Suggestions audio processing (Input ADC, buffer, DSP, output PWM)

[stef]

Hello everyone, I am trying to implement audio processings algorithms with my nrf52DK.

What i need to do in order : 
1- Sample my audio signal ( It will be guitar sound ,so fmax= 1200 Hz , let's say fs=2400Hz  )
2- Push my sampe into a FIFO
3- Start my algorithm with data inside FIFO ( Low pass, high pass, phaser etc..)
4- Change the duty cycle of my pwm 
5- Go back to 1

Im starting with the SDK, i see many differents ways to use ADC and PWM (SAADC from what i saw in documentations).

So i wouldike to know if you have any suggestion on :

1. What's the best way to configure my ADC to 2.4Khz ?
2. I see in SADC example that a ADC buffer aldready exist, but i wonder how it work. When the SAADC interrupt, is it when the SAADC sample a single value or a n=buffer_size value ?
3. I see in pwm examples that it work with sequences, but i do not want a sequence to be played, i just wouldlike to ouptut a pwm signal with a duty cycle proportionnal to my values. Should i use a sequence of 1 value and then change the value ?

How can i make my pwm and SAADC synchronized, i mean doing the five steps above in order (in SAADC interrupt) ?

Thanks a lot, any help is welcomed :) 

[stef]

OK i think i understood for 1 and 2, thanks a lot.
Concerning the PWM, you advise me to use a sequence (with the length of my buffer size for exemple), and to update the values periodically (inside the SAADC interrupt ? or using separate timer).
Thanks

[haakonsh]

You can use a separate TIMER to control the triggering of the PWM's TASKS_SEQSTART[n] task. This task starts outputting a sequence of PWM samples on the selected pin. 

The idea is to find how much time is required to process the SAADC samples and update the PWM sequence in RAM, and then to fire the PWM regularly based on this offset in time. You can, for example, start the second TIMER on the SAADC's END event, and have it trigger the PWM sequence start task after x amount of time. 

All TIMER's run on the same clock source so there's no drift between the individual TIMER peripherals. 

You really have to read the PWM — Pulse width modulation spec and PWM driver API ref, and try out the PWM Driver Example.