DMR on FRS/GMRS (seemingly Voice Inversion, not DMR)

[WRZK526]

48 minutes ago, nokones said:

462.5375 MHz is an allocated Business Radio Service freq and being used in a digital trunk system with 100 watt stations on Pinal and Mingus. The AZ GMRS 550 station on White Tanks already operates narrowband. If the other 550 stations operate narrowband and the mobiles and portables set their channels for narrowband, more than likely you wouldn't have any adjacent channel interference from the 462.5375 freq.

In addition to narrowband emissions, if people would use a real radio with a real receiver and not a cheap POS Pandaland Radio with a receiver on a chip, more than likely you wouldn't have that problem.

I reread this and realized I missed the part about one being on Mingus. Thank you this sounds about what is happening. I plan on contacting the owner of the repeaters and asking his opinion. 

[WRZK526]

35 minutes ago, SteveShannon said:

If I were you, I would contact the nearest ham radio club and ask if they would be interested in helping you identify a source of interference.  Or you could ask the FCC, but the ham club would probably be more responsive.  It might be as simple as a nearby cheap baby monitor. The fact that it’s ongoing makes it easier to find. Once you’ve identified the source of the interference, let the FCC know.

I will try this, thank you. 

[Lscott]

52 minutes ago, nokones said:

If the other 550 stations operate narrowband and the mobiles and portables set their channels for narrowband, more than likely you wouldn't have any adjacent channel interference from the 462.5375 freq.

That's assuming the radios have the proper narrow band IF filters. Some of my Kenwood HT's switch between the wide and narrow band IF filters dependent on the selected channel bandwidth. Others might do it in software using a DSP chip.

Some radios simply go the cheaper route. They restrict the modulation deviation to 2.5KHz in narrow band, then double the audio gain to make up the difference. This is done still using the wide band IF filters to save money. 

[nokones]

1 hour ago, WRZK526 said:

Thank you! Once I am north of Paulden I lose that signal and do not get it in Flagstaff. I start getting it again going down I-17 when dropping into the Camp Verde area then lose it going south when you climb Mingus on I-17. I talk frequently to the Mt. Eldon 550 repeater, the Porcupine repeater both at .550. Given the line of sight and how the mountains are I can assume it is coming from the Cottonwood area. I know there is a big cement plant in Clarkdale? 

It is coming from the Mingus site over looking Jerome.

[TNFrank]

A DMR signal picked up on an analog station would sound like a fast series of clicks. Since Encription isn't allowed what you're probably hearing is a scrambled signal. Many radios can scramble the signal making it sound odd. 

[Lscott]

42 minutes ago, TNFrank said:

A DMR signal picked up on an analog station would sound like a fast series of clicks. Since Encription isn't allowed what you're probably hearing is a scrambled signal. Many radios can scramble the signal making it sound odd. 

There is usually no confusion if it's a DMR signal. Due to the TDMA nature the RF pulses. One active slot for 27.5 milliseconds, the inactive slot nothing for 27.5 milliseconds. There is a dead period between slots of 1.5 milliseconds reserved for transmitter power amp ramp up and down time, slot syn on repeaters etc. 

See attached page out of DMR spec's for slot timing.

Part_1_-_DMR_Air_Interface__AI__protocol_-_ETSI_TS_102_361-1_V2.-job_17.pdf

[WRTC928]

2 hours ago, TNFrank said:

A DMR signal picked up on an analog station would sound like a fast series of clicks. Since Encription isn't allowed what you're probably hearing is a scrambled signal. Many radios can scramble the signal making it sound odd. 

If I understand the rules correctly, scrambling isn't allowed either, but so many inexpensive radios can do it now that it was inevitable some people would start using it. Unless you're using it to plan a heist, probably nobody will care enough to try to crack it. If I heard it, I'd likely assume they didn't want me to know their grandmother's cornbread recipe.

[TNFrank]

I didn't understand it that way. Encryption isn't allowed but it's ok to scramble your communication.  I'll see what I can find out.

