Working on a DAC and would like to provide both BNC and RCA connector options for the single S/PDIF input. Can they be in simple parallel, sharing a common 75 ohm termination resistor, with one connector always left open? Or is there some reason this is not advised? TIA
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Ideally you shouldn’t do that. Each connector has an impedance characteristic which you will mess up by paralleling them.
To be “correct”, you should use a 75Ω BNC connector rather than a 50Ω one (the latter is more common for RF and instrumentation). I don’t think you’ll find a 75Ω RCA jack easily so we just have to shrug your shoulders. This is why the better DACs use a BNC jack rather than RCA. And then you would use a proper 75Ω cable to make the connection.
If you must have BNC and RCA, a better solution is to use a pulse transformer on each input, then feed the two into separate inputs of a S/PDIF receiver chip.
To be “correct”, you should use a 75Ω BNC connector rather than a 50Ω one (the latter is more common for RF and instrumentation). I don’t think you’ll find a 75Ω RCA jack easily so we just have to shrug your shoulders. This is why the better DACs use a BNC jack rather than RCA. And then you would use a proper 75Ω cable to make the connection.
If you must have BNC and RCA, a better solution is to use a pulse transformer on each input, then feed the two into separate inputs of a S/PDIF receiver chip.
It would depend how close together those two inputs were - a single termination means there's going to be a stub at the unconnected input. If the stub's short enough then no matter.
Thanks.
I'm struggling with the responses so far. If I build carefully, I can get the shared termination resistor about 1 cm away from either plug. If I look at a typical assembled commercial DAC PCB, the resistor is probably 1 cm from the plug. Is this really significant in a short wire implementation?
I'm struggling with the responses so far. If I build carefully, I can get the shared termination resistor about 1 cm away from either plug. If I look at a typical assembled commercial DAC PCB, the resistor is probably 1 cm from the plug. Is this really significant in a short wire implementation?
I wouldn't worry about it.
A 1 cm distance corresponds to a delay of about 33.356 ps in vacuum, 72 ps for an inner PCB trace, so 144 ps for a round trip on a PCB inner trace. As long as the rise and fall times of the S/PDIF signal are much greater than that, the stub will not create any problems.
The rise and fall times of S/PDIF signals are normally at least 5 ns. In fact, for the professional variant AES3, 5 ns is the minimum to be spec compliant.
A 1 cm distance corresponds to a delay of about 33.356 ps in vacuum, 72 ps for an inner PCB trace, so 144 ps for a round trip on a PCB inner trace. As long as the rise and fall times of the S/PDIF signal are much greater than that, the stub will not create any problems.
The rise and fall times of S/PDIF signals are normally at least 5 ns. In fact, for the professional variant AES3, 5 ns is the minimum to be spec compliant.
No, according to the ancient version of IEC60958-4 that I have a pdf file of, edition 2.1 from 2008, section 8.2.2.4, the rise and fall times have to be between 5 ns and 30 ns.
The problem with a BNC and RCA tied to the same terminator and SPDIF input is that someone might leave cables connected to both inputs (neglecting the stub issue already discussed). It's another reason to use an adapter, although an adapter may just be something to get lost.
Another thing to consider is that some dac chips and some SPDIF receivers can support more than one input. In that case the connector/terminator problem might be solved more properly.
Another thing to consider is that some dac chips and some SPDIF receivers can support more than one input. In that case the connector/terminator problem might be solved more properly.
Yes, that is correct information. When both cables are attached it will be messed up, no sound thinking technician would do such a thing.
The adapter or the separate input are both standard solutions that work correctly. Or keep it even simpler: replace all SPDIF RCA connectors of all the devices to 75 Ohm BNC.
The adapter or the separate input are both standard solutions that work correctly. Or keep it even simpler: replace all SPDIF RCA connectors of all the devices to 75 Ohm BNC.
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