Scan speed versus sensitivity

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Scan speed versus sensitivity

Post by VK3KYY » Mon Mar 22, 2021 10:32 pm

The current scan speed is for each channel / frequency to be sampled for 30 milliseconds (DMR simplex is sampled for 60 milliseconds because DMR simplex is only transmitted for 30 milliseconds every 60 milliseconds)

However, because of the way the Rx chip works, the sensitivity of FM signal detection is only around 60% of its maximum if the channel / frequency is only held for 30 milliseconds

Every time the frequency is changed the FM detector output value from the Rx is cleared, and the value slowly increases with a normal C R charge curve, only reaching 100% after 160 milliseconds

This results in the scanning not always stopping on a channel / frequency even if the signal is relatively strong.

The sensitivity is also controlled by the squelch, because the scan will stop only when the squelch is opened.

Even allowing for adjustments to the squelch to improve the scan sensitivity, always using the 30 millisecond step time, is not going to be ideal, as some signals may have fading or have other fluctuations which means that they can't be detected in 30 milliseconds.

So I'm currently investigating adding a setting to allow the scan speed to be be controlled as a setting, with my current test range of 30 ms to 480 milliseconds in 30 millisecond steps.

There the scan step time does not need to be larger than about 160 milliseconds in order that the Rx is at maximum sensitivity, but potentially longer settings may be useful for fluctuating signals.

Which brings me to a question..

Would it be useful to allow very slow scanning e.g. more than 480 milliseconds per channel / frequency ?
Are there any occasions when perhaps holding of the channel for as long as 5 or 10 seconds would be beneficial?

User avatar
W1AEX
Posts: 126
Joined: Sat Nov 16, 2019 9:00 pm
Location: Connecticut, USA
Contact:

Re: Scan speed versus sensitivity

Post by W1AEX » Wed Mar 24, 2021 1:54 pm

Hi Roger,

A user setting option for scan speed is a very interesting idea. Both of my GD77 units are used heavily in scan mode for FM listening (ham, public service, marine, and the local c-med frequencies) and they scan at a rate that is lightning fast, which is a good thing in my opinion. At the current scan rate it does miss calls now and then but on the next pass it will usually stop and catch the call. I have often thought it would be nice to be able to drop the scan speed to possibly reduce the occurrence of that behavior. So... the range from 30 ms to 480 milliseconds in 30 millisecond steps would be an asset in my opinion. I cannot think of a situation with my uses where anything longer than 480 milliseconds would be useful but perhaps others will weigh in on that.

By the way, both of my GD77 units are purring along happily with the 3/21/2021 developmental version.

73 and many thanks, Rob W1AEX

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Scan speed versus sensitivity

Post by VK3KYY » Wed Mar 24, 2021 8:44 pm

Thanks Rob

I will post a test version on github later and put a link here

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Scan speed versus sensitivity

Post by VK3KYY » Wed Mar 24, 2021 9:49 pm

Hi Rob

Try the version in this zip

https://github.com/rogerclarkmelbourne/ ... d_test.zip

(JA versions are Japanese font only)

Scan step speed is in the 'Options' screen

Default is 30 milliseconds


BTW.

VFO Dual watch in this version now operates in power saving mode. However during normal scanning the radio will not enter power saving, as the main power saving is by shutting down both the Rx chip and also the DMR chip, and the DMR controls the frequency calibration signal

User avatar
W1AEX
Posts: 126
Joined: Sat Nov 16, 2019 9:00 pm
Location: Connecticut, USA
Contact:

Re: Scan speed versus sensitivity

Post by W1AEX » Thu Mar 25, 2021 3:23 am

Thank you Roger. It works like a champ! I loaded it just before heading off to bed so I have not given it a real run yet but I tried it with a setting of 210ms and it stopped perfectly with a moderately weak FM signal first thing. The implementation is very slick. I will give it a good run tomorrow with one of my GD77s at the default 30ms and the other using higher settings to note any difference in catching the signal on the first pass. First impression is that the scan step speed option is a nice feature to have.

73, Rob W1AEX

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Scan speed versus sensitivity

Post by VK3KYY » Thu Mar 25, 2021 4:16 am

Rob,

Thanks

It would be interesting to know what step speed / time is needed to give more reliable signal detection.

It could be that 60 or 90 milliseconds is enough.

Anyway, let me know you you get on.

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Scan speed versus sensitivity

Post by VK3KYY » Thu Mar 25, 2021 10:49 am

Rob

I noticed the voice prompts were screwed up in the last zip I posted

This version has a fix for that problem

https://github.com/rogerclarkmelbourne/ ... est_v2.zip

Nothing else has been changed

User avatar
W1AEX
Posts: 126
Joined: Sat Nov 16, 2019 9:00 pm
Location: Connecticut, USA
Contact:

Re: Scan speed versus sensitivity

Post by W1AEX » Thu Mar 25, 2021 3:19 pm

Thank you Roger. I installed the second version and both GD77s are now scanning the same analog zone which has a mix of different public service agencies (police/fire/ems) along with four marine channels. One radio has the 3/21/2021 firmware version that is fixed at the 30 ms scan step and the other has the scan speed option firmware that is set for 120ms. When I am certain that 120 ms is reliably stopping I'll go to 90 ms and try that, then will move to 60 ms the 90 ms option is reliable. One variable with my two radios is that one is vintage July 2018 and the other is about 1.5 years newer vintage December 2019. I guess that might make a difference in how they behave.

Thanks for the opportunity to test this!

73, Rob W1AEX

User avatar
m1dyp
Posts: 601
Joined: Sat Nov 16, 2019 8:03 am
Location: Hertfordshire, U.K.
Contact:

Re: Scan speed versus sensitivity

Post by m1dyp » Thu Mar 25, 2021 4:54 pm

thanks, works great at 90

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Scan speed versus sensitivity

Post by VK3KYY » Thu Mar 25, 2021 9:05 pm

Hi Rob

I doubt the age of the radios would make any difference as neither of them is that old.

Post Reply