Raspberry Pi I2S to SPDIF Hat

Discussion in 'Digital: DACs, USB converters, decrapifiers' started by Michael Kelly, Apr 30, 2016.

  1. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    The Raspberry PI can provide all the I2S clocks, but they are not very good due the fact that the PI internal clock is not an exact multiple of the audio frame rate. Luckily the Pi can be set to accept external clocks. This allows us to create exact clocks for 44.1Khz and 48Khz based frame rates. On the 502DAC this is done by the PCM5122 and the two very low jitter NDK oscillators. We also bypass the WM8804 internal clocking circuitry and this reduces the clock jitter to below 1.5ps. No flip-flop or re-clocking is needed.

    As for drive strength, the I2S header on the 502DAC was put there for test only. Not to drive other devices off-board.

    Regards,
    Michael
     
  2. damaged-goods

    damaged-goods Acquaintance

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    Following the previous discussion on connectivity I'd love a real AES out and I2S via hdmi.

    I'm using the original 502dac from early on and it works flawless, no troubles at all. One of my best audio purchases.

    I'd prefer a black case though because the whole setup has too many lights, I've painted mine black.
     
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  3. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    We tried some colored cases with stenciling on them, but the results were poor. We tried etching as well, but that drove the cost up too much. I don't think a $25-30 case would be too popular!

    Regards,
    Michael

    P.S. What do you mean by "real AES"? We have balanced (XLR via adapter) and single ended (BNC Coax) AES already.
     
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  4. damaged-goods

    damaged-goods Acquaintance

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    I'd prefer a regular XLR/AES out without adapter just for aestheic reasons.
     
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  5. lehmanhill

    lehmanhill Acquaintance

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    Thank you. I wondered how that worked. Clearly no need for a flip- flop out. All of a sudden my curiosity about the competition has disappeared.

    Understood. And there is no reason for us to expect anything more. But you know we are going to play around and try it. ;-)
     
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  6. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    Folks,
    I am doing a new digital shield with analog expansion. This will incorporate the digital section of the 502DAC, but using an XLR for AES, and an RCA for coax. No BNC, but opto remains. There will be an expansion header for add-on DAC's. The I2S bus will come to this header and it is buffered if you want to do your own thing. Additionally I have added balanced I2S via another header. However, this would not be available when an DAC expansion is present.

    One question, do you guys have a sense of what is preferred: I2S over HDMI or RJ45? In both cases the driver would be a differential LVDS transmitter. I am leaning towards RJ45 since you can make your own cables pretty easily, but there is no ground unless the cable is shielded. HDMI gives us better signal integrity, but you can't do much with it if you needed to breakout the signals. Also, on a PI Shield I worry that people will confuse it for the PI HDMI output. The layout I am working on would place it on the same edge as the PI HDMI, so it's a real concern.

    Cheers,
    Michael

    Some 3D renders of the initial design:

    [​IMG]

    [​IMG]

    Also one of the analog add-ons, AKM 4493 DAC with LME49600 Headphone Amp:

    [​IMG]
     
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  7. earnmyturns

    earnmyturns Smartest friend

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    There are more DACs that take (some form of) I2S over HDMI. Although each vendor seems to like a different pinout, so you'd need a way to configure pinout for a particular DAC type. As for RJ45, the only DACs I know that use it are higher-end Metrum models.

    This might be useful.
     
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  8. Yethal

    Yethal Rando

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    Hi Michael,
    The renders look great!
    Based on them it looks like the shield will use it's own power supply. Does that mean we'll have to use two power supplies (the way digione signature does it) or will there be an option to pass through the power from the hat board to the Pi (the way usbridge does).
     
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  9. neogeosnk

    neogeosnk Friend

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    I2S, seems like more manufacturers are jumping on the I2s bandwagon.
     
  10. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    That’s good feedback thanks. I will review and see if I can

    Cheers,
    Michael
    The 5V supply an power the shield and analog add on as well as the PI. There is a jumper to allow the pi to be powered separately.

    Cheers,
    Michael
     
  11. dBel84

    dBel84 Friend

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    I like the AES option instead of the adapter, also assuming that the AK headphone amp version will have "active RCA" for powered speakers?

    ..dB
     
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  12. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    That’s good feedback thanks. I will review and see if I can come up with a simple method to support as many as possible.

    I would still like to hear from folks that own these devices that support I2S over HDMI. That way I can get a sense of which ones I can’t leave off!

    Cheers,
    Michael
    I am not sure if I understand. Powered speakers only require standard line level input. Which would be satisfied by the dac outputs without the headphone amp.

    Cheers,
    Michael
     
  13. earnmyturns

    earnmyturns Smartest friend

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    I used to own Allo USBridge>USB>Singxer SU-1>I2S-HDMI>Holo Spring until I switched to an all-Metrum setup (Ambre>Onyx). I slightly prefer the new setup personally, but the Holo Spring and other Holo DACs get a lot of positive notes on various boards.
     
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  14. HumanFly

    HumanFly Acquaintance

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    I think he wants to use the volume control on the headphone-amp to control the volume coming from the speakers. Otherwise you need another device to change the volume. (if the speakers don`t have one, like my Exite 14A`s).
    Very nice to hear that you are working on a new board. Any upgrades(over the 502 used as a transport into a DAC) in the audiopath you would like to share?
     
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  15. Azteca

    Azteca Friend

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    Standard XLR AES, alright! That is honestly the only thing that I felt could be improved upon, but I understood space was at a premium. I am pinching my pennies right now but it's on my "to purchase" list.
     
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  16. Azteca

    Azteca Friend

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    I am a Roon user and would like to know if anyone is using a Pi with a 502DAC as a Roon endpoint? What distro/OS are you using? How do you like it?
     
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  17. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    It is my understanding that most if not all players support software volume control. I know it gets a bad rap, but all my internal testing shows no degradation in the measured signals. I freely admit this is not music, so I defer tom those who have better hearing than me!

    Cheers,
    Michael
     
  18. Michael Kelly

    Michael Kelly MOT: Pi 2 Design

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    My take is that I will support HDMI using two variations. One supports PS Audio and the other supports Gustard. Note that any devices that use HDMI with non-differential signals are not supported at all.

    The selection mechanism will be pop/no-pop resistors to keep cost low and signal integrity high. See below (the header enables 5V to the HDMI if needed):

    [​IMG]
     
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  19. Greg121986

    Greg121986 Almost "Made"

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    This is exciting! I also love the idea of AES with a full size XLR output.

    For now, I use a Singxer SU-1 KTE. It is feeding an Audio-GD DAC that I retrofitted with an HDMI I2S input after removing the old RJ45 input. The HDMI I2S for the Audio-GD is the same setup as I had with a Holo Spring DAC, which is also the same as the PS-Audio standard. PCM and DSD work perfectly on both devices, without making any changes to the Singxer dip switches.
     
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  20. dBel84

    dBel84 Friend

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    It had more to do with the output controlled by the volume pot.

    Looks great either way

    .. dB
     
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