When it comes to audio, there are two types of compression and both are widely misunderstood, sometimes even by audio engineers. To briefly sum it up:
Data compression is used to reduced the size of computer files. Sound compression is used to affect the apparent loudness, energy level, or impact of sounds.
This post is Part 1 of 2 from Data Compression vs. Sound Compression. Today I’ll be explaining data compression and its two different subcategories. To learn about sound compression, stay tuned for Part 2.
Two Types of Data Compression
As written above, data compression (also known as file compression) means the size of the original audio file gets reduced. Depending on the type of data compression, though, sound quality may also be reduced.
When sending final mixdowns of a Fix Your Mix project, clients receive a .zip file and an MP3. Both of these files utilize data compression, however .zip format is lossless (temporarily compressed) while .mp3 format is lossy (permanently compressed). Both are necessary for different applications.
Lossless Data Compression
(Common file format extensions include .zip, .rar, and .sit)
Lossless data compression is temporary, which means that sound quality is not reduced. Once the file is decompressed (“decoded”), it goes back to its original file format and file size. Popular lossless data compressors include WinRAR, WinZip, and Stuffit Expander, as well as good old fashioned operating systems including Windows XP and Mac OS X.
Pros:
- Highest sound quality possible.
- Allows you to compress multiple files into a single file. This can be useful for internet transfers since web browsers do not allow you to download entire folders at once.
Cons:
- Files cannot be played back directly by audio players: they must first be decompressed (“extracted”) to their original format by the operating system. This may take up to several minutes depending on the speed of your computer.
- File sizes usually not as small as lossy formats (e.g. MP3)
Common Uses:
- Sending to a CD replication plant.
- Sending to a mastering engineer.
- Sending to a video production company for sync licensing to film or video.
Lossy Data Compression
(Common file format extensions include .mp3, .m4a, and .wma)
Lossy data compression is permanent, meaning sound quality is reduced. Popular lossy audio encoders include iTunes, which uses a proprietary codec, and LameBrain, which uses the LAME codec. Most DAW programs will export directly to lossy formats, however this option costs extra for ProTools.
Pros:
- Files can be played directly by audio players. (Files are decompressed by the audio player itself rather than the operating system.)
- Plays on iPods and other portable audio playback devices.
- Smaller file sizes than lossless formats.
Cons:
- Lower sound quality.
Common Uses:
- Posting to Myspace, Facebook, etc.
- Posting on a website or blog.
- Email attachments.
- Playback via iPods, cell phones and other portable audio playback devices.
The MP3 format consists of data compression and data compression only. I’ve heard several old school engineers mess this one up so allow me to reiterate:
Myth: MP3 encoders compress both the sound (like a compressor/limiter would do) and the data (to reduce file size).
Truth: The MP3 format is entirely, 100% data compression. No sound compression is involved.
In the future, I’ll be writing about sound compression in depth and the quality of MP3s.
Additional notes:
- WAV and AIF are both lossless file formats, however no data compression is involved so they are high in file size but may be played back instantaneously like an MP3.
- This section of this post regarding lossy data compression deals specifically with audio formats, but there are lossy compression formats for images and video as well, such as a .jpg file.
- The algorithms which comprise data compression formats are known as “codecs.” Some of these codecs excel in certain applications and not in others. Some have no advantages at all and were developed solely for branding purposes. For an example of the latter, there is no technical advantage to compressing audio to .wma format–it was developed so that Microsoft could force people to listen to music in their Windows Media Player.
so then how exactly does data compression of the lossy variety change the sound? can you give examples?
Here is a nifty little site that does some spectral comparisons of uncompressed and lossy-compressed data files.
http://www.avisoft.com/compression.htm
Lossy compression algorithms vary across the board, but they all function primarily by attempting to reduce file size by reducing the fidelity or removing altogether parts of the audio spectrum deemed unnoticeable to the human ear. For example, this is sometimes accomplished by decreasing bit-depth at ultra high frequencies or frequencies that are masked by big sounds. Reducing bit depth can contribute to distortions and unwanted harmonics as waveforms become more step-wise in their formation.
Aside from that, we all know, the human ear can “perceive” much more than it can “hear” so even the best lossy compression has some tangible effect on sonic quality and therefore is built on a faulty premise.
The real question becomes whether or not that change in sonic quality is enough to damn the compression algorithm altogether. The increasing trajectory of .mp3 files toward becoming the standard format would indicate that the loss of fidelity is not significant to the market on the whole.
Similarly, distortions in spectral content are not always a bad thing. The aural exciter is an example of people adding selective harmonic distortion and enjoying the consequences in so-called professional “hi-fi” audio circumstances. The problem with lossy data compression algorithms is that the spurious harmonic additions tend to be random and therefore the outcomes are unpredictable; whereas the aural exciter has a fairly uniform end result.
One thing I often wonder, and am reminded of reading this article is this: As I am about 20 years old and have spent about half my life listening to the majority of my music on mp3, does this effect how I hear and evaluate sound when I am mixing. It may change my expectation and my concept of what things should sound like particularly when it comes to high freequency content in mixes. I would also take this a little further to say that since I have done the majority of my music listening on earphones, and bad IPOD ones at that for the majority of my life (purely due to circumstance) will this change my approach i.e. I may not like too much bottom end in mixes as I don’t hear it so much in my earphones etc.
This mp3/portable earphone music player gransition may have a similar impact to the change from vynal to CD. As vynal physically couldn’t allow alot of bottom end due to the cutting of the actual physical waveform into the vynal, that effected how mix engineers approached things. I’m not sure if it is due to my largely earphone based listening experience, or my love of 1990 albums like Nirvana’s nevermind and Radiohead’s ok computer, but I do think modern music is becoming bass heavy, often to a negative effect.
p.s. apolagies for the long comment, its my first time coming across this site, I love it, and I had alot to say on this subject!
hey mr keith,
you forgot to mention lossless formats specific to audio, such as flac, ape, and the recent mp3hd (same extension *.mp3, but combining both lossless and lossy versions in a single file, ie. playable on ipods as lossy mp3).
peace
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