I asked about PCM at another forum and a pretty sharp feller broke it down for me in a manner even I can understand.

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"PCM is the most simple way of digitising sound. It's what CD players use, it's what PCs use, it's what everything digital uses (except SACD). It works by measuring the position of the speaker/microphone thousands of times a second, and storing that position to a certain accuracy. On a CD sound is sampled 44100 times a second to an accuracy of 16 bits.

On a CD, this data is stored raw - 44100 16-bit samples a second. That means 44100*16=705.6kbits per second. Then times two because it's storing two channels for stereo: 1.4Mb/s.

Raw uncompressed PCM for a 5.1 channel film would be pretty bulky: 48000*16*5= 3.8Mb/s, plus a bit for the LFE. No way could you fit that onto a 35mm film print. And it would be rather a lot on a DVD too. Hence Dolby Digital, et al.

Dolby Digital, DTS, and all these other formats are just different ways of compressing PCM to take up less space on the disc or film print. Dolby Digital, DTS, MP3, AAC etc do this by throwing away information they think you can't hear.

Dolby Digital can squeeze 5.1 sound down to 0.4Mb/s or so - a factor of ten less space than raw PCM. The decoder takes the Dolby Digital compressed bitstream, and turns it back into raw PCM. It won't be exactly the same as it started with, but it shouldn't sound too much worse (to human ears, at least - an alien with a different hearing system might think it sounded dreadful. But then they wouldn't be convinced by our 3-colour TV either.).

The simplest way to think of it is that Dolby Digital and DTS are "MP3 for surround sound". They get the data down to a manageable size.

Now, DVD-Audio and the new HD formats have extra space for audio. So they can store either raw PCM, or they can use a lossless compression system. (DVD-Video can actually use raw PCM, but this is rare except for some 2-channel tracks on music discs, as there's usually not enough room).

These lossless systems typically can compress PCM by a factor of up to 2 or 3. Nowhere near as small as Dolby Digital, but you lose nothing in the process - the decoder gets back the original PCM undamaged. These systems are MLP (used on DVD-Audio), Dolby TrueHD (basically MLP with a few extensions) and DTS HD Master Audio.

With these lossless systems, they are all totally equivalent in sound quality, so anyone who debates whether PCM, Dolby TrueHD or DTS HD Master Audio sound better needs their head examined. They can only differ in how much they compress, or extra facilities like better downmixing, dynamic range reduction, etc. The important point when comparing soundtracks will be what resolution they're at (48kHz/16-bit, or 96kHz/24-bit, etc).

Super Audio CD is different - it uses a totally different system from PCM, called DSD. This is hard to explain, but basically it's performing 1-bit sampling at 2.8MHz. Again, this data is bulky (2.8Mbit/s per channel) so it can be compressed losslessly, using a system called DST, which typically compresses by a factor of three. DSD has pros and cons over PCM.

Now, in a sane world, all these compression systems would be handled inside the player, and interconnects between components would just use PCM or DSD. But before HDMI and i.Link came along, there were no digital interconnects that could handle 6-channel audio. So as a fudge, the Dolby Digital and DTS bitstreams were squeezed into the 2-channel space of S/PDIF and TOSlink, and the receiver had to have decoders (as well as the player, for its analogue outputs). This fudge only worked for the highly compressed bitstreams. There's no room in S/PDIF to pull the same trick for the lossless formats.

But now we have HDMI and i.Link, with their support for multiple channels of hi-res PCM/DSD, there's no need to send a bitstream across them. The player can do the decode, and the receiver can just accept raw PCM or DSD."

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