Converting toslink output to coaxial

According to the SNES and Saturn digital output mod on GameSX, you can choose a coaxial connector rather than a toslink transmitter, using only a couple of resistors.

https://gamesx.com/wiki/doku.php?id=av:snes_sp_dif


Is it safe to assume that the same mod could be applied to all toslink outputs, or is it limited to the CS8405 chip for some reason? I would like to change the toslink output on my CD player to coax instead, as I would rather use that input on my external DAC.

 

Yes that will work fine for 5v TTL toslink. If you have any 3.3v TTL toslink devices then the voltage divider will need to changed.

 

Excellent thank you. So I just measure peak to peak on the input pin on the toslink transmitter with my scope, to figure out what it uses?

 

Link says coax should be dropped down to 0.5v

If the source was 5v with that voltage divider, you would get 1v.
if using 3.3v ish, you get 0.7v

I suspect the above is intended for 3.3v stuff, not 5v.

 

Thank you

Correction: I did not want to use peak to peak (Vpp) to measure this, as it makes little sense. Vmax is what I needed instead.

So I measured the pins on the transmitter. It has 4 pins, where two of them gives pretty similar waveforms, one is 5v and the other is about 3,3v.

I set the acquire mode to Peak (possibly what I was thinking of earlier), and the memory depth to max.

Below are some shots of pin 1 and 2:



They both look pretty identical, so I hunted down the service manual:


And I can see that pin 2 is connected to a pin called 'EMP" on the digital signal processor. I take it that's the pin I want to use.

Can I leave the toslink transmitter in, and just drill a hole next to it for the coax connector, and expect both to work if I tap the signal from pin 2. Or should I remove the transmitter instead?

 

Tried it with the toslink transmitter still installed, used pin 2, and a 470 and 75 ohm resistor as voltage devider. Got around 600mv on the output, which may have been a bit too high but should be within working specs, and it doesn't work :/

Could it be pin 1 instead?

 

Tried lowering the output voltage a bit using pin 2. Didn't work.
Tried pin 1 (with a different voltage divider of course), didn't work either.

Do I have to remove the Toslink port for this to work? :/

 

It looks OK to me - there is also going to be a 75R termination at the far end of the coax, so the effective impedance of the lower end of the potential divider is (1/(1/75)+(1/91)) = About 41R - which would give you about 550mV

 

Forgot about the termination resistor

 

So assuming it's pin 2:
If the input is 3.8v and I use a 330ohm resistor (R1) to connect the signal to the RCA plug, then I would need a 150ohm resistor to GND (R2), to adjust for the 75 ohm termination?

EDIT: Tried it, but it didn't work either

 

Yeah, that would give you 500mv

 

Okay, what if I took the signal from the collector on the PNP transistor marked Q223 (see the attached picture from the datasheet).

Probing that, gives a pretty nice looking square wave ranging from 0v to 5v. That makes a lot more sense than the pins on the toslink transmitter where the difference was around 2v, and never dipping anywhere near 0v.



One thing I can't quite figure out though. Looking at the schematic it would appear as if the collector is connected to GND. But measuring with my multimeter that's not the case. Can someone enlighten me here?

EDIT:

Alright, that actually worked. Neato!

And the toslink output works still as well.

Used a 330ohm resistor as R1 and a 75ohm as R2. Should give about 510mv (after 75ohm termination).

However, the voltage on the collector pin has been halved to about 2.5v. And the output signal looks pretty messy (captured without the 75ohm termination).

Collector pin after the mod:


Output (without termination):


I wonder if this will cause any harm, or will cause massive jitter, making the music sound poor?

Install pic here:


EDIT2:
Output with termination (connected to my AVR):


Looks absolutely terrible, and doesn't resemble squarewaves at all. I'm surprised it even works

Here is how I have it wired up, any suggestions would be much appreciated


Player is a Sony CDP-X222ES.

 

Been reading up on this a bit, and I have a few ideas. Please correct me if I'm wrong.

I want to tap the signal from the collector side of the transistor, as that has the cleanest squarewave pattern by far (see first picture in last post).

Then I'm thinking of doing of doing one of the following:

1. Using a buffer to amplify the signal. This should leave everything prior to the buffer unchanged, and then I can do the voltage divider on the output of the buffer > RCA plug. My worry here, is that the voltage divider will still mess up the signal, just like it did without the buffer.

2. Use a buffer (or other IC) as a logic level converter and input the 0-5v signal, and converting it to something that will end up being 0-0.5v at the RCA with the 75ohm termination in the end. Not entirely sure what would need to be done to the output, to counteract the effects of the 75 ohm termination though.

I still need to do some more reading on how to use the buffer as a level converter, but I'm hoping that would produce a much cleaner result than my previous attempts.

My concerns using a buffer is if it will pass though the squarewave without added noise when level converting, the end result should be as flat as the input. And what about speed? Are these chips fast enough?

I have some old HCF4050BEs on hand. But I need to order stuff from Mouser soon anyway, in case they are a bad choice.

 

Maybe use an opamp? Just set the resistors to give you 1v output and put a 75ohm resistor on the output. The 75ohm to ground at the other end will half it to 0.5v

 

Thank you for the suggestion. I will do some reading on that tomorrow and hopefully try it out too

 

I tried reading up on the opamps, but I just can't wrap my head around how I would achieve a gain of 0.2. As far as I could find it's not possible, at least not using a single channel opamp. But maybe it's possible if you use multiple inverting and non-inverting channels in a row?

At any rate, I tried running the signal straight through the opamp and it noticeably colored the signal which resulted in very rounded corners. Probably not what I'm looking for.

I hooked up the buffer IC and immediately noticed that it cleans up the signal quite nicely. Again I tried using a voltage divider on the output to achieve 500mV, but again this produces a very messy signal. I'm guessing there's no way around using low value resistors here, as I have to take the 75ohm termination into account, right?

This is much harder than I expected

 

Have you tried using the output of the buffer ic to power a transistor, then have the divider after this?

Maybe buffer doesn't have enough current on the output to do what you want.

 

I haven't tried that. I have packed it away for today, but maybe I'll try tomorrow.

This is the last screenshot I took. I THINK that's the end results I got, but I'm not sure. I remember it being much worse than this :/


Signal > Buffer > voltage divider (682 to RCA socket, and 100ohms to GND) and connected to my AVR so the 75 termination was in place.

Must have calculated something wrong, because the output is only about 350mV. Did work though.

I would like to clean it up. This is the output of a Sony DVD-player's Coaxial output (without 75ohms termination):

Looks much better.

Maybe I could run the result from the voltage divider through the buffer one more time, to straighten it out?

One thing that worries me. Is 1V output (without termination) low enough to not cause any damage if the GND connection on the cable is disconnected during use? The inputs on my AVR are probably expecting 0.5-0.6V at the most?

 

Alright I tried with the transistor like you suggested. I ran the signal through the buffer just to sharpen it up a bit. Got it hooked up like this (needs the 75 ohm resistor on the output, but I didn't get that far):



Using the signal to switch the transistor on and off, and then feeding 5V converted to approx 1V into the transistors collector.

Haven't read too much up on this, but not having a resistor at R1 at the Base results in an output signal that peaks at 4V. I tried various resistor values here, and while that did lower the output voltage, the signal got shifted higher. What I mean is that what should be 0V was instead 600mV. Below is a screenshot where I uses a 4K7 resistor as R1.

The edges and slopes look pretty messy which may be a problem. I would like them as straight as the toslink-signal.

Did not hook this up to my AVR because of the voltage offset.

 

Put R1 in too.