Interference problem on a wireless mic system - Page 1
Forum section: electronics / beginners

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 04, 2018, 09:57:25 pm
content: Hi, I'm trying to help solve an interference problem someone is having with their 100v line PA system. It is in a small auditorium with a half a dozen ceiling mounted speakers in two rows and using two VHF wireless microphones. The length of the auditorium is well within the maximum range of the mics. The problem is there are several fixed spots in the auditorium where the microphone starts to ring and whistle and the mic radio signal drops significantly. The spots in question seem to be almost halfway between any pair of speakers which seems more than co-incidence. They have tried various locations for the receivers etc and nothing seems to have made any difference.One thing I looked at was the line output from the radio, which is unbalanced. The first image shows the audio signal but it appears swamped by the hash and the scope FC is getting confused. One might perhaps expect the amp to filter this out and the hash is too high frequency to be audible but I was wondering whether it might be being passed and causing some kind of intermod effect via feedback from the speakers?I would also like to understand what I am seeing here as there seem to be several frequency components. The second image shows the output with mic switched off and the line quiet. In the following images I have tried to separate out the various components. I am guessing that the 38khz wave is SMPSU noise, but what might the 9MHz and 19MHz noise be?The other curious thing was that when microphones were switched on and in proximity to the scope, the probes were picking up a signal approximately one third (about 60-70Mhz) of the carrier frequency. Would this be a harmonic?Moreover, if this is down to the wall wart, what do I do to filter it?

author: ogden
author_rank: Super Contributor
timestamp: May 04, 2018, 10:42:23 pm
content: What microphones/receivers (models) used? What radio frequencies used?Whistling is justacoustic feedback.Someone shall consider introducingfeedback eliminator.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 05, 2018, 01:19:58 am
content: Do the wireless mic receivers have some sort of small recessed adjustable 'squelch' type control on the back?If yes, it pays to get a small screwdriver and play with that whilst in the "several fixed spots in the auditorium where the microphone starts to ring and whistle and the mic radio signal drops significantly"you may get lucky and nuke your problem, or help minimise itWatch out for loud runaway hash noise during extreme or unsure adjustments, best to have a compressor limiter in the chain on hard limit to be sureand check all the cabling too, many unbalanced cables today are crap, a single wire with a carbon shield layer inside the jacket that crumbles when flexedGood luck

author: Brumby
author_rank: Unknown
timestamp: May 05, 2018, 04:12:04 am
content: First thing, as has been mentioned, is that the ringing and whistling you are getting is more than likely audio feedback. At any one of those points you describe, I would guess that if you change which direction the microphone is pointing, you are going to find orientations where the problem is worse and others where it is not so.Next, I would like to ask if you have the earth lead from your scope connected properly. If it is not connected at all, then you are going to have everything connected to the probe acting as an antenna picking up who knows what. Those screen shots look like the sort of crap you could get. Before making such a connection, ensure there is no significant voltage difference between the scope earth and the earthing point on the device under test. Use a Lo-Z voltage range on your meter, if you have one.I am also curious as to the transmit frequencies used. These should be marked on the microphones and the receiver.One last point - harmonics are always multiples of a given frequency, not fractions. That doesn't exclude there being some other explanation, though.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 05, 2018, 08:31:50 am
content: Thanks for the replies. All good points and here are some answers to the questions raised:Quote from: ogden on May 04, 2018, 10:42:23 pmTrantec S3.2. 173MHz and 174MHz.Quote from: ogden on May 04, 2018, 10:42:23 pmWell perhaps I was a bit inaccurate when I described it as whistling. Its more like a howling warbling echo effect that threatens to break into a whistle at any moment. The amp does have feedback filtering so could it possibly be the effect of the amp attempting to deal with it accoustic feedback?What I don't get however, is why this affects the strength of the radio signal? The signal dips by a couple of LEDs (there are I think 5 or 6) when this problem is encountered.Quote from: Electro Detective on May 05, 2018, 01:19:58 amYes, they do indeed but adjustment does not solve the problem.Quote from: Electro Detective on May 05, 2018, 01:19:58 amThe cabling looks like proper microphone cable, looks of good quality two copper core screened with rubberised (silicone I think) insulation. However, the mic has an unbalanced output employing a 6mm mono jack, but the amp is designed for balanced mic inputs which are wired inside the amp casing using screw connectors. I have not had a look at the the inside of the amp and the specifics of the wiring, so I'm not sure at this point how the connection is made. Unscrewing the jack cover at the receiver end I can see that the screen is disconnected and the two inner cores are being used to connect signal and earth. The length of the receiver to amp lead is about 2m.Quote from: Brumby on May 05, 2018, 04:12:04 amThe orientation of the microphone does indeed have some impact although sometimes upgright cancels it out, other times horizontal does. Pointing at or away from the speaker seems to have no bearing, although if you put the microphone right up to the loudspeaker (maybe less then 6 in away) then you get the sharp audio feedback whisle. In normal use, I would estimate that it would be more than 1.5m or more away.Quote from: Brumby on May 05, 2018, 04:12:04 amI confirm that I did have the earth connected via the croc clip. It is possible perhaps that earth lead picked up the 173/174MHz transmission from the Microphone, but I was surprised by the level of 38khz noise. The readings were taken with the amp switched off and disconnected from the receiver. Instead, plugged in a spare 6mm jack and connected the probe to the jack contacts.Quote from: Brumby on May 05, 2018, 04:12:04 amYes, that’s a fair point although looking at it again, the frequency readings are not even a fraction of the transmission frequency. I wonder whether the FC is just not making proper sense of the mix of noise and showing frequencies incorrectly?. It should be easy enough to test this by having everything turned off except the mic. presumably then I should only see the frequency radiated by the mic transmitter.

