Re: [Discuss-gnuradio] bell 202 modulation

[Marcus Leech]

Matteo Campanella wrote:
> Hello, I am trying to find the best way to implement a bell 202 modem in
> gnuradio. The problem I am facing is that I need to produce two tones, one
> for mark and one for space, and I have to switch from one to other
> depending on which symbol I am sending. The duration of the tone (or tone
> switch) must be set exactly according to the baud rate of 1200 I have to
> produce.
>
> I was thinking to use the nco class to generate the tones, and to output n
> samples per symbol at a certain fixed sample rate that would be enough for
> the 1200 baud, let´s say the standard audio 48000.
>
> Has anyone better suggestions to work this out?
>
> MC
>
>   
There's a dialtone example.  The relevant lines are:

       src0 = gr.sig_source_f (sample_rate, gr.GR_SIN_WAVE, 350, ampl)
       src1 = gr.sig_source_f (sample_rate, gr.GR_SIN_WAVE, 440, ampl)
       dst = audio.sink (sample_rate, options.audio_output)
       self.connect (src0, (dst, 0))
       self.connect (src1, (dst, 1))

I think what you'd want to do is stick a "multiply" in the signal chain 
for each
 generator, and then combine the outputs of the generator with an "add".
 Generate two different copies of your bitstream, one which is 
"straight up", and
 the other is inverted.  Feed this to the multipliers on each signal path.

Effectively, the ones and zeros turn on and off the relevant signal 
sources by
 multiplying them by one or zero.