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!
Thanks, I know that but I am asking why not just change the 110ohm resistor to a 75ohm and leave the 1:1 transformer???
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
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
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...
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.
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.
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
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
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
Must go figure out the wiring.
Fran
Hi Brian. I would think that the extra .105v isn't going to do squat as it doesn't even approach AES voltage spec? Since it works then the receiver must be able to take low level signals. Indeed from what I can make out the datasheet says -.3v to .8v for low in (I'm a newbie though and maybe not even reading the right thing, also if this is the case I don't even understand how it could be a negative v!) So at the end of the day there seems to be no point in the xfrmr?
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