Saturday, February 09, 2013

AES/EBU to ‘SPDIF’  - ghetto style

For interfacing AES/EBU to S/PDIF, I've built a simple transformer out of a small ferrite core, I was planning to convert the 110Ω symmetric AES/EBU signal coming out of a surplus ADC/DAC (I love you, eBay ;-) ...) to the S/PDIF of my computer interfaces (and vice-versa).

There's a nice text about all this over at RANE, but normal people just shell out the 100€ for two of Neutrik's transformers in a box.

The screenshots below show the output of the transformer, terminated by 75 Ohm (or by the audio-interface S/PDIF input, looks identical) when driven by the AES output. I wanted to hide everything in the XLR connector shell, and hence the ferrite beads are pretty small. Therefore they saturate visibly at 32kHz fs, but it works fine for this signal direction. At 96kHz everything looks perfect.

From Chris’ Miscellanea

From Chris’ Miscellanea

From Chris’ Miscellanea

The other direction works not as nicely: All of my audio-gear has a pretty high output impedance on the S/PDIF outputs! With a open output, I get something like 3-5VPP of signal swing, but terminated it goes down to 500mV (just as the spec says). With the saturating ferrite beads, the high-impedance outputs cannot provide the additional current, and the ideally square-shaped output degenerates to tiny spikes of both polarities and is completely unusable. The not-very-pretty-but-working-solution: Just connect the SPDIF to AES 1:1, but keep cable lengths short. (no scope, screenshots, sorry).

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