On Monday, June 16, 2014 04:19:04 you wrote:
> Hi,
> 
> The attached file contains the results of an experiment in recording
> signals of short duration.
> 
> Comments welcomed.
> 
> Regards, Edgar
> Moonah, Tasmania.

Interesting experiment. Two sudden ideas...

1- Usually NDB are AM modulated, so you can find a continuous carrier with two 
keyed sidebands +- about 1000 Hz. Carrier and sidebands are phase correlated.

2- The keyed pattern is periodic at defined timestamps (but some NDB are keyed 
by a mechanical keyer and the period may have some jitter). 
Using multichannel averaging software on the recorded .wav file you can add 
many correlated sound pattern with uncorrelated noise. 

http://123.physics.ucdavis.edu/week_4_files/narrow-band.pdf

May be Paul Nicholson vlfrx-tools can do something.

Regards
Claudio IK2PII



 
 
 
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