SDS100/SDS200: Decoding speed or signal strength?

[Facsimile]

I have a SDS200 and DSDPlus FL. They are both connected to a multicoupler and therefore the same antenna. I have locked both receivers on to the same P25 talkgroup and site. More than occasionally, DSDPlus will start outputting conversation before the SDS200. Other times, the SDS200 will output conversation before DSDPlus. When I experience this delay, it's usually three to four seconds and most of the time enough to only hear one side. Is this due to signal strength? My SDS200 RSSI usually reads about -70 on the control channel but once the transmission starts, it will jump to -95 +/-5. Why does this happen? Shouldn't the signal strength be the same if it's coming from the same site?

Curious as to everyone's thoughts.

Thanks

 

[jtwalker]

I don’t have an answer for you, but just to clarify that going from -70 to -95 is a fairly significant drop in signal strength. Maybe signal strength changes from control channel to voice channel? Not sure, just throwing that out there as a possibility.

 

[dave3825]

Two different objects, one that relies on pc to decode, and the other, uses it’s hardware and software to decode. Both process in the same manner but different hardware/software in both will behave slightly differently and at different speeds based on resources being used at the time. Window does stuff and is idle. My dongles are sometimes quicker than the scanners and vice versa. As for as the 200 rssi levels, I notice more fluctuation on the scanner then I do watching the spectrum in DSDPlus. The levels of the control and voice channels are usually never exactly the same. My local system, some voice channels peak, significantly higher than others even though the license states they all output, the same amount of power. It could be due to the different frequency wavelengths but I’m not an expert.

 

[Ubbe]

Curious as to everyone's thoughts.

If you state what system and what site it might help. Then someone that can monitor the same system can check for you what happens with the signal strength.

It's common to divide up transmitters to several antennas and something like 4 channels to each antenna can be expected, so if the site use more than 4 frequencies then the signal strength can shift depending of the channel and antenna used as you can have reflections from one antenna that happens to cancel out the signal more than from the other antenna. In most digital system they have several receive antennas some 10ft from each other configured to be used in a diversity system that picks the best antenna and will usually increase sensitivity some 6dB from mobiles/portables. Radio waves works in the same way in the other direction, from the site to your receive antenna.

For SDS scanners they use an automatic gain system, that cannot be disabled, that reduce the gain and show lower signal strength if they see too high signal levels in a 10MHz window, the channel width for TV and satellite signals that the receive chip are designed for.

Having a 3-4 sec delay from one receiver are not normal when two receivers monitor the exact same control channel. For Uniden scanners they can have a tendency to ignore control channel information occasionally but not doing it for 3-4 seconds and certainly not happening to DSDPlus. It sounds as if the two receivers are monitoring two different sites. But I'm sure you have double checked that both receivers are locked to the same frequency, so it has to be something else going on.

/Ubbe

 

[nessnet]

I assume the dongles are RTL-SDRs?
The important thing to remember, is that with RTL-SDRs, you can crank the gain way up - IE: you can control it. With the Uniden SDS radios, as Ubbe said, the gain is automatic and due to the overly 'wide' chip they use, the AGC can possibly kick in and bring the sensitivity down.

Also, double check the SDS to make sure it is on just one control channel.
Remember, you scan sites, not trunk groups.

Lastly, maybe try SDRTrunk.
No batch files to $#@! with and it may have a better P25II decoder(?)

 

[dave3825]

Also, double check the SDS to make sure it is on just one control channel.
Remember, you scan sites, not trunk groups.

So when scanning a single site, and locked on a cc waiting for a voice grant, what’s the 200’s rssi displaying? The cc or the voice channel it switches to?

 

[nessnet]

So when scanning a single site, and locked on a cc waiting for a voice grant, what’s the 200’s rssi displaying? The cc or the voice channel it switches to?

Both.
When 'listening' to the control channel, it displays the RSSI of the control channel.
When a channel grant comes in and the radio then tunes to that voice freq, that is the RSSI displayed.

Whatever the tuner is tuned to at the time.

 

[dave3825]

That’s exactly what I thought. Just sucks on systems whose voice channels are either different wattages or degraded due to reflections.

