AY emulation testing

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Pegaz
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Re: AY emulation testing

Post by Pegaz »

djnzx48 wrote: Fri Oct 12, 2018 3:04 am Yeah, I agree that targeting the actual machine is preferable. The reality is though that not everyone owns real Spectrums, and many people are forced to use emulators as their only option. Personally I'd like to write programs for platforms that people will run them on, and I'd say Spin, SpecEmu and Spectaculator together probably make up the majority of emulators in use, without considering clones.

Even if you ignore emulators you still have to deal with compatibility issues, such as the contended memory banks being different on the +2A/+3. I know emulators aren't official Spectrum models, but they're in wide use so it's reasonable to want to support them.

And it's not just a binary decision. You can develop for the Spectrum primarily but still have emulation/additional hardware in mind, for example with ULAplus.
I completely agree.
Emulators are very important and the best of them already behave practically as real Spectrum.
Their benefits will be even greater in the future, as there will be less and less working Spectrum around.
For me, every Spectrum is equally important , whether its based on emulator, fpga or some other non-original hardware.
Otherwise, even Amstrad models might not be considered as original hardware too.
With the emulators, Spectrum will drastically extend its life span and user base, without them it will only become just a piece of hardware in the museum.
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Ast A. Moore
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Re: AY emulation testing

Post by Ast A. Moore »

djnzx48 wrote: Fri Oct 12, 2018 3:04 am Yeah, I agree that targeting the actual machine is preferable. The reality is though that not everyone owns real Spectrums, and many people are forced to use emulators as their only option. Personally I'd like to write programs for platforms that people will run them on, and I'd say Spin, SpecEmu and Spectaculator together probably make up the majority of emulators in use, without considering clones.

Even if you ignore emulators you still have to deal with compatibility issues, such as the contended memory banks being different on the +2A/+3. I know emulators aren't official Spectrum models, but they're in wide use so it's reasonable to want to support them.

And it's not just a binary decision. You can develop for the Spectrum primarily but still have emulation/additional hardware in mind, for example with ULAplus.
Sure. Perhaps, I didn’t express myself clearly. I didn’t mean to say you should go out of your way to write software that will only work properly on real hardware. Quite the opposite, in my book, it would be a mistake to go out of your way to write software that will only work properly on a particular emulator, even if it happens to be the most popular emulator.

Yes, it’s difficult enough to create workarounds for all the models of the real Spectrum. Aside from RAM contention differences, here are a few other quirks of the real hardware:

1. 48/16K Spectrums display colors differently from all the subsequent iterations. Darker colors exhibit smaller differences between their bright and non-bright variations (e.g. bright blue and non-bright blue look virtually identical). I’m not aware of any emulator that ever attempted to implement this behavior.

Image

2. The 48K/128K/+2 Spectrums are affected by something commonly referred to as the ULA snow effect, if the I registered is pointed to a particular range of values and the CPU is accessing contended memory. Some emulators can reproduce this effect to a certain degree of accuracy. None (that I know of), reproduce it 100 percent correctly. Many, simply ignore this quirk altogether. Note: At least one Spectrum game—Vectron (1985)—used it deliberately as a special effect.

3. Timing differences between all three Spectrum architectures—48K/128K/+2A. These have been mostly accurately implemented in many emulators, especially those that are being actively maintained.

4. Reading from certain ports (e.g. any port with Bits 1 and 15 reset) will cause the 128K/+2 machines to reset/hang. Some emulators don’t implement this behavior, or implement it incorrectly.

5. The floating bus behaves differently on the +2A/+3 than it does on other models. Since we’ve extensively studied and fully described its behavior in late 2017, only SpecEmu and Spectramine have correctly implemented it.

6. Issues 1 and 2 of the Spectrum 48K return different values when port $FE is read from. This causes some games that use simplified (and incorrect) keyboard polling routines to not work on later Spectrum models. Some emulators do offer an option to enable this behavior, but not all.

