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INTRODUCTION
The Series 2 Digital Audio Adapters are IBM AT compatible add-on boards which convert high fidelity
analog signals to digital data for storage to, and retrieval from, disk.
The Series 2 adapters sample two channels of audio from 6.25 kHz to 50kHz with 16 bit resolution. They
incorporate Sigma Delta technology with 64 times oversampling, providing superior fidelity at greater
than 90 dB signal-to-noise ratio.
ABOUT DIGITAL AUDIO
In professional circles, digital audio has been with us for over 10 years. With the advent of the compact
disk in 1983, digital audio has become commonplace as a consumer item. Few will argue that digital
audio has afforded an order of magnitude improvement in overall sound quality and signal-to-noise ratio
over the best analog systems which preceded them. But just what is digital audio, and where and how is
it used?
It is possible to use digital data transmission techniques to transmit digital audio signals by wire or radio.
However, this practice has not yet become common due to the extremely wide signal bandwidth required
to transmit real-time digital audio signals. For the present, digital audio techniques seem largely confined
to the recording and playback of music and other audio signals where, in a few short years, digital audio
technology has all but replaced the previous analog record/playback techniques. In the present decade
we will see digital audio technology replace analog technology in most signal processing functions in
both the professional and consumer markets. It is also likely, particularly with the advent of fiber optic
cables, that digital audio technology will be utilized in the transmission of real-time audio signals on a
widespread basis.
But what is digital audio?
In essence, digital audio is a technological process whereby an analog audio signal (produced when
sound waves in the air excite a microphone) is first converted into a continuous stream of numbers (or
digits). Once in digital form, the signal is extremely immune to degradation caused by system noise or
defects in the storage or transmission medium (unlike previous analog systems). The digitized audio
signal is easily recorded onto a variety of optical or magnetic media, where it can be stored indefinitely
without loss. The digitized signal is then reconverted to an analog signal by reversing the digitizing pro-
cess. In digital audio record/playback systems, each of these two functions is performed separately. In
digital audio signal processing systems (where no record/playback function occurs) both analog-to-digital
and digital-to-analog conversion processes occur simultaneously. A variety of techniques are possible,
but the most common method by which audio signals are processed digitally is known as linear pulse
code modulation, or PCM. Let's take a brief look at how PCM works.
Converting an analog signal to digital is a two-stage process, sampling and quantization. This is
illustrated in Figure 1. At regular intervals, a sample-and-hold circuit instantaneously freezes the audio
waveform voltage and holds it steady while the quantizing circuit selects the binary code which most
closely represents the sampled voltage. Most digital audio is based on a 16-bit PCM system. This means
that the quantizer has 65,536 (2
16
) possible signal values to choose from, each represented by a unique
sequence of the ones and zeroes which make up the individual code "bits" of the digital signal.