wings

wings

Removed & from line 107 and program compiles and runs ok.

Regards,
wings

I haven’t tried these values, but I hope this helps.

Thanks

For defining which wavetable you want to use, you can either use a
digital pin (for 2 different waveforms) , ‘n’ digital pins (for 2^n
different waveforms) or a analog signal with different waveforms
mapped onto it.

If you only use two wavetables, you can use one digital pin for this purpose:

(…)
// … main loop
if (digitalRead(waveSelectorPin) == HIGH)
waveSelector = SINE_WAVE;
else
waveSelector = SQUARE_WAVE;
(…)

If using more than 2 wavetables and you need the digital pins for
other stuff, just use an analog pin.

(…)
// main loop
int waveSelector_analog = analogRead(waveSelectorPin);
if (waveSelector_analog < ANALOG_TOP_SINE)
waveSelector = SINE_WAVE;
else if (waveSelector_analog < ANALOG_TOP_SQUARE)
waveSelector = SQUARE_WAVE;
....
else (waveSelector_analog < ANALOG_TOP_TOOTHSAW)
waveSelector = TOOTHSAW_WAVE;
(...)

The values ANALOG_TOP_TOOTHSAW, ANALOG_TOP_SQUARE, ANALOG_TOP_SINE,
etc , depend on the number of wavetables you use, and how you map them
in the analog signal. for instance, using 3 tables you'll have:
ANALOG_TOP_SINE = 1023/3
ANALOG_TOP_SQUARE = 1023/2
ANALOG_TOP_SINE = 1023/1

so values between [0, 1023/3[ are for sinewave selection, [1023/3,
1023/2[ squarewave selection, and [1023/2, 1023] for toothsaw wave.

(please note that this is one way of doing this.)

Now you should use the 'waveSelector' value on the ISR to select the table:

ISR(TIMER1_OVF_vect)
{
static uint8_t osc = 0;

// Send oscillator output to PWM
OCR1A = osc;

// Update accumulator
phaseAccumulator += phaseIncrement;
index = phaseAccumulator >> 8;

// Read oscillator value for next interrupt
switch(waveSelector)
{
case SQUARE_WAVE:
osc = pgm_read_byte( &squareTable[index] );
break;
case SINE_WAVE:
osc = pgm_read_byte( &squareTable[index] );
break;
….

case TOOTHSAW_WAVE:
osc = pgm_read_byte( &toothsawTable[index] );
break;
}
}

hope this hels,
regards

By the way i started tweaking here and there, and produced an 8-note keyboard with an octave switch succesfully.
Now i just wanted to add a waveform switch: i added a square wave wavetable to the sketch and have it working by changing the value in
osc = pgm_read_byte( &squareTable[index] );

but still i can’t find how to make it dynamic, i mean, having a trimpot or something like that to make me switch between different wavetables…

i tried different ways and googled the ISR(TIMER1_OVF_vect) part of the sketch to try and understand that part better, but i’m stuck, so that’s the reason i’m writing here.

Maybe you can have any suggestion or explain how this part of the code, where the sineTable is used, works?

Thanks a lot in advance and keep up the good work!

alessandro

You can decrease the sampling frequency to gain more processing time. Keep in mind though that the sampling frequency should be at least the double of the maximum frequency you want to generate (Nyquist theorem).

To decrease the sampling frequency, you can either change the PWM mode to 9 or 10bit PWM mode, thus gaining more time for background processing or you can select a different clock source to TIMER1 instead of using no prescaler. Each possibility has drawbacks.

If I do recall properly, the Atmega168 TIMER1 has 4 clock sources with prescaler: clkio/8, clio/64, clkio/256, clkio/1024. If you use clkio/8, TIMER1 will overflow every 7.815kHz (16000000/8*256) which allow you to reconstruct signals up to 3.9kHz which isn’t that good for audio applications.

With a 10 bit fast PWM no prescaling, the sampling frequency would be (16000000/1024*1) 15.625kHz thus allowing analog reconstruction of signals up to 7.8125 kHz. However this would require the need of 10 bit lookup tables instead of 8bit tables thus increasing the memory footprint.

“If I try to add some more, for example volume, it all breaks due to timing errors.”
You can add a potentiometer at the output stage to control the volume, no need for CPU power for that.

If you need another oscillator, you can use OCR1B to generate a waveform output on OC1B pin, using the same timer. This is a good option for example, if you want stereo output.

“I found that using a prescale lowers the load on the processor, but i cannot see in this code how to keep the waveforms correct.” – I forgot to answer this one.

If you use a prescaler you’re changing the sampling frequency. For the sketch to continue to work properly you need to recompute the dds ‘resolution’ according to the sampling frequency.

resolution = 2^16 * 2^16/ (samplingFrequency)

cheers
Hope this helps,
mouro

Nice work.
Regards,
Patrick