Dolby Pro Logic Surround
This sound standard uses 4 discrete channels. There is a front-left, front-center, front-right, and a
limited frequency ambience channel for the rear. The rear channel is made to be played equally through
2 rear speakers. The rear channel is limited to frequencies between 100Hz and 7000Hz, and this is
enough because this channel is only to add ambient noise. All main sound comes from the front 3
speakers. A subwoofer is often used by connecting it as a crossover to the front
channels.
Dolby Pro Logic sound is encoded into a stereo signal and can be played through normal speakers,
with minimum distortion, although a difference can be noticed. How this is done is that the center
channel is added equally to the two stereo channels, canceling itself out. The rear ambient signal is
added to the stereo signal by dividing itself into two sounds perfectly 180 degrees out of phase with
each other. This encoding suffers limitations, by is still capable of producing realistic sounds. Any
distortion or errors can usually be compensated for by robust decoder analysis.
Being able to be encoded into a stereo signal is a major advantage to Dolby Pro Logic. This means it
can be played and stored in regular stereo formats, such as VHS or radio. When played, either
through a sound card or other hardware, the stereo signal is sent to a pro logic decoder before reaching
the speakers. The pro logic decoder separates the stereo signal into its proper 4 discrete channels for
playback.
Dolby Digital 5.1
Dolby Digital, also know AC-3 is top of the line for audio recording and playback. It uses 6 discrete
channels and provides true surround sound. It is called 5.1 because their are 5 full frequency channels;
front-right, front-center, front-left, rear-right, and rear-left, and one low frequency channel which
operates at frequencies between 3Hz and 120Hz. The low frequency emitter is specifically designed
for subwoofer output, and provides truer bass sound because the low frequencies are not overpowered
by high frequencies.
Dolby Digital is a compressed digital signal, which uses a lossy compression sceme. Most sound cards
are able to downsample the digital 6 channel signal into 2 speakers, and some aren't.
Computers are currently not able to process and playback 6 channel AC-3 or DTS, but instead have
to output the signal to a device that can. The signal has to be output as a digital signal because it is
compressed and specially encoded, so the sound card must have a digital S/PDIF connecter
which is connected to an external decoder.
Digital Theatre Sound (DTS), is a very similar AC-3, but is a higher quality signal. It is what
is used in almost all theaters, and used in some high quality audio recordings. DTS is used in most
theaters, and AC-3 is used for high end audio. AC-3 is used in DVD movies and is planned to be used
in the upcoming High Definition TV.
Quad Output
A 4 discrete channel sound system is used for 3D games. To support Quad speakers, the game must
use either the EAX or A3D API's. Because computer sound is made in respect to a 3
dimension rendered world, it is very easy to position sound on a 4 speaker system. There is a discrete
channel for each direction, two left and right speakers for the front and 2 for the rear. A subwoofer is
often used as a crossover between the two front speakers.
This is setup is not only efficient, but also economical. There is no external decoder needed because
the signals are never encoded like the Dolby audio. Sound is output from the sound card using 2
ordinary stereo outs, one for the front, and one for the rear.
