ADC - word clock vs MCLK

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fripholm

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I'm currently trying to wrap my head around the datasheets for the PCM4222 and its evaluation board PCM4222EVM.

On this board, MCLK is derived directly (through a buffer) from the external word clock input when the clock source is set to EXT.  When set to INT, two different VCOs at ~22 and ~24MHz are used which is much closer to what I would expect, frequency-wise.

When I scope the WC output of my audio interface, it's more or less a square wave with a frequency that matches the current sample rate, e.g. 44.1kHz. But according to the datasheet of the PCM4222, the MCLK is usually 128 or 256 times the sample rate which should be much higher in the MHz range (~5.6 or 11.2MHz in this case). LRCLK on the other hand matches the sample rate and as far as I can see there is a flip-flop (U21) that halves this to be used as SCKI.

So, how is it that this evaluation board works with an MCLK frequency that is much lower than needed? What am I missing?
 

abbey road d enfer

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fripholm said:
I'm currently trying to wrap my head around the datasheets for the PCM4222 and its evaluation board PCM4222EVM.

On this board, MCLK is derived directly (through a buffer) from the external word clock input when the clock source is set to EXT.  When set to INT, two different VCOs at ~22 and ~24MHz are used which is much closer to what I would expect, frequency-wise.

When I scope the WC output of my audio interface, it's more or less a square wave with a frequency that matches the current sample rate, e.g. 44.1kHz. But according to the datasheet of the PCM4222, the MCLK is usually 128 or 256 times the sample rate which should be much higher in the MHz range (~5.6 or 11.2MHz in this case). LRCLK on the other hand matches the sample rate and as far as I can see there is a flip-flop (U21) that halves this to be used as SCKI.

So, how is it that this evaluation board works with an MCLK frequency that is much lower than needed? What am I missing?
The master Clock input is not designed to receive WC; it needs a 4 to 24MHz clock, as explained in table 4 of teh data sheet.
 

abbey road d enfer

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fripholm said:
Aahh, I see  :-[

I guess, I saw 'BNC connector' mentioned and thought, this has to be for word clock - as this connector type is (also) used by WC...

If I wanted to use WC as sync master, this needs to be multiplied somehow, right? PLL?
that's usually how it's done.
 

fripholm

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Aahh, I see  :-[

I guess, I saw 'BNC connector' mentioned and thought, this has to be for word clock - as this connector type is (also) used by WC...

If I wanted to use WC as sync master, this needs to be multiplied somehow, right? PLL? Any other ways?
 

mhelin

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I need to put a pair of wordclock outputs on my ADAT ADC board (just a small adaptor PCB to supply I2S clocks and power to recycled E-Mu 1820M ADC board) and I've bee wondering why use the optical ADAT with the coaxial WC as those mostly seem to be unbalanced and relying to clean ground connections so you will loose all of the benefits of the optical interface. As the Rosendahl nanoclock manual says it "Similarly to analogue audio signals producing "hum" affected by different ground potentials, digital
transmission lines can be worsened/ modulated by ground loops. Good studio grounding is essential, not only for word clock signals. To isolate ground potentials galvanically and for interfacing coaxial to balanced AES11 lines (XLR 3-pin) we recommend Neutrik Digital Audio Impedance Transformers NADITBNC-F / NADITBNC-M."

No I'm wondering if I should really use a pair of transformers in the WC output circuit. Usually there seems to be none, for an example the Symetrix GENx6 doesn't have any in the input or output:
http://www.symetrix.co/kb/GENx6_sch.pdf

There are some clock distribution amplifiers which have transformer isolated WC inputs, though. Anyway I'm considering using Murata DA102C transformers (http://www.farnell.com/datasheets/2259213.pdf). They are also MUCH cheaper than the Neutrik problem solvers.
 

abbey road d enfer

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mhelin said:
No I'm wondering if I should really use a pair of transformers in the WC output circuit.
Actually you need only one for galvanic isolation.

There are some clock distribution amplifiers which have transformer isolated WC inputs, though.
The designers of these products consider they are not concerned with the consequences on audio at the system level.

Anyway I'm considering using Murata DA102C transformers (http://www.farnell.com/datasheets/2259213.pdf). They are also MUCH cheaper than the Neutrik problem solvers.
The Neutrik gizmos are expensive because they are enclosed in the adapter case. You don't need that.
Go for the Murata.
 

mhelin

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abbey road d enfer said:
Actually you need only one for galvanic isolation.
The designers of these products consider they are not concerned with the consequences on audio at the system level.
The Neutrik gizmos are expensive because they are enclosed in the adapter case. You don't need that.
Go for the Murata.
Thanks, I will do so.
 

Rochey

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The master Clock input is not designed to receive WC; it needs a 4 to 24MHz clock, as explained in table 4 of teh data sheet.
I complained like HELL when this was designed like this. I was met with a message of "A pll would lower the performance - this is an eval kit, not a system"

Seemed fair at the time. Technically still does. People constantly try and put the eval kit in a 19" rack case without understanding the system.
 

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