Paul B. Webster wrote:

>Ken Webster wrote:
>
>> Oh .. actually I just thought about it .. I suppose that XORing with a
>> pseudorandom sequence would probably just add a lot of noise but still
>> allow the voice to be heard if it was loud enough
>
>  I don't think so.  XORing with a PR sequence should be pretty
>effective - at scrambling.  You see, the PR sequence *fills* the
>bandwidth with noise - the desired signal cannot be louder than this, in
>effect the signal modulates the noise rather than the other way around,
>and modulated noise is - still noise!


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.

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).

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?  Then on the
receiver you subtract the PRN and end up with a somewhat noisier version of
the signal you started with.

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?

>
... (snip)
>  In the case of the PRBNG sequences used to hash data, there is a trick
>to resynchronise the register, but I haven't figured it out yet.  Anyone
>seen a good explanation of this?


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 .. when your recovered signal has
a low amplitude (even for just a brief interval) then you must be in phase.

If you can't guarantee that your signal has quiet intervals but you have
some bandwidth to spare then you could group data into packets consisting of
signal sent at a higher sample rate with a period of silence inserted.  Then
when you get your PRN generator in phase on the receiving end you should see
near-silence at regular intervals.

The only "tricks" I am aware of are to try many phases of the PRN sequence
simultaneously, processor time permitting, by keeping a window of the PRN
sequence and trying multiple indexes within the window simultaneously.
Otherwise it could take a long time to find the correct phase if the PRN
sequence is long.

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

Cheers,

Ken