DIY iPod digital out

Jan 22, 2008 at 8:11 PM Thread Starter Post #1 of 85

joneeboi

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I'm dreaming of a white Christmas where DIY-ing iPod owners can enjoy digital I2S output from their iPods. From the little research I did, the Opus DAC can input I2S, but I'm wondering how to carry this signal to the DAC. I've searched and searched but couldn't find anything on DIY I2S cables anywhere. I gather that the wires can't be too long and need some sort of shielding. Should I just open up an old CD player and yank that I2S cable instead? My plan is to pull the signal from the Wolfson (funnest soldering job ever) and send them to the unused pins on the dock (funnest soldering job ever), which are pins 7, 14, and 17. I'll need a 4th one, so I'm guessing I can steal one from, say, pins, 8, 9, or 10. Apple iPod dock interface pinout and signals @ pinouts.ru tells me that these are used only on iPod photo, so I guess these are fair game too, right? From one of the many iPod-implemented Wolfson datasheets (see diyMod post 1), the data accepted are master clock, bit clock, DAC data, and DAC left/right audio interface. I think usually I2S has only 3 lines, but for some reason the iPod has 4. What does the Opus DAC take in for its I2S module? Essentially we're swapping the iPod's (4G in this case) Wolfson for the Opus Dac's. Has anyone else looked into the matter?

Any help is appreciated.
 
Jan 22, 2008 at 10:40 PM Post #3 of 85
I2S in my experience is four wires, but *I think* I2S links are sometimes three when your MCLK is a system clock coming from somewhere else, but I could well be wrong.

In this situation however you want all four. As the Opus uses Wolfson 8740/1s then I'm pretty sure it'll want those four.
 
Jan 23, 2008 at 12:43 AM Post #7 of 85
Quote:

Originally Posted by joneeboi /img/forum/go_quote.gif
Should I just open up an old CD player and yank that I2S cable instead? .... From one of the many iPod-implemented Wolfson datasheets (see diyMod post 1), the data accepted are master clock, bit clock, DAC data, and DAC left/right audio interface. I think usually I2S has only 3 lines, but for some reason the iPod has 4.


Don't open an old cdp... all you would find is some poor ribbon wire. I2S wire have to be kept short (less than 20cm is good) and if you're going outside of the box, I'd go shielded. CAT5 could be ok I guess.

Traditionnal I2S was 3 wires, for old DAC chips like the tda1543 (bck, data, lrck). Now that the DAC integrates digital filters and the like, you have to add the mck (ou sck) line. The WM8740 used in the opus certainly needs the 4 lines.
 
Jan 23, 2008 at 12:49 AM Post #8 of 85
I guess Wadia does it too

http://www.head-fi.org/forums/f7/wad...-ipods-281975/

This would be interesting to have, I imagine many users with portable DAC/AMP combos though it seems a limited amount have optical/coax in would appreciate this..

I might have to follow this thread :P
 
Jan 23, 2008 at 1:44 AM Post #9 of 85
I must have closed Firefox before posting my response. =T

We could go the Wadia way, but as far as DIY goes, that's selling out. :P It's pricier than DIY, plus it isn't as personalized as we can make it. This guy from OverlordXenu's thread seems to have gotten it right. Serves our purpose, small footprint. One drawback is that it would have to draw power from the iPod. Then again, while we are fabricating our own iPod docks, we might also want to add charging capability to the iPod while the transceiver steals power from it. I can't imagine this kind of setup being portable, but some have carried T-amps and K1Ks around too. The biggest hurdle would be taking the signal from the iPod's Wolfson and sending it to the 30-pin dock. Further, internal layouts may restrict generation-compatibility. For instance, the 5G/5.5G iPods have that metal frame that screws into the front plate, for those of us who have seen the innards, so routing the wires to the dock would suck. I find it'd be easiest with 3G and 4G iPods, as it seems Apple hadn't shrunken that layout just then. There's plenty of room indoors for those iPods, so signal retrieval wouldn't be so bad. For future reference, I soldered some navships stranded SPC to the iPod dock before but only used 2 strands (documented in diyMod thread). After you get the hang of it, you feel like you can solder 100 Alien DAC PCM2702 chips with a 10W soldering gun in the dark. Anyone who'll try routing wire-wrap through that 5G/5.5G frame to the dock can report on the fit. Also, I don't suppose making this fit the Alien DAC for the ultimate in portability is viable either, is it?
 
