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I am designing a small digital synthesizer. Because of space constrictions, I can only use 3.5mm TRS connectors. RCA is already too big.

If possible, I'd like to include not only an analog output, but also a digital output over S/PDIF (coax).

I know that S/PDIF needs 75 Ohms impedance. Would it be possible to run the signal through a mini TRS jack without problems? The connection on the PCB from the IC would of course be impedance matched, and I would solder the TRS plug directly onto a 75 ohms coaxial cable, with a cinch connector after a few centimeters.

Would a 2.5mm TRS maybe be better? Or does it not work at all?

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This was actually quite commonly done back in the day.

In fact, some soundcards had a little auto-detect feature where if you plugged in a mono 3.5 mm plug, they would output S/PDIF, while if you plugged in a stereo one they output analog. Likely the switching was actually implemented in driver software, but it was fairly transparent to the user - somewhere I still have the little 3.5mm mono to RCA adapter that shipped with these.

As for distortions due to impedance discontinuity this does happen, but while you can just barely see it on a scope, normal equipment is not bothered. But early in may career I spent a while trying to debug a product which seemed hyper sensitive to various distortions - a product manager was amazed to watch as I sat there with a Y adapter at one end of a two meter cable changing out various terminating resistors that I had soldered to RCA plugs and alternately made the product work and not work. Ultimately went so far as to calculate a S/PDIF frame with various distortions and load it into an arbitrary waveform generator to demonstrate some of the specific susceptibilities. Turned out however, that the DSP vendor had given us a bad binary-blob for their proprietary chip which mis-operated the clock recovery PLL. When they finally sent one that used the hardware correctly the system stopped being sensitive to the various minor imperfections we'd been discovering in a long list of computer and DVD player outputs.

So in conclusion: it was done, and with equipment that isn't fundamentally broken it works fine.

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  • \$\begingroup\$ Wow, that sound like some very hard to find bug. Thank you for your answer! I also assumed it would not matter in normal use cases... \$\endgroup\$
    – Spatz
    Feb 26 '19 at 22:24
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The S/PDIF data rate is 3.072 Mbps max (@ 48 kHz sample rate), and with biphase encoding, that means an analog bandwidth that's also around 3 MHz. At these frequencies, and given the maximum cable length of ~10m, the small impedance "bump" created by a TRS connector is really not going to cause any noticeable problem.

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    \$\begingroup\$ It depends of course on the sampling rate - I have to deal with up to 192kHz (12.288 Mbit/s) on various physical interfaces passing S/PDIF formatted data. Like the answer says, it will most likely work just fine over 2.5mm or 3.5mm TRS connector, as RCA connectors are not matched to 75 ohms impedance either. That said, some devices have 3.5mm TRS plugs that also include optical transmitter for having optical output. \$\endgroup\$
    – Justme
    Feb 26 '19 at 19:58
  • \$\begingroup\$ Also, to be honest, aside from select Apple product managers, nobody actually likes conversion dongles that are "one small plug, 5cm of cable, then the standard plug"; I'd recommend really thinking about using the standard connector directly on the board, iff you really want to offer a standard interconnect. \$\endgroup\$ Feb 26 '19 at 20:04
  • \$\begingroup\$ In fact, if your device has an unused "underside" or so: Maybe it'd work for you to design your PCB with a hole so that a panel-mount RCA connector can be directly screwed through the board? \$\endgroup\$ Feb 26 '19 at 20:08
  • \$\begingroup\$ @Justme: Actually, a standard RCA plug is a surprisingly good match for 75-ohm coax. Long ago, ham radio operators used it with success on low-to-medium powered HF rigs. \$\endgroup\$
    – Dave Tweed
    Feb 26 '19 at 21:23
  • \$\begingroup\$ Thank you for your answers. The sampling rate will be 48kHz/24bit, so I hope TRS will not cause any problems. I could not find a source for 3.5mm jacks with optical connections, do you know where to get these? The overall device height is 11mm, so unfortunately RCA will not fit in neither dimension. That's also why a dongle is necessary, but there will be other IO, too, so the dongle would be optional... \$\endgroup\$
    – Spatz
    Feb 26 '19 at 22:23
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You might find a SMB or SMC a better match, since they're available in 75 ohm impedance for ~US$5 each.

SMB Connector (Wikipedia https://en.wikipedia.org/wiki/SMB_connector#/media/File:SMB_Connector.jpg)

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  • \$\begingroup\$ Depending on the details of your implementation, I'd likely second this, not because of the impedance match, but because on an audio device someone is bound to eventually plug your S/PDIF out into an audio in. \$\endgroup\$ Feb 26 '19 at 20:53
  • \$\begingroup\$ These connectors are rather expensive, and I'd rather use standard cables. But the dongle will probably use 2.5mm, to prevent plugging in the wrong IO. \$\endgroup\$
    – Spatz
    Feb 26 '19 at 22:25

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