Hi Everybody:
Things in life have finally calmed down for me and though I never quit playing with my measurement toys, family stuff pretty much kept me too busy to do more than that. Anyway, I needed a wireless measurement system and the lovely Lectrosonics TM400 system isn’t workable anymore now that they have chosen for some unknown reason to build a 300Ω load into their plug-on transmitter. That’s too low for many line level devices to drive, much less a measurement microphone.
I noticed a thread in the Audio Measurement and Testing section that asked about using Line 6 wireless for measurement and I think this belongs here instead.
Oddly this sent me to the opposite direction in the market - the super inexpensive Line 6 XD-V75 WiFi system with a beltpack. Though more than twice the cost of this wireless system, the battery powered Earthworks M30BX is an ideal mic to use given its accuracy. A phantom powered mic will need a portable battery supply such as the Countryman AXBPM. It supplies 18V which is fine for the majority of measurement mics.
The XD-V75 has very good build quality for the money. It provides the highest quality wireless transmission I’ve ever come across and does so at a very reasonable 2.9ms latency. WiFi makes it legal everywhere but comes at the expense of low channel count. Battery life is excellent. The beltpack can handle +6.5dBV before clipping with new batteries, +5.4dBV from about 3/4 battery life to empty. Battery, RF strength and scanning as well clipping indicators are accurate and well designed. The manual is unusual - it’s well written. This isn’t the system I’d use for mission critical stuff where avoiding interference is life and death or for more than about (8) channels.
SetupShure’s WQ302 TA4F mini XLR to 1/4" TS phone plug is a great cable to use to interface a self-powered measurement mic to the beltpack. Simply cut off the phone plug and solder on a female XLR. Shield goes to pin 1, center conductors (there’s two of them for some reason) to pin 2, then a jumper between pin 1 and pin 3:
Before:
After:
MeasurementsLog swept sine at max undistorted drive level:
Log swept sine level plots from max to max -80dB:
15ms tone bursts, 24 per octave, at max undistorted drive level:
The above plot is a
very tough test for a wireless system. This basically simulates the quickest acoustic impulse you’ll find in the real world and the XD-V75 continues to behave like a wire. Here’s what a tone burst looks like at 914.2Hz:
These are the selectable EQ curves available in the beltpack. I guess this would be helpful with a live sound system that didn’t have a console. The funny curve designations come from the manual (SF = speech filter, IF = instrument filter):
A bonus measurement - completely useless in this case - but pretty. This is a wavelet analysis where the time axis has been recalculated to show cycles at each frequency. Wavelets are like the tone bursts shown earlier, the plot shows what would have happened had a single tone burst at a single frequency been injected into the system and then a plot was made of how it died down over time. This is most useful for resonance studies where the DUT continues to make sound after the stimulus is removed (not good). That does not happen in this wireless system but what does happen (that you can easily see in the phase plot) is that above 6kHz the output begins to lag behind the lower frequencies:
Post ScriptThat’s a pun! :) A related post
I made a long time ago is helpful for more background on all this trouble we go to avoiding the best solution - a mic cable.