Urgent help with existing Radio Mics

[howartp]

Hello.

 

I've got two sets of Radio Mics - in Channel 30 (School) and Channel 38 (Church) - plus four in channel 70.

 

I've used these exact sets together for at least 8 shows if not 12, at two venues. (One set is owned by each venue, and I use them together for big shows).

 

I've brought the channel 38 mics into school ready for Act 1 and Act 2 rehearsals tonight and tomorrow; Dress Rehearsal this Sunday.

 

I'm seeing random, but fairly consistent, RF activity on the receivers for channel 38 even though the beltpacks are off, and three of the channels on the sound desk are receiving actual audio signals. I listened into two of these on my headphones last night and was amazed to hear the producer speaking down on the stage through the noise; further listening led me to two of the channel 30 mics (in use onstage) 'leaking' onto two of the channel 38 frequencies.

 

Now I know about intermodulation, but Audio Technica have produced the frequency plan I'm using and I've used it since 30/38 came available at DSO without problems.

 

Any ideas what could be happening? Why am I now seeing tonnes of interference and spurts of actual audio on the wrong frequencies?

 

The frequencies I'm using, with Audio Technica 3000 series beltpacks, are as follows: The three-stars are interfering with each other, including audio. The one-star is interfering with noise only. Several others are seeing silent RF that isn't producing AF.

 

Channel 30

543.250 ***

543.550 *

544.200 ***

544.650

547.850

548.300

548.950

549.250

 

Channel 38

606.500

607.350

607.700

608.350

608.700

611.300 ***

611.650 *

612.300

612.650 ***

613.150

613.500

 

Channel 70

863.375

863.875

864.125

864.625

 

Peter

 

 

 

[pete10uk]

I'm almost certain that you can't use ch 30 anymore, at least not without a specific licence, but I may stand corrected, I thought that was just a tempoery frequency to use over the DSO?

 

If you can hear audio from within your room you must have a mic turned on, at or there about's the same frequency or a modulation issue. The intermittent peak in the signal is normal when you have modulation and you can sometime hear other mics when the mic set to that receivers frequency is switched off, however when you turn it on, I at least haven't suffered any issues, it always seems to latch on to the strongest signal, obviously if you have the correct frequency mic right out on the RF limits and a modulating mic close by you may end up with issues.

[Shez]

According to Arquiva, ch.30 is available in Keighley, although on a coordinated basis of course. Presumably you've got a licence for the venue and dates you're using them?

[paulears]

The intermodulation products of transmitters in one band and receivers in a different one can be very complex, and is often made worse by transmitters that don't have very tight filtering on the output. There is always a chance that AT gave you a set for 30 and a set for 38, but didn't check cross band sproggies at all? I'm guessing you probably didn't declare the ch30 frequencies for the PSME license, so you could always experiment a bit with the troublesome ones?

 

I'm surprised you selected ch 30 kit - it's certainly available, but access is coordinated and quite specific

These licences require users to book on a location by location, date by date basis. Frequencies are booked anywhere between 15 minutes to one year, however annual, long term of fixed site access is usually only permitted for the permanent resident or owner of the site.

[howartp]

Yes, I'm licensed for both 38 (roaming) and 30 (site-specific).

 

To clarify, I have all receivers turned on.

 

If I turn on beltpack number 17 (543.550) but leave beltpack 6 (611.300) switched off, I can hear beltpack 17 on both desk channel 17 (clear) and 6 (in between noise).

 

Same with 20/9 and something/8.

 

I spent the morning re-doing my mic plot without beltpack 6 and 9 in use, which I've managed, but I only noticed beltpack 7 this lunchtime - I will try your idea of it grabbing the strongest signal this afternoon (rehearsal in an hour) by having all beltpacks turned on (they will be in use which is the best way of testing them).

 

Peter

 

 

 

There is always a chance that AT gave you a set for 30 and a set for 38, but didn't check cross band sproggies at all?

 

I'm surprised you selected ch 30 kit - it's certainly available, but access is coordinated and quite specific.

 

AT did the plan knowing both would be in use together.

 

I did some research in BR and elsewhere as to how to best get as many channels as possible working together; the response from BR at the time was poor so I had to go for what I could. Bear in mind we are a school and a church, so we don't have pro-theatre budget for this.

 

Pete

 

 

 

 

 

[cedd]

How close are your beltpacks to your receive aerials? What receive aerials are you running?

Licensing aside, you may be overloading the front ends of your receivers. Cross-channel interference is pretty common in such cases, and you still hear audio because your offending transmitter is broadcasting pilot tone. How does the RF look with all TX's off? Anything there?

 

Try switching to lower gain antennae or moving them further away.

[Paul TC]

I've used these exact sets together for at least 8 shows if not 12, at two venues. (One set is owned by each venue, and I use them together for big shows).

 

I've brought the channel 38 mics into school ready for Act 1 and Act 2 rehearsals tonight and tomorrow; Dress Rehearsal this Sunday.

 

Have you used these sets together at this venue before and how long ago ?

 

In looking at RF sources it is not just your equipment you need to look at for inter-mod, you need to be aware of any other transmitters that may be in the area nearby.

 

As an example some years ago when working in Private Mobile Radio we had a UHF transmitter that every time it keyed up came through on another UHF receiver that was well separated in frequency. However, the frequency difference happened to be the same as the local MF transmitter of the local BBC station up the road.

