SPDIF coax digital output topologies

[Vinnie R.]

All,

Lately I've been trying to investigate the different types of
spdif coax output topologies. I want to modify or replace
the one in my DV-45A (used as a transport with a dAck)
to work the best with the CS8414 receiver input that has
a 75-ohm resistor from input to ground (impedance matching),
and 0.01uF cap in front of the RXP and RXN pins. (as recommended in http://www.cirrus.com/en/pubs/proDatasheet/8413-4.pdf
page 35)

I've seen some topologies that use hex inverters to buffer the signal, some that use pulse transformers, inductors in series, different resistor values that 'combine' to make 75-ohm-ish output inpedance, etc.

I recently stumbled across this one:
http://www.benchmarkmedia.com/appnotes-d/cdjeep.asp

Has anyone used this circuit to replace a typical hex-inverter
buffered circuit? I am trying to figure out what the schottky
diodes are used for. I don't see how the output voltage would
be true 0.5Vpp SPDIF. Then again, does it have to be for
the CS8414 to work properly?

My DV-45A uses a transistor in an emitter-follower config for buffering, and then a 68-ohm resistor, followed by a 47uF electrolytic cap, and then to the phono plug with a 100k resistor to gnd. If my dAck already has a cap in front of the RXP and RXN
inputs, is a dc blocking cap necessary in the digital out circuit? Maybe transmitting the signal via coax cable with dc on it is not desireable?

I have the feeling that when using an external dac, the digital out circuit of the transport has a noticeable influence on the sound. I do hear some differences with different transports.

Thanks for all your input,

Vinnie

[Guido Tent]

Originally posted by Vinnie R.

I have the feeling that when using an external dac, the digital out circuit of the transport has a noticeable influence on the sound. I do hear some differences with different transports.

Thanks for all your input,

Vinnie

Vinie

Yes, that quality is of importance, but not only determined by the output circuit

The diodes are for protection

I do not like parallel inverters

A galvanic isolation at the CDP output helps

Maybe you get inspired by

http://www.epanorama.net/links/audiodigital.html#spdif

differntial drive of the 8412/14 is also benneficial

http://members.chello.nl/~m.heijlige...tml/dactop.htm

may give some hints

best regards

[Vinnie R.]

Hi Guido,

Thanks for your reply and links.

For the circuit shown in this link:
http://www.benchmarkmedia.com/appnotes-d/cdjeep.asp

Why would you want to use schottkys for protection? Doesn't the cap offer protection? Also, the output is not going to be 0.5Vpp spdif, right? Is not using a buffer okay if the coax cable is <= 1m ? Is simpler better in this case?
Why the two 150ohm resistors instead of one 75-ohm resistor?

As far as using transformers for isolation, I find that they slow down the rise/fall times of the spdif waveform and worsen the SNR. The nice squareness to the waveform becomes more pulled apart (going through inverters seems to this as well).

I can't seem to find enough threads discussing the details of the spdif out circuit and modding it. If anyone has done work on this, I'd love to hear about it.

Thanks again,

Vinnie

[Lovan]

Using two 150 ohms resistors makes a more exact 75 ohm load. Thus 4x300 ohm would make more exakt and so on.
But beware that noise goes up as the value of the resistors does. ..

I think there has been a lot of discussions about using and why not using transformers. Search the archives. Jocko and Fred seems to have(had) a lot to say about them

[Jocko Homo]

In theory, 2 150R in parallel will give 75R.

In practice: no. The gate driving this has an intrinsic impedance, usually between 20-30R, depending on the series. You need to factor that in.

Slowing down rise/fall times:

Some stuff that Philips makes really does this: to reduce EMI. It makes for a rotten output. A high quality transformer will not.

A lot of handwringing about SNR, CMRR, and the like gives rise to silly notions that the transformer should have loose coupling between the windings. Bad idea. They must be tightly coupled to get the leakage inductance down.

The main trick is to get the reflection coefficient down as low as possible. A "rho" of only 0.1 has an audible effect. And that is fairly low.

So......further discussions???

Jocko

[jwb]

As long as you are modifying the transport ... dump S/PDIF and hack in something else. I think I whistled this tune a few times ....

See High Resolution Multi-Channel Digital Interface if you have no idea what I mean.

[Vinnie R.]

Originally posted by Lovan
Using two 150 ohms resistors makes a more exact 75 ohm load. Thus 4x300 ohm would make more exakt and so on.
But beware that noise goes up as the value of the resistors does. ..

I think there has been a lot of discussions about using and why not using transformers. Search the archives. Jocko and Fred seems to have(had) a lot to say about them

Yes, but one 150 ohm resistor is in series, and one is going to ground. I see a voltage divider when I look at that schematic, where the output is 1/2 the input. Why not use just a 75 ohm resistor in series?

Also, do you see a benefit of using that scheme instead of buffers, transformers, etc. Is simpler better for a short run application?

Also, why use a transformer? Being that the receiver of the dac has a cap before the RXP and RXN pins, why is more isolation needed? I'm not seeing the complete picture.

-Vinnie

[Tieftoener]

[Q]Yes, but one 150 ohm resistor is in series, and one is going to ground. I see a voltage divider when I look at that schematic, where the output is 1/2 the input. Why not use just a 75 ohm resistor in series? [/Q]

You must consider the model for a voltage source in calculating impedances. In AC signals (digital signals), a voltage source (buffer) looks like a short circuit to ground. So, when looking at the output impedance of a circuit with the above description, the two 150 impedances appear to be in parallel, thus giving a 75ohm total impedance.

But, as Jocko metioned, real-life is never as "perfect" as theory and simulation - there is gate resistance in the buffer to consider also. I wasn't aware that it was as high as 20-30 ohms as he mentions, but I sure wouldn't argue with the guy seeing as he has so much experience.

One very well could use just a 75 ohm in series (or 30-40 ohms to get total series impedance from what Jocko says) - but I'm guessing the output of the buffer is 5V... and they probably wanted to knock down the output a bit so as not to overload a reciever. They do this a lot in video buffers to... they typically have a gain of 2 (6dB) (I suppose to maintain signal integrity -less noise - through the amplifiers). The output then has the above 150ohm split voltage dividers. Just something I've noticed and thought I'd mention...

[pieris]

Although I agree that SPDIF is quite flawed, we can't all build our own serial digital physical layer. So, I've looked into the Pioneer circuit and propose this solution. I hope it's not daunting; it should improve the output characteristics to the point that the SPDIF driver is no longer a limiting factor.

Comments are welcome (and if you can maintain the output voltage and slew rate without using the diodes or adding another transistor, I'd like to know).

http://www.pmb.net/projects/cdplayers/dv45a_spdif.html

[Jocko Homo]

Just glanced at that......I'll read it later when I have time.

Don't see anything that makes it great..........probably works......

Jocko