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Old 13th June 2010, 07:19 PM   #1
jkeny is offline jkeny  Ireland
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Default SPDIF balanced output

I want to investigate the possibility of using a differential SPDIF line driver on the output of a device to:
- eliminate the need for a SPDIF output transformer as these require all sorts of contortion to get them right & I don't have the equipment or time or knowledge to do so.
- provide a purely resistive 75ohm load to the DIT for better impedance matching rather than the inductive/reactive load that a SPDIF transformer presents.

I'm using a DIT4196 Digital transmitter which has differential output line drivers. I'm not talking about AES, 110ohm balanced, I still want to use existing SPDIF 75 ohm cabling & the DAC will have standard SPDIF input circuitry - transformer receiver, etc. Would I lose out on the common mode noise rejection if not having twisted pair? How much of a drawback would this be?

Firstly, why isn't this done more often? Am I missing something? I know probably grounding issues would be a problem if not using a balanced driver but in this case are there other gotchas?

I know there are T-pad calculators for working out the resistive network needed to apply some attenuation & keep it 75ohm but they apply to single ended output - how do I adjust for balanced output?

Would a resistive divider be too much load for the DIT4196 line drivers? The datasheet specifies their characteristics as being able to output from min 2.6V to max 3.3V at 30mA when powered from 3.3V

Last edited by jkeny; 13th June 2010 at 07:24 PM.
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Old 13th June 2010, 07:34 PM   #2
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Reasons you don't see it more often:

- active solution is at least as complex as passive; if you don't have equipment/experience to check a passive solution, you probably are in the same situation with an active one;

- a xformer with a resistive termination on the secondary looks like a resistor to the DIT, difficult to get it simpler and more reliable...

jd
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Old 13th June 2010, 08:03 PM   #3
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Quote:
Originally Posted by jkeny View Post
I know there are T-pad calculators for working out the resistive network needed to apply some attenuation & keep it 75ohm but they apply to single ended output - how do I adjust for balanced output?

Would a resistive divider be too much load for the DIT4196 line drivers? The datasheet specifies their characteristics as being able to output from min 2.6V to max 3.3V at 30mA when powered from 3.3V
Take a piece of paper, write down letter T. Then write down another letter T over previous, but turned upside down. After that rotate this piece of paper for 90 degrees, until you see letter H. This is what you want

LINK

Google for H pad attenuator or use this link H pad calculator.

As Jocko pointed out several times, you will have to look at datasheet for your device to find out output Z & use it when calculating resistor values.

For output loading just use Ohm law. If your device output 3.3V, with 100R resistor to ground there will be 33mA current through this resistor. If max. output is 30mA, you are overloading it and need to increase resistor to decrease current.

Last edited by stormsonic; 13th June 2010 at 08:08 PM.
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Old 13th June 2010, 08:04 PM   #4
jkeny is offline jkeny  Ireland
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Quote:
Originally Posted by janneman View Post
Reasons you don't see it more often:

- active solution is at least as complex as passive; if you don't have equipment/experience to check a passive solution, you probably are in the same situation with an active one;
I'm not talking about an active stage - just a resistive divider Pi-pad (or H-pad) on the differential output line drivers of the DIT - I don't see why I need equipment for this, I've already calculated the output impedance of the DIT so this will be taken into account (more than most SPDIF transformer output stages do!)

Quote:
- a xformer with a resistive termination on the secondary looks like a resistor to the DIT, difficult to get it simpler and more reliable...

jd
AFAIK, I think this is way too simplistic a statement :
- firstly you are assuming that the transformer can handle the frequencies involved here in a completely transparent manner & pass the signal unadulterated?
- secondly transformers have an inductive kick in them - this has to be catered for, I believe
- there are probably other issues with them
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Old 13th June 2010, 08:17 PM   #5
jkeny is offline jkeny  Ireland
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Quote:
Originally Posted by stormsonic View Post
Take a piece of paper, write down letter T. Then write down another letter T over previous, but turned upside down. After that rotate this piece of paper for 90 degrees, until you see letter H. This is what you want

Google for H pad attenuator or use this link H pad calculator.
Yes, stormsonic, I was posting ast the same time as you & figured that it must be a H-pad.

