Fire truck intercom interference problem

Thread Starter

chrischrischris

Joined Feb 18, 2012
303
Hi.

I'm a member of a volunteer fire brigade. For the past 3 or 4 years we have had endless issues trying to get an intercom functioning clearly between the front cab of the truck and the "rops" (the bench seat behind the cab - outside the truck). Clear communication between all member on turning out to a job is vital.

From the cab, the driver or OIC (officer in charge) can speak freely into a goose neck mike that hangs off the ceiling, connected to an amp of some sort and wired back to the rops. In the rops there is a hand held mike with a button we press if we want to talk, and a speaker to hear the cabin when we let go of the button. As the engine revs up and down, so does the "whine" and static. To hear the crew in the cab, the volume has to be turned up so high that it doesn't allow us to hear each other in the back when they're not communicating with us. Our district maintenance officers have been unable to fix it to date.

There are countless trucks across the brigades with the same issue. Head office has said new trucks will use a digital intercom, but that doesn't help us now and our district maintenance officers don't have a fix.

I'm wondering if anyone can give me any advice on how I can test what the problem is with this setup, or if I could make a change to fix it. If not, I'd be happy to design/build an intercom from scratch run on cell batteries if that helps against the interference. All suggestions welcome.
 

MrChips

Joined Oct 2, 2009
22,071
Here are my thoughts:

This is a fairly straight-forward AF amplifier:

12VDC power, two microphones, one amplifier, two speakers.
There is a switch that has to change over from inside to outside microphone, and outside to inside speaker.
There is no way to do this without using long wires unless you redesign the system significantly.

I would assume that the interfering noise is originating from the spark-plug ignition system and this is at RF.

The first question is, if the noise is heard on the outside speaker, is there similar noise on the inside speaker?

There are some possible fixes (1) add filters to remove the RF noise and (2) examine how the microphones are wired and switched and mitigate noise pickup on the microphone cables.

Edit: Albert has asked the same question.
 

MrChips

Joined Oct 2, 2009
22,071
Without receiving the answer this would be my attempt to fix the problem.

(1) Install RF filters in the appropriate locations (still to be determined).

(2) Install microphone preamps at both mics in order to increase the mic signal and reduce the output impedance. A voltage divider will be needed at the input of the amplifier to bring the voltage back down to the required level.

(3) Change the cabling between the microphone preamp and the amp to shielded twisted pairs (three pairs needed, one for the mic, one for power, one for switching.
 

MrChips

Joined Oct 2, 2009
22,071
Since we are talking about "countless trucks across the brigades" this would be a nice piece of contract work that could be of financial benefit to a small startup.
 

MrChips

Joined Oct 2, 2009
22,071
And one more thought:

Since it must already be very noisy inside the cab of a firetruck (and outside as well) you can install noise cancelling microphones that will cancel engine and road noise and amplify voice only. That would be the deluxe upgrade at extra cost, of course.

Edit: I more I think about this, this calls for a DSP (digital signal processor) at the microphone. And since the voice signal is now digitized, you may as well send the digital signal to the audio amp. With a DSP you would not need a PTT (press to talk) button. The DSP can sense who is speaking and will automatically send the signal to the appropriate loudspeaker. A DSP solution can be produced for the same cost as an analog solution. The additional mark up is really a marketing ploy for the perceived superior value.
 

Thread Starter

chrischrischris

Joined Feb 18, 2012
303
Here are my thoughts:

This is a fairly straight-forward AF amplifier:

12VDC power, two microphones, one amplifier, two speakers.
There is a switch that has to change over from inside to outside microphone, and outside to inside speaker.
There is no way to do this without using long wires unless you redesign the system significantly.

I would assume that the interfering noise is originating from the spark-plug ignition system and this is at RF.

The first question is, if the noise is heard on the outside speaker, is there similar noise on the inside speaker?

There are some possible fixes (1) add filters to remove the RF noise and (2) examine how the microphones are wired and switched and mitigate noise pickup on the microphone cables.

Edit: Albert has asked the same question.
Thanks for your quick reply.

