SN76489 polarity

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Michel Gerritse
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SN76489 polarity

Post by Michel Gerritse » Wed Apr 29, 2020 8:27 pm

It has been almost 2 decades since I released something to the public. Some might remember me as "Gerrie", author of some old Sega emulators (I lost access to my old mail address, so I created a new account). I recently decided to continue a project which I have been working on and off in the past 20 years or so, as with the worldwide COVID-19 pandemic I have more spare time to spent. More about this project later, if someone is interested.

The past week I have been working on a new SN76489 emulator. I think I have pretty much nailed it, however I do have a couple of questions. As I lost all my documentation and notes, and the fact that I don't have hardware to run tests with I thought it might a good idea to ask here:

Is the SN76489 capable of outputting a negative polarity ?
I've checked various sources but couldn't find a definitive answer; Some emulators do have a negative output, others don't (MAME for example).
I know the output of the noise LFSR is always positive, but I was wondering what the attenuator does when it gets a flip-flop of 0 from the tone channels. (the flip-flop flipping between 0 - 1 when the half tone period reaches 0).

TmEE co.(TM)
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Re: SN76489 polarity

Post by TmEE co.(TM) » Thu Apr 30, 2020 9:39 am

There's no negative polarity on TI/VDP PSG and most other PSG type sound chips. You do get stuff settle below ground level when you highpass filter the final chip output as it is done on all hardware outside the sound chip itself.

On the TI/VDP PSG, outputs start high and toggle between 0 and a higher state.
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Michel Gerritse
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Re: SN76489 polarity

Post by Michel Gerritse » Fri May 01, 2020 3:13 pm

Thank you for that. I was already suspecting it but got it now confirmed.

For now I will leave my SN76489 core as it is and move on to the next thing. I'm sure there is still a bug or 2 to fix, but I will look into it later once my audio engine is able to resample streams above 200kHz so the SN76489 can render at its native clock speed.

Stef
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Re: SN76489 polarity

Post by Stef » Fri May 01, 2020 7:33 pm

Hey Michel :) Long time we didn't heard about you !

Michel Gerritse
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Joined: Sat Apr 25, 2020 7:25 pm
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Re: SN76489 polarity

Post by Michel Gerritse » Sat May 02, 2020 6:56 pm

Hey Stef, good to see you! Yes, it has been way too long but real life kicked in. When you have a ton of things to do emulation was not my top priority.
I hope I have some more spare time to spent so I can work on things I've been dying to do the past years.

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