Cyclic Background Signal on Trunking System

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 5:44 am

On a trunked radio, what would cause a cyclic background signal to be heard on one trunked system but not another?

Bill_G · Post by **Bill_G** » Mon May 02, 2011 5:48 am

Elaborate.

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 6:17 am

Bill_G wrote:Elaborate.

Trunked system 1: communications come up on a talkgroup. In the background you can hear, at a very noticeable level, a flutter that sounds like a subaudible tone. This is most noticeable when someone stops talking and before the carrier drops. It happens on all talkgroups.

Trunked system 1: there is no flutter / subaudible tone in the background on any talkgroup. All you hear on each talkgroup is the normal audio.

Both trunked systems are Smartnet systems.

Bill_G · Post by **Bill_G** » Mon May 02, 2011 7:15 am

So, you are monitoring two systems. Is system one simulcast, and are you within the overlap of two or more sites? Is system two non-simulcast, or is it simulcast but you are well within the footprint of a site?

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 8:09 am

Bill_G wrote:So, you are monitoring two systems. Is system one simulcast, and are you within the overlap of two or more sites? Is system two non-simulcast, or is it simulcast but you are well within the footprint of a site?

No, neither system is simulcast. I had only the one system programmed at first (the one with the problem), and programmed the second system (the one without the problem) as a test to see if the problem existed there as well. These are two entirely different systems in different locations.

Bill_G · Post by **Bill_G** » Mon May 02, 2011 8:44 am

Will another receiver exhibit the same growl on the same system?

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 11:28 am

Bill_G wrote:Will another receiver exhibit the same growl on the same system?

I don't have another of the same model of radio that I can test, but there is no problem with a different model radio that I tested. I'm wondering if what I'm hearing is the connect tone bleeding through.

Bill_G · Post by **Bill_G** » Mon May 02, 2011 2:34 pm

So, a different model of radio hearing the same system does not have the growl. I think I agree with you that you are hearing the connect tone, but it may be isolated to just this one portable. Most systems use 105.8hz, but the one in question may be using something different, and for some reason the high pass filter isn't working well enough to keep it out of the receive audio.

jackhackett · Post by **jackhackett** » Mon May 02, 2011 4:11 pm

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 4:14 pm

Bill_G wrote:So, a different model of radio hearing the same system does not have the growl. I think I agree with you that you are hearing the connect tone, but it may be isolated to just this one portable. Most systems use 105.8hz, but the one in question may be using something different, and for some reason the high pass filter isn't working well enough to keep it out of the receive audio.

Thanks. I don't really think that it's a high pass filter problem because the connect tones are 83.72 and 76.6 (right next to each other), but I'm pretty sure now that it is the connect tone that I am hearing on one of the systems - why I hear the connect tone bleeding through on one system but not the other is still a mystery.

Unless it has to do with Smartzone. The system where I can hear the tone is a Smartzone system whereas the system where I do not hear the tone is a Smartnet system. The radio that hears the tone on the Smartzone system is a Smartnet (H37) radio and the radio that does not hear the tone on the Smartzone system is a Smartzone (H38) radio. Doesn't a Smartzone system transmit a connect tone to subscribers whereas that is not the case in a Smartnet system? Maybe only the Smartzone flashed radio is designed to filter connect tones from received audio?

Post by **xmo** » Mon May 02, 2011 4:55 pm

Connect tone is irrelevant.

Connect tones are transmitted inbound by mobiles and portables. The outbound sub-audible signal transmitted by the fixed-end is called low speed data.

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 5:10 pm

xmo wrote:Connect tone is irrelevant.

Connect tones are transmitted inbound by mobiles and portables. The outbound sub-audible signal transmitted by the fixed-end is called low speed data.

Thanks. Do you know why I would be hearing it in the background?

d119 · Post by **d119** » Mon May 02, 2011 6:59 pm

A couple of possibilities:

1. Does it occur on ALL conversations on the system, or just occasionally? This would indicate a single channel, maybe two out of alignment.

2. Perhaps the system was never aligned properly to begin with and all channels are broadcasting connect word (low speed data) at high deviation.

It's a system-side issue, not a subscriber-side issue. Unless it's causing problems, I'd just leave it alone. By chance is this a 900MHz system? This is much more common in 2.5kHz deviation 900MHz systems due to the expansion of the audio performed in the subscriber.

