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Re: [Discuss-gnuradio] bell 202 modulation

From: Matteo Campanella
Subject: Re: [Discuss-gnuradio] bell 202 modulation
Date: Wed, 29 Mar 2006 09:59:32 +0200 (CEST)
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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

is there any way to do that by using an already made block?


> On Tue, Mar 28, 2006 at 11:51:03PM +0200, 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
> Use the gr.frequency_modulator_fc block to generate complex baseband FSK.
> You'll probably want to use gr.bytes_to_syms to in front of it, or in
> the more general case: gr.packed_to_unpacked followed by
> gr_chunks_to_symbols_bf.
> Eric

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