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Old 12th August 2019, 12:26 PM   #1
tcpip is offline tcpip  India
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Default Input selector for SPDIF inputs

I don't know whether this has been asked before, but it'll be strange if it hasn't.

I am thinking of designing (or even buying off the shelf) a simple 3-way or 4-way relay based input selector module and using it for selecting from 3 or 4 digital SPDIF inputs, and then feed the selected output to a DAC.

I intend to mount the input selector board really close to the rear panel, and then the DAC within a few inches of the input selector board's output. The RCA sockets will be panel mount types, and I'll have a few inches of cabling from those sockets to the inputs of the relay board.

The relay board will not have any active or passive component in the signal path, other than the relays.

I intend to use a DPST or DPDT relay for each SPDIF input, so that I can switch both the ground line and the signal line.
  • Will this work? Do you see any problems?
  • If I use the Toslink modules from Twisted Pear, can I wire one Toslink module's output signals in parallel with the wires from an RCA socket, so that a user can choose whether he wants optical or coax digital for each input? No switches to select between Toslink and coax -- just hard-wired in parallel. Will this work?

Last edited by tcpip; 12th August 2019 at 12:32 PM.
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Old 13th August 2019, 02:46 PM   #2
tcpip is offline tcpip  India
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Anybody?
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Old 14th August 2019, 06:33 AM   #3
Oystein is offline Oystein  Netherlands
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I think mechanical relays are not the most suitable for high speed digital signals.

For your input circuit build something like this:
(please note that the shown MAX3280 should be better replaced by a 26LV32)
Click the image to open in full size.

After the individual inputs use a 74xxx series digital ports to enable / disable each input.
An "AND" port would be an easy implementation but please realize all signals here are 3,3V (not 5V as common TTL is)
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Old 14th August 2019, 06:44 AM   #4
tcpip is offline tcpip  India
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Ok. Understood, at least partly. Thanks.

A question about the Toslink input. What signal, if any, does it output when there is no optical cable connected to its input? Is it feasible to keep it continuously connected to an SPDIF receiver circuit block, while I receive an input via the coax cable?
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Old 14th August 2019, 07:39 AM   #5
Oystein is offline Oystein  Netherlands
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The Toslink receiver (like Toshiba TORX 147) keeps output low when no signal.

I am not sure if it pulls it low active (and therefore puls SPDIF down too) so still better use a digital switch to avoid interaction between the inputs.
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Old 14th August 2019, 10:47 PM   #6
BrianDonegan is offline BrianDonegan  United States
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Input selector for SPDIF inputs
This is what our 4:1 Mux/Demux module does. No relays.

4:1 Mux/Demux Switching Module
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Old 15th August 2019, 01:51 AM   #7
tcpip is offline tcpip  India
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Quote:
Originally Posted by Oystein View Post
The Toslink receiver (like Toshiba TORX 147) keeps output low when no signal.

I am not sure if it pulls it low active (and therefore puls SPDIF down too) so still better use a digital switch to avoid interaction between the inputs.
I was suspecting as much. Can't just wire them up in parallel. This is not the analog world.
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Old 15th August 2019, 01:56 AM   #8
tcpip is offline tcpip  India
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Quote:
Originally Posted by BrianDonegan View Post
This is what our 4:1 Mux/Demux module does. No relays.



4:1 Mux/Demux Switching Module
Yes, I'd asked you about these a few years ago and you had explained. I was wondering whether I can build a relay based switching board. I was also thinking of more than 4 digital inputs. 4 digital inputs seems like it's more than enough for any normal household, but the split between optical and coax seems to cause problems for many of my friends. They always complain that they either have too few optical or too few coax inputs in their DAC or amp.

That's why my exploration started with the thought: can I have effectively just 4 inputs but support both coax and optical for each input, so that some people may use most of their optical inputs and others may use most of their coax?

Can I use your 4-way board and still somehow connect both optical and coax to each of the ways? Can I use a simple DPST switch at each input on the rear panel, for instance, and "up" will mean coax and "down" will mean optical?
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Old 15th August 2019, 03:54 AM   #9
tcpip is offline tcpip  India
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On second thought, DPST does not seem like a good idea -- will DPDT work?

Due to some strange coincidence, when I looked for DPDT toggle switches with
  • panel mount
  • gold plated contacts
  • flatted lever
  • ON-ON actuation
I get only one choice from both Mouser and Digikey: this NKK switch

I didn't realise this was such a unique combo of features. Expensive.

What are B1 to B4 on the silkscreen image of the 4:1 switching module?

Last edited by tcpip; 15th August 2019 at 04:12 AM.
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Old 15th August 2019, 04:24 AM   #10
BrianDonegan is offline BrianDonegan  United States
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Input selector for SPDIF inputs
Optional ufl connectors
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