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SubjectRE: PC speaker

On Jun 12 2007 13:08, David Schwartz wrote:
>
>As far as burning out a speaker goes, if you can drop the frequency to zero
>(DC) and get continuous current through the speaker, that could burn it out.
>This makes several assumptions, many of which may not be true on modern PCs:
>
>1) It assumes the speaker is a conventional coil speaker, not a piezo
>element. (This is certainly true on some PCs, although it's increasingly
>false on newer PCs.)
>
>2) It assumes the speaker is DC driven. (I'm pretty sure this was true on
>the original IBM PC. Not sure about newer computers.)

Most likely. AC is hardly any good inside a computer :)

>3) It assumes you can configure the circuitry that drives the speaker such
>that it will stay on. (No idea.)

A crystal will drive the frequency, which can be set with outb(0x42).
Whether the crystal's oscillating signal is connected to the speaker
can be controlled with outb(0x61) IIRC [or just see
drivers/input/misc/pcspkr.c].

>4) It assumes the current will be sufficient to burn out the speaker. (I
>know it will get very hot on older machines, whether it will burn out --
>might even depend on the exact speaker model.)

Since you can set the x86's crystals frequency from 1193182 to 18 Hz
(PIT_TICK_RATE / 1 to PIT_TICK_RATE / 65535) [*], you can never really
bust it. But even then, what would a speaker do it was constanly given
+5V? (I _suppose_ the other level is 0V, not -5V -- makes for easy
design.) That's IMO just like a sound file with volume(x) = 1, nothing
spectacular if you ask me.

[*] The line "if (value > 20 && value < 32767) in pcspkr.c looks a bit bogus.


>On at least some older computers, you could burn out the hardware that
>drove the speaker this way. I think it was either the Pet or the Apple
>][ (didn't work on all machines, depended on how much current the
>speaker drew and other odd factors). I witnessed an Apple ][e blow out
>an I/O chip when it crashed with an output (that was supposed to be
>pulsed) left in the on position.


Jan
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