LF: CW S/N abilities

[Andy Talbot <andy.g4jnt@googlemail.com>]

A serious appeal to all the CW and QRS(S) ops on this reflector.  

 

I'm interested in what can really be achieved in terms of readability of weak CW .  For the calculations mentioned earlier I made the assumption the best listeners can read 20 - 30WPM in 10dB S/N in a 30Hz bandwidth, without repeats, which equates to -9dB S/N in the standardised 2500Hz. It is possible to detect a tone significantly lower than this, (I can positively detect a 600Hz tone at -25dB S/N in 2500Hz, but wouldn't know immediately if it was switched on/off)  but first-time readability is essential.   If anyone can measure S/N accurately and have measured your CW receiving capability, it would be interesting to get confirmation or otherwise that this assumption is not too-far-out. 

Same applies to QRS(S) although this I suspect is less amenable to any additional processing gain from learning and experience.

 

S/N can be determined from any spectrogram software offering  a spectrum rather than waterfall display.    Note the height of the tone spike above the average noise level;  this is the S/N in the bandwidth of the FFT Bin   Determine this value if it is not stated as a 'resolution' or any similar name. 

If not given specifically, resolution BW can be calculated from Sampling rate divided by FFT size.  Eg 11025Hz sampling with 4096 point FFT gives a resolution of 2.7Hz.

The normalised 2500Hz value is then obtained from S/N [2.5k] =  S/N [measured]   -  10.LOG (Resolution BW / 2500).  

If you can't measure S/N, record the marginal you've 100% copied and I will measure it.

 

Please be honest about this.  If some CW experts clain they can out-perform any appliance operator, lets see the figures that prove it.

Andy
www.g4jnt.com