1.3 Quantization, Binary Data and Word Length

When a PAM pulse is quantized it is converted into a PCM word of a certain length or resolution. The number of bits in the word determines the accuracy with which the original analog signal may be represented a larger number of bits allowing more accurate quantization, resulting in lower noise and distortion. This process is covered further in Chapter 2, and thus will not be discussed further here; suffice it to say that for digital audio word lengths of up to 24 bits may be considered necessary for high sound quality, whereas for digital video a smaller number of bits per sample (typically 810) is adequate. Since the number of bits per sample is related directly to the signal-to-noise (S/N) ratio of the system it can be deduced that the S/N ratio required for high quality sound is greater than that required for high quality pictures.

Although the resolution of audio samples may be greater than that of video samples, the sampling rate of a video system is much higher than that required for audio. Thus the total amount of data required per second to represent a moving video picture is considerably greater than that required to represent a sound signal. (In all cases it is assumed that no form of data reduction is used.) This has important implications when considering the requirements for different types of digital interface.

In computer terminology, eight bits is a byte , and this is the unit of storage often used in computer systems, even though data may actually be handled in word lengths considerably longer than eight bits. A binary word is sometimes confused with a byte , but a word can be of virtually any length, whereas a byte may only ever be eight bits. The bit with the greatest 'weight' in a binary word (the leftmost, or the highest power of two, when written down in conventional form) is called the most significant bit , and the bit with the least weight (2 = 1, which is normally the rightmost bit) is called the least significant bit . The following example illustrates the point:

Take the eight-bit binary value '01011101':

	*MSB*														*LSB*
Binary			1				1		1		1				1
Binary weight	2 ⁷		2 ⁶		2 ⁵		2 ⁴		2 ³		2 ²		2 ¹		2
Decimal weight	128		64		32		16		8		4		2		1
Decimal equivalent		+	64	+		+	16	+	8	+	4	+		+	1 = 93

A kilobit (Kb) is 1024 bits (2 ¹⁰ bits), and a kilobyte (Kbyte) is 1024 bytes. A megabit (Mb) is 1024 kilobits. Confusingly, in communications terminology a data rate of a kilobit per second represents 1000 bits per second, not 1024 bits per second.