[Asterisk-Dev] Re: Handling AST_FORMAT_SLINEAR at 48000Hz

[Tony Mountifield]

In article <20051117024659.E88354 at xorpc.icir.org>,
Luigi Rizzo <rizzo at icir.org> wrote:
> On Thu, Nov 17, 2005 at 10:28:47AM +0000, Tony Mountifield wrote:
> > 
> > The low-pass filter sounds complicated. How would that compare with, say,
> > taking the mean average of every six 48kHz samples to produce one 8kHz
> > sample?
> 
> it's just not the same thing and will give rise to all sort of
> frequency aliasing effects - those that the lowpass filtering is supposed
> to remove.
> quick example: consider a 9khz signal - that is clearly not audible
> on a 4KHz channel (8000 hz sampling rate). Yet if you sample it at
> 48khz and take the average of every 6 samples, these averages will not
> be constant (because the 9khz signal does not have a period of 6 samples)
> and so you'll get an output signal that will appear as 1KHz.
> This is the aliasing effect.

That makes sense - thanks for the explanation!

Cheers
Tony
-- 
Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
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