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tiroth 10th March 2003 01:53 PM

Problem with SPDIF comparator (Elso)
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I've implemented Elso's comparator-input S/PDIF reciever as seen below except with 10nF input cap and 100k resistor to ground at the noninverting input. I am running it single-ended.

The comparator does pass the S/PDIF signal, but not cleanly. There is a lot of hash around the transitions. If I run it through a buffer it comes out clean. I'm using the usual rat's nest wiring in testing and I suspect that at least part of the problem is that the comparator is picking up and triggering on reflections. Still, this is not ideal. A circuit using 74HC04U triggers cleanly using identical wiring.

I tried connecting a pot tied high to the inverting input and biasing it above ground, but this was not fruitful. I continued to see hash up until the comparator turned off completely. Does this circuit need some hysterisis?

Some background on setup: This circuit is contructed using SOIC package comparator, on a ground plane, bypassed by 100nF and 10uF ceramics, and fed by on-board regulators powered by a lab supply. The rails look completely clean. The outputs are kept well away from the inputs.

Any ideas, Elso, others? Is this a problem with the circuit or with my implementation?

Fred Dieckmann 10th March 2003 02:35 PM

Is this a problem with the circuit or with my implementation?


"A circuit using 74HC04U triggers cleanly using identical wiring."

And will sound better in my experience. The SPDIF calls for a 0.5 volt level and you are driving it at logic levels and most likely overdriving the the hell out of the input of the comparator. Comparators are designed to get logic level out puts out of small input signals. Also excesive speed can cause triggering from noise on the interface. A limter amp with a 74HC04U followed by a
74HC86 phase spliter sounds much better in my experience. High speed comparators can cause even very experience designers fits.

For a little more background:

Elso Kwak 10th March 2003 03:22 PM

Comparator Question
Hi Fred and tiroth,
Fred, glad you are back!
I don't know what is wrong with my circuit.
I have received negative and very positive feedbacks on it.
To me it seems some CDP's have a lot of noise on the ground, making the trippoint for the comparator rather vague. In this case I would put a 2k pot at the inverting input. The pot connected through 820 Ohm resistors to both supply rails. Or use a transformer. Don't ask me which one is the best! Or add hysteresys as Fred suggests. Also note the comparator is a very high speed device that needs proper powersupplydecoupling as close as possible to the device. I got very clean SPDIF signals using my interface with Philips CD-650, Sony CDP-M27 and Sony CDP-X33ES.

Fred I am also glad you answered my old question: which circuit is better? I see you prefer the HCU04 and the 74HC86.
The comparator is not overdriven too much as the 5V max. from the output of the CDP is halved by the arrangement of the 75 Ohm resistors. 3V is the max. input on the comparator. Yes I have blown one up with 5V!:rolleyes:

tiroth 10th March 2003 04:47 PM

Hmm...frustrating. I will probably cut the comparator out then and piggyback a small board to replace it. I figured comparator should be better than the 04U but experience says no.

Elso, my first thought was ground or rail bounce, but my bypass caps are right on the pads of the comparator, and the bypassing is well in excess of what should be required. I also tried varying the trip point without much success.

I think the comparator is just too fast and too sensitive. Jocko told me that, I should have listened.

Thanks for your input.

tiroth 10th March 2003 04:50 PM

This is the best circuit I have found to date:

Elso Kwak 10th March 2003 04:53 PM

Comparator Question
Hi tiroth,
I am still puzzled as the DAC side of my circuit also works with the Sony SCD-1, Sony CDP-XA30ES and Genesis Digital Lens SPDIF output, I just remember. (Not my own equipment BTW)
May I ask what CD-player you use?:confused:
And Jocko should have told me! We exchanged emails about the SPDIF interface!!:redhot:

Elso/ not using SPDIF anymore....;)

tiroth 10th March 2003 05:16 PM

I tested with a Philips DVD-825. The output of this player isn't especially clean and is probably a good interface test for that reason. I didn't try any other sources.

tiroth 10th March 2003 05:20 PM

Bypassing SPDIF entirely is definitely a good idea, but I wanted to make an interface that was universally compatible. Plus using SPDIF lets me test one of the major underlying goals of my DAC: jitter rejection.

vuki 10th March 2003 08:08 PM

instead using comparator
wouldn't SN75176 make the most elegant solution for driving a receiver input?

PhilW 10th March 2003 10:05 PM

how then to drive cs8412 differentially?
I bought a comparator with a view to making Elso's circuit to drive my home-made dac. At present I am driving the cs8412 with a home-wound pulse transformer which Elso's circuit was meant to replace.

From the previous posts in this thread it seems that using the comparator is not as straightforward as it sounds. But how do you use the 74HC04U circuit to drive the 8412 differentially??

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