cs8416 SPDIF receiver - best input config ??

[soundcheck]

Hi folks.

SPDIF. One more time. Please be patient with me! I'm more a software guy.
(And I'm not gonna sell what might comes out of this here. )

I'd like to revise my SPDIF receiver. It's a CS8416 configured as proposed
in the datasheet, with 0,1uf in series and 75R in parallel connected via RCA.

The target is to get the characteristic impedance of input & output matched as best as possible to 75R.

I've done some SPDIF reading lately. Believe me right now I'm more
confused then before.

I found that mod was looking like a feasible solution.

From what I read a pulsetransformer (Newava S22160) should be a good choice if connected to a noisy source such as a PC. In my case it is the Squeezebox Touch. I need to have a look at that output too. At least the RCA needs to get removed.

I'll swap all connectors with BNC. Direct soldering on one side of the link is another option that appeals to me.

I have a couple of general questions:


1. Why do I find sometimes just a 75R in parallel and on other occasions
a voltage devider.
2. Just 75R seems to me as if the impedance of that 8416 chip would be
neglectable!?!? In the link above it's been said that the AK4584 shall be added to the equation with 30R. Any ideas???

3. How do I find out the output impedance of that CS8416 chip. Couldn't find it in the datasheet.

4. Does anybody has the formula to calculate the devider and the resulting overall impedance.

5. I see in that above linked example a lower resistor value on the primary of the transformer and the higher value in series on the secondary. Why is that?


I think that'll be it for the time being. I'd really appreciate your advise.


Please let me also know, if I'm completely off-track or if something is missing.

Cheers

[singa]

Hi,
If you would download Cs8416 all your questions will be answered as
it shows the few ways to interface with 8416.Simply put, 8416 is recieving
spdif signal from eg. cd player where industry standard for the line impedance
is 75 ohm for consumer standard equipment so you see 75 ohm across the
secondary (single ended,one end to ground) of pulse tranny.Now if it were the professional interface it would be 110/115 ohm or there abouts.There is no need to fiqure out 8416 input impedance as the design engineers have done it for you.

A 75 ohm resistor is the cheapest and easiest way followed by transformer
coupling for pro standard because in all likelyhood they are running balanced mode for signal line like long cables in a recording studio.

If you see a voltage divider it's because they want to reduce the signal
amplitude and if you see the voltage divider the signal is taken from the lower
resistor at the top end and the other to ground.So the resistance seen by
8416 input is roughly the resistance of the lower resistor.You can search
online for a voltage divider calculator or caluculate manually ,it's not hard.

You should not have any problems it is straight forward. Singa.

[soundcheck]

Originally Posted by singa

Hi,
If you would download Cs8416 all your questions will be answered

Are you sure? I doubt it. I had and have that piece of paper in front of me.

Originally Posted by singa

as the design engineers have done it for you.

Reading all what's been written over here and elsewhere, I doubt that.

Originally Posted by singa

If you see a voltage divider it's because they want to reduce the signal
amplitude and if you see the voltage divider the signal is taken from the
lower

It's not that easy I guess to calculate the impedance. Beside that divider, there's the sink and then there are capacities and new inductivities.
all together - incl. parasitics, there'll be a nice mix of everything.

Not to forget that also the frequency plays a big role here.

[stormsonic]

only partially off-track. There are two different things:
- TX side (transmit-out)
- RX side (receive)

TX side in your case is Squeezebox Touch.
CS8416 is receiver chip, it belongs on RX side.
S/PDIF is communication protocol and physicial interface between TX and RX side.

[Bill Fuss]

Originally Posted by soundcheck

Hi folks.

SPDIF. One more time. Please be patient with me! I'm more a software guy.
(And I'm not gonna sell what might comes out of this here. )

I'd like to revise my SPDIF receiver. It's a CS8416 configured as proposed
in the datasheet, with 0,1uf in series and 75R in parallel connected via RCA.

The target is to get the characteristic impedance of input & output matched as best as possible to 75R.

I've done some SPDIF reading lately. Believe me right now I'm more
confused then before.

I found that mod was looking like a feasible solution.

From what I read a pulsetransformer (Newava S22160) should be a good choice if connected to a noisy source such as a PC. In my case it is the Squeezebox Touch. I need to have a look at that output too. At least the RCA needs to get removed.

