I have not followed the entire thread of this discussion though the mention of
'scrambled audio' leads me to suggest the following.

If you want a VERY simple scrambling system, then do a simple 'frequency
inversion'. The simplicity of this is that the encoder is the decoder as well!

The other great thing is, that as long as you are not bothered by changes in
phase of the audio, then just take an ADC reading and multiply every other
sample by -1 and this will frequency invert the signal. I would guess a PIC
could manage this even on fairly high bit rates.

regards,
SW.

____________________Reply Separator____________________
Subject:    Re: interesting concept
Author: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
Date:       30/03/99 09:30

Guy,

That seems like a good idea.
The problem is that you DON'T have a "perfect" analog channel. What will you
do with noise? That will affect the transmitted signal. The receiver will
have a different signal than the transmitter intended.
In "normal" telephony, you may hear "clicks" or other strange noises, but
that is not critical, only annoying.
But if you transmit "scrambled" audio, then what will happen if the signal
is altered in transit? You will FFT, descramble and reverse FFT a different
signal, and I think that the output from that will be meaningless...

Sorry to play the devil's advocate...

Regards,
Andres Tarzia
Technology Consultant, SMART S.A.
e-mail: atarzia@smart.com.ar

-----Original Message-----
From: Guy Sirton [mailto:dansoft@MAIL.INTER.NET.IL]
Sent: Tuesday, March 30, 1999 10:57
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: interesting concept


Hi,

[...deleted...]

I've had this idea regarding voice encryption and since I'll probably
never do anything with it here it is:
1. Sample a voice segment
2. FFT
3. permutate the result according to your key (shuffle it around)
4. Reverse FFT
5. Output
6. Generate next key.

The resulting signal is still within the same band (i.e. no out of
band frequencies are added), synchronization can be achieved either
by timing from the beginning of the session or by "marking" each
segment with some frequency domain code.  Decoding is ofcourse the
reverse permutation.

Anyone got code for FFT on a PIC?

What do you think?

Guy - mlsirton@inter.net.il