Ken Webster wrote:

> Well, if this is true (and it does sound reasonable to me after
> thinking about it), then wouldn't this do the trick?  Simply XOR with
> a pseudorandom sequence on the transmitting end and then XOR with the
> same sequence on the receiving end to cancel most of the noise.

  It will indeed "do the trick", but is impractical for the reasons I
and others have mentioned; it introduces a large high-frequency (wide
bandwidth) component, and necessitates a more reliable link than a DAC
to transmit and ADC to receive.

> Sampling errors would merely add a little noise (or perhaps a lot of
> noise .. I just realized that some small errors could cause the result
> of the XOR on the receiver to wrap around from near zero to near 0xff
> or vice-versa .. depending on how frequently this happens the
> recovered signal could be quite noisy).

  That's right.

> What about limiting the amplitude of the voice signal to a signed
> 7-bit value and likewise limiting the amplitude of the PRN to 7-bits
> and adding the two (rather than XORing) so that the noise is
> superposed and never causes low values to wrap around to high values
> or vice-versa?

  If you think about it, adding and XOR-ing are almost the same.  But
the real problem is still that it is the PRBS *itself* that contains
these "wrap-arounds" (rapid change of MSB) already.

> If large errors are introduced by the bandwidth limits of the channel
> (limits on slew-rate, etc.) then couldn't you run the PRN through a
> lowpass filter and still get good results?

  If you low-pass it and combine with the signal, it will no longer
fully mask the signal. :)

> If you did the above (adding PRN at transmitter and subtracting it at
> receiver) then the signal amplitude should be continuously very high
> unless you are synchronized.  As long as your original data has some
> silent or quiet parts then you should be able to slew the phase of the
> PRN at the receiver until the average amplitude drops

  That's close to the method.  I think there's something sneaky in the
maths; if you XOR (auto-correlate) the PRBS with itself out-of-phase,
the resultant has, I suspect, some interesting properties.

> This sounds fun to try!  I'd be interested in hearing about the
> results if anyone tries this.

  The theory has as I say, already been nutted out.  I'd just like to
see a good article...
--
  Cheers,
        Paul B.