Wireless Wireless mic settings

[Ocsac]

Having done mostly live music, I was recently given the opportunity to fill in running theater sound for a county school system arts center while they were between employees. They liked my work enough to give me a contract. This forum has been a great help in navigating RF management. (They had 18 wireless mics, but no RF management. I knew I was going to run into that one day.) I have several deca... I mean, a number of years as an electronics tech, including RF, but never had to coordinate 16 frequencies. Thanks to the forum for steering me to WWB, etc. I think the RF improvements I'm making helped get the contract. Many thanks to forum members here for past discussions on that topic.

A curious situation I found is that all their Shure SLX1 transmitters, with settings -12, 0, or mic, are set to '0'. The directors, as well as the students involved in theater tech are trained to use '0'. This results in preamp gains in the 20 - 30 dB range. I've experimented with the 'mic' setting, which lowers the gain needed to around '0'. Is this a six-of-one / half-dozen-of-another situation? Are there pros and cons to each setting? Can anyone think of a reason they would not use the 'mic' setting on the transmitter?

 

[DrewE]

I'd think generally you'd want the "mic" setting. If using that causes clipping or distortion on loud sounds, then return it to the 0dB setting. This is basic gain staging stuff, to yield the best signal to noise ratio. (In particular, it's about maximizing the modulation of the RF carrier; the exact signal level between the receiver and the mixer is not nearly as important so long as the input gain on the mixer can be adjusted suitably.)

I'd be curious if anybody knows why the 0dB setting was chosen. If it's something like "using mic always causes feeback," then they were just not using the input gain or faders on the mixer appropriately. If, on the other hand, it's something like "it sounds terrible at the climax of the Big Solo Number in the second act of The Show," then the lower gain setting may well be entirely appropriate and necessary.

 

[Ocsac]

I like the signal-to-noise angle. I may make the switch. Have two tech rehearsals this week. If all goes well I'll leave it for the recorded performance, then 6 (reduced capacity) live shows.

 

[macsound]

On the gain adjustment, I always had the transmitter set fairly low to prevent clipping. But also know that elements come in different output levels so there might be some tweaking involved there too.

On choosing frequencies, it's hard to choose without "looking" at your available spectrum, but something I learned in the past 5 or so years which I never realized before is choosing frequencies in different parts of the wave. Not sure if its true but I try to abide by it.
For example 500.000 and 501.375 aren't off by 1 whole mhz, so they're less likely to interfere. Again, not sure if thats true.

 

[FMEng]

On choosing frequencies, it's hard to choose without "looking" at your available spectrum, but something I learned in the past 5 or so years which I never realized before is choosing frequencies in different parts of the wave. Not sure if its true but I try to abide by it.
For example 500.000 and 501.375 aren't off by 1 whole mhz, so they're less likely to interfere. Again, not sure if thats true.

If only it were that simple. The channel groups written by the manufacturer, and software like Wireless Workbench choose frequencies to avoid mathematical relationships that create intermodulation products. Using those frequencies will work better than any random picks you can make.

Let me try to explain. Intermod products are new signals that appear when multiple carriers are mixed together either in a transmitter's output stage, or in a receiver's input stage. Intermod products can cause interference just like any other carrier. A simple example with three transmitters:

Transmitter A 501.25 MHz
Transmitter B 506.9 MHz
Transmitter C 504.9 MHz

Intermod = A + B - C = 501.25 + 506.9 - 504.9 = 503.25 MHz

If 503.25 lands within the bandwidth of another receiver, it could have interference. A + B - C is only one kind of product, and there are several. As the number of channels increases, the number potential intermod products grows exponentially. You might get away with random picks with a small number of systems, but not when you run a bunch together.

I once had a combination of five, Telex VHF mic systems. Running 4 of the 5 together worked great. When I fired up the fifth, I started getting dropouts and hearing bits of TV station sync buzz. (Analog TV video carriers had a distinct sound when demodulated to audio, called sync buzz.) It turns out the five frequencies, plus the local channel 11 TV station mixed together and landed on one of the mic channels.

 

[Ocsac]

Made the switch for rehearsal tonight with zero issues. No audible evidence of clipping.

I once had a combination of five, Telex VHF mic systems. Running 4 of the 5 together worked great. When I fired up the fifth, I started getting dropouts and hearing bits of TV station sync buzz.

Interesting to me that you mention dropouts. I've been wondering what intermod interference might sound like. The situation I inherited had some low-hanging fruit: In addition to randomly assigned radio frequencies (6 of them in the J3 band which is two-thirds occupied by TV in our locale), they had 16 receivers in a rack, all with individual antennas, under the stage. Transmitter antennas tucked between the transmitter and the body of perspiring dancers. We cleaned up enough noise and drop-outs that I have no idea which steps took care of what.

 

[mbenonis]

More than you'll ever want to know about intermodulation: https://service.shure.com/s/article/what-is-intermodulation?language=en_US