author: Benta
author_rank: Super Contributor
timestamp: May 05, 2018, 09:23:35 am
content: One thing to keep in mind is that many wireless microphones have been rendered useless after the introduction of DAB. 173 and 174 MHz is smack in the DAB band. I'm not sure about the status of DAB in the UK, though.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 05, 2018, 11:46:57 am
content: Interesting. I have just done a bit of reading on DAB frequencies in the UK. According to Wikipedia the UK uses 174.928MHz (block 5A) up to 229.072MHz (block 12D). According to Electronics Notes, block 5A is for "local ensembles" and is not in use by any stations so this should theoretically provide sufficient separation from the higher mic frequency of 174.500MHz. Of course, the Wikipedia information may not be current.References:https://en.wikipedia.org/wiki/Digital_radio_in_the_United_Kingdomhttps://www.electronics-notes.com/articles/audio-video/broadcast-audio/digital-radio-audio-broadcasting-dab-frequencies.phpThe manufacturers manual states that the mic uses a single fixed frequency between 165MHz and 230MHz, the IF bandwidth is 100kHz with a 10.7MHz IF. Although the upper 90% of that does co-incide with the DAB frequency range, the frequencies of these particular mics seem to be clear of the bottom of that range. I suppose one way to be sure whether anything else is using those frequencies is to have a listen with a scanner, but I don't have one at present.Manufacturers user manual:https://www.connevans.info/image/connevans/s3_2.pdf

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: May 05, 2018, 01:28:47 pm
content: Quote from: WaveyDipole on May 05, 2018, 11:46:57 amThere has to be a way to program / select the frequencies you are using. Otherwise: Throw those mics away.I am not particularly informed about how strong the regulations in UK are enforced regarding frequency use - here in Germany you will have to pay a hefty fine if you get caught using wireless frequencies you are not supposed to without a proper license.For UK:https://www.canford.co.uk/Technical/Article/UKLegalRadioMicFrequencies..maybe a little OT though.

author: ogden
author_rank: Super Contributor
timestamp: May 05, 2018, 06:26:47 pm
content: Quote from: WaveyDipole on May 05, 2018, 08:31:50 amIt could be radio multipath phenomena as well. Supposedly now you know "the spots". If it is possible to reposition receivers - do it. Move receiver to another spot at least 3..6 feet from current position and redo "weak spot finding" tests. In case of radio/multipath problem, "weak" spots shall move as well.

author: Mr. Scram
author_rank: Super Contributor
timestamp: May 05, 2018, 06:57:01 pm
content: Quote from: Benta on May 05, 2018, 09:23:35 amThis is exactly what I wanted to say. We have some older wireless microphones that suffer a lot from unwanted signals.

