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Old 27th November 2012, 12:28 PM   #4651
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Default CD PRO question

Hi guys,
while I'm studying TDA1541 and SAA7324 timing I have a question.
I own 1 CD PRO2M module, 1 TDA1541 S1 and 1 8.4762MHz ultra low phase noise OCXO. I play 44.1/16 bit material only, so I would like to put all in a box to build a simple integrated CD player. I would like to use the same OCXO either for the transport than for the DAC chip. I'll take I2S signal directly from CD PRO to feed TDA1541.
The CD PRO uses SAA7324 as servo/decoder to output I2S; there is a poor 8.4762MHz crystal on the board I want replace with OCXO clock. it seems SAA7324 is configured to provide 2 words of 24 bit at 4fs, so BCK is exactly 8.4672MHz (24*2*4*44.1), so I think I can remove the crystal and inject the clock coming from OCXO.
About TDA1541, I would like to implement it in NOS mode, so my question:
is it right to divide OCXO clock by 4 (2.1168MHz) for DAC chip BCK?

Andrea
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Old 27th November 2012, 02:02 PM   #4652
JOSI1 is offline JOSI1  Germany
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Hello Andrea,

it is right to divide OCXO clock by 4 (2.1168MHz) for DAC chip BCK.
If your OCXO has 2 clock outputs use one for the divider by 4 and the other via an inverting/non-inverting buffer for the SAA7324 clock input (do not exceed 3.3V level).
Take care that the SAA7324 doesn't output the I2S-signals with a fixed phase relation
to the 8.4762MHz input clock (there are 3 or 4 posible phases after power up).
So it can happen that you have re-power the CD-Pro until you get a correct audio signal.
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Old 27th November 2012, 02:49 PM   #4653
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Josi1,
thanks for reply.
Quote:
Originally Posted by JOSI1 View Post
Hello Andrea,
it is right to divide OCXO clock by 4 (2.1168MHz) for DAC chip BCK.
If your OCXO has 2 clock outputs use one for the divider by 4 and the other via an inverting/non-inverting buffer for the SAA7324 clock input (do not exceed 3.3V level).
Unfortunately my OCXO has only one output, so I have to use it for both SAA7324 and TDA1541.

Quote:
Take care that the SAA7324 doesn't output the I2S-signals with a fixed phase relation
to the 8.4762MHz input clock (there are 3 or 4 posible phases after power up).
So it can happen that you have re-power the CD-Pro until you get a correct audio signal.
That could be a little problem.. I see 7324 has a power-on reset input (pin 38), so I should reset it until I detect a correct audio signal at the output. Any idea?

Andrea
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Old 28th November 2012, 06:09 AM   #4654
JOSI1 is offline JOSI1  Germany
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Hello Andrea,

I didn't follow this problem since it occurs very rarely in my system.
Furthermore I keep my system powered most of the time.
Please let me know if you have any findings concerning the reset pin.
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Old 28th November 2012, 01:06 PM   #4655
SSerg is offline SSerg  Russian Federation
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Hi to all,

Here are some explanations towards my table and table itself translated in English.
The measurements are executed for two samples of the microcircuits (the conditional number 1 and 2). The frequency f – a frequency of the switching DEM-cell. For built-in multivibrator and capacity of the external capacitor 470 pF she forms 200 - 250 kHz.

Sensibly expect that for pins 8, 9 (23, 22) filtering resistances are equal, in the same way either as for pins 10, 11 (20, 21). The difference of the voltages at pins 8, 9 (23, 22) and 10, 11 (20, 21) allows to estimate the resistance of filtering resistors. This difference for greater precision is measured at the other limit of the measurements. The result was provided in table.
After uncomplicated calculations we get that for pins 8, 9, 22, 23 filtering resistances are 1,0 kOm, but for pins 10, 11, 20, 21 – 1,6 kOm.
Attached Files
File Type: pdf table.pdf (40.7 KB, 148 views)
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Old 28th November 2012, 10:39 PM   #4656
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Quite insightful Sserg. What are you conclusions on implications?

-to achieve same cutoff of the ripple attenuator the external filter caps caps should be 2 different sets (one for the 1k resistors, one for the 1.6k)?

- what else?
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Old 29th November 2012, 07:46 AM   #4657
SSerg is offline SSerg  Russian Federation
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Hi, studiostevus,
Quote:
Originally Posted by studiostevus View Post
- what else?
The required capacity depends from resistance of the resistor not only, but as well as from ripple frequency and values of the bit currents.
Coming out of these values, possible estimate the caps capacity correlation required for alike filtering actions for different bits.
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Old 29th November 2012, 09:24 AM   #4658
oshifis is offline oshifis  Hungary
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Note that a current generator is different from a resistor in that the voltage drop is not proportional to the resistance... So concluding to the internal resistance from the measured voltage drop can be misleading. I recommend a dynamic measurement method instead, e.g. by measuring the RC time constant. It can also be measured by applying an external adjustable current source and measure the variation of voltage caused by a small current change.
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Old 29th November 2012, 10:18 AM   #4659
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Quote:
Originally Posted by SSerg View Post
Hi, studiostevus,


The required capacity depends from resistance of the resistor not only, but as well as from ripple frequency and values of the bit currents.
Coming out of these values, possible estimate the caps capacity correlation required for alike filtering actions for different bits.
That's a possibility indeed, but forgive my ignorance... What would be the point, as long as the filtering is sufficient to filter out the switching ripple, no?

The only further benefit i can think of is aligning the rc filter's phase shifts between the different bits (1-16)... Not sure what the benefit of that would be though (can't hurt for sure)
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Old 29th November 2012, 01:26 PM   #4660
SSerg is offline SSerg  Russian Federation
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Hi, oshifis
Quote:
Originally Posted by oshifis View Post
Note that a current generator is different from a resistor in that the voltage drop is not proportional to the resistance...
Call attention that I compare the voltages for resistors within one and same DEM cells; they get the current from one and same current generator, so I factor out current generator.
My estimation of the resistance is not perfect, but it is enough motivated.
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