How can I generate a smooth continuous sine wave

Question

I am trying to build a single frequency active noise control system to
control a tonal acoustic disturbance. In order to do this I need to be able to
drive a set of loudspeakers with a single frequency waveform which will be
continuously adapting at each sample.
The control system will ideally be implemented to control low frequency
tones, 700-1000Hz, and thus sampling rates of below about 4000Hz should be
sufficient for the task.
However, when trying to continuously generate a waveform at an update rate
of 4000 updates/sec the resulting sine wave output from the DAQ card when
observed on an oscilloscope looks very much like a digital signal (a sine
wave containing large "steps" in the voltage), as if there
has been no
smoothing in the DAC. Playing this through a loudspeaker results in a very
"buzzy" sound and not a single frequency tone. Only when the update rate
is much higher >50000 updates/sec, does the signal start to sound like a
single frequency tone.
I would much rather be able to implement the control system at a sample
rate of 4000Hz than >50000Hz. How do I therefore guarantee a smooth analog
sine wave output at the lower sample rates? Is this a labview problem, or
a hardware problem?

Answer 1

Is this a labview problem, or
a hardware problem?"
Problem?
Well it actually the nature of the beast. As a rough rule of thumb, the signal starts to become smooth when the update rate is at least 10X the highest frequency present in your signal.
The human ear is amazing in its ability to detect (hear) higher frequency components. This is the buzz you hear.
If you want to keep your update rate low, then you may be able to get away with a low pass filter on the output from your DAQ device. An appropriately size cap and resistor may be all you need.
Ben
Ben Rayner
Certified LabVIEW Developer
www.DSAutomation.com
Ben Rayner
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