Which tone comes first?

Which tone comes first?

[Arno Verhoeven]

Hi,

With 1200bd AFSK NRZI a logical 1 is transmitted as a constant tone, and 
a logical 0 as a change in tone.
I'm wondering which tone comes first when we start a sequence. Should it 
start with 1200 Hz or start with 2200 Hz? Or doesn't it make any difference?

-- 
73, PE1ICQ  Arno Verhoeven
--------------------------------
AX.25  pe1icq@pi8zaa.#nbo.nld.eu
smtp   pe1icq@pi8zaa.ampr.org
inet   pe1icq@amsat.org 

Re: Which tone comes first?

[Dave Platt]

> With 1200bd AFSK NRZI a logical 1 is transmitted as a constant tone, and 
> a logical 0 as a change in tone.
> I'm wondering which tone comes first when we start a sequence. Should it 
> start with 1200 Hz or start with 2200 Hz? Or doesn't it make any difference?

It really doesn't matter.  Use whichever you wish, or switch
deterministically or randomly on each individual packet transmssion.
At the receiving end, all "knowledge" of the actual tone being
transmitted is wiped out by the first stage of the NRZI detector,
which simply compares current-tone to previous-tone via an XOR
arrangement.

The only time it _might_ make a difference is if you're using a
sideband transmission system, and want to set up a convention that the
first tone burst is at one frequency or the other so as to allow for
automatic per-packet fine tuning.  I don't know of anyone who actually
does things that way, though.  On FM packet, this doesn't apply, since
the modulation is symmetrical around the carrier frequency.

Re: Which tone comes first?

[Arno Verhoeven]

On 2006-06-11 21:30, Dave Platt wrote:
>> With 1200bd AFSK NRZI a logical 1 is transmitted as a constant tone, and 
>> a logical 0 as a change in tone.
>> I'm wondering which tone comes first when we start a sequence. Should it 
>> start with 1200 Hz or start with 2200 Hz? Or doesn't it make any difference?
>>     
>
> It really doesn't matter.  Use whichever you wish, or switch
> deterministically or randomly on each individual packet transmssion.
> At the receiving end, all "knowledge" of the actual tone being
> transmitted is wiped out by the first stage of the NRZI detector,
> which simply compares current-tone to previous-tone via an XOR
> arrangement.
>   
Thanks. That's what I thought. But I'm having some problems with a 
particular setup using a PIC to genrate the tones. I will have to play 
with it some more...

-- 
73, PE1ICQ  Arno Verhoeven
--------------------------------
AX.25  pe1icq@pi8zaa.#nbo.nld.eu
smtp   pe1icq@pi8zaa.ampr.org
inet   pe1icq@amsat.org 

Re: Which tone comes first?

[Kevin Dawson]

On Sun, Jun 11, 2006 at 09:41:30PM +0200, Arno Verhoeven wrote:
> On 2006-06-11 21:30, Dave Platt wrote:
> >>I'm wondering which tone comes first when we start a sequence. Should it 
> >>start with 1200 Hz or start with 2200 Hz? Or doesn't it make any 
> >>difference?
> >
> >It really doesn't matter.  Use whichever you wish, or switch
> >deterministically or randomly on each individual packet transmssion.
> >  
> Thanks. That's what I thought. But I'm having some problems with a 
> particular setup using a PIC to genrate the tones. I will have to play 
> with it some more...

Some years ago, I had a project using a CML modem and found that it
needed a few transitions for bit sync.  It didn't matter what was
sent - just the timing was needed.  That may or may not be the problem
in your case, of course, but if you can arrange for it, it might help.

Kevin