Re: [Discuss-gnuradio] bell 202 modulation

[Matteo Campanella]

100011010 should become
  1        0         0           0      1    1       0        1       0
space-space,mark-mark,space-space,mark-mark-mark-mark,space-space-mark,space

with the arbitrary decision on the first bit, that could be either space
or mark, as there is no previous state. space is 1200Hz and mark is
2200Hz.
I have already encoded this on a DDS based on PWM on a PIC 16F628, but I
had control over REAL time there.
I have just written a block to do the differential encoding, that is, to
obtain the correct -1,1 sequence, but my doubt is how to apply this to the
freq modulator in order to obtain exactly the two tones I need for mark
and space, and the correct bit timing, as I do not have control over
time...

MC

> Bel 202 does NOT use Manchester  encoding.  The individual bits are NOT
> sent bilevel but opposite polarity.  Bel 202 very much as energy at DC
> after demodulation which Manchester would eliminate.  It does have
> differential encoding but that is not Manchester.
>
> 100011010.....
> would be transmitted as
> ?10010111....
>
> This is nothing to do with biphase elements.
>
>
> Bob
>
>
>
> 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
>>
>>
>>> 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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>>
>>
>
>
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