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Old 3rd February 2010, 03:55 AM   #1
nonsub is offline nonsub  Canada
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Default Spdif > aes

Can anyone kindly confirm this. Assuming the DAR (in this case CS8416) accepts either protocol, can one go from SPDIF RCA 75ohm cable > 1:1 transformer > 75ohm termination > DAR and be considered "proper?" I am seeing other solutions leaving the the termination at 75ohm but changing to a 1:1.21 xformer. Doesn't make sense to me the layman? Thanks!
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Old 3rd February 2010, 11:29 AM   #2
AndrewT is offline AndrewT  Scotland
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Originally Posted by nonsub View Post
Can anyone kindly confirm this. Assuming the DAR (in this case CS8416) accepts either protocol, can one go from SPDIF RCA 75ohm cable > 1:1 transformer > 75ohm termination > DAR and be considered "proper?" I am seeing other solutions leaving the the termination at 75ohm but changing to a 1:1.21 xformer. Doesn't make sense to me the layman? Thanks!
that 1:1.21 transformer converts the impedance from 75ohm to 110ohm.
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Old 3rd February 2010, 03:06 PM   #3
nonsub is offline nonsub  Canada
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that 1:1.21 transformer converts the impedance from 75ohm to 110ohm.
Thanks, I know that but I am asking why not just change the 110ohm resistor to a 75ohm and leave the 1:1 transformer???
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Old 3rd February 2010, 03:36 PM   #4
Bfpca is offline Bfpca  Canada
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Hi


If you terminate the 75ohm coax with 110 ohms it will not be matched in impedance. The transformer with a 1.21 ratio multiplies the impedance at the input by the square of the ratio so 75ohm x 1.21 squared - 1.4641 X 75 = 109.8 ohms. So the source sees a properly terminated 75 ohms even though the transformer secondary is terminated with 110 ohms.

If you use a 1:1 transformer than the proper termination is 75 ohms for spdif.

Brian
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Old 3rd February 2010, 04:00 PM   #5
nonsub is offline nonsub  Canada
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....If you use a 1:1 transformer than the proper termination is 75 ohms for spdif.
Thanks Brian, fellow cdn! OK so I am CORRECT then... 75ohm coax > 1:1 > 75ohm resistor is FINE right? So I wander why ppl are bothering changing to 1.21 and leaving the 110ohm this is an expensive route...
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Old 3rd February 2010, 04:03 PM   #6
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For the short cable lengths that most of us use the impedance mis-match between 75 & 110 Ohms won't make any difference. Once the cables are 30 or 40 feet long or more then impedance matching becomes important.
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Old 3rd February 2010, 04:14 PM   #7
AndrewT is offline AndrewT  Scotland
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So I wander why ppl are bothering changing to 1.21 and leaving the 110ohm ...
the PCB might be laid out to have a characteristic 110ohms impedance.
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Old 3rd February 2010, 04:43 PM   #8
nonsub is offline nonsub  Canada
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the PCB might be laid out to have a characteristic 110ohms impedance.
Yeah (in this case we are talking about Behringer DCX2496) it has a 110ohm resistor. Surely just replacing the resistor with a 75 ohm is a lot easier and WAY less expensive than leaving it and replacing the 1:1 xfrmr with a 1:1.2 one, that's my point/question.
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Old 4th February 2010, 10:28 AM   #9
Bfpca is offline Bfpca  Canada
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Hi Folks

The only reason I can think of is that the voltage input will be higher using the 1:1.21 transformer. With .5v in you would have .605v out of the transformer. This may become an issue with sources that have low levels, I'm not sure. The input Z is set by the resistor and I believe the recievers are usually differential so I am not sure what the intent is. I notice that it seems to be common to use the 1.21 transformer so it may be more ideal with the input stage of the reciever , not sure. I am using spdif coax in through a DEQ 2496 and no problems. One other thing is that the DCX / DEQ have AES inputs and may be optimised to work with AES signal levels/ impedances. It would be interesting to look at the waveforms going into the receiver to see what it looks like.

Cheers
Brian
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Old 4th February 2010, 10:35 AM   #10
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Most interesting. I have a DAC with both SPDIF and AES inputs, and it would be very handy to use the AES as a second SPDIF input.

Must go figure out the wiring.

Fran
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