Intellivision PSG Timing Question

[AlyoshaTAS]

Hello again,

 

I have a question about how the PSG is clocked.

 

My emulator currently has the PSG working, and it sounds correct. It passes the MTE201 test cart and the sounds are correct compared to what I hear from youtube as well as other emulators. Game sounds all sound correct.

 

But in order to achieve that, I have to only clock the PSG during the visible part of the frame. Is this correct?

It doesn't quite make sense to me that this would be the case, but anything else I try sounds way off, and doing it that way sounds perfect.

 

I see that the chips enable pin is tied to the System ROM chip somehow, are these things related? Is there something I'm missing?

 

I appreciate any insight, thanks!

 

 

 

 

[mr_me]

There are people more knowledgeable than myself that could answer these questions; you might have better luck in the programming subforum.

 

Looking at the schematic ( http://spatula-city.org/~im14u2c/intv/tech/master.html), it looks like the clock signal starts at the colour chip than goes to the STIC before going to the PSG. The STIC does go into a different mode during vertical blank but I would be surprised if that clock signal is effected since it also goes to the CPU. Maybe there is another explanation that produces the same result.

 

Regarding the PSG pin connected to the system rom, I see in the schematic that those same pins are also connected to the 8-bit ram. Looks like the rom chip is responsible for making the 8-bit ram work. Why is the PSG connected to the 8-bit ram? The Intellivision PSG is a slightly modified version of the standard General Instrument chip. That pin 26 in the standard chip specifications is described as test2 and not to be used as tie-point. Maybe it was repurposed for the modified intellivision chip. Dont know if its documented somewhere. Looking at the ram chip spec it looks like it turns on the ram chip for read/write but the rom chip can do the same thing

[+nanochess]

You're having it wrong, probably your frame output is faster than it should be because the PSG is clocked all the time exactly the same as MSX (for NTSC)

[AlyoshaTAS]

Thank you for the replies.

 

I had a couple of bugs in the audio timing and now I can run it for the entire frame and it sounds much better.

 

Some games still have problems like Star Wars: Empire Strikes Back, but it's at least much better then it was.