Thank you Patrick for the explanation. No doubt the mode has solid theoretical resons for what it edits. Intercomparing various digital modes has some merit-- like for a Ph.D thesis or marketing.
However as a user, the present value edited isn't a number that is too useful. It apparently tells me nothing about how far below the RX noise the signal I'm copying really is. That's unfortunate. Making contacts with large -db values seems to overstate the mode's capabilities in this regard. It's kind of like the expensive receivers which show S9 signal levels for weak stations-- making the purchaser happy he spend the extra money. Then later he finds out that the manufacturer has calibrated his S meter in 3db (or less) increments and his receiver is perhaps only marginally better. For me these 6 + minute/QSO's are agony. I'm really after some indication of what the agony is really buying in signal reception. 73 de Brian/K3KO --- In digitalradio@yahoogroups.com, "Patrick Lindecker" <[EMAIL PROTECTED]> wrote: > > Hello brian, > > The S/N referenced to a bandwidth is used to compare modes under a noise environment criteria. > A minimum S/N of 0 dB means that with an equal power (let's say 1 watt) of signal and noise (noise distributed over a 3 KHz band so with a density of 0.33 W/KHz), the signal transmitted will be decoded. > In an other mode with a minimum S/N of -10 dB, you will need only 0.1 Watt of signal for 1 Watt of the same noise to decode the text transmitted, and so on. > > >In fact, if I'm interested in hearing really weak signals, using the 2.1KHz filter allows weaker CW signals to be heard -- > The ability to decode a weak (CW) signal is a psycho-acoustic problem. As far as I know, reducing the bandwidth helps down to a limit (perhaps 500 Hz?). > > 73 > Patrick > > > ----- Original Message ----- > From: Brian A > To: digitalradio@yahoogroups.com > Sent: Saturday, April 21, 2007 2:07 PM > Subject: [digitalradio] Re: S/N Multipsk figures -- JT65A vs Olivia and others > > > I'm perplexed by the edited dB figures. > > On JT65A HF it doesn't make any sense that the values are -5 or -6 db > when the signal is strong and moving the S-meter to s5 or s6. > > Here we use a 300Hz filter and the audio output is adjusted to read > about 0db with no signal. What I would have expected is the db value > would be referenced to this receiver noise floor value. > > Secondly, I can hear and copy the CW ID at edited values of -20db or > so. That also makes no sense. This threshold should be around -10db > or so below the RX noise floor. This audible threshold is pretty much > independent of whether one uses a 2.1 KHz filter or the narrower 300HZ > filter. In fact, if I'm interested in hearing really weak signals, > using the 2.1KHz filter allows weaker CW signals to be heard -- > presumably due to less attenuation in the wider filter. This only > works of course if there is no signal within the filter passband which > starts AGC action. > > AVC use no doubt confounds things for stronger signals. However, > there is no option on the IC706 to turn of AGC. Audio output is pretty > much linear (as per ARRL BPL studies) below the AGC threshold. I > estimate that threshould to be about S2 for the 706. > > So just what does the edited db value mean? It certainly does not > represent how far the signal is below the RX noise floor. > > Since RX gain is unknown, it can't represent some absolute value of > voltage. > > 73 de Brian/K3KO > > --- In digitalradio@yahoogroups.com, "Patrick Lindecker" <f6cte@> > wrote: > > > > Hello to all, > > > > Comparizon with S/N Multipsk figures. > > > > According to JT65 specifications, this mode decodes with few errors > down to -23 dB, with a normalized band of 2.5 KHz. > > All Multipsk figures are normalized with a band of 3 KHz. -23 dB in > 2.5 KHz is about -24 dB in 3 KHz band (-23.792 dB exactly). > > > > This figure of -24 dB can be compared to Olivia 250-8 which has a > minimum S/N of -14 dB. So JT65 is 10 dB better or 10 times better. > > But of course JT65 is much slower that Olivia 250-8. > > > > The only modes which are close to JT65 are: > > * THROBX: Lowest S/N: -18,5 dB for the 1 baud, -17.5 dB for the 2 bauds > > * PSKAM10: Lowest S/N : -19.5 dB > > > > In conclusion JT65 is better (under S/N criteria) that any modes in > Multipsk. > > > > 73 > > Patrick > > > > > > > > > > ----- Original Message ----- > > From: Tony > > To: digitalradio@yahoogroups.com > > Sent: Saturday, April 21, 2007 9:25 AM > > Subject: [digitalradio] Path Simulator tests -- JT65A vs Olivia > and others > > > > > > All: > > > > I used Pathsim to compare the sensitivity of JT65A > > vs MFSK, PSK31 and OLIVIA using AWGN to alter the > > SNR. I ran direct-path with no ionospheric > > disturbance. > > > > The chat modes decoded with error-free print down > > to -12 to -14db SNR. The JT65A mode decoded > > at -27db SNR (signal inaudible). > > > > Assuming the Pathsim white noise measurments were > > accurate, I think it's safte to say that JT65 is > > capable of decoding much weaker signals than the > > others. Would be interesting to see how it does > > with simulated ionospheric disturbances. > > > > 73 Tony - KT2Q > > >
