Scan priority

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VK3KYY
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Scan priority

Post by VK3KYY » Sun Apr 04, 2021 12:02 am

There have been some requests to implement priority scanning, where some channels have priority, however there has also been some confusion about what is technically possible.

None of the supported radios have 2 receivers in them, so its not possible to monitor more than 1 frequency at the same time.
Hence if the scan has stopped on one channel, the radio can't monitor another 'priority' channel without the Rx stopping on the current channel for at least 100 milliseconds.

Hence in VFO Dual watch, it would not be possible to apply a priority channel without causing Rx breakup while the radio changes frequency to check the 'priority' channel.

IMO, having the Rx drop out for 100 milliseconds every second, would be quite annoying, and I would definietly not want this happening during scanning.


The other way however that Scan Priority could work, would be for 1 or more channels to be checked more often than the channels which were not marked as 'priority' channels.

In this case, the firmware would effectively maintain 2 lists of channels to scan. One list would contain the channels marked as priority, and the other list for non-priority channels.

This however is where things get complicated.

Imagine that channels 1,5 and 10 are priority, and channels 2,3,4,6,7,8 and 9 are non priority.

One method to prioritise the scanning of the priority channels is to scan channels like this


1,2,5,3,10,4,1,6,5,7,10,8,1,9,2,5 etc

However this works best when there are only a few priority channels e.g. just channel 1, as the scan would look like this

1,2,1,3,1,4,1,5 etc

However, if there are 5 priority scan channels, the degree of priority of those channels would deminish

The alternative stategy is to scan all priority channels, then scan 1 non-priority channel

e.g.

1,5,10,2,1,5,10,3,1,5,10,4 etc



However, whether any form of priority scanning is necessary is quite debatable, as the maximum scan speed is around 20 channels per second, an entire zone can be scanned in around 4 seconds, so only very short transmissions may be missed during scanning.

ok1pt
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Joined: Mon Jul 20, 2020 3:38 am

Re: Scan priority

Post by ok1pt » Sun Apr 04, 2021 5:45 am

VK3KYY wrote:
Sun Apr 04, 2021 12:02 am

Hence in VFO Dual watch, it would not be possible to apply a priority channel without causing Rx breakup while the radio changes frequency to check the 'priority' channel.

IMO, having the Rx drop out for 100 milliseconds every second, would be quite annoying, and I would definietly not want this happening during scanning.
Hi Roger!
Motorola professional radios, which also have only 1 receiver, do exactly this. If you listen to a non-priority channel, they drop RX in regular intervals to scan up to 2 priority channels. I don't know the duration of the silence during priority scan, but I believe it's 60 - 80ms, so 100ms isn't much worse.
The period is configurable in the CPS, if I remember correctly it's configurable between 1 - 5 seconds.
I believe that if the radio will not pop, click or make any other annoying noises during the priority scan, a short silence for every 3 seconds is fully acceptable.

With regards & 73,
Pavel

VK3KYY
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Location: Melbourne, Australia

Re: Scan priority

Post by VK3KYY » Sun Apr 04, 2021 5:51 am

In theory DMR can be detected in 60 milliseconds, but the S/N detector in the RF chip is very slow, and for good detection of DMR 80 to 100ms is essential

And when the radio returns to the original frequency it will again take 60 + milliseconds before the DMR chip re-synchronises, so it could be 200ms before the audio is resumed on DMR.

FM would be better but only by perhaps 50ms.

Probably Motorola radios have better hardware for this functionality.

G4EML
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Re: Scan priority

Post by G4EML » Sun Apr 04, 2021 9:43 am

All the radios I have come across that implement priority scan work as Pavel described. They only have one or two priority channels and those are constantly checked at a preset interval. If the scan stops on a non-priority channel the audio is briefly interrupted every second or so while the priority channels are checked in rotation. If the priority 2 channel has activity it will be interrupted every second while the radio checks the priority 1 channel. In this way a call on the priority 1 channel will always be heard.

For commercial use this is quite important as the priority channel might be an emergency call. For amateur use it is probably less so.

Colin.

VK3KYY
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Location: Melbourne, Australia

Re: Scan priority

Post by VK3KYY » Sun Apr 04, 2021 10:05 am

Colin

Thanks. I miss understood what scan priority normally does.

In that case I'm not sure how much point there would be in spending a lot of time and effort implementing this for Amateur radio.

As these radios are relatively cheap, the better option to for people who need to monitor multiple channels is just to buy another radio.

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