Re: [Discuss-gnuradio] bell 202 modulation

[Robert McGwier]

Matteo:

Normally I do not play the game this way but let me suggest that you do 
the following thing so we can get beyond this to give people the correct 
information.

Open up Google.  Type in BEL-202, hit enter.  Please tell me WHOSE NAME 
and DSP assembler code you see on the first several entries and many of 
the remaining entries.  My code and name are not there by accident as 
the most referred to pages in the world on BEL-202.

The data is not Manchester encoded.  It is not split level in the middle 
of the bit.  I think your safest bet is to assume that the people who 
are in this group are not idiots.


100011010... is transmitted as
x10010101...

The Bel-202 standard allows transmission up to 1800 bps.  If it were 
encoded biphase or Manchester, the channel would need to be > 3600Hz 
wide.  This was done back in the ancient days before equalizers, and 
easy DSP so one could not mitigate for channel edges.  There is no split 
phase data in BEL-202.

Take it or leave it.  I have left this conversation as of now.

Bob






Matteo Campanella wrote:
> 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
>
>   


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