author: The Soulman
author_rank: Super Contributor
timestamp: May 05, 2018, 07:28:05 pm
content: Is there a setting to reduce rf power from the microphone?Could indeed be multi-path, or rf signal is being picked up by the speaker-system and getting in the amplifiers feedback loop.

author: ogden
author_rank: Super Contributor
timestamp: May 05, 2018, 07:40:40 pm
content: Quote from: The Soulman on May 05, 2018, 07:28:05 pm173/174MHz getting in the audio amplifier to create feedback loop? - impossible due to zillion reasons.

author: Towger
author_rank: Super Contributor
timestamp: May 05, 2018, 08:05:20 pm
content: I have experienced interference from the lighting control computer with old mikes on those frequencies. The computer was beside the mixer and receivers and would breakthrough from time to time. Of course they went on got new mikes on the 2.4ghz band. Hit head against wall.

author: ogden
author_rank: Super Contributor
timestamp: May 05, 2018, 08:18:58 pm
content: Quote from: Towger on May 05, 2018, 08:05:20 pmThis is completely different. Lighting computer transmit antenna was very close to receive antenna of mikes. If receiver of mikes does not have proper preselector filter after antenna, then it's LNA can be saturated by out of band interferer (of lighting control computer TX). This is not the case OP describes.

author: Towger
author_rank: Super Contributor
timestamp: May 05, 2018, 08:40:51 pm
content: It was from the computer's bus. The lights are hard wired with multiple cat5 up to the stage, where they change over to traditional DMX/110ohm cable.

author: ogden
author_rank: Super Contributor
timestamp: May 05, 2018, 08:49:24 pm
content: Quote from: Towger on May 05, 2018, 08:40:51 pmWhatever. Your case was static interference, OP describes spatial effects which is more like acoustic feedback or RF multipath.

author: chris_leyson
author_rank: Super Contributor
timestamp: May 05, 2018, 09:03:31 pm
content: QuoteFrom the description it's probably aucoustic feedback and has nothing to with it being a wireless mic. There will be some extra delay between the mic and driving amp because of the delay through the wireless harware so that will change the audio feedback characteristics. Turn down the mic gain to fix the problem or maybe use frequency shifting hardware between the mic and drive amplifier. Frequency shifting was and maybe still is a technique for reducing or eliminating howl around.RF multipath will just give you peaks and drop outs but in practice multipath is not an issue. The mic is most probably FM so you wouldn't really hear any variation unless the signal strength drops to below the FM demodulator capture range and then it will drop out. The RF part of the audio feedback loop is insignificant in terms of phase delay, basic physics and common sense will tell you that.Try it with a wired mic and see how that works, you will probably get the same result depending on how you set the overall gain in the audio path. Stabillized feedback amplifiers was published by Harold Black from Bell Labs over 80 years ago and still these days most so called electronics engineers haven't got a clue about feedback.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 05, 2018, 09:49:26 pm
content: Quote from: chris_leyson on May 05, 2018, 09:03:31 pmThanks. Looking at adjusting the mic gain is one possible option to consider. The manufacturers user manual has some guidance on this, in summary reducing the gain to the point that the peak light never comes on. As far as we are aware, we have never seen the peak light come on although we will ask the sound technician to monitor this. Different positions have been tried for the receivers around the sound console, although the suggestion now is to move the receivers into the ceiling space. This would be a fairly costly process as it will require mains supply to be provided and due to the increased length (est 20-25m), balanced line wiring of the receiver to amplifier connection, which, in turn will, I think, need some kind of unbalanced to balanced converter at the receiver end.I do know that the amplifier has some form of accoustic feedback prevention built in, although I'm not sure what type.My question still remains though. If this is an accoustic feedback issue (and I accept that this is likely to be the case) then why does the radio signal also drop significantly? Surely the radio signal should not be affected by the accoustic feedback loop? I'm asking because this has been asked of me and I do not yet have a convincing answer to give them.I perhaps should also point out - in case it is relevant - that the rostrum microphone is wired as are a number of other microphones in use, including two "roving" microphones on long leads that were used prior to the wireless ones, and these never give a problem.

