Frequently Asked Questions - Wireless

FAQs - Wireless

The output level is adjusted in the DSP and not by an output attenuator. Since the original design was seen as a "venue" (auditorium, etc.) unit originally, lower output levels were not seen a necessary or a desirable feature and would add cost to the user. The problem for sound mixers is that if the level were to be adjusted lower in the DSP, then the output D to A noise would become a problem.

The other thing is that we didn't realize was that some of the pro gear had a hole in their ability to handle input levels. It amazes me that line levels are so high and mic levels so low that they don't overlap.

As a band aid, we have a cable that has an attenuator at the mixer end to reduce ground loop noise and can be set for -20, -30 and -40 dB of attenuation. 

See MCAXLRATTEN

Deviation is the measure of how far a frequency modulated RF carrier can change frequency in response to a signal such as audio. The amount of deviation is limited to a maximum value by regulatory agencies or it can be limited to a maximum bandwidth that the signal can occupy centered on the carrier frequency. For instance, the FCC specifies a 75 kHz peak deviation and a maximum occupied bandwidth of 200 kHz. 

FM is a form of spread spectrum modulation since the occupied bandwidth is greater than the bandwidth of the audio signal. For instance, at full modulation, a 1 kHz test tone broadcast by an AM station would occupy a little over 2 kHz of bandwidth but as wideband FM modulation it occupies more than 150 kHz of bandwidth. This additional occupied bandwidth has "process gain" just like any spread spectrum signal and suppresses interfering signals and noise. The greater the deviation, the greater the noise suppression effect. In general, 75 kHz deviation systems have over 3 dB better noise performance than 50 kHz systems, all other things being equal. With a compander in the system, the 3 dB RF link improvement due to the wider deviation sounds like a 6 dB improvement to the ear. There is a downside to the wider deviation and that is at very low levels of RF, the wider deviation loses its advantage over the narrower deviation systems and actually has a disadvantage. However, this occurs only when audio signal to noise ratios are at 12 dB or lower, which is effectively useless for wireless microphone purposes anyway.

Here is a URL that will take you to the Part 74 rules and regs regarding wireless microphone frequencies in UHF.

Please note that movie producers are defined in this as: "Motion picture producer. Motion picture producer refers to a person or organization engaged in the production or filming of motion pictures."

Part 74 then lists all the frequencies that auxiliary low powered stations can use , some of which are 470 to 806 and 944 to 952 MHz. Then movie producers are listed as one of the groups that can use low power auxiliary stations (wireless mics are one type station). All is well, right?

Then here is the gotcha:
"(d) Cable television operations, motion picture and television program producers may be authorized to operate low power auxiliary stations only in the bands allocated for TV broadcasting." 

TV broadcasting, however, is the UHF range of 470 to 806 MHz, some of which is gone or disappearing. Specifically, 944-952 MHz is not TV broadcast. Therefore, 944-952 is licensable only to broadcast entities but not to the groups in (d) above, including movie producers.

Frankly, the rules are confusing and seem to say that 944-952 is usable by movie production but then that section quoted above takes it away. 

Hope this makes the fog more palpable.

For a typical two wire lavaliere mic that specifies 5 Volts, the manufacturer is actually assuming that the transmitter has a 5 Volt supply in series with a bias resistor of 1k to 5k, depending on the brand of transmitter. The actual voltage at the mic will be 5 Volts minus the drop across the bias resistor. For example, a mic that is listed to draw 500 uAmp would produce a 2.5 Volt drop across a 5k bias resistor. The mic would only see 2.5 Volts (5V minus 2.5V drop). A different mic that pulled only 100 uA would see 4.5 Volts. So for most all transmitters, the voltage to the mic is all over the map. Generally the mics still work, because they actually can handle a wide range of voltages.

All the current Lectro transmitter models, such as the LMa, have a servo input that regulates the bias voltage to exactly 4 Volts under any condition of bias current. The voltage is set to 4 Volts by using the pin 2 to pin 4 wiring. This allows us to handle a wide range of microphones with any current draw with no concern about excessive voltage drop across the bias resistor and is unique to the Lectro transmitters. We chose 4 Volts because this was a typical design voltage and all the professional lavaliere mics we looked at worked very well at that voltage. The one exception is the tiny Countryman B6 and E6 models which require 2 Volts at high current. For the Countryman mics Pin 4 is NOT connected to Pin 2 and this sets the servo input to a regulated 2 Volts which is ideal for those lavaliere mics.