[SteveShannon]

19 minutes ago, TNFrank said:

I didn't understand it that way. Encryption isn't allowed but it's ok to scramble your communication.  I'll see what I can find out.

https://docs.fcc.gov/public/attachments/FCC-10-106A1.pdf
Here’s what the FCC says about scrambling on GMRS:

20. Recently, several GMRS and FRS radios have been certified with an optional voice

“scrambling” feature that purports to add a level of privacy to communications within a particular group

of users.35 We believe that voice-obscuring techniques, which go beyond the ubiquitous, standardized

tone squelch, are inappropriate for these services. Specifically, we believe that these voice-obscuring

techniques could thwart the channel sharing protocols in these services and the ability to communicate

during an emergency. To ensure there is no future confusion on this matter, we propose to clarify our

rules to specifically prohibit voice obscuring or scrambling in the GMRS, FRS, and CB Radio Service,

and to provide that any such equipment with those features will be prohibited from receiving a grant of

equipment certification for operation under Part 95 of our rules. We seek comment on this proposal and

whether other Part 95 Services should prohibit voice “scrambling.

” We invite commenters to address

whether there are alternatives that may allow voice altering features while still addressing the concerns

identified above. We also seek comment on how to phase-out the marketing and sales of existing

equipment. Should we impose a specific cut-off date or dates? Should the same date apply to the

manufacture, import, and sales of devices? Should we allow existing inventory on shelves to be sold or

should it be removed? What are the trade-offs of the various approaches?

[TNFrank]

I guess that's that then.

[dugcyn]

have not read all of this thread but do want to point out. we recently had interference from a licensed station bleeding into gmrs frequencies. a very experienced radio operator kindly contacted them and let them know. I do not know the details or conversation but it sounded like they had a defective repeater causing this and soon after shut it down. 

am posting because only got a few posts into this reccomending not to contact owner. I think if done well that should be the first move to correct interference. i am not saying direct contact but most cases you can e-mail etc. this may be more effective than FCC in some cases.

if by chance i caused interference this would be my preference and would be much more motivated to help my community vs the government request. IDK

[Northcutt114]

4 hours ago, TNFrank said:

I didn't understand it that way. Encryption isn't allowed but it's ok to scramble your communication.  I'll see what I can find out.

For what I have seen firsthand, it's not really "scrambling" anything. It's just adjusting the frequency you transmit on by a nominal amount. Kind of like when you're not dead on in HF and the pitch of the voice is weird until you  zero beat it. You set the amount of Hz deviation on both radios to the same. You sound weird and artifact-y to each other and mis-pitched and weird to others not using the "scramble" feature. But nothing's really scrambled. At least that's how it works on the Quansheng UvK6.

 

 

[Lscott]

59 minutes ago, Northcutt114 said:

For what I have seen firsthand, it's not really "scrambling" anything. It's just adjusting the frequency you transmit on by a nominal amount. Kind of like when you're not dead on in HF and the pitch of the voice is weird until you  zero beat it. You set the amount of Hz deviation on both radios to the same. You sound weird and artifact-y to each other and mis-pitched and weird to others not using the "scramble" feature. But nothing's really scrambled. At least that's how it works on the Quansheng UvK6.

 

 

What they usually mean is “voice inversion”. 

https://en.wikipedia.org/wiki/Voice_inversion

 

[nokones]

Will this voice inversion or any type of transmission whereas, the voice is not clearly modulated, meet the Part 95, Subparts A & E, related to the emissions requirement or frequency accuracy that are outlined in the respective subparts?

[SteveShannon]

8 minutes ago, nokones said:

Will this voice inversion or any type of transmission whereas, the voice is not clearly modulated, meet the Part 95, Subparts A & E, related to the emissions requirement or frequency accuracy that are outlined in the respective subparts?

Scrambling is audio processing that’s done before modulation, so it should have no effect on either the transmitted emissions type or frequency accuracy.  Then at the other end, after demodulation at the receiver, the audio is processed again to invert the audio, which restores something close to the original sound.

[nokones]

4 minutes ago, SteveShannon said:

Scrambling is audio processing that’s done before modulation, so it should have no effect on either the transmitted emissions type or frequency accuracy.  Then at the other end, after demodulation at the receiver, the audio is processed again to invert the audio, which restores something close to the original sound.

