What is PCM ?

In the digital domain, PCM (Pulse Code Modulation) is the most straightforward mechanism to store audio. The analog audio is sampled in accordance with the Nyquest theorem and the individual samples are stored sequentially in binary format. The wave file is the most common format for storing PCM data.

Interchange Format Files (IFF)
It is a "Meta" file format developed by a company named Electronic Arts. The full name of this format is ElectronicArts Interchange File Format 1985 (EA IFF 85). IFF lays down a top-level protocol on what the structure of IFF compliant files should look like. It targets issues such as versioning, compatibility, portability, and so forth. It helps specify standardized file formats that aren't tied to a particular product. The wave file format is based on the generic IFF format.

The WAVE File Format supports a variety of bit resolutions, sample rates, and channels of audio.

The WAVE file format is based on Microsoft's version of the Electronic Arts Interchange File Format method for storing data. In keeping with the dictums of IFF, data in a Wave file is stored in many different "chunks." So, if a vendor wants to store additional information in a Wave file, he just adds info to new chunks instead of trying to tweak the base file format or come up with his own proprietary file format.


There are three chunks that are required to be present in a valid wave file:

'RIFF', 'WAVE' chunk
"fmt" chunk
'data' chunk

All other chunks are optional. The Riff wave chunk is the identifier chunk that tells us that this is a wave file. The "fmt" chunk contains important parameters describing the waveform, such as its sample rate, bits per sample, and so forth. The Data chunk contains the actual waveform data.

An application that uses a WAVE file must be able to read the three required chunks although it can ignore the optional chunks. But, all applications that perform a copy operation on wave files should copy all of the chunks in the WAVE.

The Riff chunk is always the first chunk. The fmt chunk should be present before the data chunk. Apart from this, there are no restrictions upon the order of the chunks within a WAVE file.

While interpreting WAVE files, the unit of measurement used is a "sample." Literally, it is what it says. A sample represents data captured during a single sampling cycle. So, if you are sampling at 44 KHz, you will have 44 K samples. Each sample could be represented as 8 bits, 16 bits, 24 bits, or 32 bits. (There is no restriction on how many bits you use for a sample except that it has to be a multiple of 8.) To some extent, the more the number of bits in a sample, the better the quality of the audio.

One detail to note is that 8-bit samples are represented as "unsigned" values whereas 16-bit and higher are represented by "signed" values.

references:
http://www.codeguru.com/cpp/g-m/multimedia/audio/article.php/c8935/PCM-Audio-and-Wave-Files.htm