I'm guessing you are talking about the receivers that pick up the Dish Network channels for free? Pansat also makes MPEG-2 receivers. If you mean the Dish Network ones I've never used them, just heard of them. Here is a site in Canada that sells them, a friend gave me this a month or two ago. www.sat-toys.com/index2.shtm1 Supposedly all the receivers are made by the same company, I don't know if that's true or not, I think I read that here.
PCM stands for "Post Code Modulation". It is a system where the signal is sampled as it is sent and resampled at the same rate at the other end. This is then formed into the original signal. Mostly used in telecommunications these days on the digital network as it enables hundreds of telephone calls to be sent on one line at the same time.
Back to the original question: DVD player gives you the possibility to convert Dolby Digital, DTS and/or MPEG to PCM before sending it via digital outputs. It is meant to be used if your receiver/amplifier is not DD/DTS/MPEG compatible.
Some further info on PCM ("Pulse Code Modulation"): It consists of a n-bits sample, produced at a certain frequency. Examples (bits/KHz for each stereo channel):
Broadcasting (NICAM): 14 bits at 32 KHz.
CD: 16 bits at 44.1 KHz
DVD-Video: often 16 or 24 bits at 48 KHz (96 KHz audio is rare but supported).
DVD-Audio: up to 24 bits at max 192 KHz. This format handles up to 6 channels at max 24 bits/96 KHz.
So PCM is a flexible format, spanning from space-/bandwidth-saving low-fi audio to audiophile music. It is also scalable, and therefore "easy" to convert and manipulate.