Analog output with Audio Expander

Nebojsa

Im using Audio expander to get input from 7 microphones and estimate angle of arrival of sound. And I wanna sent that angle ( posible values - 50, - 46,..., 42, 46) as voltage from 0V to 5V via analog output on Bela to Arduino analog input (which takes 0V to 5V voltage). I have converted my angle values (from - 50 to 46deg) to range of 0 to 1 float, because float 0 is 0V and float 1 is 5V for Bela when sending via analog output (correct me if I'm wrong). Also I'm using analogWriteOnce function and pass same value for every analogFrame, for example if my audioBlocksize is 512 I send same value with analogWriteOnce for 256 frames. Question here is do I get const voltage on output or should I do it something different, because I wanna create const voltage so Arduino can read it without error.

Now hardware part: I have read the blog for Audio expander and I think I did everything right but just wanna check here. I have used analogoutput pin 0 on Bela expander which is on same place as on Bela. Also in Settings I have dissabled all audio outputs. I have connected grounds from Bela and Arduino on protoboard so there is no difference there.

Did I do everything right or should I change something, because angles that Bela sends and angles that I recive on Arduino are almost the same, difference is in 4deg which is my angle resolution. Maybe I should scale angle values from 0 to 0.92 (if it caps there, I have read somewhere)?

giuliomoro

Nebojsa Question here is do I get const voltage on output or should I do it something different, because I wanna create const voltage so Arduino can read it without error.

you get a constant voltage, it's an actual DAC, not a PWM output.

Nebojsa I have used analogoutput pin 0 on Bela expander which is on same place as on Bela. Also in Settings I have dissabled all audio outputs. I have connected grounds from Bela and Arduino on protoboard so there is no difference there.

sounds good

Nebojsa Maybe I should scale angle values from 0 to 0.92 (if it caps there, I have read somewhere)?

the nominal output level is 0V to 5V. Whether it can actually reach 5V cleanly depends on the actual power supply that you are using. In many cases, the 5V supply has a lot of ripple and goes as low as 4.8V or 4.7V, so if you need a cleaner voltage you may want to use a smaller value as your maximum, such as 0.95 or 0.92.

What are you using the Arduino for?

Nebojsa

giuliomoro thank you for reply. My power source is USB from computer, so I will try with expecting 4,7V and 4,8V. Well Arduino is connected with servo motor and on top of motor i glued laser. So after Arduino recives angle it tells servo motor how much to rotate with activated laser, with this I'm pointing laser at angle where my sound source is (It's really cool 😃).

giuliomoro

sounds good, what algorithms are you using to detect the direction of the source on Bela?

Nebojsa

giuliomoro well right now CAPON AOA method. But 2 months ago I implemented simple delay and sum algorithm. Difference is that CAPON can scan different frequencies in sound, delay & sum algorithm is just simple wideband AOA method. Ofcourse there is a big price in processing, my delay and sum algorithm was only using 15% of CPU, where now with CAPON is at 50% when i scan from 2000Hz to 5000Hz in increments of 30Hz. In both cases I used mic_array of 6 microphones.

giuliomoro

nice .. I was wondering if you would be able to drive the servos directly from Bela, avoiding the Arduino in the loop. What's the PWM carrier frequency and resolution you are using? Would the 3.3V output from Bela be enough to drive your power electronics?

Nebojsa

giuliomoro I'm using SG90 servo motor, as far as I see it can operate at 4,8V or 6V.

giuliomoro

Hmmm it seems that you should be able to operate it from the analog outs of Bela (bypassing the audio expander). The pulse width values are between 1ms and 2ms, so you'll get about 22 possible different positions when driving it from the Bela analog out. Not sure how many you get from Arduino.

Nebojsa

giuliomoro Nice, I didn't tought of that. Well I don't know how much is possible but right now I using 50 different positions.

giuliomoro

right, in that case to use it on Bela you'd need a small external circuit that can increase the output level from 3.3V to 5V and then you can use it from the Bela digital outputs with 44 positions, or many many more using PRU PWM (see this example https://github.com/BelaPlatform/Bela/blob/master/examples/Extras/pru-pwm/render.cpp ).