Re: Name that modulation...

John Hadstate wrote:
On Jul 17, 9:06 am, cb...@xxxxxxxxxxx wrote:
On Jul 16, 4:57 pm, "John E. Hadstate" <jh113...@xxxxxxxxxxx> wrote:

Start with logic 0s and 1s. Convert them to pulses that look
roughly like Gaussian bell curves each going from near-zero to
maximum to near-zero in one bit time, 0s going negative, 1s
going positive. Use said negative-and-positive-going pulses to
frequency-modulate a carrier.
What is the name of this type of modulation? If my description
of the mechanics of this modulation scheme is not quite right,
what is the name of the modulation that would produce the same
apparent result as described above?
John,

The other answers are good, but i would be a bit more general. In my
mind what you describe is
a subset of the CPM continuous phase modulation family. Assuming that
the 0s and 1s you describe are actually being mapped to a binary +/-1,
which is evident when you describe negative and positive going pulses,
then all that remains is for you to describe exactly the frequency
pulse g(t) which for CPM can be GMSK, Raised cosine, shaped raised
cosine (which all give you the Gaussian shape-like curve you
describe). You then drive the output (with suitable scaling) through
a freq modulator.

The resultant for single h is

s(t,a) = exp(j * phi(t,a) )

where

phi(t,a) = 2 pi h sum_i a_i q(t-iT) and q(t) = int_{-infty}^t g(b) db

and a_i are the binary mapped symbols

I guess all i would do is define the freq pulse shape g(t) that you
observe, and you have everything else.

col

Thanks. Given the noise and distortion, I can't really tell whether
the original frequency-modulating pulses are Gaussian, sine-squared,
or some other similar shape. Mainly, I was looking for acronyms like
GMSK, SFSK, etc. that point in the right general direction.

The positive and negative fixed-amplitude modulation would be ordinary FSK if the transitions were abrupt. Slowing the frequency transitions by shaping the pulses reduces out-of-band splatter. Modulating with square pulses and bandpass filtering the modulated carrier will produce the rounded pulses you observe upon demodulation. Make the bandpass filter narrow enough and you have MSK.

Jerry
--
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