) while keeping the same subjective quality level. Of course, this "properly" is kind of a black magic component.  But even the simple W(z)/H(z) trick did well for me. (I derived W(z) by feeding OggEnc with mono pink noise). But the shaping strength could be softened for those too scared of psychoacoustics.

Quote from: jcoalson on
Quote from: SebastianG on
As a matter of fact I recently (couple of weeks ago) read the FLAC file format spec again to check wheter I should give it a try ...
I've been working on supporting variable blocksize properly; currently thespec is ambiguous in some cases... stay tuned.

Cool!. Could you clarify the "Notes" paragraph in the frame header section, please? What blocksizes are allowed if it's a variable length block stream? I'd use "1000-1111 : 256 * (2^(n-8)) samples" but it looks like they are not allowed.

Cheers!
SG
 
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