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Thread: Audio Recording / Logging to SD card --> microSoundRecorder

  1. #76
    Senior Member DD4WH's Avatar
    Join Date
    Oct 2015
    Central Europe
    Hi Garth,

    Walter has already answered your questions, he was faster than me :-).

    One additional thing:

    * I do not recommend the use of USB power banks any more
    * the #define SLEEP_SHORT with the according ShortSleepDuration works for some types of power banks, but some power banks do not wake up, because the current from the Teensy is still too low
    * I purchased an ANKER PowerCore 13000 about a year ago, it works very well and wakes up, BUT then I purchased exactly the same model again a few weeks ago, and it looks exactly the same from the outside, but that does not work at all and is not waking up . . . seems they radically changed the battery management system

    So, I now soldered battery holders to all my prototypes (3 x AA batteries = 3.6 to 4.5 Volts depending on your battery type, connected to Vin and GND) and I do not use power banks for recording any more. This has two additional advantages:

    * much more reliable
    * current draw is drastically reduced, because it seems that a significant proportion of the current drawn was consumed by the power bank alone, and not the Teensy
    * AA batteries can be purchased everywhere worldwide, even in remote areas in the tropics and can be taken with you and help a lot if you do not have the means to recharge your power bank
    * for longer periods, you could even use C or D battery types to allow for several weeks of unattended recording

    I also put this information into the WIKI:


  2. #77
    Senior Member DD4WH's Avatar
    Join Date
    Oct 2015
    Central Europe
    Filtering of supply voltage for microphones

    It is vitally important to filter the supply voltage for the mics you are attaching to the microSoundRecorder.

    Test with digital mic ICS43434 (which has a maximum achievable SNR of 65dB):

    * I took the supply voltage directly from the Teensy 3.3Volt pin. --> leads to low frequency noise in the range 15 to 250Hz. Measured SNR of my recording is about 50dB.

    * I took the supply voltage from the Teensy 3.3Volt pin, a 100F electrolyt cap to AGND and a series resistor of 1000 Ohms. This RC filter has a lowpass cutoff for the supply voltage of 1.6Hz, so suppression is 20dB at 16Hz and 40dB at 160 Hz etc. The measured SNR of my recording with the RC-filtered supply voltage: 65dB ! Goal is thus achieved: I can record everything the mic is able to pick up and that means, the noise generated by the Teensy is lower than the internal mic noise.

    So, with a very easy supply voltage RC filter, you can achieve a massive increase in SNR with your mic.

    Keep an eye on the max current you need: with 3.3Volts and 1000 Ohm resistor, you achieve a maximum current of 3.3mA, which is more than enough for that mic (current of 0.5mA max), but you will not be able to supply a preamp with that current ;-).

    Have fun with the microSoundRecorder,

    Frank DD4WH

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