 

Wideband distribution amps may not be helping, are you using separate antennas and distribution for each band ?

[howartp]

How close are your beltpacks to your receive aerials? What receive aerials are you running?

Licensing aside, you may be overloading the front ends of your receivers. Cross-channel interference is pretty common in such cases, and you still hear audio because your offending transmitter is broadcasting pilot tone. How does the RF look with all TX's off? Anything there?

 

Try switching to lower gain antennae or moving them further away.

We're using Audio Technica ATW-A49 antenna with DA49 distribution (plus one older distribution unit DA860(?) that only does Channels 68-70).

 

Stage to aerials is about 10-12m off the top of my head. Can't remember whether there is RF with all TX's off or not; usually when I'm there I have TX's in ch30 on for rehearsing, so I rarely see it with everything off. I'll check.

 

I've used these exact sets together for at least 8 shows if not 12, at two venues. (One set is owned by each venue, and I use them together for big shows).

 

I've brought the channel 38 mics into school ready for Act 1 and Act 2 rehearsals tonight and tomorrow; Dress Rehearsal this Sunday.

 

Have you used these sets together at this venue before and how long ago ?

 

In looking at RF sources it is not just your equipment you need to look at for inter-mod, you need to be aware of any other transmitters that may be in the area nearby.

 

As an example some years ago when working in Private Mobile Radio we had a UHF transmitter that every time it keyed up came through on another UHF receiver that was well separated in frequency. However, the frequency difference happened to be the same as the local MF transmitter of the local BBC station up the road.

 

Wideband distribution amps may not be helping, are you using separate antennas and distribution for each band ?

 

Yes, I only use this equipment at two venues (School and Church); last time was July. (Feb, July and November are the big events)

 

Peter

 

 

 

[dmxlights]

Keep the receivers beside the sound desk if you can take one of the mics causing trouble to the stage and see if it still inter fears with the other.you could be getting interference if the pack is close to the receivers

[howartp]

Keep the receivers beside the sound desk if you can take one of the mics causing trouble to the stage and see if it still inter fears with the other.you could be getting interference if the pack is close to the receivers

 

Unfortunately both of those conditions were already satisfied - the receivers are all by the sound desk where I can read the battery and signal levels, and when I was listening in to the two offending ones during rehearsal yesterday they were on stage. :(

 

Both rehearsals seem to have gone ok without a problem on the third interference channel so I'm leaning towards Pete10UK's comment that the receivers will lock onto the strongest signal. Might even chance turning the other two back on tomorrow and see how they go; if so I've got two spares to use cos I've redone my micplot without them. :)

 

Peter

 

 

 

[Just Some Bloke]

Just out of interest, I'm wondering why you would want any of the beltpacks turned off during a rehearsal or performance. Wouldn't you leave them turned on all the time and mute them from the desk?

[Mr Steve]

There are AT 3000 series mics listed in Shure's Wireless Workbench 6 software. You could enter your mic details in to that and check that your frequencies will work together / or use the frequencies which it suggests for that brand.

[paulears]

If the receivers are physically distant from the transmitters the ability to hear a transmitter on a different frequency is greatly reduced because the signal strength is much lower, which suggests either a strong spurious signal or very poor out of band rejection at the receiver. Intermod mixing would be somewhat random as people moved about. A solid continuous signal suggests more like a real fault condition?

[howartp]

Just out of interest, I'm wondering why you would want any of the beltpacks turned off during a rehearsal or performance. Wouldn't you leave them turned on all the time and mute them from the desk?

These weren't full rehearsals; just mic-check / level-checks for some songs, and me learning who the soloist's are.

 

There are AT 3000 series mics listed in Shure's Wireless Workbench 6 software. You could enter your mic details in to that and check that your frequencies will work together / or use the frequencies which it suggests for that brand.

Ooh, I'll grab a hold of that - thanks.

 

If the receivers are physically distant from the transmitters the ability to hear a transmitter on a different frequency is greatly reduced because the signal strength is much lower, which suggests either a strong spurious signal or very poor out of band rejection at the receiver. Intermod mixing would be somewhat random as people moved about. A solid continuous signal suggests more like a real fault condition?

I've been through Act 2 and full Dress Rehearsal with no further issues on the third pair of frequencies that are effecting each other, so it is likely the issue of having some beltpacks turned off during smaller rehearsals.

 

Thanks for everyone's help on this.

 

Peter

 

 

 

 

[Rob_Beech]

The set of frequencies doesn't exactly play well together.

 

But it really ought to work in practise. I hope you've managed to get things running with all the TX switched on.

I'm surprised you were not having issues on your 611.650 not your 611.300.

 

For reference, a plot of

 

542.250 MHz

543.700 MHz

546.100 MHz

543.125 MHz

565.350 MHz

554.725 MHz

562.725 MHz

555.900 MHz

616.500 MHz

615.400 MHz

611.225 MHz

608.400 MHz

620.050 MHz

627.525 MHz

629.300 MHz

620.975 MHz

606.400 MHz

623.200 MHz

626.900 MHz

 

Appears to be more pleasant but I appreciate this is academic as you have coordinated frequencies and you don't want to have to purchase more in 39 and 40 aswell.

 

Rob