Bad audio quality through SCART from Genesis (using headphone out) – OSSC 1.6

[SirRockALot]

That makes no sense, the sync stripper lives at the SCART end of the cable, so the interference would have been added by then.

I wrote ‘composite leaks into RGB inside the SCART switch’. If you plug the shielded cable directly into the TV/OSSC you’d see no problem, but going through the switch, it does. So here it makes a difference even inside the SCART head. Note that you can also get cables where the stripper is installed on the console side.

[Harrumph]

yes, you can plug in any other audio source to A1 and it gets mixed together with audio coming from SCART and sent over HDMI.

Ah I see, then I was mistaken. So when the switch is in AV1 out mode, it works exactly like before in that there is a direct connection between the SCART audio input and the AV1 out. IIRC, feeding audio input from both directions causes problems because you get incorrect impedance, or something to that effect, (I remember seeing some post about this when people had problems trying to route component source audio via the AV1 jack while concurrently having a SCART cable attached, which is why the switch was implemented).

But the other problem I see is the cable you’re using (I didn’t check the link until today). I had assumed the audio wire from headphone socket connected directly to the SCART head, but it seems it meets up with the DIN connector and then travels up the cable together with all the other wires (I see that retrogamingcables.co.uk also does this so I guess it’s a common solution). This risks introducing coupling noise on the line.

So one solution is to get a cable that is constructed such that the audio lines travel separately all the way up to the SCART head, I am pretty sure this type of MD1 cable is available somewhere.

A potential alternative solution that doesn’t require you to buy a new cable, is to depopulate the audio input pins on the scart head (i.e. desolder the audio wires coming in there), and run the audio separately to AV1 as you have already done.

[BuckoA51]

I wrote ‘composite leaks into RGB inside the SCART switch’. If you plug the shielded cable directly into the TV/OSSC you’d see no problem, but going through the switch, it does. So here it makes a difference even inside the SCART head. Note that you can also get cables where the stripper is installed on the console side.

Using sync strippers introduces other potential problems and quality issues too though, so generally they’re only recommended where clean csync is required and where getting this from the console itself is not possible.

[jonlad]

Harrumph – I have already mentioned that I tried running a separate 3.5mm to 3.5mm audio cable from the MD to the 3.5mm AV1 In jack. This is a completely separate cable from the RetroGaming scart cable. There is still the same distortion.

I have tried headphones from the MD headphone jack and there is no distortion.

Only time distortion is present is when audio (either via separate 3.5 jack cable or via RetroGaming scart/audio cable) is running through the OSSC.

Has anyone else tested this using the games affected above?

[SirRockALot]

Using sync strippers introduces other potential problems and quality issues too though, so generally they’re only recommended where clean csync is required and where getting this from the console itself is not possible.

I only use it for the PAL GC, there’s no csync/luma option available and my switch has trouble with sync-on-composite :/ I’ve heard before that sync strippers can cause issues. Could you explain what kind of issues can be seen?

[Zoomer88]

I only use it for the PAL GC, there’s no csync/luma option available and my switch has trouble with sync-on-composite :/ I’ve heard before that sync strippers can cause issues. Could you explain what kind of issues can be seen?

I don’t see exactly how sync strippers are relevant to this thread especially since the issue seems not even to be related with cables.

[BuckoA51]

Cables can certainly be a factor in such things, though perhaps not likely given the research you’ve done with this.

I’ll see if I can make some audio captures of 1.5 vs 1.6 vs direct connection, I was going to do that today but I’ve hurt my back so can’t bend to hook up all the cables.

I’ve heard before that sync strippers can cause issues. Could you explain what kind of issues can be seen?

Technical explanation from Voultar:-

An LM1881 is going to generate a certain amount of line delay. And based on how many uS that line-delay is, the raster will start drawing the screen “too late” causing a horizontal shift in the picture. That’s on top of all of the nasty reflection problems and other things you have to take into account. If you have a device that requires a logic level sync signal, you should use what the console provides. An LM1881 should really only be used if you don’t have access to this.

[marqs]

Does the issue persist if you connect an external audio source to 3.5mm connector of AV1 while MD is connected to AV1 scart (assuming scart audio pins are not connected on that cable)? It might be worthwhile to test other way around too (e.g. a VGA video source with only MD’s audio connected to 3.5mm jack of AV3).

[SirRockALot]

Technical explanation from Voultar:-

An LM1881 is going to generate a certain amount of line delay. And based on how many uS that line-delay is, the raster will start drawing the screen “too late” causing a horizontal shift in the picture. That’s on top of all of the nasty reflection problems and other things you have to take into account. If you have a device that requires a logic level sync signal, you should use what the console provides. An LM1881 should really only be used if you don’t have access to this.

Interesting, thanks! Obviously it’s stupid to use a sync stripper if there are no image artifacts or compatibility problems or native csync is available, but this gives me a reason to remove the stripper from my PAL GC cable once I get a better SCART switch.

[jonlad]

@Marqs – When you say external, do you mean separate to the MD? You mean from another source?

I have tested the MD audio into AV1 using a separate audio cable whilst MD is connected to AV1 scart and still have the same issue.

[esp]

I’m also having the same issue with my OSSC 1.6. My 1.5 with audio mod sounds fine with the same console and cables, but I’m getting audio distortion on the Mega Drive with 1.6 over HDMI, even though A1 out sounds normal.

@marqs I connected the headphone out from the Mega Drive to A3 in and used a VGA source, but the distortion is still there.
I could record samples if that would help.

[marqs]

@esp: aside from the fact that v1.5 addon uses different audio ADC, the only relevant difference is that v1.6 board does not have pre-ADC antialias filter. It’d kind or ironic if MD’s “high definition” audio contains significant amount of out-of-band high-frequency components / noise to require an antialias filter while other systems sound fine without it.

[Zoomer88]

@marqs On the other hand it might be related to the fact that we’re using headphone out with a headphone amp meaning audio signal is different from other consoles with non amplified line out. Thus requiring antialiasing. Hence probably the fact that some people with Genesis do not experience this issue – they’re using model 2 Genesis with line out on the miniDIN connector.

Could that be the case?

[marqs]

I understood that the same issue occurred with standard line-level output from DIN. It’s certainly not ideal to connect headphone output to a high-impendance input even though usually the result is acceptable. That brings to my the original Nintendo DS where I had to add some additional load resistance to its headphone output to avoid distortion on low volume levels while using certain headphones or active speakers.

[Zoomer88]

@marqs Actually, I don’t believe we have testimonials that the same happens with line out. You see, with model 1 Genesis routing audio from headphone out is considered the only viable option since miniDIN on model 1 is wired only for mono out. Even the official documentation for sega cd model 1 suggests using headphone amp or internal adio lines (audio pins on sega cd connector inside the sega Genesis console) and special cables are supplied with sega cd for that.

All aftermarket scart rgb cables designed for Model 1 would have this additional minijack dangling from the miniDIN connector, while audio pins in miniDIN itself usually not wired at all.

Also this separate headphone circuitry is considered to be superior quality (in addition to being stereophonic of course).

So it’s most likely that those reporting in this topic are extracting audio from Genesis the same way I do it.

Ironically this headphone amp on high-def midel 1 genesis is the only way (well, at least easily accesible way) to have the best audio from genesis, for model 2 and other iterations have other errors and bugs where audio is considered and reqire rather heavy hardware modifications, let alone tracking the proper modifiable model 2 revision (google genesis ccam or megaamp).

So all in all, this is a hardware limitation on OSSC 1.6’s part, I believe? There’s no hope of fixing it in software? Would it be possible to replace 1.6 with 1.5 with audio mod?

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