- Nov 6, 2006
Neither one will work because a receiver can only demodulate one signal at a time, and the two signals will cancel each other out at the receiving end, This is true no matter what brand or product you are using. A dual or quad receiver is so named because it has two or four discreet receivers built into a single chassisWell .. a receiver 'receives' and by law, it has to accept any and all communications on said frequency - so theoretically, it should receive both. If you have one of these units, you could easily test this. You potentially might see 1 overpower (block) the other from being able to communicate / transmit clearly.
If the GTD mics have been working well for sometime (even in the 600MHz range) then the issue may be caused by your own mics or other equipment. Have you changed or added anything lately? Added any other wireless equipment (not microphones) nearby?
A device need not be on the same channel to cause interference. The first mic you turn on has in addition to it's nominal freq.spike - a series of upper and lower harmonics which will also spike (to a lesser strength). When you add a second mic it behaves the same way - AND in addition the two transmitters will also interact with each other to create additional multiple order inter modulation products. This condition accelerates with each additional transmitter because the phenomena is not linear - it's logarithmic. It's not enough to simply add a mic on a different channel - you have to choose each time a channel that will not land on any significant byproduct of the full deployment (all gear in use.) This is essentially what a mic receiver is trying to do when in "scan" mode - so it's important to leave each previously set transmitter "ON" while scanning for the next device.
About a third of what some wireless mics used in the 600 MHz band is still okay to use. For example, a Shure QLXD L50 can be reconfigured as L50A to be in compliance. Another third of the 600Mhz now requires a license, and the remainder is completely off limits. If you have the ability to set the mics consistently below 635Mhz then at least do that.. Your systems are low power and should perform just fine when indoors. It's outdoors that you will be exposed to other high power sources (if any) that might interfere.
Back to the mics:
Antenna orientation matters a great deal. Whip antennas should be vertical, however - if your users tend to hold the mic horizontal try putting one antenna at 45 deg. For a body pack - try to get the antenna exposed and on the side of the body facing the receiver. Try each mic individually with the other transmitter turned off - then compare that performance to what happens when BOTH are turned on. Watch for other wireless devices - such as those brought by videographers and caterers. Re-scan if necessary.
Preventing and dealing with issues:
You can easily start by checking your mics against known RF sources in your area. Try using Shure's free RF Frequency finder and Wireless Workbench software to find the best available frequencies for your location.
I would not buy into the SLX product unless you find a steal on used gear. It's discontinued for a reason. (Used gear, unfortunately has nearly doubled in price recently because of supply chain issues.) QLXD has the added benefit of network setup. If you connect multiple receivers to a hub you can do a system scan that coordinates all receivers simultaneously to clear and compatible frequencies.
With bargain mics avoid the common mistake of thinking that two mics with a different band identifier (such as H8, H6, or J10, etc.) would not interfere with each other. These designations are often more like Bingo cards - operating with the same set of numbers but simply shuffling them in a different arrangement.