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OK, so PCM and S/PDIF are the formats while optical and coaxial are the transfer mediums. Gotcha. (Duh, I forgot all about coaxial being the transfer medium...I bought this radio shack digital converter that converts "PCM" to "optical," and this seemed odd to me as I am used to seeing coaxial to optical.) Thanks for the responses!

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Just a bit of confusion I've found - SPDIF usually means coaxial connection. Occasionally, I have seen an optical connection referred to as SPDIF, but I'm pretty sure this is wrong. Although the actual signal sent is identical (substitute light pulses for electrical pulses), the method of transmission is part of the SPDIF standard. The proper name for optical is TOSLINK.

If you've ever seen a graph of a sound wave, then you are most of the way there in terms of understanding what PCM is. The wave is broken down into samples (or points) which make up the wave. Each one of these points has an amplitude and a time. Usually, amplitude is on the up/down axis and time increases as you go from left to right. To make things simple, it is assumed that the time between each sample is constant - this way, only the sampling rate/frequency has to be stored along with the amplitude information. The amplitude is simply the position of the sound wave in the up/down direction.

So, a PCM square wave could look like this:

-5, -5, -5, 5, 5, 5, -5, -5, -5, 5, 5, 5, -5, -5, -5...

Of course, if you open up a .wav file, it won't look very much like this. If you open up a .wav file of a sine wave in notepad then you might see a pattern. Also, there is a bit of header information which says stuff like sampling rate, sample size (the number of different values each sample could have) and for .wav files, it also specifies that the file is PCM encoded since a .wav file can use any type of encoding - including ATRAC3 and MP3.

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