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  1. #1

    Touch I2S-out and Clock-in mods

    I have completed modding the Touch to accept clock signals from my DAC and return I2S outputs to the DAC, and thought I'd share them for anyone interested.

    First, the obvious: do NOT try this unless you are comfortable with SMD mods and you already have clock signals from your DAC (22.5792 and 24.576 MHz) and your DAC accepts I2S.

    I have used LVDS format to conduct all signals to and from the Touch. This allows the use of reasonble length cables (I currently use a one meter cable, but have successfully used a 2 meter cable.) I have found a DVI (dual-link) connector and cable setup to be ideal. The connector mounts easily to the case and can support all 5 shielded twisted pairs needed (2 for the clocks and 3 for the I2S signals). RJ-45 and HDMI only support 4 twisted pairs.

    The actual Touch mod is quite straightforward. First you need to remove the existing 22.5792 and 24.576 MHz crystals and their associated 4 capacitors.
    I mounted a 65LVDS9637 dual receiver on a small 8-pin SOIC mounting board and connected its inputs to the DVI connector and its outputs directly to pins 3 and 11 of the 74LVC04U chip next to the crystals. Next I mounted a 65LVDS31 quad driver on a 16-pin SOIC mounting board and connected its inputs to the I2S signals entering the DAC at the very convenient test points labeled TP 260 (Bit Clk), TP 267 (Word Clk) and TP 269 (Data) and its outputs to the DVI connector. There are many ground pads available, but I had trouble deciding where to take 3.3V. I traced out a spot that was directly connected to pin 16 of the 74LVC04U on a capacitor near the test points and used that. I used ferrite beads to connect the power to the LVDS chips and each one is bypassed with SMD ceramics.

    The attached schematic shows the connections I used. Keep the single-ended connections to the Touch board short and as close to the board as possible. The twisted pairs going to the DVI connector may be longer. I used a 19g solid core wire to ground the DVI connector to the Touch. This gives it some mechanical stability while it is loose from the case. I used 31g wire-wrap wire for all other connections.

    I have attached photos. There is not a lot of room inside the Touch, but just enough for these SOIC mounting boards and the DVI connector.

    I am EXTREMELY pleased with the result. The real key to this is bringing in low-jitter clocks from the DAC to the Touch. Jitter generated inside the Touch is now unimportant, since all I2S signals are reclocked near the DAC chip (I use Tent Clocks and pico-gate LVC logic). Actually the hard part in all of this is setting up your DAC to really use low jitter clocking. My Touch is now a slave to my DAC, completely asynchronous from the computer, similar to asynchronous USB DACs. (The real avantage in this system compared with the asynchronous USB rigs is the availablilty fo the whole squeezebox networking concept.)

    Hope some of you find this interesting.
    Jack
    Last edited by JIJ3; 2010-11-07 at 13:01. Reason: photos added

  2. #2
    Senior Member Ryssen's Avatar
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    Yes,it┤s interesting.
    Waiting for the pics.

    Edit:Where did you get those pcb┤s for the LVD┤s?
    QNAP TS-219P+,Firmware v4.0.2(020130726),WD2002 X 2
    SqueezeBox Duett
    CS4398-->Lundahl DAC

  3. #3
    The 8-pin was from ebay:
    http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT

    The 16-pin was from Digikey: 9161CA-ND,(although 9162CA-ND is superior).

    These are the ones I had on-hand.

    There are lots of interesting such boards on ebay

  4. #4
    Senior Member Turnandcough's Avatar
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    Very interesting indeed.
    I've been wanting to do the I2S thing for a while now and I have an extra DAC board collecting dust in the closet.
    So far your post is the biggest piece of the puzzle that I've stumbled upon. Unfortunately I'm still a bit uncomfortable and would probably need a step by step with detailed pics before attempting it.

  5. #5
    This is really not the sort of project that is amenable to step-by step instructions. It depends on your level of experience with this sort of thing. I would start by seeing whether you are able to open up the Touch for a look-see and then get it back together unmodified and still functional. This is and of itslf is not easy (there is another thread on doing this, with guidance from John Swenson, http://forums.slimdevices.com/showthread.php?t=78244). Be aware you run the distinct risk of ruining your Touch.

