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-   -   reducing spdif signal level (http://www.diyaudio.com/forums/digital-source/58092-reducing-spdif-signal-level.html)

Sjef 28th May 2005 01:58 PM

reducing spdif signal level
 
Is there anyone who knows a proper method to reduce the signal strength of a sp/dif output.

I currently have the following problem.

I'm using a Monarchy Audio 48/96 DIP/upsampler between my cd player and DAC and that works just fine. Now I also want to use the DIP between my PC and the DAC. I have a USB/spdif convertor wich works pretty good. The problem is that the USB/spdif convertor has an output voltage of 5 volt. My DAC has no problem with that and according to the manual the dip/upsampler should also be able to handle this input level, but in practice everythings start to destort like crazy, so I guess the input is overdriven.

Could this signal be reduced by simply putting a voltage divider made from two 75 resistors at the input or would it be a better method by using a pulse transformer ? Does anybody know of a good sp/dif transformer to make a galvanic seperation between the PC and the DIP to prevent earthloop problems ?

Jocko Homo 28th May 2005 03:36 PM

HUH???????

I see no reason why increasing the SPDIF input level would cause distortion. No, it is not being overdriven.........something else must be wrong.........like DC offset, and the RX can not handle it.

As for your other questions.............search for posts by me. There are lots of them.

Jocko

Sjef 28th May 2005 05:18 PM

I have just tried to block any DC with two 10nF coupling caps. The DIP locks in but still a lot of distorsion. After that I have tried to put the computer behind a 230V isolation transformer, didn't help either. Measured if there was any potential difference between the PC and the DIP, but there was none (at least not with the isolation transformer in place) So the only difference between the USB/SPDIF convertor and the CD player I can come up with is the difference in signal level. The CD player puts out a 0,5Vpp while the USB/SPDIF puts out 5Vpp. Both are 16bits/44.1kHz fixed point. The DIP/Upsampler has no problem with either my CD player, my DVD player or my MiniDisc player. only with the computer. My DAC doesn't have any problem with the computer output at all.

Any other suggestion what this problem might be ?

AndrewT 28th May 2005 06:03 PM

Hi,
I attach table showing levels, as you already know the maximum level into s/pdif is usually 600mV.

I have added comma separators to allow you to regenerate the columns.

Just pad down the PC output by about 18db = /8.

, AES3, AES3id, S/PDIF,
Interface, Bal, Unbal, Unbalanced ,
Connector, XLR-3, BNC, RCA ,
Impedance, 110 ohms, 75 ohms, 75 ohms,
Output Level, 2-7 Vp-p, 1.0 Vp-p, 0.5 Vp-p ,
Max Output, 7 Vp-p, 1.2 Vp-p, 0.6 Vp-p ,
Max Current, 64 mA , 1.6 mA , 8 mA ,
Min Input, 200mV, 320mV, 200mV ,
Cable, STP, Coax, Coax,
Max Distance, 100 m , 1000 m , 10 m ,


AES3 = aes/ebu = 958-4
AES-3id = newer standard for long line
transmission
S/PDIF = spdif = sony phillips = 958-3

AndrewT 28th May 2005 06:08 PM

Hi,
after I posted it dawned that the PC may be putting out an AES3 signal since the voltage matches.

You can adjust the level of AES3 to match s/pdif but the bits in the preamble do not match and this may be confusing the DAC.
have a read of the rane site for more info. http://www.rane.com/note149.html

Sjef 28th May 2005 06:09 PM

Padding down in the digital domain doesn't change anything to the voltage level of the digital output, that will always be 5Vpp.

A lot of spdif receiver nowadays are capable of receiving 5Vpp signal level. The Monarchy Audio DIP 48/96 even boosts the output to 5Vpp.

AndrewT 28th May 2005 06:30 PM

Hi,
padding down is to reduce the signal level.
I can not suggest any method to reduce levels in the digital domain. Sounds like some complex computer task to achieve that.

carlosfm 28th May 2005 11:10 PM

Quote:

Originally posted by Sjef
I have just tried to block any DC with two 10nF coupling caps. The DIP locks in but still a lot of distorsion. After that I have tried to put the computer behind a 230V isolation transformer, didn't help either. Measured if there was any potential difference between the PC and the DIP, but there was none (at least not with the isolation transformer in place) So the only difference between the USB/SPDIF convertor and the CD player I can come up with is the difference in signal level.
I've seen this happen with a very cheap DVD player connected to a (good) dac.
Your USB/SPDIF converter must be really plagged by jitter, or it's just so bad because it uses the (very noisy) power from the PC, through the USB port.
The "lock" light on the dac lights up, but it is not really locked, the chip it trying to lock the signal but it can't achieve that, it's intermitent.

Sjef 29th May 2005 12:48 PM

still strange why my DAC (it's not realy a DAC it's a DEQX processor) perfectly locks in on the USB/SPDIF convertor. The signal from the convertor has even a lot less noise than the digital out off the cd player. And yes the convertor is powered by the usb bus powerline. I have tried to power it with it's own battery powered supply but surprisingly enough this didn't help and it sounded worse than powered via usb, so I left in on the usb power.

Jocko Homo 29th May 2005 04:18 PM

Maybe you are looking in the wrong place........
 
If I understand the problem, your DAC works with this USB thingie ok, but distorts when you insert the "upsampler" thingie in the line. Correct?

In that case...............the problem lies within. I would not put any of those gimmicks in my system to begin with. Lots of reasons. The main one being they usually make things worse.

Jocko


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