Re: Waveform horror! Is a 44.1 kHz sampling rate sufficient?

On Sun, 20 Sep 2009 14:54:26 -0700 (PDT), Michael
<mrdarrett@xxxxxxxxx> wrote:

I used Audacity to generate a 19 kHz tone, planning on burning it to a
CD and playing it in my car stereo for... um... well, let's not get
into that. I then zoomed in on the waveform to take a peek, prior to
burning the waveform. What horror!

http://mrdarrett.googlepages.com/distort.jpg

The waveform is severely distorted. I could see where the points
mathematically
would be correct, but given the sparse sampling points, the speaker
would not be instructed to swing rail-to-rail at, say, between 15.0000
and 15.0005 seconds.

A FFT gives a spread of frequencies centered around the 19 kHz, but,
yuck!

I tried various other frequencies: 9 kHz gives a pretty distorted
waveform as well.

What are the implications as far as accurate sound reproduction at a
44.1 kHz
sampling frequency, as used by CDs?

Michael

An audio player converts the digital samples to voltages using a DAC,
and the DAC is followed by a lowpass filter. The filter effectively -
almost intelligently - interpolates the points into a smooth curve.

Shannon's Sampling Theorem says that if you lowpass filter a signal to
below F/2, sample it at frequency F, and run the resulting samples
through another filter of below F/2, the resulting output signal is a
perfect copy of the input for all signals that make it through the
filters. It looks like hell halfway through, but it works.

Burn your CD and play it through a stereo system and look at the
waveform on a scope. A 19 KHz sine wave will look fine.

John



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