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

[SirRockALot]

For what it’s worth, I have a VA4 MD1 and use the stereo minijack breakout cable from RGC and I can’t hear any audio issues whatsoever. Since I use quite a long cable with a SCART extender and go through a switch there’s a bit of coupling between video/audio (some high pitched noise on certain high contrast images), but no digitization artifacts or any kind of distortion / clipping I can hear. Sounds exactly like what comes out of the headphone jack. I have the same kind of cable for the AES, no issues either.

[Ali-Pacha]

Received my OSSC today, I’ve played a lot through options (thanks the junkerhq wiki), and I’m absolutely stunned by the picture I’m able to get…except jailbars, but I knew it beforehand 😀
BTW, I add another testimony to this sound issue, high pitch ringing / distortion when sound passes through OSCC 1.6 ADC. RGC packapunch cable, very first one japanese unit (HAA-2510, then jailbars). Absolutely no problem when OSCC is an analog sound passthru, from the SCART as well as from the headphones jack of the console.

Ali

[marqs]

I have a model 1 VA1 board over here now and have investigated the issue a bit. Distortion appears to be a common flaw in model 1 VA0-VA2 boards due to excessive pre-amp gain. In my tests distortion was most apparent when actual headphones were directly connected to the socket. Aside from various amp replacement mods, the issue can be also fixed with simpler methods described in this thread.

Below are samples from before and after applying the fix:

Sample 1 (ossc with AA-filter, vanilla VA1)
Sample 2 (ossc without AA-filter, vanilla VA1)
Sample 3 (ossc without AA-filter, VA1 with reduced pre-amp gain)
Sample 4 (ossc without AA-filter, VA1 with reduced pre-amp gain and tweaked VREF)

[jonlad]

Only time I hear it is via OSSC HDMI, headphones/OSSC analogue/MD 3.5mm jack to AVR all sound fine. Maybe I have a different model 1 to yours but still get the distortion via the OSSC.

Marqs – what does this mean for those not as knowledgeable as yourself? Is there a possible fix/fw change without having to hard mod the MD/OSSC or is that our only option?

[Zoomer88]

I’ve just checked the topic there and they’re talking about fixing va0-va2 amps, but I’m experiencing the issue on va4 and va5 (only throgh ossc though, not through PC’s line-in or AVR

[marqs]

Reading through the thread, it’s hard to find any consistency on which MD1 models are affected and which aren’t:

* There are 3 reports (Ali-Pacha?, Damn-Deal-Done, me) on distortion with VA0-VA2 boards
* Distortion is reported on VA4 and VA5 boards (Zoomer88)
* No distortion is reported on a VA4 unit (SirRockALot)
* Distortion disappears when audio is tapped before power amp (VA?) (esp)

Since that pre-amp gain mod brings VA0-VA2 boards closer to VA3-VA6. it’s surprising to hear some VA3-VA6 have the issue as the mod seemed to get rid of distortion as demonstrated on the clips I linked. I wonder if anyone has experienced distortion via DIN output which would suggest that MD’s CXA1034 headphone amp is the common factor (with 10% THD it’s far from hi-fi for sure)? There are still tweaks that can be done with OSSC v1.6 audio ADC (e.g. doubling of internal sample rate) which might help without any hw modifications, but without knowing the root cause it’s hard to get forward.

[BuckoA51]

Hey Markus, I can confirm I have the issue here on my VA5 Megadrive 1. It’s subtle, especially compared to how bad that VA1 was I sent you, but it’s still there. If you aren’t overly familiar with Megadrive you could easily miss it.

If you like I can make a recording comparing OSSC 1.5 + Audio and OSSC 1.6 with the VA5 too? If you need to borrow my VA5 to play around with I can arrange that for you.

[marqs]

Could you first check if distortion is present if audio is tapped from DIN connector (e.g. via DIN5-RCA cable if you have one)?

[BuckoA51]

I do and I believe it was, but I will test again to be sure. I made some recordings using both cables, but whatever compression or filtering the capture cards doing seems to have squashed out all the strange noise, typical!

[James-F]

Model1 VA6 same distorted noise.
When I unplug the 3.5mm jack from the Genesis and plug into a phone and play music the sound is fantastic and low noise.

Seems like an impedance mismatch on the OSSC side for very low impedance outputs like headphones drivers from the Genesis.
Adding 1k ohm resistor in series to the signals on both channels might fix this.

EDIT:
Yep that 1k resistor in series did the trick.
Dead silent and distortion free.

[marqs]

How about adding a parallel load (e.g. 32ohm) like I suggested a few pages back?

[BuckoA51]

On my VA5 I tried the audio using a mono SCART cable that takes the sound from the DIN socket. There was still distortion and it seemed over loud.

[James-F]

Yep a 330ohm as a load fixed the audio too.

As a matter of fact, just by touching the 3.5mm tips with my finger (me as a load) makes things much better.
I would have to assume that any resistance creates a reference voltage from a current source for the OSSC ADC, I would also have to assume that the OSSC has no analog circuitry before its ADC..?

As you can see in the schematic, the headphone output of the MD1 expects a speaker load.
Any ADC expects a Voltage on a Load.

Model1 Audio Schematic
CXA1034PM

[BuckoA51]

Ok so we need to get all this information together, if I’m understanding this correctly, for VA0, VA1 and VA2 consoles, perform this mod:- http://www.sega-16.com/forum/showthread.php?27397-VA2-Preamp-fix

For VA3 +, add a resistor? Please can you provide a photo of where to install this?

[James-F]

MD1 VA0-2 have its internal amplifier clipping from too much gain, this has nothing to do with the OSSC.
If I had to assume, the OSSC has no load in any form before its ADC, so for the unusual MD1 VA0-VA6 “speaker”-output one would have to add a load… any load, which can be added to the scart plug itself.
1k on each line would be good.

Model1 Scart Plug
Just solder 1k resistor between pin 2 & 4, and pin 6 & 4.
Note that Pin 2 and 6 wired to the tip and ring of the 3.5mm plug, pin 4 is ground.

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