|Use||Digital signal processing|
lossyWAV is a new free lossy pre-processor for PCM audio contained in the WAV file format. It reduces bit depth of the input signal, which, when used in conjunction with certain lossless codecs, reduces the bitrate of the encoded file significantly compared to unpreprocessed compression. lossyWAV's primary goal is to maintain transparency with a high degree of confidence when processing any audio data.
- 1 History
- 2 Indicative bitrate reduction
- 3 File identification
- 4 Quality presets
- 5 Supported input formats
- 6 Codec compatibility
- 7 Using lossyWAV
- 8 Frequently asked questions
- 9 External links
lossyFLAC is an idea started by 2Bdecided at Hydrogenaudio, utilising the wasted bits feature of the FLAC lossless codec with the aim of transparently reducing audio bit depth (making some lower significant bits (LSB's) zero), consequently taking advantage of FLAC's detection of consistently-zeroed lower significant bits within each single frame and significantly increasing coding efficiency. In this way the user can enjoy audio encoded using the same codec (which may be all important from a hardware compatibility perspective) at a reduced bitrate compared to the lossless version.
Subsequently, lossyFLAC proved itself to work with other lossless codecs, so the application name was changed to lossyWAV.
Since then, Nick.C has heavily developed and built upon lossyWAV, with valuable tuning performed by halb27 at Hydrogenaudio.
Indicative bitrate reduction
It must be stressed that lossyWAV is a pure variable bit rate pre-processor. Bits-to-remove from the audio data are calculated on a block-by-block basis (codec-block length = 512 samples, 11.6msec @ 44.1kHz) using overlapping fast Fourier transform (FFT) analyses of at least two lengths (default quality preset (-q 5) = 64 & 1024 samples). After some manipulation, the results of each FFT analysis for a specific codec-block are then grouped and the minimum value used to determine bits-to-remove for the whole codec-block. Bit removal adds noise to the output, however the added noise has been pre-calculated and its level will be at or below the noise floor of the codec_block in question. Each sample in the codec-block is then rounded such that the first <bits-to-remove> lsb's are zero. In this way the wasted bits feature of FLAC et al is exploited.
|lossyWAV Test Set||Version||FLAC -8||-q 10||-q 9||-q 8||-q 7||-q 6||-q 5||-q 4||-q 3||-q 2||-q 1||-q 0|
|53 sample "problem" set||1.0.0b||784 kbit/s||654 kbit/s||626 kbit/s||596 kbit/s||565 kbit/s||534 kbit/s||501 kbit/s||470 kbit/s||447 kbit/s||408 kbit/s||366 kbit/s||329 kbit/s|
|lossyWAV Test Set||Version||FLAC -8||--insane||--extreme||--standard||--portable||-q 0|
|55 sample "problem" set||1.0.1o||784 kbit/s||652 kbit/s||580 kbit/s||504 kbit/s||421 kbit/s||321 kbit/s|
lossyWAV-processed WAV files are named with a double filename extension, .lossy.wav, to make them instantly identifiable. e.g. ".lossy.flac" would indicate an audio file which was processed using lossyWAV, and subsequently encoded using FLAC.
The --correction parameter is used when processing to create a correction file which is named with the .lwcdf.wav double filename extension. When "added" to the corresponding .lossy.wav, using the -merge parameter, the original file will be reconstituted.
Combinations of lossyWAV with each specific encoder are referred to as lossyX, where X is an abbreviation of the lossless codec name. Combination names are listed in the "known supported codecs" section below.lossyWAV inserts a variable-length FACT chunk into the WAV file immediately after the FMT chunk. This takes the form:
fact/<size>/lossyWAV x.y.z @ dd/mm/yyyy hh:mm:ss, -q 5Where the version, date & time and user settings are copied. Additionally, if a lossyWAV FACT chunk is found in a file, the processing will be halted (exit code = 16) to prevent re-processing of an already processed file.
The -check parameter can be used to determine whether a file has previously been processed without trying to process it, exit code = 16 if already processed; exit code = 0 if not.
- -q 10 to -q 8: Highest quality presets, disc space-saving alternative to lossless archiving for large audio collections, considered to be suitable for transcoding to other lossy codecs;
- -q 7 to -q 6: High quality presets, disc space-saving alternative to lossless archiving for large audio collections;
- -q 5: Default preset, generally accepted to be transparent;
- -q 4 to -q 0: DAP quality presets of reducing bitrate with reducing quality preset number, for usage on a compatible DAP. 
New quality preset synonyms introduced at 1.0.1j/k (and -q <n> quality parameter moved to advanced settings):
- --insane: (-q 10)
- --extreme: (-q 7.5)
- --standard: (-q 5)
- --portable: (-q 2.5)
All tuning has been performed on quality preset -q 5 with higher presets being more conservative. Quality preset -q 5 is generally accepted to be (and from testing so far is) transparent. If you find a track which -q 5 fails to achieve transparency after processing, please post a sample (no more than 30 seconds) in the development thread.
Supported input formats
- WAV: 9-bit to 32-bit integer; 1 to 8 channels; sample rate ≥ 32kHz PCM. Very high sample rates (>48kHz) have not been extensively tested. Tunings have been focussed on 16-bit, 44.1kHz samples (i.e. CD PCM).
|Codec||Supported||Encoder parameters||Combination name|
|FLAC||Yes||-5 -b 512 --keep-foreign-metadata||lossyFLAC|
|MPEG-4 ALS||Yes||-l -n512||lossyALS|
A comparison of portable media players is here, which shows FLAC and WMA Lossless compatibility among listed players. Any player supported by Rockbox can use FLAC or WavPack files after installing Rockbox.
lossyWAV 1.0.0b, Copyright (C) 2007,2008 Nick Currie. Copyleft. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful,but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. Usage : lossyWAV <input wav file> <options> Example : lossyWAV musicfile.wav Quality Options: -q, --quality <n> quality preset (10=highest quality, 0=lowest bitrate; -q 5 is generally accepted to be transparent) default=-q 5. Standard Options: -c, --check check if WAV file has already been processed; default=off. errorlevel=16 if already processed, 0 if not. -C, --correction write correction file for processed WAV file; default=off. -f, --force forcibly over-write output file if it exists; default=off. -h, --help display help. -L, --longhelp display extended help. -M, --merge merge existing lossy.wav and lwcdf.wav files. -N, --noclips set allowable number of clips / channel / codec block to 0; default=3,3,3,3,2,1,0,0,0,0,0 (-q 0 to -q 10) -o, --outdir <dir> destination directory for the output file(s). -v, --version display the lossyWAV version number. Special thanks: David Robinson for the method itself and motivation to implement it. Don Cross for the Complex-FFT algorithm used. Horst Albrecht for valuable tuning input and feedback.
Example Foobar2000 converter settings
Example flossy.bat file called from Foobar2000
@echo off z:\bin\lossyWAV %1 %3 %4 %5 %6 %7 %8 %9 --below --nowarnings --quiet z:\bin\flac -5 -f -b 512 "%~N1.lossy.wav" -o"%~N2.flac" del "%~N1.lossy.wav"
Frequently asked questions
- Question: Is it VBR?
- Short answer: Yes.
- Question: Is it transparent?
- Short answer: Almost certainly.
- Question: Is it lossless?
- Short answer: No.
- Question: Will it ever have a CBR mode?
- Short answer: No.
- Question: Why should I use this?
- Original lossyFLAC thread Where David Robinson (Replay Gain developer) introduces the method and a MATLAB implementation.
- Current development thread You will find the latest release candidate and latest beta version in post #1 of this thread.