Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › PCM2702 USB DAC Revision B
New Posts  All Forums:Forum Nav:

PCM2702 USB DAC Revision B - Page 14

post #196 of 670
Quote:
Originally Posted by roibm
kinda late but I'll give it a shot anyway...
It gives I2S directly from USB not from the piece of scrap called sp/dif.
As about another project, I'm on the non OS boat, already built stuff, just wanna get rid of sp/dif once and for all.
If you're speaking of the PCM2706(-7), I think you missed my point. The way it uses the USB is just as crappy as spdif. USB can be just as bad as SPDIF.

Picking USB to avoid spdif is just going from Charybde to Scylla, if you cannot configure your device in asynchronous mode. And you cannot do that with the PCM2*** serie.
post #197 of 670
There are a few points worth making here.

The problems with S/PDIF are intrinsic to its design. Even with as close to a perfect a specified S/PDIF feed as one could imagine, one with no jitter, wonderfully perfect waveforms etc etc, the actual nature of the beast leaves you with intrinsic problems. The most fundamental is the difficulty in recovering the clock without it being contaminated by signal correlated jitter.
Now there is a lot of experience with this, and to a pretty reasonable extent it is solvable. But not without serious effort.

USB audio is an almost utterly different beast. But it too has intrinsic issues. The worst of which is the lack of any reference clock. So we must recover a clock from the received data here too. This leads to problems, but for different reasons to S/PDIF.

I2S is a better encoding of digital audio, in that is separates the clock from the data. So once we have a clean clock - and getting that clean clock may be quite a trial - we should be able to avoid it becoming contaminated with jitter - especially signal correlated jitter. But the I2S output from any receiver, be it an S/PDIF in a conventional system, or a USB device such as a PCM2706, will only be as clean as the internal capabilities of the receiver. If it were S/PDIF the likelihood of signal correlated jitter on the I2S clock will be very high, with a USB receiver, more likely just lots of auto-correlated noise in the jitter spectrum left over from the vagaries of the USB transmission.

If you have not read the lovely story of the design of the PCM270x series, you need to. Understanding why the SPACT design does what it does is important to understanding the tradeoffs discussed.

http://www.planetanalog.com/showArti...cleID=12801995
post #198 of 670
Very good discussion of the jitter problem, Francis_Vaughan
post #199 of 670
Thread Starter 

A quick update

I placed an order with The PCB Shop last week. The order is currently in the production stage. The boards will be shipped to me on 25th of November 2005.

The bad news is the cost of the board will be a higher than I originally specified. Apparently I missed 2 things:
* The boards can only be delivered by courier service for registered users
* Second legend is required.

I will confirm the final price when I get invoiced. It should be around £5.9 - £6


I still have a few boards unallocated. If you would like to join prototyping, it is not too late. Just drop me an email or PM.
post #200 of 670
Why are CL and CR so large? 1/2(pi)(Hz)(R), assuming that you want a corner around 4Hz (which limits all phase distortions to under 40Hz) and (worst case scenario) the input impedence of your headphone amp is 10K, 4uF is still plenty big. I would think a film cap would be a good choice here. With a 50K input, 1uF is even plenty.
post #201 of 670
Alf....YGPM
post #202 of 670
IIRC, you also have to factor in the input cap of the following amp. So you don't want the output cap to be too small
post #203 of 670
Alf,

YGPM
post #204 of 670
YGPM!!!
post #205 of 670
what is the motivation for not using SMD caps?

is it so people can swap out parts easily?
post #206 of 670
Gah, what now? Group buy is already happening?

Did i miss it? Can i still get one?

Where is the latest schematic/board layout?

What are the details?

So many questions - so little time, and i hope i havent missed out! :P

Rob.
post #207 of 670
Quote:
Originally Posted by frdchang
what is the motivation for not using SMD caps?

is it so people can swap out parts easily?
In my (limited) experience, smd electrolytic caps are more expensive, don't offer as much choice, don't really save any board space, are harder to solder than smd resistors or ceramic caps. This is just for me.
post #208 of 670
Board is still in a prototyping phase. Small amount of boards were order to check the quality and functionality of the design. Alf still had a few boards left over and was asking for some help. Don't worry Rob, you didn't miss anything

Quote:
Originally Posted by robzy
Gah, what now? Group buy is already happening?

Did i miss it? Can i still get one?

Where is the latest schematic/board layout?

What are the details?

So many questions - so little time, and i hope i havent missed out! :P

Rob.
post #209 of 670
I agree with 00940 about SMD electrolytics; for DIY purposes they have no advantage, and have limited choice, especially for those of you wishing to put "exotic" output caps on (Nitals for me ). They're a pain to solder (try one of P-As boards), and take up more board space for equivalent capacitance.

Alf, personally I would ditch the second legend; just post a concise picture of the layout for people... I'd rather pay less and have to think (a little)
post #210 of 670
I'd second that...having a good quality website as you do with the layout posted should be fine.
New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › PCM2702 USB DAC Revision B