[Elecraft] K3 2.7kHz filter DSP FC

[David Woolley (E.L)]

Paul Christensen wrote:
> Bruce, as Barry points out, the K3's audio response really needs the 
> ability to extend down significantly lower than the current ~ 250 Hz 

> we're not concerned with the broadcast of music in our service, then the 
> male voice (lowest relevant fundamental of 70 Hz) and certain digital

Billions of people are quite happy with the 300kHz cutoff used by the 
public telephone system (a total of 3.1kHz between 300 and 3.4kHz).  The 
critical frequency for speech communications is the lowest formant 
frequency; it is the resonances in the vocal cavity that convey speech, 
not the fundamental.

The lowest formant in normal speech (formant 1 for "u") spans about 200 
to 400Hz, so 250Hz probably is a reasonable compromise; it will include 
the peak and only slightly distort the lower edge.


> modes in SSB mode (e.g., PSK31) may be used to establish the lower 
> response threshold.

When you use digital modes with the receiver configured for SSB, you are 
really using the audio path as a final IF of about 1kHz.  As long as the 
  audio passband is flat across the actual digital signal, it doesn't 
matter what its centre frequency (final final IF frequency) is.

That's basically the strategy used by telephone modems (except 56kbs 
ones, which play tricks with the digitisation of the signal).  The 
faster ones use a carrier of about 1800Hz (nominal channel centre is 
1850 Hz).


-- 
David Woolley
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