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.
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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.
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.
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.
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Thanks Andrew,so then a standard 1in -3out CATV splitter may suffice?
Yes all the DAC's have a 75ohm res at their input.
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
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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.
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If it does you're still going to reduce the signal level which raises the jitter in the detected output.
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This is certainly a concern and the reason for my initial hesitance for using one.Might I be better off constructing a purpose built switch box instead.Ie.using 4 x 75 ohm female BNC connectors,toggle switch in a small metal box?
A good thought but you might find it useful to source all at once. If you're building a switch box it's not a big leap to skip the switch and do the DA in the same box. Then again the switch box may be all you need and it requires no power - unlike the DA.
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Certainly having all outputs active would be preferable to having to operate a switch.If I could find a suitable design for a DA I would give it strong consideration.
Well I went hunting and came up with this
Jan98 Project
The general idea is right but to fix it:
1:replace C1 with a wire
2:delete R3 and connect opamp pin 3 directly to R2
3:change R2 value to 100 ohms
4:change R4 and R5 to 1K
5:change R8 and R9 to 10 ohms
Items 1-3 remove the variable gain and AC coupling
Item 4 sets the gain to 2
Item 5 changes the insane resistor values. Even slammed to the positive rail, how do you get 2 volts into a 50 ohm load with 2700 ohms in series? Answer: You don't. Either it's a typo or the author never tried this.
Q1 and Q2 need not be those devices if you can't find them. 2N3904/2N3906 2N4401/2N4403 would do fine. The CA3100 can be subbed as well. The 3100 is a 38 MHz amp which is about as slow as you want to go. You want 40-100 Mhz and 24 Volt minimum.
Ignore the audio section
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Thanks stratus for taking the time to find that circuit.I also came across the data sheet for the Analog Devices AD811 video line driver opamp.The circuit shown on page 14 looks elegantly simple. http://www.analog.com/static/imported-files/data_sheets/AD811.pdf
Truly fine amplifiers. I hesitated mentioning them because the Digikey price is $7 which puts some people off. ADI recommends driving up to 2 back-terminated lines with an 811 which means you should use 2 but in fact you'd probably be OK with one. The 40dB isolation between outputs is only for 2 outputs and will be somewhat lower when running 3. This is a bit of a problem with analog signals but less so with digital like SPDIF.
I did the analog section of this machine which is brimming with AD811s - about 85 of them. FYI there is a 3 output DA in this machine where I used a separate 811 for each of the 3 outputs. I wanted as little interaction as possible.
Film Systems - Nova - Real-time multi format film scanner
Be aware that the 811 is a Current Feedback amplifier and the feedback resistor determines the bandwidth. You then tweak the gain by setting the input resistor in inverting mode or the resistor to ground in non-inverting mode. None of this just raise the feedback resistor for more gain. ADI shows recommended resistors for a given power supply voltage. I found their recommendations to be spot on. Even if you run it as a unity gain buffer you MUST use a feedback resistor - no direct connecting of output to '-' input that we do with voltage feedback amps. Once you get your head around the restrictions of CF amps, they are very powerful - particularly the 811.
Read the data sheet carefully as they will answer just about any question you might have. I found their data sheets to be about the best I have encountered and their chips ALWAYS do what they say they will. I've never been let down with ADI chips and they are always my first choice.
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How do you "split" the output from the 811?
You have not addressed the main query.
Do the three 75 ohm resistors in the link terminate each coax correctly? What if the output impedance of the 811 is not zero?
What about your concern that joining three resistors together would not be an effective termination for a HF coax?
All you have done is amplified the signal. the splitting remains the issue.
You have not addressed the main query.
Do the three 75 ohm resistors in the link terminate each coax correctly? What if the output impedance of the 811 is not zero?
What about your concern that joining three resistors together would not be an effective termination for a HF coax?
All you have done is amplified the signal. the splitting remains the issue.
$7 for a one IC solution,that works for me.I remember building a Jung designed line stage 15+yrs ago that utilized the 811 as an output buffer.I'ts a great audio device but I recalled Jung mentioning it was purpose designed for video use.I dug up the data sheet and bingo there on pg14 was exactly what I needed.I think I will scale it back to two outputs instead of three and will configure it for unity gain.
I see you do this as profession,who is your clinentel if I may ask?
Thanks for your input.
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