Badly distorted audio through headphone jack on 9.2/MiSTER Pi I/O boards

[CyberAkumaZero]

I don't have a non-MiSTER Pi DE-10 or 9.2 I/O board so I can't verify if it happens on others too, I have seen conflicting reports that it's on all 9.2 boards or only MiSTER Pi versions. This is happening even on the revised MiSTER Pi I/O boards which fixed a previous audio issue mostly to do with the PlayStation core.

When playing audio through the analog headphone jack on many MegaCD games the sound is heavily distorted. The effect differs from game to game, and does not appear to occur on the boot screen. Some games like Sonic CD have scratchy audio in the intro, less so once in-game but still occasionally happens, others like Keio Flying Squadron the distortion is so bad it's completely incomprehensible both in the intro and in-game, in a few games audio essentially completely cuts out a few a few moments.

The audio is fine over HDMI (apparently TOSLINK too but I have no TOSLINK devices to test), it's only the headphone jack where it is an issue.

The problem also seems to be related to the video output for some reason despite there being no issue with the analog video output itself. Turning on Component Blend seems to instantly mitigate 99.99% of the issue, but there is still the occasional scratch. Others have reported that setting vga_scaler=1 or using anything other than RGB mode on the analog video output also mitigates the issue.

There have been discussions about it here:

https://misterfpga.org/viewtopic.php?t=8792
https://misterfpga.org/viewtopic.php?t=8479
https://www.reddit.com/r/MiSTerFPGA/comments/1hbs0jy/sega_cdcore_garbled_analog_audio_via_mister_pi/

[birdybro]

Sounds like a marginal timing issue. I think the issue was resolved in the PSX core when Robert shifted the phase of one of the PLL.

[CyberAkumaZero]

For the PSX core, wasn't it found to be a physical defect in the Taki version of the 9.2 boards? I recall him RMAing them to replace some capacitors that had the wrong values, though I don't know if the PSX core also had an issue and the software was updated in response to it.

Likewise I don't know if this is due to the core or the board, the weirdness of it fixing itself depending on video settings makes me think it might be the core.

[birdybro]

Actually looks like that was just a build only distributed in the discord and might be unrelated. Either way, the behavior you are observing where the audio issues go away when you turn on composite is usually the kind of weird issue you see with a marginal timing problem.

[birdybro]

I ran 10 compilations in a row, this was seed 6 with the best quality of fit. Does it make the issue go away or is it still present?

MegaCDseed6.zip

[CyberAkumaZero]

Tried it quickly on the games I mentioned. The issue is significantly mitigated, but it still sounds like there is the occasional scratch or pop during the audio. Reminds me of when I turned on Component Blend to mitigate it. Sonic CD's title screen sounded fine, the intro FMV that plays if you wait at the title screen had a few scratches, as did the intro the Keio Flying Squadron. I'll do more testing on it.

I have seen others report that the audio becomes better or worse when adjusting some of the analog video settings such as the gamma or contrast, setting it higher seemed to make the audio worse. The contents of the image being sent over analog appear to effect the audio in different ways, even the type of image being sent (RGB, S-Video, composite, etc).

[birdybro]

Yeah, this lends more credence to it being a marginal timing issue, thanks for testing! Hopefully this will point Sergey or Sorgelig in the right direction (or whoever else wants to take a stab at it). These kinds of issues can be difficult to solve.

[CyberAkumaZero]

No problem, hope someone can figure out why this is happening.

Doing some additional testing, it's definitely effected by what is currently being displayed on the screen, and I don't just mean the composite blend. Two games that I found were good examples of this in opposite ways were Popful Mail and Shadow of the Beast II.

Using the original core:
In the case of Popful Mail in the intro it first starts out sounding fine, but then gets heavily distorted. I realized that the screen when the intro first starts is almost entirely black, and the parts that are not are showing darker/grey colors. It wasn't until brighter colors started appearing in the intro that the audio started to distort. Furthermore opening the menu actually made the effect worse, so much so that it was still pretty badly distorted even with Component Blend enabled while the menu was open.

Shadow of the Beast II was almost the opposite. There was distortion in the intro, but what was interesting was when I got in game. In this one, opening the menu made it better, not worse, and I noticed that the audio was pretty distorted when I first load into the game standing near a tree, but if I walk left a little to make the screen scroll the effect diminishes greatly, walking back to the tree brings the distortion back. And again, Component Blend mitigated it heavily but it was still an issue.

In the Seed6 core the distortion was heavily mitigated but not gone completely.

[paulb-nl]

Since it seems the video content is causing this issue I am curious if it is one or more of the RGB colors causing interference onto the audio signals.

I made some gamma filters that filter out only red, green or blue. Please try them and see if there is any difference.
gamma_rgb_test.zip

[CyberAkumaZero]

It didn't seem to make any difference, but then again I admit I am not sure if I was using them right. I put them in a subfolder in the Filters folder, then after launching a game went to Video Processing, changed filter from Nearest Neighbor to From File, and tried toggling between the three files. Switching between the no red/blue/green files didn't seen to have any effect on the image, I could still see red/blue/green colors when the filter to remove them was selected.

Speaking of which though, I am pretty sure red is part of the problem, the intros of both Keio Flying Squadron and Popful Mail helped with this.

Keio starts out with a series of sepia-colored screens that heavily distort the audio, then after a minute it suddenly cuts to a green scene in a forest where it's mostly green and the audio clears up considerably, then it shows red colors again with a fire and a character in red clothing and the audio distorts again.

Popful starts out with a mostly black screen and the intro in grey colors, then shows a scene almost entirely in green, the audio is mostly fine in both of these scene, but then as the image scrolls down showing red the distortion starts, though, it feels like it gets worse once the image scrolls down further and it shows both red and a sepia/peach-like colored scene.

Maybe it's more a mix of red and other colors instead of pure red, or might be where the red colors are on the screen since the distortion in Popful did not start until the red had scrolled up a little, not when it first appears.

[paulb-nl]

They are gamma filters so you need to use them at the Gamma Correction menu item below the horizontal and vertical filters.
Also make sure vga_mode=rgb in MiSTer.ini

[CyberAkumaZero]

Will these only work through VGA out? Or the HDMI port too?

[paulb-nl]

They affect both HDMI and VGA.

[CyberAkumaZero]

Ok, after trying them out it was hard to determine what was happening because the results are not consistent from game to game, also doesn't help that opening the menu to change them can effect the audio sometimes too.

I tried to find scenes that I could just leave it at for a while while switching the filters so that too much movement doesn't effect it, this wasn't possible with every game I tried though.

For the most part, filtering out the blue was the worst result. Either it did nothing, or fixed the audio a bit but it was still clearly garbled. In those cases filtering out either red or green seemed to work, even though from what I tested scenes that are mostly green tend to not have much of a problem.

I think maybe it's not pure red or green but some gradient between them that might be the issue, but every game seems to behave differently so it's hard to pin it down. One game the only difference between two scenes was that I picked a character that was mostly grey/white instead of blue and his tiny world map sprite was enough to cause the issue when the previous blue sprite in the same scene did not.

Perhaps it might even be a sharp change between some red/blue color to another? That might explain why Composite Blend tends to fix it if it smooths out the transition between the two colors instead of it be an immediate change from one pixel to another.

[birdybro]

It's not as directly related to graphics as you are assuming. It's just randomly the case that when you adjust composite blend it's changing the timing of logic elsewhere.

[paulb-nl]

@sorgelig Do you have this issue with analog audio on this core with your boards?