 

[Facsimile]

Thanks for all of the info everyone. I wanted to follow up with the answer specifically for this case. I used a SDS100 and got much closer to the transmitter and subsequently better reception. I had zero instances of delay over the course of 30 minutes. I might just be on the edge of the site I'm trying to listen to on my SDS200.

 

[nessnet]

Thanks for all of the info everyone. I wanted to follow up with the answer specifically for this case. I used a SDS100 and got much closer to the transmitter and subsequently better reception. I had zero instances of delay over the course of 30 minutes. I might just be on the edge of the site I'm trying to listen to on my SDS200.

Set your filters on that site to off (if not already) on the 200 and see if it makes any difference.
If it is a distance/attenuation issue.

If it is the AGC kicking in on the 200, (you have a nearby source of RF), then fiddling with the filters may give you better results.

 

[Ubbe]

-70dBm are not on the edge of the coverage. -110dBm are.

-95dBm are are also a good signal, most of my monitoring are at that level and the strongest signal that I monitor are -80dBm.

Uniden uses a preamplifier in their SDS scanners. The SDR receiver used, as also most dongles, are not sensitive enough and have a high internal noise figure and they add some 6-10dB gain so should receive better than a SDR dongle.

You can see how a SDS receiver works from my tests. SDS100/SDS200: - Global Auto Filter Vs. Global Attenuation

/Ubbe

 

[Facsimile]

Set your filters on that site to off (if not already) on the 200 and see if it makes any difference.
If it is a distance/attenuation issue.

If it is the AGC kicking in on the 200, (you have a nearby source of RF), then fiddling with the filters may give you better results.

Filters are set to off as well as the global filter. I imagine there is a ton of RF interference around me. Three monitor set up as well as computer. sitting between is the SDS.

-70dBm are not on the edge of the coverage. -110dBm are.

-95dBm are are also a good signal, most of my monitoring are at that level and the strongest signal that I monitor are -80dBm.

I just verified - my control channel sits at -78 dBm. When there are transmissions, the dBm will always jump to -102 to -105.

 

[IAmSixNine]

I was going to suggest you play with the filters on that system/site and see if that helps but i believe that was already mentioned. I had to move away from global filter as some systems around me work better with wide normal and others with invert. But ultimately i do find that having filters on and testing them always work better (for me) than having them off. But this is part of the fun of owning these. Tweaking them till they work great.

 

[Facsimile]

I was going to suggest you play with the filters on that system/site and see if that helps but i believe that was already mentioned. I had to move away from global filter as some systems around me work better with wide normal and others with invert. But ultimately i do find that having filters on and testing them always work better (for me) than having them off. But this is part of the fun of owning these. Tweaking them till they work great.

Is there a general rule to follow for filters on the SDS or is it truly just random trial and error, then just pick the best?

 

[nessnet]

Is there a general rule to follow for filters on the SDS or is it truly just random trial and error, then just pick the best?

Filters are 100% dependent on what the RF environment is in a specific location - what works one place may not work in another.
So, the only way to see which one works (if any work), is to experiment.

There have been great filter tutorials posted here in the past.

 

[Ubbe]

So, the only way to see which one works (if any work), is to experiment.

And don't forget that after trying all filter settings to set IFX to the frequencies with issues and then try all filter settings again. Filters are set per site or conventional department but IFX are set per frequency and are global, it sets IFX to whatever system that uses that frequency.

Most Uniden scanners has two IF filters and then has a IFX function, the received frequency, lets say 850MHz, are mixed down to a lower frequency that can be something like 360MHz or 280MHz and IFX then force a switch between those frequencies and can totally change how the scanner handles interferences.

/Ubbe

 

[JoeBearcat]

going from -70 to -95 is a fairly significant drop in signal strength. Maybe signal strength changes from control channel to voice channel?

That is probably the case. Remember that trunked systems often use the same antennas for several transmitted frequencies and the pattern of the antenna relative to any particular frequency may be different from any others. Then you add things like reflections into the mix and a widely varying mix of signal strengths is possible - especially the farther you get away from the site. If you move the antenna a foot or two you may find the exact opposite relative strength. It's also possible the voice channel is using a lower power (intentionally or due to wear).