author: Brumby
author_rank: Unknown
timestamp: May 06, 2018, 12:32:11 am
content: I'm only guessing - but there might be an AGC control somewhere in the mix and when you get a high volume signal (such as when you go into feedback), the level is automatically dropped. My only reservation with this is that I've never encountered such a control on the RF transmit power. On the audio signal, yes, but not the transmitter.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 06, 2018, 02:30:02 am
content: Have you tried the mics separately, with either one totally disconnected, i.e. no hookup to anything including power ? same deal?Perhaps try power the receivers via a battery source or a sine wave UPS disconnected from mains if possibleThis one is a whopper, and I've had some that made no sense to knowIn some cases the clients were just over it so wired mics came back into play, and a few wireless models tested till a good one worked as a roverIn most cases there was too much electrical and electronics stuff going on and off at different times, plus phones and wireless stuff going on to lock down the problem.Not that there would be any assurance of a solution anyway...

author: jmelson
author_rank: Super Contributor
timestamp: May 06, 2018, 03:28:39 am
content: Quote from: Brumby on May 05, 2018, 04:12:04 amSimple test. Go to one of the bad spots, observe the whistling. Place your hand over the microphone. If the whistling stops, it is very likely to be audio.Many microphones are "noise canceling", or cardioid pattern, meaning they attenuate sounds from behind the mike, and only allow sounds from in front of the mike. There is no way a mike can tell whether a sound coming in the front is from a speaker or a person. So, you always want the speakers behind the mike. That is how the monitor speakers bands use work. They are placed on the floor, so they are roughly pointing at the exact back of the mikes.If a performer grabs a mike and walks around or turns away from the speaker, then the mike starts to pick it up.The only way to combat this when walking around the room is to have the mike VERY close to the mouth, so first, you can turn the mike gain down, and second, the person's head shields the mike from the speakers.Jon

author: Bassman59
author_rank: Super Contributor
timestamp: May 06, 2018, 04:07:31 am
content: Quote from: jmelson on May 06, 2018, 03:28:39 amSometimes, placing your hand over the microphone exacerbates the feedback.QuoteCardioid (and super-cardioid and hyper-cardioid) mics are NOT "noise canceling." Like a directional antenna, the pattern has nulls, where the pickup of sound is attenuated. But it's not noise cancellation like those Bose headphones.

author: Bassman59
author_rank: Super Contributor
timestamp: May 06, 2018, 04:13:17 am
content: Quote from: WaveyDipole on May 04, 2018, 09:57:25 pmThe problem is simply acoustic feedback. Rule #1 of microphone use: don't walk in front of the speakers with a live mic. You'll likely get feedback.Ceiling speakers are a problem if the person using the mic is walking under them. The problem is worse if the mic is an omni-directional lavaliere and it's worn on the lapel and the gain is cranked. A skilled mix person actively riding the mic channels is necessary.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: May 06, 2018, 09:07:38 am
content: Is there any sort of acoustic treatment of the room / venue or just a bunch of speakers pointed towards the audience?If it is indeed acoustic feedback there probably are lots of things you can do to make the room much more enjoyable to listen to.Bass traps, diffusors, and absorbers can be built on a budget with materials from the hardware store (depending a little on how big the problems of the room are), together with proper speaker placement this works wonders

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 06, 2018, 01:02:48 pm
content: Yes, the problem happens with just a single microphone on, although both receivers were turned on as well. I will repeat the test with one receiver turned off and also try the hand test later during the week.There is no accoustic treatment of the room that I am aware off.We have found today that careful orientation of the telescopic antenae and the receiver units seemed to help considerably. Following the manufacturers guidance to keep the telecopic whips at right angles to each other as well as placing the receiver at right angle to each other made a big difference although it did not completely eliminate the problem. I have also been advised by the sound technician that the peak light on the unit did not come on at any time, including when the problem did arise, so we do not appear to be peaking the gain. Of further note was that one of the telescopic antenae is broken and does not extend to full length, which propably does not help. There was some talk of replacing it. Is it worth trying alternative antenae, and if so, then what sort? I notice that many kits come with leads and separately mounted antenae.One other thing that might be of note is that the receiver has an A and B LED. We orignally thought that these indicate channels, but according to manual these are related to diversity and indicate which antenna is active at the time. The sound tecnicians have observed that the light usually switches from A to B just as the problem clears. From what I have seen, the light on the other receiver switches simmultaneously and they tend to stay opposite to each other, i.e. if receiver 1 is on A, receiver B will be on B and vice versa. I will do some more testing around that to confirm it.