PC Speaker: Difference between revisions

The PC Speaker is the most primitive sound device available on PC compatible systems. It is characterized by the distinctive "beeps" and "squeaks" that it can be made to produce, and is therefore sometimes referred to as the "PC Beeper" or the "PC Squeaker".

The speaker itself has two possible positions, "in" and "out". This position can be set through bit 1 of port 0x61 on the [[Keyboard Controller]]. If this bit is set (=1), the speaker will move to the "out" position, if it is cleared (=0) then the speaker will move to the "in" position. ItMoving isin importantand toout noterepeatedly thatproduces theaudible speakertones willif notthe instantlyspeed jumpof torepetition (the desiredfrequency) position.is Ratherwithin itthe willrange movethe towardsspeaker can reproduce and the desiredhuman positionear can hear. Also, a single movement ("in" or "out") overmakes a shortclick periodsound ofbecause it's so timefast. ThisThus, allowsa thefrequency speakerwhich is too low to producebe actualheard beepsas anda whistlestone insteadmay ofbe justheard clicksas a rattle or buzz. (In fact, any frequency produced by this system also produces higher frequencies; look up "square wave harmonics" if you're interested.)

The PC Speaker can be connected directly to the output of timer number 2 on the [[Programmable Interval Timer]] by setting bit 0 of port 0x61 (=1). In this mode, when the timer goes "high" (=1) the speaker will move to the "out" position. Likewise, when the timer goes "low" (=0) the speaker will move to the "in" position. By changing the frequency at which timer 2 "ticks", the PC Speaker can be made to output sound of the same frequency.

This mode is very popular because it is easy to program and because it is asynchronous from the rest of the computer's operation, meaning that it takes very little CPU time.

In the absence of a real sound card, the PC Speaker can be used to output low quality digital sound. As stated earlier, the speaker itself has only two possible positions, "in" and "out". More positions are needed though in order to play digital sound. Typically, 256 positions (8 bits) are considered adequate to play comprehensible audio. One popular method used by many early PC games to overcome this limitation is called pulse-width modulation. This technique uses the physical properties of the speaker to allow it to output the (relatively) complex sounds that exist in 8-bit audio.

You also need a way to actually change the speaker's position. This could be done simply through bit 1 of port 0x61, but this option is too slow to be practical. Instead, it is best to (re)program timer 2 to properly set the position of the speaker for you while you wait for the next interrupt (or do other, more useful things with any spare CPU time).

When the PIT interrupts you, you need to program PIT timer 2 so that it puts the PC Speaker in the "out" position for a fraction of 60 microseconds. For 8 -bit audio, the number of microseconds can be calculated like this:

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This will work on real hardware and in QEMU if it started with <code>-soundhwaudiodev pa,id=speaker -machine pcspk-audiodev=speaker</code>. The code also changes the PIT timer 2 frequency, so you will have to reset that when you're done "beep"ing :)

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//Play sound using built -in speaker

//make it shutupshut up