ezDAC v.1.5 Builders Thread

[6h5c]

Post nr. has a point:

"1) Any conversion can increase the chance of distortion, so if its unnecessary, don't use it."

As far as I know, the problem with USB is that the clock or I2S signal (essentially the same) is generated by the PC. If the PC is jittery (most are...) the I2S is jittery. This is then sent straight to a DAC chip. The PC is the master, and the USB DAC is the slave.

With an ASRC in between, the input data is resampled and clocked out to the DAC chip with a NEW ASYNCHRONOUS clock. A zillion new samples are inserted between the samples of the input data and interpolated. The ASRC's input port that receives the I2S signal is set to slave, but the output port that's connected to the DAC chip is set as master, and is timed through a fresh on-board XO or some external clock. The output samples are clocked-out to the DAC chip by the new local clock.
So whether you use a CS8416 or a PCM27xx as I2S source shouldn't matter.

There's a great thread about ASRC here. The AD1890 datasheet is also very useful.

Regards,

Ray

[paulb]

The question for me is whether to convert USB to S/PDIF, or to I2S. I'm designing a USB converter board for the ezDAC and have assumed (well, up until now) that I would use S/PDIF. I don't agree with the "any conversion can increase distortion" - these are digital signals, remember. I think the ASRC can probably handle any introduced jitter. Still, I may go with I2S. My goal is to have both S/PDIF and USB inputs to the DAC.

[6h5c]

Originally Posted by paulb

I don't agree with the "any conversion can increase distortion" - these are digital signals, remember.

That's where the snag is. S/P-dif itself is not a digital signal, only the data embedded in the signal is. Transport is analog, over a coaxial (or optical) cable, with BMC modulation. If the cable is not properly terminated, reflections and interference occur, which create jitter.
Less conversions means less jitter.

Sidestep: the data coming off an audio CD is also not digital . The HF signal inside a CD player is pure analog. That's probably why the sound is affected by certain modifications, like damping the CD or transport, and improving the power supply to the servo's.

Furthermore, the ASRC does not eliminate jitter, it attenuates jitter. So less in, means less out.

So I'd say go USB > I2S.

Ray

[ezkcdude]

Originally Posted by paulb

The question for me is whether to convert USB to S/PDIF, or to I2S. I'm designing a USB converter board for the ezDAC and have assumed (well, up until now) that I would use S/PDIF. I don't agree with the "any conversion can increase distortion" - these are digital signals, remember. I think the ASRC can probably handle any introduced jitter. Still, I may go with I2S. My goal is to have both S/PDIF and USB inputs to the DAC.

Hey, guys. First, let me say Congrats! to Ray. That's awesome. Hope you get a little sleep in...

Just thought I'd chime in with my two cents (or maybe less, you decide). If one is thinking about designing a USB->SPDIF converter, is it really going to be any better than a commercially available converter? There seem to be several available, and it would save a lot of time (and probably money).

[paulb]

There are a few available; the ones I remember are the Hagtech and Twisted Pear. The Hagtech is fairly expensive, certainly more than mine will cost me. Also, mine will have a relay on the board to switch between USB and external S/PDIF. The Twisted Pear modules are great, they seem like good guys running that site. Their converter board (PCB only) is $14US. But once you start buying a few of their modules, the price goes up quickly. I've rigged up 'other people's boards' with the necessary other parts and pieces and it's never quite as nice as a custom solution. Using the ezdac PCB with a custom companion PCB will work nicely in my preamp.
If you count my time as $$, sure, they would be cheaper. But then I wouldn't be doing any DIY at all.

I am leaning towards the I2S output, depending on how nicely it works out to interrupt the signal on the ezdac PCB, and whether my chassis layout will allow me to place the boards closely. S/PDIF is intended for longer distances than I2S and I would bet it has better noise immunity on its input. You certainly wouldn't want to run I2S between enclosures, not a concern in my case. My primary input will be S/PDIF from a CD player, but I want the flexibility to bring in a signal from my music server PC as well.

[paulb]

I looked a little further and noticed the voltage swing for S/PDIF (at least according to wikipedia) is around 0.5 to 1V, probably similar to video signals. I2S has a logic voltage swing of 3V or so. So, if you're running it inside a chassis and not on a long cable, I have to say that I2S looks better.
If I ever calculate the labor cost of my DIY projects, a big chunk of will be rework. Looks like I'll be pulling the PCM2704 out of my design and putting in a PCM2706.
I can post more information on the board as it comes along if there seems to be interest. Or, if ezkcdude prefers I not pollute this thread, I can start another.

[TICUL38]

[/QUOTE] Looks like I'll be pulling the PCM2704 out of my design and putting in a PCM2706.[/QUOTE]
... good choice!

[/QUOTE]Or, if ezkcdude prefers I not pollute this thread, I can start another.[/QUOTE]
... bad choice!

Louis

[TICUL38]

.... went with Ray's suggested reviews of the AD1890 Data Sheet, and the
discussion at this thread re. the ezDAC's approach to converting bits to analog:

Asynchronous Sample Rate Conversion

Now I really know how deep I can not go, and knee-deep will do me fine for effort to assimilate/assemble a DAC for computer audio output.
So, will have to continue leaning on the experts hanging onto this thread!

Anxious for get my hands on the boards, and to set a BOM.

Louis

[6h5c]

Originally Posted by ezkcdude

Hey, guys. First, let me say Congrats! to Ray. That's awesome. Hope you get a little sleep in...

Thanks Evan! She's so beautiful.... and so noisy! Not much sleep last night (we just woke up), I guess mum ate something she didn't like. It's a learning curve...

Originally Posted by paulb

You certainly wouldn't want to run I2S between enclosures...

Actually, that's not a problem, and a perfect way to eliminate two I2S > S/P-dif (and back) conversion steps. There's PS Audio for example, with their Perfect Wave transport and DAC. They use HDMI connectors for the I2S interface. Very clever, as this is suitable for high resolution signals and widely available.

It would be my choice if I had plenty $$$

Ray

[merlin2069er]

I've got a couple of quick questions;

I got my transformers in the other day - 2x12 & 2x15.

I'm going to use 1 of the inputs & outputs from the 2x15 to power the opamps. What should I do with the other set of input/outputs from the transformer? I guess I should cap them and tie them off.

What about a fuse for the ezdac /ezdual? I've got 2 of Ray's 9v power regs and the ezdual. I've looked at Ray's BOM and he's using I think 100ma. (not sure if that's a slow / fast blow fuse). Might I need a fuse that can draw more power without blowing?

Thanks,
JG