I'll swap all connectors with BNC. Direct soldering on one side of the link is another option that appeals to me.

I have a couple of general questions:


1. Why do I find sometimes just a 75R in parallel and on other occasions
a voltage devider.
2. Just 75R seems to me as if the impedance of that 8416 chip would be
neglectable!?!? In the link above it's been said that the AK4584 shall be added to the equation with 30R. Any ideas???

3. How do I find out the output impedance of that CS8416 chip. Couldn't find it in the datasheet.

4. Does anybody has the formula to calculate the devider and the resulting overall impedance.

5. I see in that above linked example a lower resistor value on the primary of the transformer and the higher value in series on the secondary. Why is that?


I think that'll be it for the time being. I'd really appreciate your advise.


Please let me also know, if I'm completely off-track or if something is missing.

Cheers

I believe you have confused yourself. The CS8416 does not need any particular termination, but the cable does.

In your mod link it shows the voltage divider resistor used with the AK chip to lower the TTL level to normal SPDIF level. It works in conjunction with the 75ohm resistor on the trafo secondary, which merely reflects the 75 ohms to the primary side, creating a voltage divider.

A CS8416 can handle a 5 volt signal so adhering to the normal SPDIF level is not strictly necessary. In the past I have reclocked the outgoing SPDIF signal and buffered it up to 5 volts with great results with a D type flip flop chip.

Why do you need the output impedance of the 8416? What are you feeding it to?

[Bill Fuss]

If you really want an improvement, eliminate the SPDIF conversions and go to the native I2S, then you can pitch the receiver chip altogether.

[soundcheck]

Originally Posted by Bill Fuss

I believe you have confused yourself. The CS8416 does not need any particular termination

Did I say that?
I said the impedance of that chip might play a role in calculating the overall impedance of that receiving end of the link.

Originally Posted by Bill Fuss

In your mod link it shows the voltage divider resistor used with the AK chip to lower the TTL level to normal SPDIF level. It works in conjunction with the 75ohm resistor on the trafo secondary, which merely reflects the 75 ohms to the primary side, creating a voltage divider.

That might be correct if you consider the impedance of the 8416 neglectable.
Otherwise you need to do the math.

Originally Posted by Bill Fuss

A CS8416 can handle a 5 volt signal so adhering to the normal SPDIF level is not strictly necessary.

That's pretty vage.


Regarding I2S: Perhaps at a later stage.
Currently I'm making quite some progress on that SPDIF interface.

[Bill Fuss]

The entire SPDIF format is pretty vague. Even a 7V p-p AES/EBU signal is only 3.5V when it gets to the input pin. The chip is not part of the transmission line, it doesn't care how the signal got to it. The 75 ohm resistor is the termination point, not the chip.

Find some reference material on impedance matching for a better understanding. The chip's input impedance is high enough that it can be disregarded, there is no magic involved.

Try the Rane Corporation website and do some reading on interfacing, pretty good stuff.

Best, Bill

[soundcheck]

Originally Posted by Bill Fuss

The entire SPDIF format is pretty vague. Even a 7V p-p AES/EBU signal is only 3.5V when it gets to the input pin. The chip is not part of the transmission line, it doesn't care how the signal got to it. The 75 ohm resistor is the termination point, not the chip.

Find some reference material on impedance matching for a better understanding. The chip's input impedance is high enough that it can be disregarded, there is no magic involved.

Try the Rane Corporation website and do some reading on interfacing, pretty good stuff.

Best, Bill

Hi Bill.

The incoming signal sees "everything" as a complex termination not just the last resistor. The question in fact is how the "rest" beside that 75R impacts that impedance.
You say (is that an opinion btw?) it would be neglectable in my case.
Lets leave it for now. (If the data sheet says so it's OK I guess.)

Fact is that e.g. the AK4584 from above example does have an impedance of approx. 20R which would have an impact.

To figure it out I probably won't get around measuring what's happening.
Which is kind of tricky if you don't have that equipment at hand.

Cheers

[Bill Fuss]

No, actually it doesn't.

You might review the AK4584 datasheet for a little more insight into the resistive divider being used in the mod, and its purpose. It has nothing to do with the transmission line operation itself.