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Old 7th October 2003, 12:46 AM   #1
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Default Digital Audio Input Selector ?????

Hi,

I just finished my DAC project and its working fine. I was testing different equipment on it and came to the conclusion that only one input is not enough.

Does anybody have any ideas on how to make a good digital audio input selector?
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Old 7th October 2003, 12:55 AM   #2
OliverD is offline OliverD  Germany
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Look here to see how I solved the problem using gates:

Digital Input Selection Board

Which solution is best depends on what comes after the input selector. If it's a ASRC or if you use reclocking in your DAC, it doesn't matter that much to pass the signal through another couple of gates. One problem with the Schmitt trigger input of the 74HCU04 is that it reflects crap back on the S/PDIF line. To solve that problem, you can use a fast comparator at the input.

A simple and yet very good solution is to use relais to switch the inputs. Make sure all inputs are terminated properly, even if not selected. To avoid grounding problems, it might be best to use transformers on all inputs (if you don't use optical connectors, that is)
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Old 7th October 2003, 01:08 AM   #3
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I also thought of the idea do use relays at all inputs.
I will be using optical inputs but at the output I want to connect it to the DAC input which has one optical input and one coaxial input.

I will disconnect the optical input and connect internaly the output of the selector to the coax input of hte DAC.
Should this connection be done with Cinch or direct with twisted pair?

What do you think of the relay solution. What would be the best way to do this?
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Old 7th October 2003, 01:13 AM   #4
OliverD is offline OliverD  Germany
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Quote:
Originally posted by promitheus
I will be using optical inputs but at the output I want to connect it to the DAC input which has one optical input and one coaxial input. I will disconnect the optical input and connect internaly the output of the selector to the coax input of hte DAC.
Sorry, I can't make sense of that. Can you clarify your setup? What will you connect to deliver the digital signal?
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Old 7th October 2003, 01:19 AM   #5
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ok sorry about that.

my DAC has one input only.
I can choose either Coaxial or Optical. I have the DAC 2000 from the Elektor magazine.

I have at least 3 devices with optical output. So I need an input selector.

I was thinking of making a 4 optical input selector with relays or digital ICs
The output of the selector I want should be coaxial SPDIF level so I can connect it to my DAC using the coax option. This connection will be made inside the box I will use. Do you think this connection between PCBs should be coaxial or can I use normal twisted pair cable?
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Old 7th October 2003, 01:46 AM   #6
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I am building a stereo system, and asked at the Nuts and Volts forum about how to make a selector switch, and was kindly given this schematic. I haven't tested it yet, but it allows 4 inputs, and an LED to indicate what input is selected. Hope this helps.

I have a higher quality picture, but I couldn't attach it here. If you want it, I can e-mail it to you.

-Mike
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Old 7th October 2003, 01:48 AM   #7
OliverD is offline OliverD  Germany
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If you have optical inputs only, then it would be better to use gates instead of relays.

The 74HC125 will do the job. It's a tri-state buffer. I connected all the outputs together as the "output bus" and used a 74HC42 BCD-to-decimal decoder to select which of the tri-state buffers should drive the bus. The Toslink receivers will deliver TTL level and I suggest driving the input of your receiver through a small capacitor.

You build the Elektor DAC2000? Don't solder in L2, IC2, C9, C8, R2, JP1. You don't need them for your setup. Alternatively, remove JP1.

Now, if the connection between input selector and DAC is longer than, say, 10cm, I'd recommend using a coaxial connection. You need to use 75 Ohm cable and put a 75 Ohm resistor between input selector output and cable (to match impedance).

If the connection is short, use twisted pair, but remove R1.
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Old 7th October 2003, 01:49 AM   #8
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Sorry, i forgot to attach it. here it is.
Attached Images
File Type: gif audio switch.gif (90.1 KB, 3219 views)
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Old 7th October 2003, 01:53 AM   #9
OliverD is offline OliverD  Germany
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Quote:
Originally posted by soundNERD
I haven't tested it yet, but it allows 4 inputs, and an LED to indicate what input is selected. Hope this helps.
Looks quite complicated to me. For four inputs, you need only one 74HC125. The BCD-to-decimal decoder is optional (to save port lines for use with a C), you could also switch the buffers using a rotary switch.
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Old 7th October 2003, 02:18 AM   #10
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Yeah I know what you mean.

Actualy I am using the upsampling converter which has a CS8420 but the input looks exactly like the original DAC does. It has a TORX173 which I wont be needing and a coax input terminated with 75 ohm. and of course the jumper to select input.

Actually the cable can be less then 5 cm if I can make a pcb to be placed next to the upsampling converter.

I will check the ICs you gave and see what I can think of.

Is there something I should be carefull of when making a pcb?
The frequencies are high till 5MHz, right?
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