Watch out when recording PCE chip soundtracks...

[ccovell]

Here's something that I discovered last night, and it surprised me:

The audio output from the 5-pin DIN jack of my CoreGrafx has rather distorted audio, potentially causing higher-frequency harmonics (and loss of bass).  This might be a defect of my unit only, but I doubt it since this seems to be a direct byproduct of the inverting amplifier used in the CoreGrafx.)

So, I tested the PCE's audio out from this port, as well as through the 5-pin DIN jack that's on the side of my Super CD-ROM unit, and here are my findings in a pic:

[thesteve]

thats a capacitor charge curve

[ccovell]

Really? On both sides of the Y-axis?  Wouldn't a capacitor lend a charge curve for positive-going waveforms, and a discharge for negative-going waveforms?

[thesteve]

nope
a cap will trend towards 0 from + and -
bigger caps makes it less, bigger loads make it more
was that running into a load?
if so what load?
note the same thing happens with video signals, as seen as a shadow to the right of something bright or luma fade left to right

[ccovell]

nope
a cap will trend towards 0 from + and -
bigger caps makes it less, bigger loads make it more
was that running into a load?
if so what load?
note the same thing happens with video signals, as seen as a shadow to the right of something bright or luma fade left to right

Okay, then that explains it a bit...  as you can see in the scale the CoreGrafx's amp has a centreline around 0(v), while the Core->CD-ROM amp's output really has no negative voltage component.

Both through my oscilloscope and as recorded through my PC's line input show the same waveforms, thus a nominal load (whatever PC line inputs have.)

[dshadoff]

Very interesting !

What is the machine driving the Super CDROM unit (without distortion) ?
Is it the same CoreGrafx unit (with distortion), or is it a different machine entirely ?

The reason I ask is because I recall some discussion several years ago, about what the differences were between the Hu6280 and the Hu6280A - and sound was one of the items somebody speculated about.

Dave

[ccovell]

It's the same CoreGrafx... it pipes unamplified audio to the Super CD-ROM unit, which does its own (better?) amplification.

I don't know which unit does the mixing of the CD sound with the PCE's sound, since both DIN ports output CD audio...  But anyway, the amp in the CD-ROM unit, though weaker, is superior.

[thesteve]

the mixing is done in the SCDR2

[Black Tiger]

If it is an aspect of all CoreGrafx units, that would suck, as it's one of the few models (only CoreGrafx 1 and SuperGrax?) that have the updated audio chip.

[ccovell]

Okay, so if the mixing of the PCE & CD audio is done inside of the CD unit, how does the stereo audio come back from the CD and out the Core's DIN plug?

If this chart is to be believed, the (mixed?) audio IN to the PCE is mono:
http://www.gamesx.com/misctech/pcebp.php

Is there some other voodoo going on with audio from each unit on the expansion connector?

Or have I been listening to mono CD audio all these years and not realized it?

[thesteve]

as i understand it you have been listening to mono
havent tested it myself

[herr-g]

Some years ago I'd experimented a bit with the SNDin (Pin 1B) on a white PCE to realize that indeed there only seems to be monaural audio coming into the PCE from the CD Redbook. The big drawback with the white PCE is that it doesn't use the SNDin at all, so on a RGB mod you can't get CD-Redbook from the console that easily.

On the Core/Super Grafx the monaural redbook never bothered me since I only use the DIN connection with my TV. My stereo equipment is always connected to either the Headphone- (Duo) or the Cinch- connectors (IFU).

Although I've never checked it I'm quite confident that the Duo variants deliver stereo redbook on the DIN connectors.

[Trenton_net]

From what I understand, the white PCE units also have RGB output issues when combined with an IFU unit too. Using a Framemister, you get unstable video with that combo (Sync issues?) but with any other core PCE unit, it comes out nice and pretty. Maybe try using another PCE unit to see if the same distortion happens with the audio? If indeed your using a white PCE, I wouldn't be surprised if some other weirdness was going on.

[Bonknuts]

Both have 'return to zero' in action, but it's much stronger on the amp inside the unit vs the cd unit amp (load/terminator on the scope side shouldn't effect this, only the levels. At least, this is how it was for video). I've noticed this as well (supercd unit vs sgx unit). Also, supposedly only the 6280a audio amps inside the console output inverted audio (mednafen author first noticed this).

[Opethian]

as i understand it you have been listening to mono
havent tested it myself

Ive tested the SuperCDrom2 setup with a CG and CGII and the DIN on the CGs are indeed mono. the only way to get stereo output is the DIN on the supercdrom2 itself.