Performance of electronic Morse decoders in decoding telegraphy masked by noise

The performance of five different electronic Morse decoders in decoding telegraphy signs (Morse code) consisting of 800 Hz pure tones masked by a narrow-band noise with a bandwidth of 185 Hz (1/3 octave 713-890 Hz) centred at 800 Hz and a broadband noise (100-2,500 Hz) was studied at 20, 40, 80, 150 and 200 signs/min telegraphy speed. In steps of 1 dB the S/N ratios were varied from high values where 100% was recognized to low values where nothing was recognized. At narrow-band masking noise with 80 and 150 signs/min telegraphy speed a S/N ratio varying from +8 to +12 dB and +8 to +11 dB respectively was required for 100% recognition. At broad-band masking noise the S/N ratios required for 100% recognition varied from -4 to +3 dB at 80 signs/min telegraphy speed and -3 to +3 dB at 150 signs/min telegraphy speed. When compared to earlier findings in our laboratory concerning human recognition of Morse signs it seems that man outperforms electronic Morse decoders at low S/N ratios and low to moderate telegraphy speeds (< 100 signs/min).