U. Glaeser

FIGURE 39.56 Simplified schematic of IBM-PC speaker circuit.
Brief History and Evolution
The early PCs could only generate simple tones and beeps. In the early 1980s, the system designers of
the original IBM-PC used the Intel 8253 digital timer to generate a series of pulses at a regular rate,
usually a square wave within the audio range of less than 20 kHz. The output of this chip drove the base
of a transistor to switch on and off a small speaker as shown in the schematic of Fig. 39.56. This simple,
cost-effective solution effectively offloaded the 4.77 MHz Intel 8088 main processor from the task of
generating a tone. The 8253 timer had three independent channels, and the system designers used channel
0 to keep track of the time of day and channel 1 to generate DRAM refresh cycles. Thus, the otherwise
unused timer channel 2 provided an essentially cost-free audio processor.
Although the most common use of this primitive audio system was to alert the user to an event, clever
programmers were able to play simple melodies using it. Eventually, they discovered how to use pulsewidth
modulation coupled with the reactance of the circuit to create a low-quality digital-to-analog
converter (DAC), enabling the playback of digitally sampled waveforms. Audio created using this method,
however, was very noisy and presented a significant load on the main processor.
Later in the 1980s, add-in cards appeared with an integrated music synthesizer capable of playing back
polyphonic music. One such early card, the Adlib soundcard, used a form of music synthesis known as
frequency modulation (FM) synthesis. As John Chowning described in 1973, FM synthesis creates
complex sounds using simple sine waves to modulate the frequency of other sine waves [1]. The Adlib
soundcard used the simple Yamaha OPL2 FM synthesis chip, which used only two sine waves per voice
to synthesize complex waveforms. It could create satisfactory, yet unrealistic synthesis of natural musical
instruments, as well as a limited spectrum of special sound effects.
The immensely popular Adlib-compatible SoundBlaster® (Creative Technology, Ltd.) was introduced
in 1989 by Creative Labs. In addition to Adlib’s FM synthesis capabilities, it added a simple method of
playing and recording digital audio encoded as a pulse code modulated (PCM) stream. Perhaps as
important to its success, Creative Labs provided software development support to computer game
developers free of charge, resulting in widespread software support for the SoundBlaster. The new PCM
audio capabilities added the possibility of using any sound as an effect in a game. This important
enhancement led to the requirement for PCM audio on all future soundcards.
PCM audio was transferred to and from the soundcard using the Intel 8237 direct memory access
(DMA) controller on the main system motherboard, as shown in Fig 39.57. The early SoundBlaster cards
could only transfer 8-bit PCM audio, resulting in a dynamic range of only about 48 dB. Later, the Sound-
Blaster 16 card added support for 16-bit PCM audio with a much better 96 dB dynamic range, using the
16-bit DMA controller of the newer computers.
As time progressed, wavetable synthesis replaced FM synthesis. Wavetable synthesis is capable of
synthesizing musical instrument sounds that are nearly indistinguishable from real instruments except
to the trained ear. It works by triggering digital recordings of notes played on actual instruments in
response to keys played on a keyboard. To synthesize the sound of a piano, the wavetable synthesizer
stores a series of digital recordings of a real piano, and plays them back on command. Although the sound
© 2002 by CRC Press LLC
Intel
8253
Digital
Timer
+5V