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  1. #21
    Senior Member vrobin's Avatar
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    Quote Originally Posted by philippe_44 View Post
    I always forget which one is the optical but if this is what you need, then all you have to do is connect a optical transducer like a PLT133/W or anything similar to one GPIO, nothing else to do.
    What you have here in this link is more sophisticated as it transcodes but I assume then this one would need to be connected as a regular DAC to the I2S bus
    Thank you for the reference, I didn't know what to look at. It looks like PLT133 is the de facto reference for this toslink female connector.

    As all version I found are either sold by 10 units or have 5Ç fee for transport... I'll look at the RCA 75ohm option if I don't want ending like this:


    You said that for the spdif rca connector only requires two resistors. Is there a schema somewhere with this resistors/value.
    Is this to create a voltage divider for GPIO output? (to get down from 3.3V to 0.6V?)

    My digital amp digital input is a bit weird (3.5 stereo jack using central part for spdif signal):

  2. #22
    Senior Member vrobin's Avatar
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    I found some diagram from ttl to coax with 2 resistor and a capacitance:
    http://reza.net/cms/index.php?page=spidf-adaptor

    I suppose changing first resistor to compensate 5v->3.3v should be enough.

  3. #23
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    Quote Originally Posted by vrobin View Post
    Thank you for the reference, I didn't know what to look at. It looks like PLT133 is the de facto reference for this toslink female connector.

    As all version I found are either sold by 10 units or have 5€ fee for transport... I'll look at the RCA 75ohm option if I don't want ending like this:

    You said that for the spdif rca connector only requires two resistors. Is there a schema somewhere with this resistors/value.
    Is this to create a voltage divider for GPIO output? (to get down from 3.3V to 0.6V?)
    If you want a PLT133 for test, I have ton of these, depends where you are.
    For RCA connector, that's just to bring down 3.3V to ~1V and then AC coupling. There are much better solution with isolation and al of course

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    Last edited by philippe_44; 2021-03-17 at 21:52.
    LMS 8.2 on Odroid-C4 - SqueezeAMP!, 5xRadio, 5xBoom, 2xDuet, 1xTouch, 1xSB3. Sonos PLAY:3, PLAY:5, Marantz NR1603, Foobar2000, ShairPortW, 2xChromecast Audio, Chromecast v1 and v2, Squeezelite on Pi, Yamaha WX-010, AppleTV 4, Airport Express, GGMM E5, RivaArena 1 & 3

  4. #24
    Senior Member vrobin's Avatar
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    Hi Philippe,

    Thank you for your proposal, I live in France, I'm not sure it will be profitable.
    If I really want to add optical toslink, I'll spent the extra money (as a music lover... I can afford a dozen dollar on a connector ).

    The TTGO T-display I ordered won't probably fit... it loooks like it was wrongly advertised has having 8MB PSRAM in the description

    Regarding the GPIO->SPDIF schema, most of the diagrams I found have the capacitance between the TTL output and the resistor while your schema has the capacitance between the divider and RCA connector... is it relevant?

    For example:

    https://sound-au.com/project85.htm

    Code:
             100nF
      TTL ----||-----330ohm-- --------- coax S/PDIF signal out
      in                     |
                          100ohm
                             |
     Ground ----------------- --------- coax signal ground
    http://www.hardwarebook.info/S/PDIF_output


    http://reza.net/cms/index.php?page=spidf-adaptor

  5. #25
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    Quote Originally Posted by vrobin View Post
    Hi Philippe,

    Thank you for your proposal, I live in France, I'm not sure it will be profitable.
    If I really want to add optical toslink, I'll spent the extra money (as a music lover... I can afford a dozen dollar on a connector ).

    The TTGO T-display I ordered won't probably fit... it loooks like it was wrongly advertised has having 8MB PSRAM in the description

    Regarding the GPIO->SPDIF schema, most of the diagrams I found have the capacitance between the TTL output and the resistor while your schema has the capacitance between the divider and RCA connector... is it relevant?

    For example:

    https://sound-au.com/project85.htm

    Code:
             100nF
      TTL ----||-----330ohm-- --------- coax S/PDIF signal out
      in                     |
                          100ohm
                             |
     Ground ----------------- --------- coax signal ground
    http://www.hardwarebook.info/S/PDIF_output


    http://reza.net/cms/index.php?page=spidf-adaptor
    All you need is AC coupling. I've never looked in details at coax electrical levels, but I think it's +/- 0.6V max. Hence AC coupling and a divider. Divider has to take into account the output level. In the case of esp32, it's 3.3V GPIO, so 1/3 divider with AC coupling does the job. If this is 5V logic output, then you need a different divider, as of your example (1/5)
    LMS 8.2 on Odroid-C4 - SqueezeAMP!, 5xRadio, 5xBoom, 2xDuet, 1xTouch, 1xSB3. Sonos PLAY:3, PLAY:5, Marantz NR1603, Foobar2000, ShairPortW, 2xChromecast Audio, Chromecast v1 and v2, Squeezelite on Pi, Yamaha WX-010, AppleTV 4, Airport Express, GGMM E5, RivaArena 1 & 3

  6. #26
    Senior Member vrobin's Avatar
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    As I wasn't able to understand the impact of the position of the AC Coupling capacitor (thanks pointing me to this concept... I know near to nothing in electronics sadly), I tried both schemas in a simulator.

    At low frequency (10/24khz) waveform sent to the RCA connector is quite different, but at higher frequency (2.8Mhz which should be the bitrate of spdif), the position of the capacitor has no impact on the output signal:

    https://www.multisim.com/content/BAf...f-ac-coupling/
    (in the link, you can see the signals)

    Anyway, thank you again for your help. ESP32 should arrive tomorrow, I have references for Optical Toslink connectors and GPIO->RCA diagrams...I should have anything I need to try squeezelite esp32.
    I'll probably need some hints again, but I'll switch to squeezelite esp32 thread.

  7. #27
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    Quote Originally Posted by vrobin View Post
    As I wasn't able to understand the impact of the position of the AC Coupling capacitor (thanks pointing me to this concept... I know near to nothing in electronics sadly), I tried both schemas in a simulator.

    At low frequency (10/24khz) waveform sent to the RCA connector is quite different, but at higher frequency (2.8Mhz which should be the bitrate of spdif), the position of the capacitor has no impact on the output signal:

    https://www.multisim.com/content/BAf...f-ac-coupling/
    (in the link, you can see the signals)

    Anyway, thank you again for your help. ESP32 should arrive tomorrow, I have references for Optical Toslink connectors and GPIO->RCA diagrams...I should have anything I need to try squeezelite esp32.
    I'll probably need some hints again, but I'll switch to squeezelite esp32 thread.
    Yes, sorry I forgot that the spdif is 75ohms impendance (I read the spec this time). So you have a divider with (~45ohms and 210ohms) = 0.176 ratio of 3.3.V, which gives the 0.6V peak-to-peak expected ratio according to the spec and the high-pass RC is far away from its cut-off when using 2.8MHz
    Last edited by philippe_44; 2021-03-18 at 17:14.
    LMS 8.2 on Odroid-C4 - SqueezeAMP!, 5xRadio, 5xBoom, 2xDuet, 1xTouch, 1xSB3. Sonos PLAY:3, PLAY:5, Marantz NR1603, Foobar2000, ShairPortW, 2xChromecast Audio, Chromecast v1 and v2, Squeezelite on Pi, Yamaha WX-010, AppleTV 4, Airport Express, GGMM E5, RivaArena 1 & 3

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