Passing from the PCM representation adopted for CD-DA to the ADPCM format used for CD-I and CD-ROM-XA two parameters are modified: the sampling frequency and the number of bits used to represent the samples. Table 1 summarizes the main features of the two digital formats .
A peculiarity of this ADPCM coding method is the ability to offer various combinations of sampling frequancy and bit/sample, to obtain different compression factors; this allows to use different combinations according to the need of disc space for other data, and ultimately to the higher or lower fidelity of reproduction required for the application. These combinations are normally known as "coding levels", as shown in Table 2.
It must be noticed that although CD-ROM-XA adopts exactly the same coding method of CD-I, only the levels B and C are available. For this reason in the following we shall refer just to CD-I.
Finally, Table 3 shows a clear representation of the reduction of information on disc obtained by using ADPCM coding levels instead of CD-DA PCM representation. Each rectangle is a 2353 bytes sectors (a sector is a logical unit on Compact Discs); the dark rectangles represent the disk sectors used for audio, while the empty sectors are those that are used by other kinds of data (video, text, etc...).
The A and B levels do not introduce any significant restriction on the audio bandwith, as the perception of the frequencies above 16 kHz is extremely reduced> The C level instead, imposes a drastic cut of the high frequencies, justified only by a coding of low frequency signals or by the absolute need of low disc occupation.
As ADPCM coded signals contain not only samples as such but also coding parameters (1 byte repeated 2 or 4 times) that must be used for reconstructing groups of (28) samples, the actual effective bit/sample used for coding is different from the number of bits used for each sample.
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