• Audio

  • 4 audio channels into BelaMini

I am looking to bring 4 channels of audio into my bela mini and I thought I would be able to get my hands on the multichannel expander but I wasn't and now I'm short on time for an install... I don't fully understand what was said on this other thread about the reconfiguring, but is there a way someone could help me do this? If I can't get it to work then I will have to use two separate belas which is not ideal.

I split this onto a new thread as it doesn't' seem to be related to the one you posted in.

So, you have one BelaMini and what else ? If you have no other hardware at hand, then the only solution is to get the two extra channels into the analog inputs. These are lower quality than the audio inputs but may still work in some cases. You'll need four or five passive components for each channel:

                 3v3 
                  |
                  R1 100k
                  |
    100n          | 4k7
in --C1-----------+-R3----+ Bela analog in
                  |
                  R2 150k
                  |
                 GND

None of the values are critical. The rationale is to use R1 and R2 to bias the voltage at BelaAnalogIn to be around 2V, which is the mid point of the input range of the analog's ADC. C1 and R4 form a high-pass filter that ensures that the input signals is AC-coupled and R3 is an optional series resistor to provide protection to the ADC in case the source is expected to have large voltage swings (e.g.: a piezo disk).
Pick R2 and R1 so that they are in the hundreds of kiloohm range and R2 is approximately 1.5 times the value of R1 . R3 should be between 2k and 10k. C1 can be as large as you want, keep in mind that the high-pass filter cutoff frequency will largely be driven by C1 and the parallel of R1 and R2. The cutoff is: 1/(2*pi*C1*(R1*R2)/(R1+R2)). With the values shown, C1=0.00000047, R1=100000, R2=150000, the cutoff is 1/(2*pi*(100000*150000)/250000*0.0000001) = 26Hz. Tweak as needed.

As mentioned, this won't provide a great quality audio input, the only easy way to get there is via a BelaMini Multichannel expander (easy), or some additional I2S codec you may have lying around (very hard because of the required programming). When is your deadline? If you are in the EU or UK, placing an order now for a BelaMini Multichannel should get it to you by the end of the week.

Okay, thank you. I just have the bela mini and I am trying to bring four individual contact mic signals into pure data, just using them as sensors basically sending messages when they pick up peaks. Because of this I think I can sacrifice audio quality. I am in London, but I need to have it by Thursday midday at the latest so unfortunately I don't think I can get the expander... I was told that they had one at my school but then no one could find it! I should be able to try the Analog input conversion tomorrow.

    saia I just have the bela mini and I am trying to bring four individual contact mic signals into pure data, just using them as sensors basically sending messages when they pick up peaks. Because of this I think I can sacrifice audio quality

    You should be fine with the circuit above and the analog inputs.

    saia I need to have it by Thursday midday at the latest

    OTOH, if you decide you need the expander and can travel to Palmers Green or Kensington on Wednesday we could organise a pickup there.

    • saia replied to this.

          giuliomoro I could meet in Kensington on Wednesday actually, that would be fantastic.

            Actually the Kensington option fell through. If you are still interested, could you do Palmers Green?

            I just got my hands on one, they managed to find it at school! Thank you for being available to make that pick up happen it is very appreciated.