Posted on July 09, 2016 at 00:17Hi, I am doing some audio programming on the STM32F4 and I am noticing that there is some distortion/aliasing happening around the sample rate. I was writing a program that reads the incoming audio into a delay buffer and reads back its contents at a slower rate, pitching down the signal, and this brings this aliasing/distortion into audible range. I have tested this out and have found that the pitch of the aliasing is a division of the sample rate based on how much I am slowing the signal down (i.e. If I play the buffer back at 10% speed with a sample rate of 48000 this will create a 4800Hz tone).
I am wondering if there is any way I can set up my program differently to avoid this aliasing. I am currently setting up sample rate via a 2 step timer set at a frequency of 48000. I use a flag to prevent steps from repeating. I have also tried implementing a simple lowpass filter, but the aliasing still occurs.
Is this an unavoidable biproduct of the onboard ADCs, or is there something I can do to get around this? Any suggestions appreciated, I just figured out how to set a lot of this stuff up so let me know if you see anything flawed in my methodology. Code below. Thanks!
Here is my while loop:
while (1)
{
int timerValue = TIM_GetCounter(TIM2);
if (timerValue == 1 && timerFlag == 0){
timerFlag = 1;
delayCounter++;
if(delayCounter >= 48000){
delayCounter -= 48000;
}
ADC2ConvertedVoltage = (ADC2ConvertedValue *3300/0xFFF);
gain = ((float)ADC3ConvertedValue)/4095;
diveSwitch = ((float)ADC1ConvertedValue[0])/4095;
filterFeedback = (1.5*ADC2ConvertedValue) - (.5*filterFeedback);
if(diveSwitch >= .5){
if(delayCounter < 48000){
delay[delayCounter] = filterFeedback;
delayCounter++;
}
if(diveCounter <= 47999 && delayCounter >= 1){
//diveIncr = .1;
castDiveCounter = (int)diveCounter;
interp = diveCounter - castDiveCounter;
output = ((float)delay[castDiveCounter]*(1-interp))+((float)delay[castDiveCounter+1]*interp)*(3300/0xFFF);
diveCounter += diveIncr;
if(diveIncr >= 0.00004){
diveIncr -= 0.000004;
}
}
TM_DAC_SetValue(TM_DAC1, output);
}
else{
delayCounter = 0;
output = 0;
diveCounter = 0;
diveIncr = 1;
TM_DAC_SetValue(TM_DAC1, filterFeedback);
}
TM_DAC_SetValue(TM_DAC2, ADC2ConvertedVoltage);
}
else if (timerValue == 0 && timerFlag == 1){
timerFlag = 0;
}
}
#!stm32 #sampling-rate #aliasin