Yes it does run off 12V from the truck and the interference does sound like it's from the ignition system as it changes pitch as the revs change. I think my first step is to see exactly how the system is wired up, document that, including the amp, with photos as well. From memory the amp is only held in with a few screws, so I can remove it to photograph inside. Tracing the wires will be a little difficult, but I'm sure I can do it and document what type they are. I'll also post a photo of the mics on both ends. Hopefully from there I can start testing options. It's late here now and I may not be able to do this tomorrow but the next day.
 

AnalogKid

Joined Aug 1, 2013
8,680
Does the sound (pitch) of the noise change with engine RPM? Ah, I see that it does, so it is some combination of ignition and alternator noise. Diesel?

I did an intercom for an ambulance company back in the ... ok, a while ago.

The least noise will have the biggest impact if it gets in through the mic input. SO - what types are the two microphones? If electret, then you have to be extra careful with the bias supply. Also, even though an electret cartridge is inherently single-ended, you can receive it into the amp as a balanced signal. This will reduce another noise source called radiated susceptibility.

The power input is another vulnerability. I suggest two L-C filters in series, one for low frequencies and one for high frequencies. An alternative is a power line filter topology with a common mode choke, but only if nothing in the intercom system is grounded to the truck chassis.

And finally - schematic ? ? ?

ak
 
Last edited:

MrChips

Joined Oct 2, 2009
22,071
Yes, photos would be a great start.
As ak has pointed out, we need to document the mic (photos), what type of mic, cables and connectors, amplifier, etc.

I have firefighter pal here locally. He is the driver. I will inquire what problems they have experienced in their trucks.
 

shortbus

Joined Sep 30, 2009
8,043
Yes it does run off 12V from the truck and the interference does sound like it's from the ignition system as it changes pitch as the revs change
Your trucks must be different than what they use in my area. Any fire truck other than a grass fire pickup truck in my area is diesel powered, so no ignition. I'd be looking at the alternator, if it changes with RPMs of the engine.
 

Tonyr1084

Joined Sep 24, 2015
5,211
I’m just thinking what is the quality of the audio in a room.. Any feedback from the team...
Sounds good here. I'm not hearing any alternator whine or ignition.

@shortbus - Yes, the alternator whine will change with engine RPM. But so will ignition noise. However, you did raise a good point - diesel does not have spark plugs. So most likely alternator whine. But automotive ignition in adjacent vehicles can produce enough radio interference as to be picked up through amplification equipment. I used to pull up to drive throughs and order at the window and at the same time key up the CB mic. My order would be heard by EVERYONE in the restaurant. LOVED doing that. That and increasing the engine idle to match the resonance between the walls. Headers are wonderful.
 

sghioto

Joined Dec 31, 2017
1,553
Sounds good here. I'm not hearing any alternator whine or ignition.
LOL. But that's a good call on your post #10. I was looking at that same component before I read your post. I would certainly recommend that filter or similar before tearing into the system.
 

shortbus

Joined Sep 30, 2009
8,043
and the injectors surely operate on more than 12v, so that's potential for electrical noise from a diesel.
Where does the higher voltage come from? Semis do use 24V but don't think it goes to the injectors. The new Corvette direct injection use near diesel fuel pressures and still only 12V injectors. The common rail injectors in most diesels is like a hydraulic pilot valve, it uses the rail pressure to both open and close the injector. Doesn't rely on high voltage to do the job. But there are piezo injector too never seen those though.
 

MrSoftware

Joined Oct 29, 2013
1,935
Where does the higher voltage come from? Semis do use 24V but don't think it goes to the injectors. The new Corvette direct injection use near diesel fuel pressures and still only 12V injectors. The common rail injectors in most diesels is like a hydraulic pilot valve, it uses the rail pressure to both open and close the injector. Doesn't rely on high voltage to do the job. But there are piezo injector too never seen those though.
Stepping up voltage is easy, SMPS, or just a coil like the ignition coils for spark plugs. I won't claim to know much about it, but this site indicates that at least some common rail injectors take 60-100v to open.
 
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