Quote:
As Jocko pointed out several times, you will have to look at datasheet for your device to find out output Z & use it when calculating resistor values.

For output loading just use Ohm law. If your device output 3.3V, with 100R resistor to ground there will be 33mA current through this resistor. If max. output is 30mA, you are overloading it and need to increase resistor to decrease current.
Great, need to do some experiments so!

Are there any issues I'm overlooking? Like the DAC end of the SPDIF cable will be treated as normal (i.e not balanced) - is the coaxial outer conductor normally connected to ground on SPDIF input? = ground issue?

Why isn't this done more often?

Last edited by jkeny; 13th June 2010 at 08:23 PM.
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Old 13th June 2010, 08:22 PM   #6
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Quote:
Originally Posted by jkeny View Post
I'm not talking about an active stage - just a resistive divider Pi-pad (or H-pad) on the differential output line drivers of the DIT - [snip]
OK, I probably misread you there. I thought you wanted an active solution.

Quote:
Originally Posted by jkeny View Post
[snip]- firstly you are assuming that the transformer can handle the frequencies involved here in a completely transparent manner & pass the signal unadulterated?
- secondly transformers have an inductive kick in them - this has to be catered for, I believe
- there are probably other issues with them
Well, the xformer would be designed for that freq range, wouldn't it?
'inductive kick', 'other issues' - possibly.
One extremely important advantage of the xformer is the galvanic isolation - no problems with ground currents and such that can lead to jitter.

jd
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Old 13th June 2010, 08:24 PM   #7
jkeny is offline jkeny  Ireland
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Differential = no ground
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Old 13th June 2010, 09:51 PM   #8
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Quote:
Originally Posted by jkeny
I'm using a DIT4196 Digital transmitter which has differential output line drivers. I'm not talking about AES, 110ohm balanced, I still want to use existing SPDIF 75 ohm cabling & the DAC will have standard SPDIF input circuitry - transformer receiver, etc.
Depends on your circuit. CS841x receiver chip will accept differential, DIR9001 only SE. Look at datasheet for your chip.
But you wrote your DAC will have transformer coupled input, why worry about ground?

Quote:
Originally Posted by jkeny
Why isn't this done more often?
What should be done more often?

Quote:
Originally Posted by jkeny
Are there any issues I'm overlooking?
Yes. Put active circuit infront your receiver chip. Jocko posted several schematics here and at Diyhifi.
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Old 13th June 2010, 10:07 PM   #9
jkeny is offline jkeny  Ireland
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Originally Posted by stormsonic View Post
Depends on your circuit. CS841x receiver chip will accept differential, DIR9001 only SE. Look at datasheet for your chip.
But you wrote your DAC will have transformer coupled input, why worry about ground?
Yep, you're right



Quote:
What should be done more often?
Dispensing with SPDIF transformer (at least one of them: transmitting or receiving?). It would seem to be so much easier (maybe I'll find out it isn't)? Allows the transmitter to see a resistive load, easier to keep impedance at 75ohm & no need for taming non-ideal transformers.



Quote:
Yes. Put active circuit infront your receiver chip. Jocko posted several schematics here and at Diyhifi.
Yes, I remember this mentioned before - a nice linear front-end - I'll search!

Last edited by jkeny; 13th June 2010 at 10:19 PM.
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Old 14th June 2010, 10:08 AM   #10
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Have a look at AES3id or its successor AES3-4-2009, the addendum for the broadcast industry where coax is king and distances are a lot longer. No reason you couldn't use a standard SDI tx/rx pair. No transformers in serial digital video.
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