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Thread: Analog-to-Digital converter IS2 - 48000Hz?

  1. #1

    Analog-to-Digital converter IS2 - 48000Hz?

    I'm planning to get a fairly decent ADC board for recording sound with the Teensy 4.0. This is the model I'm looking at:

    https://www.audiophonics.fr/en/devic...fast_search=fs

    Is there any possibility to capture at 48000Hz via IS2 with it? Or will it work at all using the secondary IS2 port of the T4? As far as I know the Audio Library only allows to record in 44100Hz 16bit, but I don't know if this limitation is still there or only applies to the Teensy Audio Board.


    Many thanks! Any information will be highly regarded.

  2. #2
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    Quote Originally Posted by domingo View Post
    I'm planning to get a fairly decent ADC board for recording sound with the Teensy 4.0. This is the model I'm looking at:

    https://www.audiophonics.fr/en/devic...fast_search=fs

    Is there any possibility to capture at 48000Hz via IS2 with it? Or will it work at all using the secondary IS2 port of the T4? As far as I know the Audio Library only allows to record in 44100Hz 16bit, but I don't know if this limitation is still there or only applies to the Teensy Audio Board.


    Many thanks! Any information will be highly regarded.
    The user chris314 used this chip. See this thread: https://forum.pjrc.com/threads/58032...to-LRCLK-ratio

    48kHz: Depends. If you want to use USB Audio with it, the answer is "no, not easy". USB Audio does currently not support 48khz. There are users who modified it., but i'm not aware of sourcecode.
    If not, you can just modify the samplerate to 48kHz by editing a file.

  3. #3
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    What's IS2 ?

    Anyway that chip has its interface speed and mode programmed by a physical resistor on the pcb, from what I
    can read on the datasheet. This probably means it only works at 96kSPS as is... What the specifications of the
    module (as opposed to the chip) are is unknown - that page just copies the chip's datasheet values and its very
    unlikely they've actually measured the real specs of the module itself - basically there's no way to know how good
    it is, or how fussy it is about power-supply cleanliness or ground noise rejection.

    What is your sound source?

  4. #4
    Oy! I meant I2S

    Thanks for your replies!

    My source will be a couple of transistor based mic preamps plugged to a pair of electret mics. As seen here: https://sound-au.com/project93.htm
    I need to record the analog signal in the highest quality possible, therefore the ADC. Any recommendations? I'm quite fond of AKM as a brand.

    Recording would be done internally to an SD card but eventually as USB interface as well.

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    The Audio shield is not the highest quality possible alas, as its grounding is not ideal and in particular doesn't
    have separate analog and digital grounds and supply rails. However it may be perfectly adequate for cheap
    electret mics (which are usually quite noisy). The preamp is likely to be the main source of distortion, and
    the shield could add some rail noise. If the line levels are high this will minimize such issues.

    BTW the project93 circuits aren't "microphone preamps", they are head-ends for driving balanced XLR cable
    at low impedance, they don't boost mic output levels to line level, which takes a lot more gain (50 to 70dB
    is typical, with at least an adjustment range of 50 to 60dB - this allows to handle both quiet inside recording
    and noisy outdoor/stage recording.

    The Audio shield chip does have a microphone input, but there's only one channel and its not very good (there's
    a spur(*) in the audio range for instance!).

    (*) spur = spurious signal peak

  6. #6
    Very good observation on the preamp circuit you bring, MarkT! I had already all parts collected and was ready to start building it, supposing that it would bring the signal to line-level :s I guess I'll have to move to this one then: https://sound-au.com/project66.htm. And build separate 9v battery box to supply the electrets, since providing clean 48v phantom power to condenser mics seems like more than a challenge and bulky for me. I have a good impression of electrets anyways.

    Back to the Teensy, I have a new Teensy Audio Board that I got for this project and just have to find the time to test it. I mostly got it for its capacity to drive digital effects... But for recording it reports to have a not-so-ideal noise floor level. Also I have the possibility to provide higher separate voltage to an external ADC and the T4 second I2S is there wanting to be used. I'll try to follow carefully the link provided by Frank B. Thanks for your thoughts!

  7. #7
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    Quote Originally Posted by domingo View Post
    I guess I'll have to move to this one then: https://sound-au.com/project66.htm.
    Despite the description saying 6 to 15dB, this appears to be the right sort of circuit, with highly variable input gain
    stage. Note the reverse-log taper pot required may be hard to find.

    Back to the Teensy, I have a new Teensy Audio Board that I got for this project and just have to find the time to test it. I mostly got it for its capacity to drive digital effects... But for recording it reports to have a not-so-ideal noise floor level. Also I have the possibility to provide higher separate voltage to an external ADC and the T4 second I2S is there wanting to be used. I'll try to follow carefully the link provided by Frank B. Thanks for your thoughts!
    Did you measure the noise floor of the line input or the mic input? The mic input is noticable poorer.

  8. #8
    I only looked at the line-in specs so far, - SGTL5000 85 dB SNR (-60 dB input), compared to -102 dB in the AKM apparently. Did not test this in real world situation to see if the diff is relevant to me, but I'm hiss intolerant and want to avoid at all cost post low pass filters. Also, my audio source are low volume environmental field recordings. Would it be loud music I wouldn't care that much.

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