I realize this is an indication of relative signal strength - but relative to what? At first I thought it might be signal to noise ratio. But I never see a positive value (> 0 db), even for local stations. I realize that JT65-HF can successfully decode signals with a negative S/N ratio. However, I don't believe every signal it decodes has a negative S/N ratio. Listening by ear, I know some the tone sequences sound well above 0 dB S/N ratio, and yet they show readings like -6dB. Can someone explain to me what this is really measuring?