7. The Z80 has its own set of quirks, and not all emulators implement its behavior accurately.

8. No emulator is, of course, capable of displaying the Spectrum output at its native 50 fps. This isn’t the fault of any particular emulator, naturally, but rather the systems they runs on: most modern computers are locked to a 60 Hz refresh rate.

The above is by no means an exhaustive list, by the way.

So, given than no emulator implements all the quirks of the real hardware 100 percent correctly, how do you find the lowest common denominator? Do you just drop features or implement workarounds so that your game runs okay on the oldest and buggiest emulator? Do you think emulator authors will be incentivized to implement any of the missing features or fix the existing bugs if developers try to go out of their way to work around the emulators’ shortcomings?

I don’t know exactly when the first Spectrum emulator was written, but I think it’s a safe bet to say it was some time in the 1990s. People have had over a quarter of a century to figure out how to accurately emulate a very inexpensive and fairly simple 8-bit computer form the 1980s. How come no-one has done it yet? I don’t know the exact answer to that question (but am aware of the technical hurdles), yet I’m pretty sure it’s partly due to the complacency of Spectrum developers. Because regular gamers are rarely very demanding. The colors are slightly off? Who cares . . . The sound isn’t quite right? Who cares . . . The timing is not super accurate? Who cares . . . As long as I can run Manic Miner or Jet Pac . . .

Now, by no means do I dismiss the efforts of emulator authors. I fully understand that it’s a tremendous task in and of itself, especially if you don’t have access to real hardware or the knowhow of reverse engineering it. I myself primarily use emulators for development and most of the testing, and only use real hardware for final testing and fine tuning. But I don’t cut features or abandon techniques or otherwise compromise my code to cater to the shortcomings of a particular emulator. Nor do I add features that will only work in a particular emulator, unless they also work on real hardware.

That’s my philosophy in a nutshell.
Pegaz wrote: Fri Oct 12, 2018 11:10 am Emulators are very important and the best of them already behave practically as real Spectrum.
Their benefits will be even greater in the future, as there will be less and less working Spectrum around.
For me, every Spectrum is equally important , whether its based on emulator, fpga or some other non-original hardware.
Otherwise, even Amstrad models might not be considered as original hardware too.
With the emulators, Spectrum will drastically extend its life span and user base, without them it will only become just a piece of hardware in the museum.
Yes, but what do those emulators actually emulate? The Spectrum or the emulator author’s idea of what the Spectrum should have been like? I’m all for the preservation of the Spectrum. (I doubt many people will share our passion to the same degree it after our generation finally kicks the bucket, though.) But let’s preserve the actual Spectrum, to the best of our abilities. While the actual machines still run and while the people who understand, or can figure out, how they work are still around.

I’m not an activist at heart, but I do feel like we need to form a lobby to persuade emulator authors to try to make an effort and work on improving the emulation, rather than adding features to their oeuvres. However, I know what most of them will say (and I completely understand them): “We’re just doing this for fun. We’re not working for you. We don’t get paid for this, anyway, and simply spend our free time to do the things we love. If you want your bloody ‘super-accurate emulation,’ do it yourself.”

A man can dream, though. A man can dream . . .
Every man should plant a tree, build a house, and write a ZX Spectrum game.

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Re: AY emulation testing

Post by Ersh »

Ast A. Moore wrote: Fri Oct 12, 2018 2:25 pm 8. No emulator is, of course, capable of displaying the Spectrum output at its native 50 fps. This isn’t the fault of any particular emulator, naturally, but rather the systems they runs on: most modern computers are locked to a 60 Hz refresh rate.
Most, if not all, PC emulators support 50hz, and I'd say most screens support that refresh-rate as well. I have a cheap ASUS a couple of years old and it handles 50hz just fine.