    If you feel comfortable just getting it opened up, look around and find the connection points shown in the schematic. I will be happy to answer questions about what connects to where and why, and where to get parts, etc.

    You probably would first start with the I2S-out connections only since this would be fairly straightforward with your existing DAC board (although the real improvemnet is with bringing in the clocks).

    I recently found some additional photos of ths kind of mod:http://www.diyaudio.com/forums/digit...touch-mod.html

    Hope this helps

    Jack

  6. #6
    Senior Member Turnandcough's Avatar
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    Thanks JIJ3,
    Just checked out the thread on DIYaudio. Lots of pics(especially on the Thai site) but I can't make heads or tails out of any of them.

    I had no problem opening the Touch to replace the RCA with a BNC connector but, as I mentioned in another post, ended up damaging the flimsy FPC connector in the process. These are some of the pitfalls of DIY and I'm confident I will eventually find a way of fixing it. Fortunately this mistake does not prevent me from using the Touch - only the LCD.
    Last edited by Turnandcough; 2010-11-10 at 19:43.

  7. #7
    Some say that the Touch is better sounding then the Reciever. Maybe it's inside dac, but i suppose not when using I2S.

    Is there any reason that the I2S from the SB touch should be better then from a SB reciever (for redbook audio)?

  8. #8
    Senior Member Phil Leigh's Avatar
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    Quote Originally Posted by hybride View Post
    Some say that the Touch is better sounding then the Reciever. Maybe it's inside dac, but i suppose not when using I2S.

    Is there any reason that the I2S from the SB touch should be better then from a SB reciever (for redbook audio)?
    Yes - it is a completely different circuit and the improvements are not just on the DAC.
    You want to see the signal path BEFORE it gets onto a CD/vinyl...it ain't what you'd call minimal...
    Touch(wired/W7)+Teddy Pardo PSU - Audiolense 3.3/2.0+INGUZ DRC - MF M1 DAC - Linn 5103 - full Aktiv 5.1 system (6x LK140's, ESPEK/TRIKAN/KATAN/SEIZMIK 10.5), Pekin Tuner, Townsend Supertweeters,VdH Toslink,Kimber 8TC Speaker & Chord Signature Plus Interconnect cables
    Stax4070+SRM7/II phones
    Kitchen Boom, Outdoors: SB Radio, Harmony One remote for everything.

  9. #9
    Quote Originally Posted by Phil Leigh View Post
    Yes - it is a completely different circuit and the improvements are not just on the DAC.
    You are properly right, but what could be 'better' quality issues only using I2S?
    Clock jitter is imo not an issue when externally clocking the SB.
    Reclocking I2S data in the DAC, like i do, eliminates most noise issues.

    Maybe a difference in digitally handling the stream, like digital volume control?
    Is it known by someone how different the streamed data is treated in both devices?

  10. #10
    Senior Member Phil Leigh's Avatar
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    Quote Originally Posted by hybride View Post
    You are properly right, but what could be 'better' quality issues only using I2S?
    Clock jitter is imo not an issue when externally clocking the SB.
    Reclocking I2S data in the DAC, like i do, eliminates most noise issues.

    Maybe a difference in digitally handling the stream, like digital volume control?
    Is it known by someone how different the streamed data is treated in both devices?
    Well for starters there is the power supply circuit...
    You want to see the signal path BEFORE it gets onto a CD/vinyl...it ain't what you'd call minimal...
    Touch(wired/W7)+Teddy Pardo PSU - Audiolense 3.3/2.0+INGUZ DRC - MF M1 DAC - Linn 5103 - full Aktiv 5.1 system (6x LK140's, ESPEK/TRIKAN/KATAN/SEIZMIK 10.5), Pekin Tuner, Townsend Supertweeters,VdH Toslink,Kimber 8TC Speaker & Chord Signature Plus Interconnect cables
    Stax4070+SRM7/II phones
    Kitchen Boom, Outdoors: SB Radio, Harmony One remote for everything.

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