So, would the audio be clear voice on the other end whereas, anyone can hear the transmission?

[SteveShannon]

17 minutes ago, nokones said:

So, would the audio be clear voice on the other end whereas, anyone can hear the transmission?

Once it has been reinverted it’s back to clear voice but in between, between inversions, it’s very difficult to understand, somewhat like listening to an upper sideband signal using lower sideband. You can hear cadence, pauses, even inflections, but it’s all jumbled.

But, to anyone listening with the same descrambler, yes, it’s back to clear voice. 

[Thanatos]

On 1/15/2026 at 4:19 PM, WSKK463 said:

That's not a question.

 

People in my area are using DMR - out of CCRs no less - and their emissions are spattered all across the band... From channel 1 through to 22. This is a daily, continuous problem.

 

What say ye?

 

I don't use DMR so I'm not certain that's the case, or if it's some form of scrambler/encryption, but I'm unable to decrypt it, and there's no CTCSS/DCS tone which the Moni key would have bypassed. A friend said it might be DMR. By continuous, I don't mean unrelenting... The breaks between their transmissions are clear static and much louder than their transmissions because my volume is up. They're not on a timed schedule. The voices change tone - clearly different people communicating. This isn't environmental or other machine interference.

Are you sure that it's actually DMR and not the scramble feature that some CCRs have?  The way scramble and DMR sound, in comparison, should be night and day.  I run scramble mode on my family's main frequencies that we use, on analog CCRs, but anyone who has the same kind of radio can unscramble it by just figuring out which of the roughly 10 scramble modes it is.  All it basically does is just garble the voice up a bit to make it sound like nonsense, not the same type of sound that DMR encryption makes...which as someone said, makes it sound like a chainsaw (not sure that's the appropriate analogy, but close enough, I guess).  My guess is that they're using a CCR with scramble on.  As I'm sure someone will point out, encryption on GMRS is against FCC regulations; however, scrambling ISN'T because it can be easily "descrambled" with any radio that has a scramble feature.

Any way it goes, chances of the FCC doing anything about it are slim to none.  Honestly, the FCC REALLY doesn't give a crap about what happens on GMRS...if they won't shut down that POS North Georgia Repeater Network, then they're not coming after much of anyone else for anything but screwing with the HAMs and/or emergency frequencies.  The ONLY thing the FCC cares about, is getting your $35 GMRS license fee, for the most part.  Hell, most of the people here who use the GMRS frequencies don't have a license, and those of us who do, never give a callsign...things are far more relaxed when in an area where there's VERY little radio traffic ANYTIME of the day...I MIGHT hear someone, besides my family, key up on ANY UHF or VHF frequency once or twice a week, and that's on a busy week...I scan all UHF/VHF frequencies 24/7, and have for the last 5 years, simply trying to hear someone, ANYONE, using the airwaves and 99.8% of the time it's just silence (all the businesses and police depts went to DMR LONG ago, and I only run analog.

[LegitByDefault]

I think I figure it out. Since I last posted, I had been trying to narrow it down and locate the source of the transmission. It was much more difficult than it should have been. I have numerous antennas. Various DF loops, Yagis, CoCo, lossy antenna, baking sheet, body shielding, even SDRSharp and DeInvert were used but had no luck with any of it. Even going to a higher elevation with a clear line of sight across the valley had no effect on strengthening the signal. My last solution was to test how I could make the signal vanish... Well, I managed to do just that. Removal of the antenna had no effect, so I knew the signal was either very close or very strong. Placing the radio in the gas oven had no effect, nor did the microwave. When I placed the radio inside my refrigerator with the antenna still removed, the signal cut out a moment after the door sealed.

I'm currently under the impression that the signal must be transmitted from a hospital pager system about 10.2mi away at 300-500 w ERP, and perhaps even retransmitted by a nearby fire station. That would explain the long hours of continuous operation. but I still don't understand why I'm hearing it. I understand how the compander works, but perhaps I don't know enough about the intricacies of the feature. With the compander turned off, I don't hear the transmission because it's well below the noise floor. I know the Yaesu FT-65 isn't the FT-60 with superheterodyne, but it also hears the transmission when the compander is on.

