Hi,
How simple (or not) would it be to get a coaxial digital output from a CD player that only has an optical output? I know there are boxes that can convert, but Id rather take the signal directly from the player if possible.
I read a thread on here about a guy wanting to do a similar thing, but to avoid ground loops with his AV kit he converted the optical output signal to an electrical one. My DAC (with CS8420 input receiver) has transformer isolation anyway, so I'd rather take the electrical signal direct without ever going optical.
Is it simply a matter of tapping off the electrical signal before the optical socket? Or what, any tricks needed?
Thanks!
Edit: Having looked through the data sheet for the input receiver in my DAC I see that the chip also has digital output pins. Does this mean that the CD player is likely to have an 'input receiver' chip such as the CS8420 driving the digital output? I that is the case I could take the feed directly from its output pins and wire it up with a 75R resistor and pulse transformer. But maybe CD players don't work like this at all?!
How simple (or not) would it be to get a coaxial digital output from a CD player that only has an optical output? I know there are boxes that can convert, but Id rather take the signal directly from the player if possible.
I read a thread on here about a guy wanting to do a similar thing, but to avoid ground loops with his AV kit he converted the optical output signal to an electrical one. My DAC (with CS8420 input receiver) has transformer isolation anyway, so I'd rather take the electrical signal direct without ever going optical.
Is it simply a matter of tapping off the electrical signal before the optical socket? Or what, any tricks needed?
Thanks!
Edit: Having looked through the data sheet for the input receiver in my DAC I see that the chip also has digital output pins. Does this mean that the CD player is likely to have an 'input receiver' chip such as the CS8420 driving the digital output? I that is the case I could take the feed directly from its output pins and wire it up with a 75R resistor and pulse transformer. But maybe CD players don't work like this at all?!
Well if you look at the optical connector, they only have three pins generally. One for supply, one for ground and the other is your SPDIF signal. The SPDIF TTL signal will be either 3.3V or 5V and you should really reduce it to 1V for coaxial. I answered a thread on how to do this here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=116951
Hope this helps.
Hope this helps.
Hi Guys,
Right, well I have got the player now, so I have gone inside and taken some pictures for you. Its a Cyrus DAD3 with Q DAC upgrade board.
Now what I want to do is add a coaxial digital output, and low and behold the PCB already has the spaces for parts to do that! Super dooper! So I take it all apart and probe around to find were things connect. Here is a picture with red line showing where things go.
And here is a circuit diagram of the same.
Now my question, is what values do I want to use for 'filling in the blanks' as it were, on the coax out? I have some pulse transformers already so thats no problem.
First there is clearly a low pass filter, I guess to reduce noise form other bits in the player. Then there is a voltage divider, so I guess the output directly from the SAA7345GP is too high for a coax out. Though in the datasheet for this chip, it says that the DOBM digital output is in compliance with IEC958, but I guess that doesn't spec voltage? The chip datasheet can be found here - http://matthieu.benoit.free.fr/cross/data_sheets/SAA7345.pdf
After that we have the transformer and then after that for some reason we have another voltage divider. Why do we need two? I can see we want a 75R resistor between the two TX outputs to set the impedance but whats the series one for?
Help on what values to use where and if any of it should just be bypassed is much appreciated!!
Also I have to wonder why they didn't implement it in the first place. Is it likely that the SAA7345GP doesn't have enough current drive to cope with all the voltage dividers? Seems unlikely they would make it not able to provide a coax out.
Right, well I have got the player now, so I have gone inside and taken some pictures for you. Its a Cyrus DAD3 with Q DAC upgrade board.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Now what I want to do is add a coaxial digital output, and low and behold the PCB already has the spaces for parts to do that! Super dooper! So I take it all apart and probe around to find were things connect. Here is a picture with red line showing where things go.
An externally hosted image should be here but it was not working when we last tested it.
And here is a circuit diagram of the same.
An externally hosted image should be here but it was not working when we last tested it.
Now my question, is what values do I want to use for 'filling in the blanks' as it were, on the coax out? I have some pulse transformers already so thats no problem.
First there is clearly a low pass filter, I guess to reduce noise form other bits in the player. Then there is a voltage divider, so I guess the output directly from the SAA7345GP is too high for a coax out. Though in the datasheet for this chip, it says that the DOBM digital output is in compliance with IEC958, but I guess that doesn't spec voltage? The chip datasheet can be found here - http://matthieu.benoit.free.fr/cross/data_sheets/SAA7345.pdf
After that we have the transformer and then after that for some reason we have another voltage divider. Why do we need two? I can see we want a 75R resistor between the two TX outputs to set the impedance but whats the series one for?
Help on what values to use where and if any of it should just be bypassed is much appreciated!!
Also I have to wonder why they didn't implement it in the first place. Is it likely that the SAA7345GP doesn't have enough current drive to cope with all the voltage dividers? Seems unlikely they would make it not able to provide a coax out.
I'm guessing that you really don't need R1 and C1. They may be there just to pass an EMI emissions regulation.
You probably don't need R4 and R5 either.
Try the values for R2 and R3 as shown in the above link provided by rfbrw.
100 nF coupling caps in the positions provided for R1 and R4 would probably work OK. Leave C1 and R5 position open. Use a pulse transformer and your SPDIF should sing.
You probably don't need R4 and R5 either.
Try the values for R2 and R3 as shown in the above link provided by rfbrw.
100 nF coupling caps in the positions provided for R1 and R4 would probably work OK. Leave C1 and R5 position open. Use a pulse transformer and your SPDIF should sing.
I can easily share the same R1 and C1 that the optical conenctor uses. they go to the same source pin on the chip anyway.
Surely the datasheet must state the output voltage of DOBM pin? Am I just not seeing it? Its a bit tiral and error to assume its output is 3V. I want to get the voltage divider right!
Also is there any point having a coupling cap after the TX, if no DC goins in, none is going to come out, is it? I have 0.22uF caps.
Surely the datasheet must state the output voltage of DOBM pin? Am I just not seeing it? Its a bit tiral and error to assume its output is 3V. I want to get the voltage divider right!
Also is there any point having a coupling cap after the TX, if no DC goins in, none is going to come out, is it? I have 0.22uF caps.
Hi,
Thanks for your replies.
You are of course right that the DAC can handle qite a wide range, but if the output was only 1V to start with I don't want to put it through a voltage divider.
However, I looked at the datasheet for the Toshiba TX176 optical transmitter and it has a lower limit of 2v for its input, so the signal must be 5v or 3v. Adding the voltage divider posted above should bring that in to a safe level for the majority of DACs.
Thanks for your replies.
You are of course right that the DAC can handle qite a wide range, but if the output was only 1V to start with I don't want to put it through a voltage divider.
However, I looked at the datasheet for the Toshiba TX176 optical transmitter and it has a lower limit of 2v for its input, so the signal must be 5v or 3v. Adding the voltage divider posted above should bring that in to a safe level for the majority of DACs.
Thanks guys, all sorted
I wrote up what I did so the other Cyrus owners can do the same if they want.
http://www.rdhworld.co.uk/punbb/viewtopic.php?id=9948
I wrote up what I did so the other Cyrus owners can do the same if they want.
http://www.rdhworld.co.uk/punbb/viewtopic.php?id=9948
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