Last updated: August 11, 1999
Stereo, Dolby Pro-Logic, and Dolby Digital are audio formats of increasing sophistication and quality.
We all know stereo is two channels of audio (left and right) and we generally connect stereo components to receivers with a pair of 'RCA-style' or 'phono' audio cables. VHS video tapes and CDs for example have two audio tracks, one for left and the other for the right.
This clever system has, for many years now, enabled homes to have a 'surround sound' home theater system without changing VCRs or cabling between components. A surround sound system generally means having. in addition to stereo speakers, a center (mainly dialog) speaker (typically placed on top of or below the television), and rear left and right speakers, placed either side of or behind the viewing position in the room. Some people also have a 'sub woofer' to add bass and impact to movie soundtracks.
Dolby Pro-Logic is clever though because the signals indicating which sounds should come from which of the 5 or 6 speakers are 'hidden' in a regular stereo audio track and so regular stereo audio cables can be used and in fact Dolby Pro-Logic sound tracks can be recorded on VHS tapes. Many rental movie tapes are recorded with Dolby Pro-Logic 'encoding' on the stereo sound track. The Pro-Logic information indicates if a sound should come from the left, center or right front speakers, or both rear speakers - Pro-Logic can't send different signals to the left and right rear speakers, they both get the same sound. This is a compromise in the format. There's no specific sub-woofer audio signal, the receiver generally filters out low bass sounds from the other signals and sends them to the sub-woofer.
In summary, Dolby Pro-Logic encodes 4 channels of audio (left, center, right, rear) in two connections.
With the release of the digital video disc (DVD), a new audio format arrived delivering far higher quality than anything before. Dolby Digital audio consists of 6 distinct signals: left, center, right, left rear, right rear, and low frequency effects (LFE or subwoofer). Dolby Digital is also indicated as a '5:1' soundtrack, meaning 5 main audio signals plus the subwoofer signal.
Each of the five main sound channels is a full 48KHz digital signal which is higher than CD even (44.1KHz).
Choosing a DVD player is tricky because it depends on the receiver you have. If you have a receiver with a built-in Dolby Digital decoder, which means it can receive the digital information for all 6 channels of audio and decode which signal is for which speaker, you can buy a less expensive DVD player without a Dolby Digital encoder in it. If you have a receiver that is described as 'Dolby Digital Ready' then it has 6 separate analog audio input connectors and you should buy a DVD with a built-in Dolby Digital decoder, which will have 6 separate analog audio output connectors to connect to the receiver. If you have a Dolby Pro-Logic receiver that is not Dolby Digital Ready or you have an older stereo receiver, buy a DVD player without the Dolby Digital decoder in it because one would not be useful to you. If you do have Dolby Pro-Logic at least, you will still get that level of surround sound from a DVD player.
If your receiver has a built-in Dolby Digital decoder however, the connection between a DVD player and the receiver is in fact simplified. Because the receiver can process the digital information, the DVD can send it directly to the receiver in a single cable, much like the way computers communicate in a network. A single digital coax or fiber optic cable is used.
I believe this has the same amount of signals (5:1) as Dolby Digital, and I'm not really sure of its advantages over the latter. I think its a different mixing method to supposedly give a more realistic theater experience. I think only Stephen Speilberg is pushing this format.