Though handhelds (maybe laptops as well?) with a dedicated screen tend to use 60hz, so for an example, on Android you're out of luck. That's what I really like about ZXDS for the Nintendo DS, it's a portable with a solid 50fps thanks to some clever code and use of the hardware.
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Re: AY emulation testing

Post by ZXDunny »

Emulator authors (or at least Woody and I, when we were developing SpecEMU and ZXSpin) worked to the best available information at the time. That is to say, we implemented and tested what was known about the Speccy, but only what we could actually get. I'm sure we all remember RamSoft and their utter refusal to release any usable information about the ULA.

That they largely got it wrong was not discovered for many years after, but even so...

We had access to a +2a and a 48k model, which we would run tests on to make sure we matched it. We updated the emulators whenever we learned something new. But until Chris Smith released his ULA book, we were really working in the dark. Coming up with test programs to see what happened when certain conditions were met, that sort of thing.

One of those was the ULA Snow. Did you know that you can use that particular quirk of the hardware to do hardware-based attribute scrolling? We found that by accident. We determined roughly what it did, and how, and implemented that. But to implement it correctly would have required CPU usage in a Windows machine that was pretty much at the limit of hardware available at the time, if not more - this was back in the days well before 1GHz was commonly in use. We'd need to emulate the CPU and ULA/Display at 1 T-State accuracy which massively blew out the time spent emulating - for a feature that one or two games used.

We ended up with a "good enough" solution that allowed us to continue with 4Ts accuracy. hey, we didn't emulate memory refresh fading either, so that's on us too.

Because we were trying to emulate a Spectrum. Not some "idea of what a Spectrum was" but the actual hardware. I'm sorry if that wasn't good enough, but we did our best at the time.
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Ast A. Moore
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Re: AY emulation testing

Post by Ast A. Moore »

ZXDunny wrote: Fri Oct 12, 2018 5:10 pm Emulator authors (or at least Woody and I, when we were developing SpecEMU and ZXSpin) worked to the best available information at the time.
Thank you! That was a very enlightening post.

I do want to emphasize (again) that in no way do I try to take a jab at emulator authors. Moreover, and I’ve stated it on several occasions, I’m immensely grateful to Mark for implementing the floating bus on the +2A/+3. I was very excited when he came on board during our investigation of the issue and was able to come up with a new build of SpecEmu pretty soon after we’d figured out the last details. I was only too happy to create a special version of my main test program (and called it Woody’s Special) to help him fine tune his emulator, and—sure enough—his very next build replicated the behavior of the +2A dead on!

The point I’m making is that no matter how you slice it, we Spectrum software developers and you Spectrum emulator authors are in many ways co-dependent. If, as was suggested in this thread, we decide to implement workarounds—or worse, drop features—in order to fit the bill of the existing emulators, progress will never happen.

What other incentive could their be for an emulator author? Sure, some people need no external motivation (to a degree). Mark implemented the floating bus not because I'd written a game that used it, but because he just found it interesting or even exciting (I think).

Strangely enough, even Cesar, the person who actually discovered the existence of the floating bus on the +2A was not interested in pursuing it further. And he’s a bloody emulator author. :) When I started my own investigation, I tagged him a couple of times in my thread, he wasn’t all that interest and said he was busy with other things (in ZEsarUX, mind!). I get that: when he first brought it up in early 2016, the response was underwhelming, to put it mildly. My only regret is that I hadn’t returned to the scene at that time, and only discovered his post a year or more later. If I approached other emulator authors about it, they too would probably say, “Great story, bro. Ping me when there are a couple dozen more games that use it. Then I’ll think about it.”

