Another Kind of Signal History?

[pauldbnut]

Looking for comments on an idea for a variant of @Simon’s excellent Signal History.

The context is Medium Wave AM BC DX – weak signals – when a carrier can be prominent, but ID relies on detecting the elusive audio.

24-hour Signal History graphs on a target channel have been very informative. However, “SNR” subtracting Noise from Level can read 25 dB or more (dominated by the carrier) without hearing anything at all. That is not a great audibility (let alone intelligibility) predictor, so what could work better for me?

I’m suggesting a narrow-band measure of sideband modulation (excluding carrier), relative to channel-local noise floor just beyond the modulation. That’s a crude analogy with measuring one person’s speech level in a room and comparing with the general hubbub.

For example, “modulation” could be measured from 100 Hz outwards to some chosen limit (e.g. 3 or 4 kHz), and “noise” across possibly 3 or 4 to 6 kHz beyond the centre frequency, depending on neighbouring channel activity. So, the two input parameters might be called (e.g.) “modulation limit” and “channel limit”.

Obviously, for best results, the limits need fine tuning on the day. It would ideal if the noise and modulation pair could be output for upper and lower sidebands separately. Including a value for the carrier would complete the picture.

Because the bandwidths are different, average spectral density per Hz would be a better metric than power.

[pauldbnut]

Forgot to include an example using standard Signal History...
The hope is for a baseline "Modulation-to-Noise Ratio" very much closer to zero, i.e. nothing distinguishable from background noise.

[jdow]

You can implement a test for this using a pair of receivers. I fear that with the average signal quality of hams that you'll have problems with their splatter upsetting your measurements. On the other paw I suspect this morphs into how Simon guestimates SNR.

{o.o}

[pauldbnut]

a pair of receivers

Hi Joanne, interesting. Glad to get some smart feedback.

(1) Not at all sure I get what you mean by "pair of receivers".
A physical pair, dedicated to day-long monitoring is um, er, well, "expensive". Still, going with that...
One SDRC instance Signal History on-carrier with (e.g.) SSB filtered ~0.5 to ~3.0 kHz for "modulation".
Another with offset (e.g.) 4-6 kHz, SSB filtered ~0 to ~2 kHz for "noise".
In principle it could even work with a single SDRC instance on a single receiver, using RX1 and RX2 - except that we have only one "global" Signal History.

(2) Splatter - hams or not in the MW BC band!
That should hardly affect modulation readings. But yes, local "noise" definitely.
It is behind my desire for both USB & LSB measurements, probably using the lower of the two for a final S(M)NR.

(3) Your other (South?) paw is asking for trouble... I'm trying to step sideways from Simon's "X-Meter" approach. Not looking for carrier peaks and noise minima.
The clue is "average spectral density per Hz" which is (?) RMS power in the bandwidth divided by bandwidth.

Honestly, I'm not expecting much to come out of this.
Simon says he's getting into retirement mode (a bit early IMO - ignored that temptation 15 years ago).
And it's probably not going to tickle his fancy, anyway. But you never know

[jdow]

A pair of SDRC receivers. You can receive several frequencies at the same time each in its own receiver. They share the front end but split at the "IF" and allow many receivers each with its own small spectrum and waterfall around it's center frequency.

In the US I have run across AM broadcast stations that should have had the FCC at their doorsteps had anybody complained. (No complaint no FCC. Complain and sometimes the FCC fairy godmother is at her desk and something happens.) And even if the transmitter's IMD products are 50 dB down (damn good) 50 dB down from a really big signal comes in as merely a big signal.

Nonetheless you can compare signal on 10.000 MHz and noise on 10.010 MHz if you wish. You can throw a third receiver (IF and demodulator) at the problem and check 9.990 MHz all at the same time.

Ah Sibelius' second symphony just ended. I can go to bed, now.

{^_^}

[pauldbnut]

Sibelius' second symphony

Haven't listened to that for maybe 50 years.
So treated myself to Orchestre de Paris, Paavo Järvi (lazy YouTube).
No other Finale like it. Thanks.

[jdow]

KUSC.org (Good. Then Jim Svjeda retired suddenly. It's still good but bot as good.)
{^_^}