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Sending a squarewave

I. Mennings

April 04, 2018 12:53

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I am using a DAC+ with a RPi 3B and am creating sounds by reading levels from a textfile and sending them through with pyaudio`s write function.

a need to use the textfiles due to customs sound. but when i try and send out a 1kHz squarewave (made a textfile for that with hard -1 , 1 levels) and i look at the output with an osciloscoop there are harmonics in the signal like the signal is buildup out of sinewaves.

 Does the DAC alter my signal and if so can this be shutdown?

the code I am using

please help

 

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5 comments

• 0
  
  HiFiBerry team April 04, 2018 14:49

  Hi,

  the DAC is an audio DAC that works in the audio signal bandwidth (which is half of the sampling rate - 22kHz in your case). You will never be able to produce a square wave with a DAC that is bandwidth-limited. A square wave would need a unlimited bandwidth.

  Best regards,
  Daniel

• 0
  
  I. Mennings April 04, 2018 16:06

  But if a send a noise like signal, wil I then get the exact signal I programmed out?

  Kind regards

  I.

• 0
  
  HiFiBerry team April 04, 2018 20:36

  Hi,

  it does not send "noise", it interprets your signal as it would be a signal with a bandwitdh of 22kHz. And this would look exactly like what you see. I recommend to ready about the Nyquist theorem and aliasing/antialising when dealing with analog/digital conversions. 

  Best regards,
  Daniel

  Edited by HiFiBerry team April 04, 2018 20:43
• 0
  
  I. Mennings April 05, 2018 08:01

  Hello,

  Sorry I mean that for the end project i need to send out a noise-like signal(superpost on a sinewave) and that needs to be accurate. the squarewave is just a test signal to easily check what my code does. thats also the reason why all signals are entered by textfile instead of using a module with preprogramed signal functions.

  and thank you for the recommendation to read about nyquist and aliasing/antialising i will go and read about that.

  kind regards,

  I.

  Edited by I. Mennings April 05, 2018 08:02
• 0
  
  HiFiBerry team April 05, 2018 08:08

  A noise signal will be accurately reproduced only if it is bandwidth-limited to 22kHz. If you just generate random numbers using a random number generator, your noise might include components of higher frequencies that will result in similar behaviour as you see it with your square wave as the DAC will always interpret it as a signal that does not contain frequencies above 22kHz.

  The Wikipedia page about aliasing might be an interesting first read:

  https://en.wikipedia.org/wiki/Aliasing

  Unfortunately I couldn't find a web source that explains this in a simple way.

  Best regards,
  Daniel