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Old 18th November 2011, 12:07 AM   #1
sfasf is offline sfasf  United Kingdom
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Default USB -> I2S circuit questions

I'm looking to get an I2S output from usb using the PCM2706/7. Not using the internal DAC. I have attached a schematic from the TI datasheet (which is set up using the internal DAC and HID functions, not I2S.. but is bus powered) and one from this thread on head-fi (i marked the relevant areas that correspond to the questions). There are a few noob questions and a few areas i'd like some clarfication--

1. My understanding is that since this is all digital stage using the USB bus power shouldn't make any difference to the ultimate sound quality, nor should the quality of the resistors and capacitors here?

2. The head-fi schematic uses chokes in the gnd and vbus section-- am i right in thinking this isn't necessary when using the chip for I2S output only / will have no effect on output?

3. Capacitors 1 and 2 are listed in TI datasheet as 10-33pF depending on the load capacitance of the crystal resonator, without going into further detail. How should i work out what values are needed? (My c.r. has load capacitance of 30pF)

4. Can AGNDR, VccR, VccL, AGNDL and HID0, HID1, HID2 all be left floating since i'm not using the DAC section of the chip?, or do they all have to be connected to ground (with capacitors in the case of VccR/VccL). Similarly can the suspend flag (SSPND) be left floating if i don't need an LED attached?

5. Am i right in thinking the FUNC3 ('I2S data input') does not need to be connected to DOUT if only using the USB->I2S features of the chip ??

6. Should VDD and DGND be connected by a capacitor (as on the TI schematic) or not (as the one at head-fi)?

Thanks for any help.
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Old 19th November 2011, 04:11 PM   #2
sfasf is offline sfasf  United Kingdom
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My thoughts are;

1. no difference
2. not necessary
3. not sure.. but i think it doesn't matter too much if somewhere in that range??
4. float them all
5. float
6. yes, connect with capacitor.

I would love to hear from someone else on this.

Another thing i saw on one schematic was 22ohm resistors connected to each line of the I2S signal before the connector. What would be the purpose of this? (and is it necessary?)

Last edited by sfasf; 19th November 2011 at 04:15 PM.
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Old 20th November 2011, 12:12 AM   #3
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Hi sfasf, the two 22Rs are for impedance matching the balanced line.
The nominal impedance is 90R +/- 15% so depending on the device connected, a pair of Rs is usually required to achieve this.
Also, note that the 1k5 pullup R is on opposing D lines in the above schematics.
D+ indicates 'full speed' and D- indicates 'low speed'. This pullup should connect to +3V3 so you might as well use the bus powered TI schematic as the Vdd pin is an internally regulated 3V3 output.

check out this usb blurb .. USB in a NutShell - Chapter 1 - Introduction

regards Rob
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Old 21st November 2011, 02:48 PM   #4
sfasf is offline sfasf  United Kingdom
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Thanks Rob.

The 22R question was regarding the I2S line, not usb. Useful info.

Good spot on the pullup R.. I guess (assuming it wasn't an error) it was made low-speed as it makes the line more tolerant?? ..but i will need full speed to support stereo 48KHz 24bit (2.25Mbit /s PCM), which the chip supports AFAICT. IE i want to use the TI way here, right??
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Old 21st November 2011, 03:06 PM   #5
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Ha, I saw I2S but 'presumed' USB
Anyway, here's a link to another thread dealing with this.
One thing I would add is place the Rs close to the I2S chip.

Rob
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Old 21st November 2011, 03:22 PM   #6
sfasf is offline sfasf  United Kingdom
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You forgot to include the link!

'close to the I2S chip'.. the DAC or the USB-I2S chip (PCM2707)??

And are they necessary?? This USB->i2S->DAC based off this chip doesn't have any (nor the datasheet).. which makes me think not?
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Old 22nd November 2011, 12:38 AM   #7
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doh!
resistors in I2S lines

They may not be necessary, but is dependent on the cabling mainly.
If you do fit them, fit them close to the pcm2707 pins 19, 5 and 17 BCK, LRCK and DOUT respectively (assuming you're doing the standard 3 wire interface).
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Old 30th November 2011, 06:50 PM   #8
sfasf is offline sfasf  United Kingdom
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I didn't realise the MCLK was optional??
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