TABLE OF CONTENTS Digital audio is the representation of audio information in digital (discrete level) formats. The use of digital audio allows for more simple storage, processing, and transmission of audio signals. Baseband audio processing includes analog to digital conversion, digital Speech compression, and channel coding.
Analog to digital conversion is a process (digitization) that changes a continuously varying signal (analog) into a digital values. The GSM system converts analog audio signals into digital form so it can be compressed and coded onto the radio channel.
A typical analog to digital conversion process includes an initial filtering process to remove extremely high and low frequencies that could confuse the digital converter. This is followed by a periodic sampling section that measures the instantaneous level of the signals at fixed time intervals and converts the measured values (sampled voltages) into its equivalent digital number or pulses.
Figure 1.12 shows how an analog signal is converted to a digital signal. This diagram shows that an acoustic (sound) signal is converted to an audio electrical signal (continuously varying signal) by a microphone. This signal is sent through an audio band-pass filter that only allows frequency ranges within the desired audio band (removes unwanted noise and other nonaudio frequency components). The audio signal is then sampled every 125 microseconds (8,000 times per second) and converted into 8 digital bits. The digital bits represent the amplitude of the input analog signal.
Digital...
