The simplist explanation for this is that if there was only one tone
present (out of the four) , IE in the extreme case, the input to the
modulator was stuck on one symbol value, there demodulator would not
know what tone that was with respect to any of the others, not be able
to derive any relative frequency error, and not be able to track the
symbol clock.
The downside of most scramblers (I dont know what is used here) is that
you can get error propagtion in a scrambler.
One bit in error will often produce multiple bit errors until the de
scrambling shift register is cleaned out.
To reduce this problem, many modems that seek to prevent long runs on a
particular value will force a few transitions every frame or so, this is
built into the scheme, so there is no error propagation, but it does
cost some overhead.
There are a few other techniques but they are the basics
Glen
On 14/06/2018 1:20 PM, Steve wrote:
I played around a bit with the bit scrambler, first to make sure it
was a two-way function, and then by running the sm1000 scrambled
result into FreeDV on my x86.
Without the scrambler, after I was done talking, the frequency error
would waver around +/- and then slide off almost -600 Hz. The SNR
would then drop down and become un-synced. Then when I started talking
again it would correct itself, and the SNR would go back to max and
sync.
With the scrambler on however, the frequency error toggled around +/-
7.5 Hz off. The SNR never dropped down.
FYI
Steve
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