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.

Whether you’ve researched production and engineering in magazines (we recommend Tape Op) or on the web, you’re well aware of the ongoing debate between the virtues of analog and digital recording. Eventually, Phil and I will discuss the merits and limitations of both, but for now I will define the two terms in order to lay the foundation for future articles including next week’s Producer Speak: “Bit Depth, Bit Rate, and Sample Rate.”

Digital audio relies on a series of points (called samples) and works similarly to film. A reel of film is comprised of a series of still photos which, when projected at high speed, gives the illusion of fluid motion to the naked eye. Your brain “connects the dots” from one image to the next. Digital audio works like film in that sound is captured via a series of samples (which could be thought of as snapshots of sound pressure levels). These dots are then connected to form waveforms:

The scale is very different, however. Though film could theoretically run at an unlimited rate of frames (images) per second, we only need to capture and play back about 30 frames per second to give the appearance of realism. A CD, on the other hand, plays back at a rate of over 44 thousand samples per second.

Analog audio does not rely on samples at all. Analog is so called because when sound is captured to an analog medium, the waveform that is created is analogous with the sound wave being captured. This means that an analog audio signal has a higher potential for quality, although analog signal decreases in fidelity (quality of exactness) each time it is copied or transferred, whereas a digital signal will retain its quality no matter how many times it is copied.

Things that are analog: reel-to-reel tape, cassettes, microphones, preamps (not including built-in analog-to-digital converters).

Things that are digital: Protools, CDs, DAT, MP3s, WAVs, DVDs, anything on a computer.

Analog to Digital converters such as the Digidesign 192 bridge the two formats together. These are sometimes called A2D or simply “converters” when also referring to Digital to Analog conversion devices.

Next week’s Producer Speak: “Bit Depth, Bit Rate, and Sample Rate.”