I'm sorry, but this is completely wrong. Compact Discs use PCM (Pulse Code Modulation) in a 16 bit, 44.1KHz sample rate for encoding. PCM is a full bit rate encoding method and has no compression. 16/44.1 PCM is the only encoding method allowed for redbook CD's. A disk that uses any other format does not meet the redbook requirements and cannot legally be called a CD, nor will it play on a standard CD player.
Dolby Digital (whether 2.0, 2.1 or 5.1) is a lossy, compressed format that throws away some of the original data to reduce the file size, then uses an algorithm to recostruct the data that was thrown away. A disk that is encoded in DD is not a CD, regardless of what it looks like. Also, a disk encoded in DD will not play on a standard CD player.
Other than both being digital formats, PCM and DD have nothing in common.