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dvanaria
August 31st, 2013, 06:08 AM
I'm trying to figure out how the PCjr and Tandy 1000 systems were capable of generating 3 distinct voices of sound. From what I read online, they were capable of generating 3 square waves of various amplitude and frequency (I believe this was controlled by a specialized Texas Instruments chip).

But how are 3 distinct sound waves transferred to a single speaker, at the same time? Are they combined together into a more complex sound wave and sent to the speaker that way?

If that's the case, there was really nothing special about the speaker in the PCjr or Tandy, just the controlling circuitry. The original IBM PC could only control pure sound waves (sine waves), one note at a time, if I understand any of this correctly.

Anyone know more about this?

mbbrutman
August 31st, 2013, 07:56 PM
The sound generator has three voices and a noise generator. The voices generate a fixed frequency. Mixing the frequencies is not a problem for a speaker; speakers do that all of the time. (Music being reproduced is infinitely more complex than this.)

That being said, the internal beeper/buzzer on the PCjr can not be used as an output. It is controlled by one of the timer channels and the output from the TI sound chip is not wired to it.

Your question is more along the lines of the physics of sound ... and I think I need a refresher on it too ...

vwestlife
August 31st, 2013, 10:34 PM
The Texas Instruments sound chip combines the output of the three square wave channels and one noise channel onto a single output pin:

http://members.casema.nl/hhaydn/howel/parts/SN76489_09_fig_4.png

This combining is done internally within the chip, so it is not possible to have each channel going to its own separate speaker, for example... although the Sega Game Gear used a variant of the TI chip that was capable of stereo audio output.