Hello everybody,

I am new to DIY audio (but I have an embedded systems background). I have some general questions about putting
Twisted Pear components together. I already took a look this PDF with some basics that was posted by one of the forum's users but it didn't help me much as it is focussed only on BuffaloII DAC based setups. I would appreciate if someone could give me a
starting point for my DAC/preamp setup which should fulfill following requirements:

Must:
- At least one optical and coxial input to chose from
- Possibility to insert an MCU board (BeagleBoard) into the signal chain for audio processing via I2S intefaces
- Not too much space requirement (my case has only 8x8" space available). Are there any heat/noise issues with stacking Twisted Pear boards by a distance of ~1-2"
- Unbalanced RCA output (since my current equipment is not balanced)
- A good value/cost/space ratio (e.g., Opus vs. BuffaloII?, output stages vs. no output stages?, dual mono vs. stereo DAC?)

Nice to have:
- 2 optical and 2 coxial inputs to chose from
- SPDIF output of the processed I2S data

Controlling the components will be done by the BeagleBoard and another ATMega MCU. So there is no need for other control components.
I am pretty unsure about the power supplies and also the output stages. How many transformers/power supplies do I need? Would it be possible to drive the BeagleBoard with a shared power supply together with the other digial audio boards (it requires 5V at a max. current of 1.6A) Do I need an output stage before/after the Ballsie Lite module?

I read somewhere that feeding the DACs via BeagleBoard I2S interface is possible and it has been done by someone (Russ White?) but it is seems to be somehow tricky. Unfortunately, I could not find out what the tricky part actually is. Is there a problem with different voltage levels? Maybe someone can provide some more details on that issue?

I know I have many questions but it would be really kind if someone could answer at least some of them.
Thanks in advance!

Best regards,
Markus

I have not even ordered the parts at Twisted Pear Audio yet since I am still in the planning phase.
In order to be sure that everything will work together I need a starting point for a configuration that fits my needs.

Concerning the BeagleBoard I2S interfaces: I took at look at the manual to see which pins are available
at the BB's expansion header. The TI OMAP MCU provides three McBSP controllers for I2S interfacing.
One is hard wired to the audio codec on the BB and thus cannot be used. Regarding the other two McBSPs
it is not possible to mux all pins of both controllers to the expansion header at the same time.
But as far as I know you only need three pins for a stereo I2S transmission: CLOCK, DATA and DATA_SELECT.
Those can be muxed to the expansion header without conflicts. Hence, I think it should be possible
to have one input and one output I2S port available for the audio processing.

But I have some doubts about this because I heard it should be tricky to connect the DAC. But I don't know why exactly.
How far did you get?

Thanks LeonvB for your reply. You're right, my questions were partially answered by the PDF guide.

Concerning the BeagleBoard xM: I already own a Rev C4 board and want to use it for that. So unfortunately, BeagleBoard xM is not an option for me. But thank your for that hint!
But if 96/24 via McBSP1/3 is possible, I'm happy. If the tricky part is just related to the software then I'm sure I will find a solution.

In order to make my questions concerning the other parts more concrete I linked a figure at the appendix depicting my currently planned layout of the usable inner area of my enclosure. It's an aluminium profile enclosure with 210*230*80mm usable space (I know it's a little bit small but hopefully it will do for me).

Some questions:
-Does anyone see a general problem with combination of these components this way (e.g. stacking of the IVY3+Buffalo+4:1MUX-RECEIVER)?
-I saw some discussions concerning issues with stacking transformers but without a convincing statement saying it is or is not a problem. So once again, is it a problem to put the small transformer on top of the bigger one? Do I have to put something inbetween to make magnetic interaction of both as small as possible?
-The user interface and control of components will be done by the BeagleBoard and another custom designed ATmega board mounted to the front panel. Currently, there is no power supply included in this layout for both. Either I will find some place on the front panel for that or it will be external (not so nice).

Thanks for your answers!

I think your plan is sound. I have used the beagleboard to do I2S output with no issues. But that was long ago, and honestly I don't even remember much of what I did. This is one of those thing you just have to take a step at a time and figure out each bit one by one.

I would probably go MUX/IVY/Buf from bottom to top, but there are other ways that would work just as well.

However, after downloading and reading the Linux package documentation I learned that the S/PDIF driver for the board is limited

21.2 S/PDIF Tx Driver

The S/PDIF Tx driver supports the following features:
• 32, 44.1 and 48 KHz sample rates. MX53 START board only support in-accurate 48KHz sample rate.

Which makes me wonder.

Russ, what sample rate did you manage to get with the BeagleBoard?

Cheers

Thomas

Both the Beagle and the Freescale board have I2C and I2S output. Exactly what you need for volume control and streaming data to the DAC. The Freescale board does have both outputs, but they are only available on the external connector which would require one to build a daughterboard to gain access. The original Beagle did not have the required pins connected to an external port, but the XM does, and it's a much simpler connector. Some developers have had the beagle running 24/192, but it requires modification in the source.