Does this sound about right? Is this normal behavior for a hospital pager system to be picked up by a radio with the compander turned on? To reiterate, I don't hear it on any radio with it off. It does suck that I can't use a nice feature without interference transmissions from a FCC prioritized source. Could it be a defective antenna on their system?

MarkInTampa, I thought something similar and tried to rule out a nearby transmitter overloading the front end. It occurred to me that if they were nearby and using a very poorly constructed radio on higher power they might not actually be transmitting one GMRS and that the third harmonic might be on 426.6125... so I popped over to VHF on 154 (iirc) and sure enough it was there too. That theoretically placed the second harmonic on 309... The odd thing was that my S-meter showed a weak signal... That had me thinking they were nearly out of range.

SteveShannon, I did end up contacting a local ham club and received no response. Unsure if their club is still active or if this particular issue is something they get requests about all too often.

TNFrank, You're right about that, for sure. After hearing what a DMR sounds like - it reminds me of a taser arcing, which isn't what I heard. Voice inversion scrambler with frequency hopping was my best guess until today. So many youtube videos claiming 'if you do this/legal workaround' and misinformation being spread elsewhere, it's inevitable that some people are going to do it.

dugcyn, Yes, I think we would all prefer a friendly contact making a request over a government official stepping in. The problem is how some people can react or even retaliate, feeling as if their privacy is being invaded or they're being told what to do. Never know how someone will take a harmless input/request. Maybe this is a defective matter.

Thanatos, The scrambler on a sloppy CCR was on my list of possibilities, and like I said earlier in the thread, I really don't care if someone is using a scrambler as long as they are considerate of others and don't behave like they own the allocated service, thereby ruining it for others. The issue was interference band-wide across the spectrum, which perceivably included business/commercial and even Federal/Military presuming the second harmonic theory... That will get the FCC's attention.

[Lscott]

1 hour ago, WSKK463 said:

I really don't care if someone is using a scrambler as long as they are considerate of others

That's why the FCC eliminated the use of any kind of voice masking technology. It doesn't mesh with the FCC's stated public shared use of the spectrum. The obvious logic being nobody else on the frequency using voice masking technology can hear other parties unless they are doing the same with compatible settings. That basically eliminates a straight forward means to dynamically negotiate sharing the frequency.  

[MarkInTampa]

1 hour ago, WSKK463 said:

MarkInTampa, I thought something similar and tried to rule out a nearby transmitter overloading the front end. It occurred to me that if they were nearby and using a very poorly constructed radio on higher power they might not actually be transmitting one GMRS and that the third harmonic might be on 426.6125... so I popped over to VHF on 154 (iirc) and sure enough it was there too. That theoretically placed the second harmonic on 309... The odd thing was that my S-meter showed a weak signal... That had me thinking they were nearly out of range.

 

If your picking it up on VHF as well, there is one more thing you can try and rule out is a strong CATV (Cable TV) leakage problem within your home caused by a loose connector, damaged coax, bad amplifier if you have one, etc. CATV is very broad band. If you have cable, try disconnecting the cable from the ground block going into your home and see if it goes away.

[SteveShannon]

2 hours ago, WSKK463 said:

Does this sound about right? Is this normal behavior for a hospital pager system to be picked up by a radio with the compander turned on? To reiterate, I don't hear it on any radio with it off. It does suck that I can't use a nice feature without interference transmissions from a FCC prioritized source. Could it be a defective antenna on their system?

My “understanding” of the compander was that the audio compression was before the RF stage and audio expansion happens after the RF stage.  My understanding must be either incorrect or incomplete, which are both very possible.

2 hours ago, WSKK463 said:

SteveShannon, I did end up contacting a local ham club and received no response. Unsure if their club is still active or if this particular issue is something they get requests about all too often.

I’m sorry that didn’t work. If it truly is the hospital pager system or ambulance two way radio system they should want to know about it.  Maybe they would even point you to the communications consultant they contract with (I doubt they do it all in-house, but it’s not impossible I guess.)
If you can’t get help from either a communications consultant or local ham radio club I would involve the FCC.  Even if the source of the interference is a high priority service they don’t get a pass on RFI. The FCC would only write them a letter asking them to explain how they intend to avoid creating RFI in the future anyway.  I wouldn’t worry video the steps needed to detect the interference for each frequency, how prevalent it is, and document the times that it’s heard.

