Pcm Audio Processing; Figure 6-2. Analog Signals - Compaq EN Series Technical Reference Manual

Chapter 6 Audio Subsystem
6.2.1

PCM AUDIO PROCESSING

The audio subsystem uses pulse code modulation (PCM) for processing audio that is applied from
external sources to the Mic In and Line In input jacks, as well as audio from an installed CD-
ROM drive. The PCM method is also used in playback of .WAV file data commonly used in
Windows applications.
6.2.1.1 ADC Operation
The Analog-to-Digital Converter (ADC) receives an analog signal and, using pulse code
modulation (PCM) converts it into digital data that can be handled by normal logic circuitry. The
conversion process consists of measuring (sampling) the analog signal at intervals to determine
the amplitude and frequency (see Figure 6-2). The frequency of sampling intervals is a
programmable parameter known as the sampling rate. The higher the sampling rate, the more
accurate the digital representation will be.
S0 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
Time
Figure 6–2. Analog Signal Sampling/Quantizing
Each sample is quantized into a digital code that specifies the voltage level of the analog signal at
that particular time. The quantizing format options are as follows:
Mono or stereo
8- or 16-bit
Signed or unsigned
6-4 Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
Quantized Values
8-Bit 16-Bit
FFh FFFF
80h 8000h
00h 0000h