SDS100 Audio "Warble" on analog trunked only?

[landradi00]

Hi folks, I finally got around to recording some audio and doing the spectrum plot from the SDS100 for analysis.

I have been experiencing a "warbling" in the background when listening to Seattle / King County Public Safety (KCERS) on Seattle Simulcast. This is a Motorola Type II SmartZone trunked system but it uses analog voice for effectively all communications of interest. I get the warbling regardless of RSSI, from -90 to -60 and regardless of noise level or location. I get the warbling regardless of power source, battery or plug.

I'm attaching here a screenshot. As you can see, after a brief no-signal period, there's a period of FM "full quieting" silence which has the warbling waveform, followed by speech, then more warbling. The warbling is obviously visible on the waveform.

I previously had thought this might be related to the known inverse-phase issue with the SDS audio out -- which by the way you can see in the waveform clearly. That is, perhaps the audio was clean but when I had it plugged into car speakers (which is when the warbling is worst) the phase issue might have been creating a perceived artifact. But in fact it shows up on the waveform straight from the headphone out jack into the computer sound card.

Also, the warbling does not show up in other modulation types. For example, broadcast FM -- although it has a smaller (tiny) hum, does not have the warble during full quieting parts. See the second screenshot for no warble broadcast FM. AM and digital modulation do not have it either. My analog-only Uniden BC330T does not exhibit the warble; see screenshot #3.

Global auto filter is "Auto" in this recording, but the warble seems to persist regardless of various settings, including "Off," which I've tried. The site (Seattle Simulcast) filter is set to Global.

Final hint here is that I can also reproduce the warble with the Port of Seattle system, which is also a SmartZone II analog trunked system.

1. Does anyone know why this warble exists?
2. Does anybody else get a similar warble under any circumstances, and if so, can you share what types of system and signals give it?
3. Any workarounds?

Thank you, fellow SDS'ers!

 

[gtaman]

Hi folks, I finally got around to recording some audio and doing the spectrum plot from the SDS100 for analysis.

I have been experiencing a "warbling" in the background when listening to Seattle / King County Public Safety (KCERS) on Seattle Simulcast. This is a Motorola Type II SmartZone trunked system but it uses analog voice for effectively all communications of interest. I get the warbling regardless of RSSI, from -90 to -60 and regardless of noise level or location. I get the warbling regardless of power source, battery or plug.

I'm attaching here a screenshot. As you can see, after a brief no-signal period, there's a period of FM "full quieting" silence which has the warbling waveform, followed by speech, then more warbling. The warbling is obviously visible on the waveform.

I previously had thought this might be related to the known inverse-phase issue with the SDS audio out -- which by the way you can see in the waveform clearly. That is, perhaps the audio was clean but when I had it plugged into car speakers (which is when the warbling is worst) the phase issue might have been creating a perceived artifact. But in fact it shows up on the waveform straight from the headphone out jack into the computer sound card.

Also, the warbling does not show up in other modulation types. For example, broadcast FM -- although it has a smaller (tiny) hum, does not have the warble during full quieting parts. See the second screenshot for no warble broadcast FM. AM and digital modulation do not have it either. My analog-only Uniden BC330T does not exhibit the warble; see screenshot #3.

Global auto filter is "Auto" in this recording, but the warble seems to persist regardless of various settings, including "Off," which I've tried. The site (Seattle Simulcast) filter is set to Global.

Final hint here is that I can also reproduce the warble with the Port of Seattle system, which is also a SmartZone II analog trunked system.

1. Does anyone know why this warble exists?
2. Does anybody else get a similar warble under any circumstances, and if so, can you share what types of system and signals give it?
3. Any workarounds?

Thank you, fellow SDS'ers!

Pretty sure that might be normal for analog Smartnet/Smartzone systems. I know back when I had a RadioShack Pro-164 I would hear a background warble while using headphones. Pretty sure my MTS2000 that was programmed for the system did the same.

 

[Markb]

Yep. Totally normal for a Smartzone system. I believe that is the connect tone that you are hearing.

 

[landradi00]

Ah geez. So, what appeared to be a "flaw" in the SDS100 is just that it's more accurately representing something that exists in the signal??

Does anyone know of a remediation to remove it from the output audio? I tried cutting it down naively with an equalizer filter below about 400 Hz and it helps, but doesn't eliminate it (and makes voices tinny too).

 

[maus92]

According to the RRDB, the connect tone is 83.72 Hz. An audio rolloff filter at about 100 Hz should knock it down. Are you hearing the tone using the built-in speaker of the SDS100? Probably not - I doubt it has the frequency response to reproduce it. I can hear the 105.88 Hz connect tone in my local SmartZone system if I run the audio through ProScan and output to studio monitor speakers, but radio speakers don't normally reproduce audio frequencies that low.

 

[landradi00]

OK, I did a bit more looking at the waveform, zoomed in. It appears to have a "mostly" sine wave with a 0.013 second period (as precise as I could get eyeballing it in Audacity). That's suspiciously close to 1/83.7 (0.012). It sounds like the connect tone is definitely related to what we're hearing in some way.

(PS I can in fact hear the "warble" out of the SDS100 speakers, though it's quiet. The warble, if you listen to the attached mp3 in the zip file in this thread, is not actually a stable subaudible tone; it's a warble, so it is audible well above the range of the SDS speaker. And if you look at the waveform, it's not actually a pure sine at 0.013 seconds, it's got a few "bigger" (longer) peaks and troughs too, almost like it's some kind of unsteady beat frequency between signals.)

That said, both listening and looking at the waveform coming out of the analog BC330T on the same system, I can't see or hear a darn thing. The BC330T seems to filter it out.

 

[maus92]

Yea, it's the connect tone. Perhaps the SDS speaker is sensitive to some degree at that audio freq, and perhaps they *should* have a hard filter, but is it a real problem?

 

[landradi00]

It definitely makes listening significantly less pleasant vs the BR330T. I subjectively assess that the SDS100 is slightly "better" than the BR330T at this trunked system, as far as being quicker (when each scanner is solely scanning that one system) to lock on to a channel with traffic. But the audio warble is just a bummer.

Re: speaker sensitivity, I think the reason the warble might have been missed (or glossed over) in QA is that the speaker on the SDS actually doesn't render the warble well. It's most noticeable with external speakers but kind of annoying in earbuds too.

 

[maus92]

It definitely makes listening significantly less pleasant vs the BR330T. I subjectively assess that the SDS100 is slightly "better" than the BR330T at this trunked system, as far as being quicker (when each scanner is solely scanning that one system) to lock on to a channel with traffic. But the audio warble is just a bummer.

Re: speaker sensitivity, I think the reason the warble might have been missed (or glossed over) in QA is that the speaker on the SDS actually doesn't render the warble well. It's most noticeable with external speakers but kind of annoying in earbuds too.

Yup, likely the external speaker and earbuds have a wider frequency response, in the case of the earbuds, to cover music. Communications speakers *usually* shape their frequency response to voice frequencies.

 

[landradi00]

Thanks for the thoughts, I guess this one goes for me in the wishlist of firmware updates for the SDS...

 

[maus92]

Thanks for the thoughts, I guess this one goes for me in the wishlist of firmware updates for the SDS...

Seems like it would be doable.

 

[Whiskey3JMC]

Nothing out of the ordinary. My county's Smartzone system sounds just like it regardless of what radio I use

 

[landradi00]

W3JMC, can you share which radios you primarily use and hear the "Smartzone warble" on? What is puzzling me is why it's not present on the BR330T (analog only trunktracker model).