Jan 23, 2008 at 1:47 AM Post #10 of 85
Quote:

Originally Posted by jsmithepa /img/forum/go_quote.gif
Since THESE GUYS sell it for 2 grands, would u DIY it if it costs u say 1/2 as much?


The $2,000 is just for their silly dock. The iPod modification costs $200, so they probably just pull the I2S to the dock connector.
 
Jan 23, 2008 at 4:38 PM Post #12 of 85
Now, I'm not a design expert of any kind just yet, but perhaps we can design our own portable, full on DAC/amp solution using some of Wolfson's better DACs. I realize there are already solutions available along these lines, but none fitting the I2S input requirement. stevenkelby, you mentioned using the Headamp Pico for its I2S input, but that DAC/amp converts USB to I2s. Unless we open it up and route some wires, we can't do it without voiding the warranty. I'm not sure how we'd go about doing this, but is it a viable solution having a portable DAC/amp running off a 9V? That's quite a huge energy loss converting down to 3.3V, so maybe running off some AAs would be wiser. Before we even go there, we should probably pick out a DAC that'll work. I'm thinking something along the lines of a portable full-on DAC, like a portable TPA Opus DAC (okay, maybe not that far). I'm looking at some Wolfson DACs over at Mouser, and a couple of the models jump out immediately. They all seem to be designed for DVD players or digital set-top boxes, but if somehow we can plow through with the design, I think the head-fi community would be better for it. I'm no Alf, cetoole, Jambo, justin w., Ray Samuels, etc., but I can offer my limited knowledge and unbridled passion and enthusiasm to this kind of project. Please, someone with actual knowledge help me out here.
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I've had a class in digital logic design, but I'm not sure how much of it actually applies here.

Some of the DACs that I think would work here.

http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8521.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8956.pdf
http://www.wolfsonmicro.com/uploads/...728_Rev4.4.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf
 
Jan 23, 2008 at 9:12 PM Post #13 of 85
Heh, I was wondering if anyone here was thinking of doing this from the interest the "iMod" thread is getting...
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I've been thinking about it ever since I got water inside my iPod a few short months back. The most obvious problem is the 0.5mm pitch QFN the Wolfson DAC comes in.

Beyond that, while searching around, I came across this (scroll down to 12-23-07) where someone took apart an iPod video and found that it had a non-standard I2S interface. That does sound like something Apple would do... It's possible that not all iPods are like that though, especially since MSB seems to be able to do it.

Myself, I was thinking of tapping the 4 I2S signals out to the dock connector, and using an external portable DAC+AMP. I have two ideas for tapping into the I2S lines, but I don't know how much of a problem signal integrity would be. Perhaps a ASRC(?) would be needed somewhere along the line.

joneeboi, you might find this interesting...
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Jan 23, 2008 at 10:45 PM Post #14 of 85
The DVD player type DACs (generally) have the plus points of better performance and hardware control, but the down sides are their power consumption. They're all around the 70-80mW though from the datasheets I have looked at, which I guess is tolerable. I noticed you haven't mentioned the WM8740/1, they're Wolfson's best DACs which are worth looking at if you want best performance possible, and the power consumption for the WM8740 is ~80mW at 3.3V. Also, the WM8740 has differential output, so you don't need any caps on the output.

I'm willing help out where I can, although I'm afraid my ipod is staying in one piece! And I also have my own project doing my head in at the moment as you can see in the Jambo DAC thread lol.
 

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