It is, thus, a vicious circle: Spectrum developers try to avoid using certain features and quirks of the original hardware because emulators don’t support them, and emulator authors don’t implement them because there isn’t enough software that uses them. The question is: how do we get out of this rut?
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Re: AY emulation testing

Post by Pegaz »

Woody did a lot more than that.
When Ramsoft people revealed new ULA feature (four bright shades), ten years ago, Woody managed to implement this feature in SpecEmu in just a few days.
https://www.worldofspectrum.org/forums/ ... rlace-demo
https://www.worldofspectrum.org/forums/ ... edirect/p1

And this is not his only contribution to a better Spectrum emulation...
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Re: AY emulation testing

Post by djnzx48 »

I'm sorry, I hope I didn't come across as ungrateful towards emulator developers. It's just hard to get motivation for using these features and quirks when the most popular emulators don't support them and they haven't been updated in years.
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Re: AY emulation testing

Post by zx81 »

Ast A. Moore wrote: 8. No emulator is, of course, capable of displaying the Spectrum output at its native 50 fps. This isn’t the fault of any particular emulator, naturally, but rather the systems they runs on: most modern computers are locked to a 60 Hz refresh rate.
That's not exactly true, my friend... :)

The frame frequency on a 48k model is 50.0801 Hz and on a 128k model is 50.0211 Hz. These frequencies are basically impossible to reach on a TV or monitor where you can't control such frequencies (50 or 59.94 Hz, not 60 Hz). To get a precise 50 Hz refresh, the emulator needs to run slowly than real thing.

FPGA-based emulators, like the ZX-Uno, are capable to control the TV frame generation, so the "recreated" 48k runs at real speed. But, because the internal clocks have various limitations, the 128k model runs 8000 Hz away from real machine.

In ZXBaremulator the 48k model runs at the correct speed. So, when you configures the HDMI mode to 576i@50 Hz, you can see the interlaced effects without any trick in code, but with a small defect caused by the slightly different frequencies between TV and emulated Spectrum (50 vs 50.0801). For 128k models, the Raspberry only have a 1-Mhz timer, and I can't get the needed precision (a 128k frame lasts 19.99156356 ms), so it emulates frames of 19.991 ms long.

In theory, you can configure a HDMI mode with exact frequencies, but isn't easy to generate the needed numbers and can depend a bit of every TV-set, so I don't want to try this way. After all, the interlaced modes are "emulated" by modern TV-sets, and every model handles this with his own method.

I'm curious about the AY quality in ZXBaremulator, if somebody can try... ;)
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Re: AY emulation testing

Post by Pegaz »

zx81 wrote: Sat Oct 13, 2018 11:15 am
In theory, you can configure a HDMI mode with exact frequencies, but isn't easy to generate the needed numbers and can depend a bit of every TV-set, so I don't want to try this way. After all, the interlaced modes are "emulated" by modern TV-sets, and every model handles this with his own method.

I'm curious about the AY quality in ZXBaremulator, if somebody can try... ;)
Yes, I also came to a similar conclusion.
Actually, there are two ways, which allow me a perfectly smooth scroll on raspberry pi, at least on my TV set.
The first mthod is using the Unreal Speccy Portable emulator, as well as the latest beta version of your Baremulator for Pi 1 / Zero.
We've already talked about that and what you've said, in such a case Spectrum works slightly slower, for that difference between 50Hz vs 50.0801 Hz.
I recently discovered another method, which also gives an excellent result with Fuse version for Retroarch frontend on Raspberry Pi.
In this case, its possible to set the exact frequency at 50.0801 Hz in options and the result is also a perfectly smooth scroll without any interruptions.
I dont know if this particular settings is really precise or not, but in reality it works great.

btw, you previously wrote that the exact frequency for 48k Spectrum is 50.0802 Hz, so I'm interested in what is the correct number?
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Re: AY emulation testing

Post by zx81 »

Pegaz wrote: Sat Oct 13, 2018 12:00 pm btw, you previously wrote that the exact frequency for 48k Spectrum is 50.0802 Hz, so I'm interested in what is the correct number?
3500000 / 69888 = 50.08012821
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Re: AY emulation testing

Post by djnzx48 »

What about the more recent LCD monitors that support FreeSync or G-Sync adaptive sync? They can be set to arbitrary refresh rates, so couldn't you obtain a proper 50Hz display using one?
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Re: AY emulation testing

Post by djnzx48 »

I've made some simple fixes for a few emulators to fix wrong-sounding AY without needing in-game workarounds. To be safe I won't distribute the patched executables here, but you can apply the patches quite easily using a hex editor. I used Frhed but there are a number of free ones available. Make sure you make backups first in case you make a mistake.

