PC SPEAKER: All PCs since the original IBM PC have included an inexpensive loudspeaker whose operation can be controlled from software. This speaker is used for audio status indications during system Power On Self Test; for a few very old games; and for user alerts in application software. This speaker can not be connected to internal audio sources and is not used by modems which come with their own speakers.

Electrically, the speaker is almost invariably connected directly to the motherboard and is driven by a signal generated by a free running programmable internal clock and gated to the speaker under software control. The speaker can also be driven directly from software, but rarely is. Because the speaker is driven by an independent clock, tone pitch is independent of CPU clock frequency. In most cases, tone length is also independent of CPU clock frequency.

The Speaker is driven by a TTL square wave -- which is very rich in odd harmonics. This produces somewhat odd sounding single tones -- distinctly odd sounding music -- and unintelligible speech. It is (barely) possible to produce somewhat understandable speech by clever use of pulse width modulation depending on the speaker's lousy bandpass to do low pass filtering. Fortunately, sound cards have long since replaced the speaker in almost all situations requiring more than simple single tones.

The principle complaint about PC speakers seems to be the lack of a volume control. A 500 ohm variable resistor hooked into either of the speaker leads will generally provide full range volume control.

Return To Index Copyright 1994-2002 by Donald Kenney.