RE: Sound with metal effect

[Manuel Jander <manuel.jander@usm.cl>]

Hi,

On Fri, 2005-04-01 at 09:39 +0200, Giuliano Pochini wrote:
> On 31-Mar-2005 Laurielle LEA wrote:
> 
> > I'm integrating ALSA to my voipclient (softphone) and
> > my problem is the sound received by the remote user is
> > with metal effect.
> 
> VoIP uses 8KHz sampling rate IIRC. What did you expect ?

Metal effect indicates bad antialiasing filters. That may be caused by
bad hardware, bad hardware setup or a very bad software emulation. 8KHz
sound obviously does not sound CD-quality, but it should not have
aliasing noise. It should just sound with lower bandwidth, specially
higher frequencies being cutoff. Standard phone bandwidth is between
300-3400 Hz.
Software resampling through removing or adding samples, and doing some
mean value cooking is far from optimal. It would be better to use a
recursive Z-tranform (or whatever math tool you prefer) synthesised
algorithm. Even a small IIR filter for postprocessing would barely
consume any CPU power. Such algorithms are well known and available.

Best Regards
-- 
Manuel Jander
Dipl.-Ing. Fachrichtung Elektronik



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