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 7:11 pm

d119 wrote:A couple of possibilities:

1. Does it occur on ALL conversations on the system, or just occasionally? This would indicate a single channel, maybe two out of alignment.

2. Perhaps the system was never aligned properly to begin with and all channels are broadcasting connect word (low speed data) at high deviation.

It's a system-side issue, not a subscriber-side issue. Unless it's causing problems, I'd just leave it alone. By chance is this a 900MHz system? This is much more common in 2.5kHz deviation 900MHz systems due to the expansion of the audio performed in the subscriber.

Thanks. This is an 800MHz radio, and yes, it does occur on ALL conversations on the system in question. On the other hand, there is no problem whatsoever with this radio on the second system which is located in a different city. Also, I have no problem with an entirely different model radio.

d119 · Post by **d119** » Mon May 02, 2011 7:31 pm

It could be the result of the simulcast being slightly out-of-phase if it's a simulcast system. Again, nothing I'd be too concerned with unless it's creating an issue for the users.

JPOPatents · Post by **JPOPatents** » Mon May 02, 2011 7:46 pm

d119 wrote:It could be the result of the simulcast being slightly out-of-phase if it's a simulcast system. Again, nothing I'd be too concerned with unless it's creating an issue for the users.

It's a 3-site simulcast system. More of an annoyance having background warble than anything else. Thanks.

d119 · Post by **d119** » Mon May 02, 2011 8:32 pm

Simulcast phase issues, guaranteed.

Will · Post by **Will** » Mon May 02, 2011 8:35 pm

You are hearing the connect tone sent by the trunking repeaters. They may be out of phase or distorted. You would need to look at the transmitters on a service monitor and scope the audio.

tvsjr · Post by **tvsjr** » Mon May 02, 2011 8:41 pm

The connect tone isn't transmitted on the output side, as far as I'm aware.

I suspect d119 is right with simulcast phase issues, or the deviation of the low-speed signalling on the voice channel is set too high.

jackhackett · Post by **jackhackett** » Mon May 02, 2011 9:39 pm

What model radios?

Would simulcast distortion affect one model and not another?

Drive around with the radio and see if the noise changes or goes away as you get into different parts of the coverage area.

I have to say, the info here is coming in drips and drops, started out as PL problem, then turned out to be trunking which doesn't use PL, first it's not simulcast, then it is, no mention of radio model although the problem seems to be model specific, people think it's a system problem, but one radio appears to work fine.. the whole thing doesn't make much sense.

JPOPatents · Post by **JPOPatents** » Tue May 03, 2011 5:29 am

jackhackett wrote:What model radios?

Would simulcast distortion affect one model and not another?

Drive around with the radio and see if the noise changes or goes away as you get into different parts of the coverage area.

I have to say, the info here is coming in drips and drops, started out as PL problem, then turned out to be trunking which doesn't use PL, first it's not simulcast, then it is, no mention of radio model although the problem seems to be model specific, people think it's a system problem, but one radio appears to work fine.. the whole thing doesn't make much sense.

Radio that hears the problem on one system but not the other is an MTS2000 smartnet flashed radio.

Radio that does not hear the problem is an XTS3000 smartzone flashed radio.

No, it's not PL, that was my error. I'm hearing the connect tone or data in the background. It flutters.

Bill_G · Post by **Bill_G** » Tue May 03, 2011 5:31 am

I was just going to say that jpo said it wasn't simulcast earlier, but now it is. He must have gotten new information. That is why I asked about it. I'll have to look at the local Mot 800 systems to see if they xmit PL or DPL on the sim channel. DPL would be very hard to keep correlated in a receiver. The question about why one receiver does it and another doesn't could boil down to a number of factors including rcvr sensitivity, antenna type, location, localized interference, IF freq, high pass filtering, and rcvr alignment.

JPOPatents · Post by **JPOPatents** » Tue May 03, 2011 6:13 am

Bill_G wrote:I was just going to say that jpo said it wasn't simulcast earlier, but now it is. He must have gotten new information. That is why I asked about it. I'll have to look at the local Mot 800 systems to see if they xmit PL or DPL on the sim channel. DPL would be very hard to keep correlated in a receiver. The question about why one receiver does it and another doesn't could boil down to a number of factors including rcvr sensitivity, antenna type, location, localized interference, IF freq, high pass filtering, and rcvr alignment.