You may have to run the editor as administrator if you're editing files contained in Program Files. Once you've opened the file, press Ctrl+G to goto the address starting with '0x' - note that all the addresses are in hex. Then enter the new values on the rest of the line. Repeat this for the other address and values. Finally, save the file and if all has gone well you should now have improved AY sound.

Here are the patches:

Spectaculator 8.0: the file to edit is ay8912.dll in the Spectaculator\drivers directory.

Code: Select all

0x2063: E9 FA 08 00 00 90 90
0x2962: 0F B6 71 52 D1 E6 8B 51 78 E9 FA F6 FF FF
SpecEmu 3.1.b120516: the file to edit is specemu.exe. (I know it's an old version but it's the newest one I could find.)

Code: Select all

0x2025:  E9 0A C1 02 00
0x2E134: 14 00 D1 E0 88 46 20 E9 EA 3E FD FF
ZXSpin 0.7s: the file to edit is ZXSpin.exe.

Code: Select all

0x9078A: D1 E0 88 46 13 8B 86 90 00 00 00 E9 0E 10 06
0xF179F: E9 E6 EF F9 FF 00 90 90 90
The results:

Image

And that's it. I also have a working fix for ZXSpin 0.666 but the executable is packed, so applying the patch is not so simple. I'll put the fix for that here if I can get it to work.
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Re: AY emulation testing

Post by hikoki »

there you have a newer SpecEmu
SpecEmu - 3.1.b160318

[mention]Woodster[/mention] google drive links don't expire ;) thanks!
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Re: AY emulation testing

Post by chernandezba »

Hi
I’ve just discovered this old thread, very interesting!
By the way, AY emulation in ZEsarUX is a custom code made my me, so probably audio doesn’t look like other emulators. Also, I don’t add audio filters, so it’s just the audio wave with my own algorithms ;)

Cheers!
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Re: AY emulation testing

Post by djnzx48 »

I made another small patch for Spectaculator. Apparently if an AY channel has tone and noise activated at the same time, the resulting output level is double what it would be if tone or noise were used independently. The patch seems to work as intended, but I can't say for sure how accurate the results are.
Spoiler
>ay8912.dll
00002063:0F->E9
00002064:B6->FA
00002065:71->08
00002066:52->00
00002067:8B->00
00002068:51->90
00002069:78->90
00002225:66->E9
00002226:8B->46
00002227:04->07
00002228:55->00
00002229:98->00
0000222A:30->90
0000222B:00->90
0000222C:67->90
00002962:00->0F
00002963:00->B6
00002964:00->71
00002965:00->52
00002966:00->D1
00002967:00->E6
00002968:00->8B
00002969:00->51
0000296A:00->78
0000296B:00->E9
0000296C:00->FA
0000296D:00->F6
0000296E:00->FF
0000296F:00->FF
00002970:00->66
00002971:00->8B
00002972:00->04
00002973:00->55
00002974:00->98
00002975:00->30
00002977:00->67
00002978:00->51
00002979:00->B2
0000297A:00->09
0000297B:00->8B
0000297C:00->CF
0000297D:00->D2
0000297E:00->E2
0000297F:00->59
00002980:00->22
00002981:00->51
00002982:00->53
00002983:00->0F
00002984:00->85
00002985:00->A4
00002986:00->F8
00002987:00->FF
00002988:00->FF
00002989:00->66
0000298A:00->03
0000298B:00->C0
0000298C:00->E9
0000298D:00->9C
0000298E:00->F8
0000298F:00->FF
00002990:00->FF
Would anyone like to confirm this effect on a real chip? Here's a .tap file I used for testing: NoiseVolTest.tap

The drums at the start of Robocop will also work.

