Intel 8254 chip

[dbalsom]

The Intel 8254 programmable interval timer powered PC speaker music for thousands of video game titles for decades, and I think it would be a useful addition. (The original PC and XT had the 8253, but they're functionally identical except for a new 'read-back' command, VGM would only need the 8254)

Although it is not itself a sound chip, the 8254 operates in a very similar way to most sound chips, functioning as a 16-bit, three channel frequency divisor. On the IBM PC, the speaker was more or less connected directly to the output of the 8253's channel #2, via a low pass filter and amplification circuit. 6-bit PWM audio was possible via bit-banging.

The 8254 has a number of selectable timer modes, each with their own quirks, and had three gate control inputs that could externally control counter operation depending on mode.

There are some "PC Speaker" VGM packs but they are essentially translations to another sound chip such as the AY-3-8910. I'd like to add VGM recording to my PC emulator but feel that native support would really be the ideal solution.

I have very accurate 8253 emulation I'd be willing to contribute. It is in rust but could be trivially translated to C/C++.

[ValleyBell]

Nowadays I have been very hesitant on adding single-channel sound chips to the VGM format in general.
This is mostly due to VGMs not being able to have more than 2 instances of the same chip. That is not a concern for this one though.
(There were a few exceptions - OKIM6258, uPD7759 and 32x PWM - but at that time I just added everything I saw used.)

The most important question is: How nice does the bit-banging/PWM audio play with the VGM's limited tick resolution of 44100 Hz?

[dbalsom]

The most important question is: How nice does the bit-banging/PWM audio play with the VGM's limited tick resolution of 44100 Hz?

I suspect it should work given VGM handles the PCM bit-banging done by such titles as Skate or Die on the Tandy 1000, via the SN76489, where the period is held at 1 to produce a constant output and the attenuation is rapidly toggled. The SN76489 even had write wait states to contend with. Even if PWM doesn't play nice, there's still tons of music to record that does no such tricks.

I wouldn't say that PC speaker music is exactly celebrated, but some of it can sound decent.
https://www.youtube.com/watch?v=1IOL4q5tDDQ

[dbalsom]

here's some 8253 emulation in C++ I wrote while testing an 8253 chip with an arduino

https://github.com/dbalsom/arduino_8253/blob/main/sketches/validate/pit_emulator.h

you're welcome to it under GPL or MIT or whatever you like