Everyone has been extremely helpful, and I see that there is no quick answer. I originally thought that I was hearing PL, but was corrected by someone on the forum who said that trunking systems do not use PL. I appreciate the correction, and I have edited the subject line in my posts to be more accurate. It was someone else who suggested that what I was experiencing was a simulcast phase issue or some system alignment problem. Another person suggested that I was hearing the connect tone. Again, I appreciate all of your thoughtful responses.

To summarize, the system where I experience this anomaly is a 3 site, simulcast, smartzone system that has not been rebanded. On that system the MTS2000 hears, in the background whether audio is present or not, a very low frequency cyclic signal that I would best describe as a "warble" or a "flutter". If someone keys up the system, that's all you hear in the background to a full quieting signal. When someone talks, the background signal is masked for the most part, but can be heard between words and when talking stops until the system drops out. When the MTS2000 is switched to a different system, which happens to be a smartnet system that has been rebanded, then communications are crisp and clear with no background signal as described.

Therefore, I was tentatively ruling out the radio. However, the background signal that I am describing cannot be heard an XTS3000, so this anomaly may be radio specific. It's just odd that the radio hears this on one system, but not another, and I was hoping to find out more information before I spent the money to flash the radio to H38 if it was related to the H37 flash.

dxon2m · Post by **dxon2m** » Tue May 03, 2011 8:04 am

John I *think* I have an idea what you mean. I have heard something similar on the UPS Trunked System in Louisville, every transmission on the system sounds like a motorboat in the background, the audio is OK, but the background is very noisy and this happens on all talkgroups.

Also, I think the warbling tone is another phenomenon, the warble is in the background, but not significant enough to actually degrade the speech quality.

I may have some audio clips that I can share with the board, hopefully it can help clear some confusion.

jackhackett · Post by **jackhackett** » Tue May 03, 2011 1:36 pm

There was a problem with the MCS2000 mobiles where they use some sort of digital signal processor to filter out the PL and there was a problem with the programming of it that let the PL bleed through, but it was limited to a very narrow frequency, so for instance 110.9 might come through, while 103.5 and 114.8 wouldn't. The problem was corrected in firmware.

I'm wondering since they're in the same family of radios if the MTS2000 might have a similar problem with it's filter.

dxon2m · Post by **dxon2m** » Tue May 03, 2011 4:48 pm

Turns out I lost my audio files when I formatted my computer, sorry.

RADIOMAN2002 · Post by **RADIOMAN2002** » Mon May 16, 2011 11:41 am

Could it be that the sub-audible tone(OSW) is set to too high in deviation on one system but not the other?

Spiffy50 · Post by **Spiffy50** » Wed May 18, 2011 10:05 pm

OSWs aren't sent on the voice channel, nor are Connect Tones. You're more likely than not hearing the low speed data that is transmitted on each voice channel (identifies what talkgroup is active on that voice channel, umong a few other things - this is how the first generation trunk tracking radios worked, and is why you needed to program all voice channels in the system).

I have had this exact same thing happen in the past with different radios. Case in point, a SmartNet (non simulcast) system on an MTX9000 sounds perfect, however on an MTX9250, you hear that warble continually. There was no way to eliminate it, that's just the way the Waris delt with the audio.

One thing to check to see if it is "for sure" this low speed data: When a voice comm finishes, just before the channel grant drops, do you hear the warble go away and hear a very quick tone of around 150Hz? If so, this is for sure what the issue is. That tone is the disconnect word that tells the subscriber radio to return to the control channel.

On the Waris radio, one thing I did that helped a little bit was turn on companding. It made the audio a little bit crappy (but still useable), and totally got rid of the warble. Not sure if you can do that on the radio in question, but worth a try.

RADIOMAN2002 · Post by **RADIOMAN2002** » Thu May 19, 2011 7:35 am

Thanks for the correction, I knew what I meant, just the wrong term. BTW I am having this very issue now, that Westchester County Fire changed CC channels. The old CC channel is now the prefered voice channel, and I now hear the low speed data, like DPL, where as on the old primary voice channel I did not.