Disabled CC filter causes problems to start reception

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ok1pt
Posts: 90
Joined: Mon Jul 20, 2020 3:38 am

Disabled CC filter causes problems to start reception

Post by ok1pt » Sun Oct 11, 2020 5:02 am

Hi!
At least in the development release from Oct. 4th, but probably in the earlier versions as well, there is a problem with disabled CC filter (not speaking about the "monitor" mode from the very latest release). When the CC filter is disabled, the following sequence happens when a regular, continuous DMR signal is appearing:
1) The green LED is illuminated.
2) Shortly after, one audio frame is played quickly several times, causing a funny sound
3) A bogus CC number is displayed on the display (obviously CC13 instead of CC1, but it possibly later changes to CC6 or CC5
4) The green LED goes dark.
5) There is a delay about 500 ms.
6) The green LED is illuminated again.
7) Another audio frame is played several times, causing another funny sound (this step is sometimes skipped)
8) The CC starts to indicate the correct CC
9) Received signal starts to play regularly.
The whole sequence takes approximately 3 seconds, thus the first words of the calling person are lost.
All this appears when receiving from the MMDVM hotspot in a simplex mode.
With regards, Pavel

G4EML
Posts: 228
Joined: Sat Nov 16, 2019 10:01 am

Re: Disabled CC filter causes problems to start reception

Post by G4EML » Sun Oct 11, 2020 10:33 am

CC filtering works differently to the other filters. It involves programming the HRC6000 decoder chip to work in a non standard way.
Normally the CC is used by the chip to confirm that it is correctly synchronised to the incoming signal. Turning off CC filtering removes this check, as a result the chip can occasionally briefly synchronise incorrectly.
The behaviour you describe can therefore be expected to happen on some signals (but not all). The only way to improve it would be to confirm in software that the CC was stable, that would result in always missing the first few words instead of just occasionally.

ok1pt
Posts: 90
Joined: Mon Jul 20, 2020 3:38 am

Re: Disabled CC filter causes problems to start reception

Post by ok1pt » Sun Oct 11, 2020 10:52 am

G4EML wrote:
Sun Oct 11, 2020 10:33 am
CC filtering works differently to the other filters. It involves programming the HRC6000 decoder chip to work in a non standard way.
Normally the CC is used by the chip to confirm that it is correctly synchronised to the incoming signal. Turning off CC filtering removes this check, as a result the chip can occasionally briefly synchronise incorrectly.
The behaviour you describe can therefore be expected to happen on some signals (but not all). The only way to improve it would be to confirm in software that the CC was stable, that would result in always missing the first few words instead of just occasionally.
Hi!
Thanks for explanation. I thought about a possibility which do you mention, because it really doesn't happen every time and also the probability is lower when receiving from a regular repeater than from a hotspot. using relatively low-precision frequency synthesizers, so the probability of improper synchronization of the receiver is more probable. So, it's not a bug, it's a feature. For a normal operation, it's better to keep the CC filter turned on.
With regards & 73, Pavel

G4EML
Posts: 228
Joined: Sat Nov 16, 2019 10:01 am

Re: Disabled CC filter causes problems to start reception

Post by G4EML » Sun Oct 11, 2020 11:28 am

Yes, I would recommend only turning off the CC filtering if you really need to.

Originally we thought that it would not be possible to remove CC filtering because it is embedded in the way the decoder chip works. But we managed to find a way of disabling it. This is not the normal way of using this chip so it’s behaviour can be unpredictable.

Colin G4EML

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