Re: Mixing with a 3000Hz tone @ 8000 sampling rate

A quick guide.

Decide on a lower audio frequency limit you can tolerate, fl (maybe 300
Hz?), and an upper audio frequency limit you can tolerate (around 3000
Hz), fh. Complex mix your signal by -(fl+fh)/2. Lowpass filter the
real and imagnary parts with a filter with a stopband that cuts off
near (fl+fh)/2. Complex mix your signal using a frequency more negative
than -(fl+fh)/2. Take the real part. Listen to it. Should not sound
good (scrambled). Experiment with the frequency of the second mix. Be
aware that if you transmit this over analog lines that they will filter
the signal, which MAY remove the original LF audio components now at
signal HF.

On the receiver, do a similar, but inverse process.

Don't do this with a real mixer.

Dirk

Stef wrote:
> In comp.dsp,
> Shafik <shafik@xxxxxxxxxxxxx> wrote:
> >
> >Im trying to simply do a voice spectral inversion. I sample the voice
> >at 4000Hz, I use an FIR filter to remove everything over 3000Hz, then I
> >multiply with a 3000Hz tone to invert, then filter again to remove the
> >upper side band.
> >
> Shouldn't you have already removed everything above 2000Hz before
> sampling at 4000Hz?
>
> --
> Stef (remove caps, dashes and .invalid from e-mail address to reply by mail)
>
> Now and then an innocent man is sent to the legislature.

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