Re: [Discuss-gnuradio] bell 202 modulation

[Eric Blossom]

On Wed, Mar 29, 2006 at 09:59:32AM +0200, Matteo Campanella wrote:
> bell 202 coding requires to send a steady tone (mark or space depending on
> the previous state) on 1 and a tone change on 0, so the tone change must
> happen in the middle of a bit.
> 
> in order to use a modulator I guess I should use a block to get a sequence
> of symbols that is twice as long as the bit sequence, that should be
> "played" twice as fast as the bit stream sequence, where bits at zero are
> represented by 1,-1 (or -1,1) and bits at one are represented by -1,-1 (or
> 1,1).
> 
> is there any way to do that by using an already made block?
> 
> Matteo

I don't have the spec in front of me, but this sounds like DPSK.

Use the same modulator, but differentially encode prior to
modulation.  You'll need to code up the differential encoder and
decoder. They are trivial.

"Digital Signal Processing in Communication Systems", by Marvin
E. Frerking, ISBN 0-442-01616-6 gives several ways to mod and demod
DPSK. 

Eric