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Old 21st July 2014, 11:36 AM   #1
Skorpio is offline Skorpio  Denmark
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Default Skipping SPDIF?

I am currently working on a DAC upgrade, and just now is the focus on the SPDIF interface between the source and the DAC.
There are many famous threads about this subject, with qualified information (BNC connectors, cable termination, loading the 75 Ohm line with L and C, signal levels, galvanic isolation, noise, pulse transformers, PCB traces, etc.).
After digesting most of the stuff I began to redraw the complete interface:
1. Digital signal from source (gate chip) TTL level
2. Level matching and impedance matching to SPDIF
3. Trafo and/or capacitor
4. Connector
5. Cable
6. Connector
7. Trafo and/or capacitor
8. Impedance matching
9. Level matching
10. TTL level to receiver chip
A transmission line is surely a good thing for sending information over longer distances, but in my case there is just 10cm (4 inches) between in and out and that got me thinking…

What if I skip all the stages between 1 and 9 and make the signal go directly from 1 to 10?
I could make a small shielded cable and a series cap and that’s it? The only thing missing would be galvanic separation, but this is not included as my setup is yet…..

Any comments or experiences?
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Old 21st July 2014, 11:47 AM   #2
udok is offline udok  Austria
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Should be no problem for short distances. 74HC logic has already an output impedance which is a good match to line impedance. Read the datasheet of your receiver chip for the recommended TTL logic interface.
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Old 21st July 2014, 12:09 PM   #3
marce is offline marce  United Kingdom
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http://www.fairchildsemi.com/an/AN/AN-393.pdf

SPDIF is not an arduous signal to transmit especially over such a short distance.
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Old 22nd July 2014, 06:11 AM   #4
Skorpio is offline Skorpio  Denmark
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Thank you for the answers an the interesting app...

The chip in the receiver end is 74VHC153 input selector/mux just before CS8416. It stated in datasheet:
An input protection circuit insures that 0V to
7V can be applied to the input pins without regard to the
supply voltage. This device can be used to interface 5V
to 3V systems and two supply systems such as battery
backup. This circuit prevents device destruction due to
mismatched supply and input voltages


This looks good....the transmitter end will be 74HCU04 (2 inverters in parallel).

I will take the signal direct from the inverter output pins and then through a 0,15uF stack-foil cap and 82R series resistor and direct to 74VHC153 pin 12.

Will the series resistor do any ham, it might just be my analog thinking....?

Both chips are supplyed from 3.3V....
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Old 22nd July 2014, 06:56 AM   #5
peufeu is offline peufeu  France
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CS84xx is the worst possible receiver, having jitter rejection of zero. Of course with this receiver everything matters like cables etc. Why not use wolfson receiver instead ?
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Old 22nd July 2014, 07:18 AM   #6
Skorpio is offline Skorpio  Denmark
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The CS8416 is already in the circuit, it is a consumer product that I am modifying :-)

One step at a time, right now I would like to skip SPDIF

Would some kind of termination resistor be a benefit at the receiver end? Say 1-10k?
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Old 22nd July 2014, 07:28 AM   #7
udok is offline udok  Austria
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The 74HCU04 gates have an output impedance which is far too high. They are intended for oscillators.
Use the normal 74HC04 (without the U). Omit the 82 Ohm or use a smaller value.
The 74HC04 have typical about 50 Ohm output impedance, which is enough.
Instead of the input multiplexer you can use the recommended input multiplexer of the CS8416 (fig 9).
Why not use the simple TTL interface circuit from datasheet fig 20?
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Old 22nd July 2014, 09:01 AM   #8
Skorpio is offline Skorpio  Denmark
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Quote:
Originally Posted by udok View Post
The 74HCU04 gates have an output impedance which is far too high. They are intended for oscillators.
Use the normal 74HC04 (without the U). Omit the 82 Ohm or use a smaller value.
The 74HC04 have typical about 50 Ohm output impedance, which is enough.
Instead of the input multiplexer you can use the recommended input multiplexer of the CS8416 (fig 9).
Why not use the simple TTL interface circuit from datasheet fig 20?
Yes, that looks interesting....but the I limit the input to only one...

I have 3 different inputs and is modifying one of them to see if this improves quality...
Thanks for the info about the gates....

Regarding the interface to the 74VHC153, do I need some dc-bias path from the pin 12 (input)? Or is that handled internally?
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Old 22nd July 2014, 09:21 AM   #9
udok is offline udok  Austria
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The capacitor between the 74HC04 output and the 74HC153 input (the 74HC is the better choice if you don't need the 74VHC speed) should be omitted. Don't forget a pulldown if you plan to disconnect the source.

The CS8416 has an 8 input multiplexer. Use the datasheet fig. 20 for each input.
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Old 22nd July 2014, 09:34 AM   #10
Skorpio is offline Skorpio  Denmark
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Hi Udok,

Thanks for the fast help :-)

I will do the following:
1. Remove 82R and serial cap from source (go directly from gate output)
2. Make pull down on 74HC153 input (10k to GND?)
3. If test is ok, try buffered 74HC04
4. If ok, look into direct connection to CS8416...(with caps)

I am a little worried by connecting two different products directly (DC), but I see no reason for it not to work :-)
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