|Subject:||LF: CW S/N abilities|
|From:||Andy Talbot <[email protected]>|
|Date:||Sun, 10 Jan 2010 23:18:43 +0000|
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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.
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