Investigation of radio mics
I started by turning on the radio mic receivers and the sound desk, leaving the body packs turned off. In that state the Sennheiser (headset) and Alto (lapel) receivers show a low and flickering level of RF reception - up to 2 'blobs' on their meters, whereas L1 and the Alto receivers showed varying, non-maximum, similar noise.
I then turned on the 3 body packs. L2 showed channel input up to 'clip', ie its maximum, on the sound desk channel meter strip. The L1 and Lapel channels showed varying and much lesser levels.
I (gingerly) sent the L2 signal to the main speakers, and it sounded like white noise - not particularly loud. But very strangely, neither the fader nor the mute button on the desk had any effect on the output volume. I do not know why that is. Matt B was there when that happened and was equally surprised.
As L2 was showing the most RF input when none was expected it, I retuned its RF frequency. For the record:
- L1 is 863.7
- L2 was 865.0 but I changed it to 831.9
- Alto is 863.025
My recollection of using seemingly identical Sennheiser receivers elsewhere was that I could choose any frequency in the free-use band (863 - 865MHz), but these ones only offer a selection of 4 or 5 different frequencies. I didn't pursue that: maybe there's a different option that allows more choice.
After I retuned L2, with the body pack off I still got white noise, but now it responded to the fader - as I would expect it to.
I then connected the headsets to L1 and L2 and wiggled them around. It was very obvious that there is a break or a short in the cable of one of the headsets near the mic end (ie remote from the jack). I confirmed that it was the headset at fault by swapping the sets between the L1 and L2 body packs, and also confirmed that the remaining headset works properly in either the L1 or L2 body packs.
The lapel mic channel on the desk shows some noise if the body pack is off and the receiver is on, but once its body pack is powered up it works as expected. The audio signal level was quite low so I turned the channel gain up about 3 notches on the desk.
The handheld (lollipop) mic is on a licensed frequency in the 606 - 630MHz band. I could not immediately see which precise frequency, but it seems to be working properly. Again I increased the gain a notch or two to get the output volume about right when the fader was around 0db. (I presume we are paying the required licence fee.)
With all of the mics, I tried them left, right and centre at the platform, at the position where the speakers stand for family prayers, at the desk and randomly around the flat area, with no problems. I didn't try the side pews.
My informal source in the AV business tells me that they have given up using the 863-5 MHz band as it is too crowded and they always now use the 606 - 630 Mhz band, which costs £135 for 2 years. This is not guaranteed to be interference-free, but like first class on the Paris Metro, you get a better class of user.
I picked up a few technical details along the way. The Sennheiser kit (and maybe the others) uses a 'pilot tone'. This is a steady audio tone with a frequency above the limit of normal hearing that is transmitted while the body pack is powered up, whether muted or unmuted. The receiver squelches (ie silences) its output unless it is hearing the pilot tone. This should mean that if the body pack is powered off, the sound desk will not receive any background noise. You'll see there is a 'Pilot' indicator on the Sennheiser receivers. There is also a 'Mute' indicator which I believe means it is receiving the carrier frequency but there is no audio other than the pilot tone - normally because the body pack mute switch is on.
Matt B suggested that we should go digital if we decide to replace the wireless mics. I've had a quick look at the pros and cons. The consensus seems to be that digital systems are improving, but the less expensive ones have significant latency - ie delay between the directly heard sound and the amplified sound. That would not work well in our situation, where many people are hearing both direct and amplified sound.
The big plus of digital systems is that you can squeeze more channels into a smaller bandwidth, but that is probably not a concern for us.
