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120Hz IRQ timer?


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While I am working with sound effects and music using the ISR sound routine and compatible routines I did some research on other systems, in particular being spurred by the complexity of the Spectrum sounds in "Jetpac." I have found references to a bunch of systems using 120Hz interrupts.


The TI's ISR routine has the shortcoming of only being able to play from VDP memory or GROM, and compatible routines are used (Spectra2, for instance) to play from CPU RAM. This being the case, I figure it would be useful to use the same routines but with a 120Hz interrupt.


But from where? Is it possible to get it from the 9901?

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It should be possible to use the 9901... there is a gotcha, though. The ROM programmers decided that end users don't need 9901 timer interrupts, so interrupts coming into the system from the 9901 are assumed to be for the cassette DSR. Jeff Brown is credited with first working out how to take advantage of it... it takes a bit of setup and you lose some cycles before you get control, but it does work and at 120Hz it might be okay. (I remember at one point Jeff was trying to use it for sampled audio but wasn't terribly happy with the results...)


Thierry Nouspikel has a write up of it here:http://www.unige.ch/medecine/nouspikel/ti99/ints.htm#Timer%20ISR


You can also poll the 9901, there are no gotchas I'm aware of there. Other than cycle counting, I don't know any other way with a stock TI to do it.

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