Compression for CDs and DVDs

Written by Donald R.J. White
March 20, 2008

Introduction

One Byte = 8 bits = 28, or 256 options or quanta levels. CDs can store 670 MB of information and DVDs (single sided) store 4.7GB. DVDs store 4700MB/670MB = seven times more capacity. For example, 50 CDs of music can be stored on seven DVDs. Compression tools can compress disk space from about three to 200 times, depending on the stored information (i.e. text, music, video, etc). Music compresses from 1.2x to 3:1 times (MP3 format for 10:1 times), and text compresses only about three times. But, video can compress enormously, depending upon if is (1) stills, like a picture gallery or a power point presentation, or (2) movies which compress the best (roughly 100 times). The amount of compression of video depends upon the acceptability level (quality control) of lossy compression techniques.

There are two compression categories: lossless and lossy compression. The first is used for data recordings where errors are not acceptable and lossy compression where some degredation may be acceptable such as for music and video. However, there a measure of allowable degradation that should be given before a lossy compression selection is made; otherwise, default conditions may be used.

Part 1: Music

The threshold of a healthy human ear frequency response is about 16kHz. If a digital format, it would require at least 2x or 32 kB of bandwidth (MP3s standard bit rate is 128kb/s). The dynamic Range in amplitude approximates 90 dB.

The average pop song lasts about three minutes in time. A CD can store up to about 20 such songs. Thus, a song, recorded in digital format, takes up about 670 MB/20 or the equivalent of about 34 MB of space on a CD. The average FLAC-compressed (one of a few methods) audio track is around 53% of the original size for a nearly 2:1 storage compression.

The best compression is on opera, which compresses to about 1/3 the size - because of all the silent gaps between words. Organ and classical are next best, followed by jazz, vocal non-opera music, mellow modern music (Bela Fleck compresses about 50%), then noisier music follows. One of the worst-compressing track is from Grand Turismo (the game soundtrack), and it compresses to about 80% of the original size (the entire disc compresses down to about 71% of the original). Game soundtracks are consistently among the hardest to compress.

MP3 uses a form of lossy data compression. It is a common audio format for consumer audio storage, as well as a de facto standard encoding for the transfer and playback of music on digital audio players. MP3's use of a lossy compression algorithm which is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners, but is not considered high fidelity audio by the elite connoisseur. An MP3 file that is created using the mid-range bitrate setting of 128 kbit/s will result in a file that is typically about 1/10th the size of the CD file created from the original audio source. An MP3 file can also be constructed at higher or lower bitrates, with higher or lower resulting quality.

MP3 is an audio-specific format. The compression works by reducing accuracy of certain parts of sound that are deemed beyond the auditory resolution ability of most people. This method is commonly referred to as Perceptual Coding. This is relatively similar to the principles used by JPEG, an image compression format.

Summary

Except for MP3 format, available compressions in music range from about 1.2 to 3 times with 2:1 being a rough average. This suggests that about 35 "average, uncompressed CD songs" can be compressed onto a CD and up to 240 (+/-) for a DVD.