SPDIF splitter

[F1 FAN]

I'm looking for input in regards to expending the single SPDIF coaxial output of my PC to three separate outputs that will feed three different DAC's.I suspect a standard 75ohm CATV splitter would work but I would be concerned about the signal attenuation with that method.Is there an active approach that could be easily assembled,without introducing any attenuation or degredation of the data?

[stratus46]

Originally Posted by F1 FAN

I'm looking for input in regards to expending the single SPDIF coaxial output of my PC to three separate outputs that will feed three different DAC's.I suspect a standard 75ohm CATV splitter would work but I would be concerned about the signal attenuation with that method.Is there an active approach that could be easily assembled,without introducing any attenuation or degredation of the data?

Not sure about SPDIF but its close cousin AES works just fine with a standard def video DA (Distribution Amplifier) which is a 5MHz gain of 2 amplifier with a 75 ohm input termination and a 75 ohm series build out resistor for each output. National, TI, Analog Devices, Maxim and others make opamps that do just fine for this. The hardest part is the case / power supply.

G²

[F1 FAN]

Interesting,so then a standard CATV type distribution amplifier may be work?

[daniloreyes]

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[marce]

Just using a y splitter will add a serious impedance mismatch to the interface, not a good idea. if both ends are terminated at 75ohms, the combined Z will be 32.5 ohms, could add alsorts of mess to the signal.

[AndrewT]

All a Y splitter that is correctly terminated for 75ohm coax, that is also correctly terminated at their remote ends, needs is simply 3off 25r resistors.
Attach a 25r resistor to the end of the core of each of the 3 coax.
Connect the other ends of all three resistors together.
Connect all the coax shields together.
The input coax sees the first25r and then it sees a pair of exits in parallel. each exit is 25r in series with the 75ohm coax. each appears as if it were 100ohms. A parallel pair appears as 50ohms.
The feed coax sees 25+[100//100] = 75ohms. It is properly terminated.

Similarly all the receive ends are terminated with 75ohms.

For a 4way ( 1 coax feeding 3outputs) the resistor values are {3*[75-25]}/4 =37r5
For a 5way ( 1 coax feeding 4outputs) the resistor values are {4*[75-18.75]}/5 = 45r

But remember to terminate the remote ends with the correct 75r load.

You can work out the attenuation at the receive end if you ground the terminating resistors and feed in a fixed signal level.

[F1 FAN]

Originally Posted by AndrewT

All a Y splitter that is correctly terminated for 75ohm coax, that is also correctly terminated at their remote ends, needs is simply 3off 25r resistors.
Attach a 25r resistor to the end of the core of each of the 3 coax.
Connect the other ends of all three resistors together.
Connect all the coax shields together.
The input coax sees the first25r and then it sees a pair of exits in parallel. each exit is 25r in series with the 75ohm coax. each appears as if it were 100ohms. A parallel pair appears as 50ohms.
The feed coax sees 25+[100//100] = 75ohms. It is properly terminated.

Thanks Andrew,so then a standard 1in -3out CATV splitter may suffice?

Similarly all the receive ends are terminated with 75ohms.

Yes all the DAC's have a 75ohm res at their input.

[stratus46]

Originally Posted by F1 FAN

Interesting,so then a standard CATV type distribution amplifier may be work?

No. A CATV amplifier is an RF device working into the hundreds of MHz with signal levels up to a few millivolts amplifying RF modulated video signals ready for a TV receiver.

A video distribution amplifier is more like an audio DA on steroids. Baseband video bandwidth is 60Hz (DC) to 5MHz operating at 1Vp-p in, 2Vp-p out for amplifying video signals before modulating onto the RF carrier. These signals are the same as the video outputs from a VCR or DVD, not the Ch3 or 4 RF outputs.

To build one is simply an opamp capable of 5MHz delivering 2Vp-p into a 50 ohm load. You said 3 outputs earlier so the load is 3 75 ohm build outs to the 3 video outputs then into the destination loading 75 ohms to ground. The amplifier sees 150 ohms (75+75) for each destination, 3 in parallel computes to 50 ohms. Since that is actually a pretty tough life for an opamp it would be just fine to run an individual opamp for each output. These will not be your TL071s

G²

[stratus46]

Originally Posted by AndrewT

All a Y splitter that is correctly terminated for 75ohm coax, that is also correctly terminated at their remote ends, needs is simply 3off 25r resistors.
Attach a 25r resistor to the end of the core of each of the 3 coax.
Connect the other ends of all three resistors together.
Connect all the coax shields together.
The input coax sees the first25r and then it sees a pair of exits in parallel. each exit is 25r in series with the 75ohm coax. each appears as if it were 100ohms. A parallel pair appears as 50ohms.
The feed coax sees 25+[100//100] = 75ohms. It is properly terminated.

Similarly all the receive ends are terminated with 75ohms.

For a 4way ( 1 coax feeding 3outputs) the resistor values are {3*[75-25]}/4 =37r5
For a 5way ( 1 coax feeding 4outputs) the resistor values are {4*[75-18.75]}/5 = 45r

But remember to terminate the remote ends with the correct 75r load.

You can work out the attenuation at the receive end if you ground the terminating resistors and feed in a fixed signal level.

For people who worry about pico second jitter issues, this is likely to introduce so much crap into the signal it will be amazing if it works at all. Mismatched transmission lines and reduced levels will rely on the signal extraction capabilities of the receive device. Certainly not the path to good performance.

G²

[F1 FAN]

Originally Posted by stratus46

For people who worry about pico second jitter issues, this is likely to introduce so much crap into the signal it will be amazing if it works at all. Mismatched transmission lines and reduced levels will rely on the signal extraction capabilities of the receive device. Certainly not the path to good performance.

G²

His math seems to indicate proper termination impedance?