Audio Sample-Curious about a "buzz" heard on a police freq

[rananthony04]

Just curious as to what is causing a "buzzing" sound that can be heard on freq 506.1750, which is used by The Baldwin Park Police Dept.[CA] The Dispatcher audio is always clear however when officers are in the field, whether they use their HT's or the car radio, the buzzing noise can be heard either immediately, or after a few seconds of transmitting. The input freq(509.1750) is unaffected (as I have toggled between the two freqs during officers transmitting), additionally, their Ch. 2 Tac freq(506.2750) is also unaffected. They only have one control point which is located at the station(BMAST-Building with MAST/ANTENNA on top) and I have noticed this noise since July. Also, I do not know if Dispatch or officers in the field can hear this buzzing. Almost forgot, I monitor with a pro-96 and 2096, and the buzzing can be heard in unison. I hope I have answered some of the questions that would/might have been asked, if not, I hope its not overkill :twisted:
Any insight would be appreciated, thank you!
The audio sample I have provided is very recent, and has been edited (to be eligible for upload in this forum, size wise) Any other questions, feel free to ask

 

[kb2vxa]

I hear no such buzz, only the PL tone on the repeater your scanner isn't filtering out.

 

[qlajlu]

That almost sounds like electromagnetic interference like they were under huge electrical power transmission lines (kinda like the hum or buzz you hear from a florescent light bulb), but to have it so prevalent on all the channels except Tac doesn't make sense. If that were the case I could understand dispatch not being affected.

 

[js_scan888]

What you are hearing is MDC-1200.

MDC1200 - The RadioReference Wiki
MDC-1200 - Wikipedia, the free encyclopedia

The first person's unit id was: 1561
The second person unit id was: 1542

 

[js_scan888]

I guess you were talking about the "hum" and not about the mdc-1200 stuff. KB2VXA was correct. The hum is the 131.1 Hz PL (CTCSS) being heard. The tone is very noisy when looking at the signal. The dispatch tone is much cleaner and just sounds like white noise. That's why the hum is more audible, it's because the PL tone signal looks like crap coming from the radios in the field.

 

[rananthony04]

THank you to all who have responded. Any ideas on what is causing the noise and why it doesnt affect the input/Tac freq and the dispatcher??
I have additional audio recordings posted on my youtube page that are better quality sound-wise. I had to really compress the audio sample I posted here earlier.

 

[zz0468]

THank you to all who have responded. Any ideas on what is causing the noise and why it doesnt affect the input/Tac freq and the dispatcher??

It sounds like the PL tone, like others have mentioned. Commercial radios, and consoles, and the circuits that connect them all together have better filtering than scanners, so they probably don't hear it. But even with the all the filtering, sometimes they DO hear it, and just get used to it.

If you don't hear it on the input, it's possible that the input PL is a lower frequency, or perhaps the PL level is a bit on the high side on the repeater output. The fact is, though, what you're hearing is typical PL hum. It's normal, and it generally doesn't bother users with real radios.

Edit: I listened again. There's something else in there, it almost sounds like a second tone. It drops on occasion, and you can hear the single tone. A recording that's not quite so busy might be helpful, but it's not easy to diagnose something like that without access to the actual system. It could be voted, with multiple receivers, and one has a touch of line hum. I don't know...

 

[NeFire242]

I'm sure the PL tone blocks out the MDC data bursts to the field units don't have to hear it.

I wonder like zz0468 said that perhaps they're using a tone remote for a voted system or something along those lines.

 

[Hooligan]

It's amazing (and sad) how so many people are responding back with so many things OTHER than what I think you're asking about. What you call a '"buzz" sounds more like a hum to me, and as you indicate, it's clearly audible on the mobile/portable units but not the dispatcher. It's not the MDC chirps & I don't think it is CTCSS, it's a blatantly obvious hum...

Since it's not also on the dispatch-side, it's not some sort of EMI your receiver or recording system is getting.

I'm aware of another PD that had a similar noise on transmissions made by their remote base on their simplex channel. Problem turned out to be with the phone circuit that was the link between the dispatch console & the remote transmit site, and was some sort of impedance mismatch.

I'm thinking the hum generated by the repeater when repeating mobile/portable signals is going to be a similar problem, not affecting dispatch because dispatch accesses the repeater via a separate circuit.

 

[js_scan888]

Don't be sad... I mentioned MDC1200 in my first response but gave the correct answer in my second response.

For whatever reason, the receiver is either not picking up the mobile units as good as it used to, or there is some sort of interference going on before the signal is retransmitted. This causes the PL tone signal to be more distorted when going back out through the repeater. The more the tone deviates from a sine wave, the more noticeable the hum will be.

It's easy to see using a program like Audacity to view the signals from the scanner. You can also use the generate tone feature in Audacity to produce a 131.1 Hz, .1 amplitude tone. You really can't hear it. Now change from a sine wave, to a sawtooth, or square wave. Instant hum/buzz sound.

Distorted PL(CTCSS) tone === hum/buzz sound.

 

[zz0468]

It's amazing (and sad) how so many people are responding back with so many things OTHER than what I think you're asking about. What you call a '"buzz" sounds more like a hum to me, and as you indicate, it's clearly audible on the mobile/portable units but not the dispatcher. It's not the MDC chirps & I don't think it is CTCSS, it's a blatantly obvious hum...

There's nothing amazing or sad about it. There are at least three different noises that are distinguishable in the recording, including the obvious PL and MDC1200. The predominant noise happens to be PL. It's that third noise that's buried in the recording that's in question, I think. What it actually is isn't so obvious.

 

[js_scan888]

The third sound is when the squad car is quickly accelerating. This can be heard 2 times in the audio. It is similiar to the first few seconds of the following audio file that he has on youtube:

YouTube - SCANNER AUDIO-Baldwin Park Police chase a Juvenile on a motorcycle

 

[zz0468]

The third sound is when the squad car is quickly accelerating. This can be heard 2 times in the audio. It is similiar to the first few seconds of the following audio file that he has on youtube:

YouTube - SCANNER AUDIO-Baldwin Park Police chase a Juvenile on a motorcycle

Nope. That's not the third sound I'm talking about.

 

[poltergeisty]

Sounds like some kind of inverter noise/ bad power supply. But my bet is on Warren's assessment. However, I'm about to plug 131.1 Hz PL into my two way and see if I hear that. I don't recall a tone being that noisy...

 

[rananthony04]

Hey thanks guys! I do not know much about radios or comms but I am slowly learning!

 

[jrholm]

I wonder if it has anything to do with something I've noticed lately. My Baldwin Park channel has recently been picking up digital noise on my 528 in the West L.A. area. I thought it was just interference from LAPD when I was that far west. Now that I got a Pro-106 I'll have to play with it. I just noticed this interference in the last month or so.