This behaviour has serious consequences if it works as described, as it affects thousands of games and demos made in the last 20 years that are based on an incorrect model. Many tunes sound very noticeably wrong after applying this patch - assuming I didn't mess up the implementation.
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Re: AY emulation testing

Post by djnzx48 »

Is anyone willing to try running the test?

Meanwhile, I found yet another bug: envelope type 13 is broken in Spectaculator. It's supposed to ramp up to the maximum level and stay there, but instead it drops back to 0, resulting in silence. You can see the cause of the problem here:

Image

The envelope level is incremented, but does not get set back to 15 when the cycle ends, causing it to overflow and become silence.
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Re: AY emulation testing

Post by Guesser »

I've said it before, but I believe the only way to get really accurate AY sound is to first determine how the DACs actually work and then calculate the resulting levels and apply suitable filtering per machine. No fiddling with look-up-table values is going to correctly emulate a +3 for instance.
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Re: AY emulation testing

Post by djnzx48 »

Guesser wrote: Fri Sep 13, 2019 12:18 pm I've said it before, but I believe the only way to get really accurate AY sound is to first determine how the DACs actually work and then calculate the resulting levels and apply suitable filtering per machine. No fiddling with look-up-table values is going to correctly emulate a +3 for instance.
Could you elaborate on that? What does a current emulator (using accurate measurements) get wrong that would require proper DAC emulation to rectify? I'm not sure that I see much value in trying to emulate the faulty sound of the +3.

I found some recordings taken from AY and YM chips, and I couldn't detect any volume differences in the tone+noise parts. Or if they are there, the difference is more subtle than claimed. But it's hard to tell without an isolated recording.

I also fixed the envelope 13 bug, here's the patch if anyone wants to apply it.
Spoiler
000021D5:C7->E9
000021D6:00->B7
000021D7:03->07
000021DA:00->90
00002991:00->C7
00002993:00->03
00002997:00->89
00002998:00->1A
00002999:00->E9
0000299A:00->3D
0000299B:00->F8
0000299C:00->FF
0000299D:00->FF
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Re: AY emulation testing

Post by 1024MAK »

Amstrad messed up the audio circuitry on the +2A/+3 PCB hence the AY audio is distorted. Note that most +2A machines use a later PCB where they corrected the circuit design.

Also, what type of digital to analogue converter does the AY actually use? And is there good repeatability between chips of different batches...

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Re: AY emulation testing

Post by Ast A. Moore »

djnzx48 wrote: Fri Sep 13, 2019 10:03 am Is anyone willing to try running the test?
Sure. Here’s the audio from my +2A(B). Recorded from the TV speaker.
Matches the output of Fuse, by the way.
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Re: AY emulation testing

Post by djnzx48 »

Thanks. So the 'noise doubling' doesn't seem to occur on a real chip.

I wonder how EvgenRU managed to observe the effect.
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Re: AY emulation testing

Post by Guesser »

djnzx48 wrote: Sat Sep 14, 2019 5:15 am Could you elaborate on that?
The different models of Spectrum all have different circuits for the AY, though the +3 is where this has the most significant effect on the resulting audio signal as the amplification is all wrong. The +3 and Black +2 circuits both have some low pass filtering too.
Last edited by Guesser on Sat Sep 14, 2019 4:47 pm, edited 1 time in total.
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Re: AY emulation testing

Post by Guesser »

1024MAK wrote: Sat Sep 14, 2019 10:49 am Also, what type of digital to analogue converter does the AY actually use? And is there good repeatability between chips of different batches...
And what effect does shorting the channels together vs mixing them with a resistor network have on the resulting signal.
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Re: AY emulation testing

Post by goodboy »

full reverse of AY-3-8910 1979 G.I. CORP
http://dlcorp.nedopc.com/download/file.php?id=3033
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