Let me recap for my own personal understanding:

1. You hear the interference on an FT-60, which is a direct conversion receiver and on an FT-65 which is superheterodyne, but only when each has had its compander feature enabled.  Without compander, neither of those radios pick up the interference…
2. You receive the signal with adequate strength on about 154 MHz, 462.6125 MHz, and possibly faintly at about 309 MHz.
3. What other RF devices have you been able to receive it on?  I think I understand that you cannot receive it on any device with the compander off. 
4. Have you tried changing modes, such as AM, SSB, or even CW if you have an all band, all mode radio?
5. Does changing the FM bandwidth affect the reception?
    

[WSEZ903]

20. Recently, several GMRS and FRS radios have been certified with an optional voice

“scrambling” feature that purports to add a level of privacy to communications within a particular group

of users.3

Well right here show they are not paying attention to their own rules.  Wonder how many “several” really is?

[LegitByDefault]

Lscott, That's been my understanding. Moreso as I learned about the type of people that generally use one, and their common lack of understanding RF theory and/or not caring about other users in a shared space. Not everyone is a considerate user, and even those who are, likely step on others toes unknowingly while using scrambling. That can happen enough as it is, when users misunderstand CTCSS/DCS and key up without at least hitting the Moni first.

MarkInTampa, no cable TV here. Just a Roku. Not even a broadcast tv antenna. If that's even still a thing.

SteveShannon. That's my understanding. All compression and expansion happens outside of the RF side, purely on the radio itself.

FT-65 is direct conversion, whereas the FT-60 is superheterodyne. I don't own the FT-60.

To reiterate the interference I'm facing, regardless of the radio used...

If the compander is turned on, SQL set to 2. The interference occasionally opens the receiver, and I hear the interference which sounds like voice inversion. Some days it opens the receiver frequently. The interference is clearly audible, as can be heard in the poor recording that I uploaded earlier, which was taken via phone mic.

The duration can range from a quick "cht" or multiples, to a few seconds long inversion. (Initially I assumed the "cht" was purely a distant station just out of range)

While compander is still on, I can also hit the Moni key while the radio is silent with no indication of a transmission being received and I hear "voice inversion" interference still ongoing. My radios just didn't stay open for it.

If compander is turned off, I only hear something cause minor 'interference' in the form of light crackling and very faint "voice inversion" if I hit Moni and it's almost indecipherable from the noise floor but clearly there. If I turn the volume up uncomfortably high, the interference stands out well enough to know I'm not imagining it. On one of the radios, I can hear a very faint "how how how how" sound within the noise floor as well.

I'm unable to RX on 309 MHz. My presumption was that frequency being the second harmonic, if the interfering station were transmitting on 154 MHz, thereby making 462.6125 MHz the third harmonic. Signal strength ranges from 1-3. I began this journey of searching for the source of interference on 462 MHz. The transmission bleeds into all of the frequencies within GMRS/FRS, and some of the other frequencies outside the service, including within 2m and in 70cm.

Receiving devices are Laptop with SDRSharp w/external USB antenna, FT-65, Retevis RA89, and once in a rare while on my son's Midland X-tra Talk LXT560 radios.

My radios are unlocked dual band FM. The transmissions are heard within 2m, 70cm, MURS, and GMRS. I haven't scanned outside of those yet but I will when I get a chance.

No. Changing the bandwidth has no effect on the interference.

 

Hopefully I covered everything without too many typos. Hard to respond using my phone.

[MarkInTampa]

On 1/23/2026 at 4:52 PM, WSKK463 said:

 

Receiving devices are Laptop with SDRSharp w/external USB antenna, FT-65, Retevis RA89, and once in a rare while on my son's Midland X-tra Talk LXT560 radios.

 

Since you have a laptop with a SDR dongle, try giving Spektrum a shot, it might help you out in scanning across a wide range of frequencies. It turns your SDR into a makeshift spectrum analyzer and works pretty good for what it is.

Download: https://github.com/pavels/spektrum