Teensy 2.0 PWM frequency

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rbino

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Hi all, this is my first post!

I've received today two Teensy 2.0 and with one of them I want to make a USB Midi to CV to play my homemade synth from my pc.
I've already tried (and succeeded) to do that with Arduino, and I had to do this
Code: 
TCCR1B = TCCR1B & 0b11111000 | 0x01;
in the setup so the PWM was ultrasonic and I could low-pass filter it.

Now, I searched but I haven't found the way how to do it on Teensy beside the "beta" function to set the PWM frequency- but will it work even on Teensy 2.0?
If it does not, how can I set the timer frequency? And which Timer controls what, both pin-wise and software-wise (like "delay" or so)?

Thanks in advance for the answer!
 
On Teensy 2.0, right now the TimerOne and TimerThree libraries are probably the simplest way to set the PWM frequency.

There's 3 main problems with using PWM to synthesize analog output.

1: Power supply noise couples directly to the output. Real DACs have a stable reference voltage. They typically have a "power supply rejection ratio" spec, indicating how much the power supply can impact the output output. Typically it's -60 dB or less.

2: PWM has limited resolution. On 16 MHz AVR, usually you'd use 8 bit PWM to get a 62.5 kHz carrier.

3: Significant filtering is needed to remove the carrier. Usually it's very difficult to remove the carrier without restricting the bandwidth. Maybe it's ok to leave some of the carrier in the audio signal for some applications? Certainly that's not ideal. A real DAC has very little high frequency output to filter away.

Still, it's possible to get voice quality audio output from PWM.

With a DAC, you can do better, but beware of spending all your time updating the DAC. You probably want one that can work with SPI, which is faster than I2C.
 
Thanks for the answers!
So, if I understand correctly, if I want a 40kHz PWM with the Timer1 library, I have to do something like
Code: 
void setup{
...
Timer1.initialize(25);
Timer1.pwm(pin, duty);
...
}
And then use
Code: 
Timer1.setPwmDuty(pin, duty);
To change the duty in my program. Is it correct? Can the 3 pins controlled by Timer1 have different PWM duties?

Regarding the DAC, I don't think this is necessary, I already tested this kind of thing with an Arduino and a PWM at about 32kHz, low-pass filtered at 100Hz; the carrier was not audible in the CV and a settling time of 10ms is good enough for my purposes.

Edit: Maybe I wasn't clear enough, my purpose is not making audio with the PWM, but controlling an analog synth. So I don't need a very large bandwidth, it's only a control voltage.
 
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