FLAC

Over the years, FLAC (Free Lossless Audio Codec) has become one of the most popular open source audio codecs.
It works like Zip; you can compress and decompress audio without loss.
As FLAC is optimized for audio, the compression is much better than with Zip.
Depending on the music the compression rate will be between 30%-50% most of the time.

 

FLAC supports linear PCM audio:

• 4 – 32 bits fixed point word
• 1 Hz to 655,350 Hz sample rate in 1 Hz increments
• 1 – 8 channels

FLAC's metadata system supports custom tags, cover art, seek tables and cue sheets.

 

The integrity of the audio data is insured by storing an MD5 signature of the original raw audio data in the file header, which can be compared against later during decoding or testing.

 

FLAC is very well supported.
The exceptions to this rule is Apple.

Apple don't support FLAC. Hence you can't use it with iTunes.

From Win10 (2015) on FLAC is supported by Microsoft.

How it works

FLAC uses linear prediction.
Since a straight line is defined by only two coefficients (y=ax+b), this is very efficient compared to all those individual samples. As you only need to store two coefficients, this saves a huge amount of space.

 

The bad news is that there is nothing linear about audio. The linear prediction is of course highly inaccurate. Compare the prediction to the actual samples and there will be a difference between prediction and sample, the so-called residue.

 

The trick: store both the coefficients + residues in a file. On playback the decoder can completely reconstruct the original signal by adding the residue to the linear prediction recreating the original sample.

 

A logical consequence is that FLAC is by definition VBR (variable bit rate).
The extremes demonstrate this nicely.

Convert a file filled with digital silence. The linear prediction is 100% accurate. Just one horizontal line is enough! The residues are zero, because there are none. This file will be incredibly small.

 

Convert a file full of random noise. There is nothing linear about it, so the prediction is 0 and the residuals are the same as the original sample. The file size is the same as the original.

 

In practice, you will encounter very different values. Music with a lot of distortion, such as punk, is compressed little because there is a lot of distortion (noise). Acoustic music, for example a sonata for violin and piano, might be 600 kb/s. A 24-bit recording will also compress poorly, because bits 20-24 are usually filled with noise.

 

Basically, FLAC's bitrate indicates how successful it has been in compressing the source losslessly.

Compression

You can choose from 0-8, where 0 is the lowest and 8 is the highest compression.

 

This "compression" parameter is a constant source of misunderstanding.
Many people think that it works like bit rates in MP3, a higher compression means more loss.

 

FLAC's "compression level" is the amount of resources FLAC is allowed to spend on finding the best possible linear prediction. The better this prediction, the smaller the residual, the smaller the file.

This affects the amount of CPU used during encoding.

During decoding (playback) you don't have to search for an optimal solution. The decoder simply calculates the linear prediction using the given coefficients and adds the residual.
FLAC is very asymmetric: encoding is slow, decoding is fast.

 

Because FLAC uses FIR Linear Prediction for the highest compression level, there is still a small performance penalty during playback compared to level 0.

 

A higher compression level affects the encoding time and to a much lesser extent the decoding time. On current CPU's you will probably have a hard time measuring the difference between level 0 and level 8, even during encoding. I/O is the limiting factor, not the CPU.

 

In practice, 5 is often recommended as a nice compromise between encoding time and file size.
Going from 5 to 8 results in a slightly smaller file.

 

When Josh Coalson started developing FLAC (2000), this parameter made sense. We are talking about PCs to anematic to run XP...

Nowadays, the CPU is no longer a limiting factor.

 

Again, regardless of the compression ratio chosen, the result is always lossless.

Bit rate

If you play a 16/44.1 track (CD audio) ripped to WAV you see a bit rate of 1411 Kbit/s.
Play the same track in FLAC format and you will see a lower value, often around 700.
This confuses a lot of people.
If you play MP3 you see a lower bit rate too so people start wondering if FLAC is lossy too.
As FLAC is compressed, the same information is stored using less bits (otherwise you can't get a smaller file….). The bit rate as displayed by your media player is the amount of bits per second reading the file, not the bit rate after expansion to linear PCM.

 

From the FLAC website:
With FLAC you do not specify a bitrate like with some lossy codecs. It's more like specifying a quality with Vorbis or MPC, except with FLAC the quality is always "lossless" and the resulting bitrate is roughly proportional to the amount of information in the original signal. You cannot control the bitrate much and the result can be from around 100% of the input rate (if you are encoding noise), down to almost 0 (encoding silence).

 

Obvious the compression and therefore the bit rate you see varies with the complexity of the signal.
You will see differences between tracks but also while playing a track bitrates will vary constantly.

In essence it is VBR (Variable Bit Rate) but lossless all of the time.

Uncompressed FLAC

From 14.1 on, dBpoweramp supports uncompressed FLAC

This sounds like we finally have WAV with excellent tagging options.

It is probably not plain PCM as some think.

 

You can create uncompressed FLAC with parameters
-0 --disable-fixed-subframes --disable-constant-subframes

Zero length

If you need a FLAC with zero length e.g. as a placeholder you can do so using:

 

flac --no-padding --force-raw-format --endian=little --sign=signed --channels=2 --bps=16 --sample-rate=44100 -o zero.flac - </dev/null

 

On Windows, just change /dev/null to NUL

integrity

Interface

You don't like typing command lines?

Try this one: FLAC frontend

 

 

1. FLAC – FLAC website
2. FLAC - Wikipedia
3. Uncompressed FLAC - J Mac
4. Ogg Vorbis I format specification - xiph.org
5. FLAC Frontend - ktficer
6. How to encode FLAC without any compression? - Hydrogen Audio