How Analog-to-Digital Converter (ADC) Works

Surround Audio: Audio Compression

When we made the math to find out how much storage space the CD-quality audio would need, we had to multiply the required storage space per two, since the CD uses two independent audio channels. You can record totally different audio streams on each channel (left and right) of a CD. They are completely independent.

So, imagine how much storage space a surround sound system would need, since they use four or more independent audio channels. If we make the math for the most popular surround sound format nowadays, the 5.1 – which is used by DVDs –, we would come to the conclusion that it would require 441,000 bytes per second of storage space, or 25 MB per minute, if CD-quality audio is used. If you take a typical one and a half hour movie, you would need 2.2 GB of storage space just for storing the audio data, not counting the movie itself!

Just one parenthesis. For the above calculations we considered only five channels of audio. The sixth channel, the subwoofer channel (a.k.a. LFE, Low Frequency Effects), would require less storage space, since we can use a lower sampling rate for it, since it is used only for lower-frequency sounds. That’s why the name is “5.1” and not “6”: the sixth channel isn’t a “full” channel. If we considered the subwoofer channel, the required storage space would be even greater.

The solution is to use audio compression, to cut off the amount of required storage space. All audio compression algorithms available on DVDs are data-loss, i.e., the output signal is not equal to the original sound. Even though experts claim that a regular user wouldn’t notice the difference between uncompressed (PCM) audio and data-loss compressed audio, audiophiles claim that they can hear the difference. That’s why for certain titles 2-channel PCM audio (i.e., CD-quality audio) is an option.

The two most popular commercial audio compression algorithms are Dolby Digital (also known as AC3) and DTS (Digital Theater System). Dolby Digital bitrate varies between 384 Kbps and 448 Kbps, although it is theoretically possible to go as high as 640 Kbps. DTS bitrate varies between 768 Kbps and 1,536 Kbps. Since DTS uses a higher bitrate than AC3, experts claim that it has a better quality than Dolby Digital, since the higher the bitrate, the less original data was lost on the compression. Just to put those numbers in perspective, a 5-channel PCM audio with CD-quality has a bit rate of 3,445 Kbps (once again not counting the LFE channel).

On the practical side, there are other differences. DVDs with DTS-encoded audio can only be played on home theater receivers capable of decompressing DTS, while all home theater receivers can play DVDs with Dolby Digital compressed audio. On movies theaters, Dolby Digital-based movies use an optical audio track with the digital data encoded, while DTS-based movies have just a control track that commands a CD-ROM that has the digital audio information stored.