The new stainless steel SMA female connector is pressed into the aluminum front panel and is an interference fit. Any possible panel to barrel gaps are filled by us with a Loctite gap filler. The antenna wire itself has an O-ring in the nut assembly that seals the wire antenna to the nut of the male connector so any water running down the antenna cannot enter the connector. Then the only possible water entry is if the SR is upside down or angled down and water runs into the inverted nut, around the threads, into gaps between the center insulator and the barrel and down into the unit. You can seal against this improbable occurrence with a small drop (dab) of Vaseline or other petroleum jelly inside the SMA connector applied right on the white insulator. The Vaseline will prevent leakage by this path totally, even if the unit is dropped into water and if the SR (and attached camera) are submerged, you've got bigger problems anyway. The Vaseline does not affect the RF at all. If the Vaseline gets dusty when the SMA is removed, just clean it with a cloth or Q-tip and some clean Vaseline.

To test receivers under field conditions is a real pain and is rarely done properly, even at Lectro (!) in the past. To do proper receiver tests, all receivers must be on the same frequency, picking up ONE, I repeat ONE (!!!), transmitter and must use the same receive antennas with 2 two way splitters giving each receiver under test exactly the same signal at the same time. This removes transmitter antenna differences, RF interference differences and receiver antenna differences. Comparisons are rarely done this way and therefore always inexact (wrong).

Here are some things that we found that loused up our two transmitter, two receiver comparison tests big time:

  1. Since transmitter antennas are rarely bent the same and therefore are at different distances from the body, the RF from the two transmitters is mismatched.
  2. Since the transmitters are at different places on the body, reflections from objects in the area are aways different.
  3. Since the transmitters are at different frequencies, the two points above are different as well as different RF noise from the environment. Keep in mind, RF that doesn't show up on the receiver's scan function can make large differences in reception when you are trying to receive weak signals, i.e., check range.
  4. If the modulation (gain) of the two transmitters is not set exactly the same, one receiver can seem to have an advantage. This would show up more when comparing different brands.
  5. The antenna placement of the receiver antennas causes just as many problems as the transmitter points made above. After all, the receiver antennas can't be in the same place simultaneously unless you use the splitter method described above.


This says that comparing two brands of receivers, transmitters, etc., is a crap shoot at worst and difficult at best. Of course, after being beaten unconscious by the sales people multiple times, I've learned to keep my mouth shut when a customer tells me how much better our equipment works than brand X. If you still want to compare two different pieces of wireless gear, I recommend multiple walks, switching frequencies multiple times, changing positions of the transmitters and moving the receivers around so that they occupy each others spots on different walks. You must also set the two transmitters up for correct or full modulation and then set the two receivers to have the same audio output level. To get valid results without the splitter setup and single transmitter described above, requires lots of tests and much walking.

Way back with the early CR185's (VHF compact receiver), when dinosaurs roamed the earth, we lost a few output stages to cheap mixers that had a single switch for all or nothing phantom power. We decided that we couldn't expect our customers to always be 100% perfect (we do expect 99.8%) and so we protected the output stages with bridge diodes. However, if the diodes were conducting and protecting the receiver, the signal was killed until the phantom power was removed. Some customers didn't realize what was happening, and returned perfectly good units to us for repair. That's when we realized that 99.8% of the customers didn't read the manual. Near perfection of another kind. My apologies to those two people who did read the manual. I also might mention here, that I never read a manual until I've got the device operating. It's an ego thing.

So, we added a resistive series circuit to protect against capacitive discharge, resistive bleeders to ground to reduce peak voltages and non-polar capacitors for protection against mis-wiring (pin 2 to pin 1, etc.). We have had no output stage failures since we put in all the gimmickry. We have lost an output stage or two when it appeared the outputs got wired up to 110Vac in some manner. This can usually be spotted by the large quantities of charred circuit board.

None of this is mentioned in an FAQ because we don't consider phantom power a problem. I will add this note, since I haven't done an FAQ in a long time and it is a good question to ask.

Best Regards,

Larry Fisher
Lectrosonics

Here's a reply to a customer who had an SMd that would only run for 6 hours on lithium batteries.

Hi Tom,
There have been four problems with the SMd battery setup. Here's the problems:

  1. The screws that hold the contact leg to the board were not tightened properly at the factory. Resolved by giving the assemblers a torque screw-driver just for those screws.
  2. The spring behind the contacts was not cut properly. The "spring" is a piece of silicone tubing which is very stable and long lived but was being cut by hand, sometimes at an angle. This can be detected by looking at the battery height above the case with the door open. Commonly one battery would stand proud of the case and the offending battery would be 0.050" below the case edge. This was resolved by cutting the springs with a machined fixture that cuts the springs squarely and at a constant length.
  3. The battery polarity protector inside the case was a hair too thick and kept some batteries with short nipples from touching the contact. The offending battery just happened to be the Eveready lithium batteries that we ship with the product. This was resolved by molding a thinner polarity protector.
  4. The diameter of the machined ring on the inner surface of the battery door, also just happened to fit inside a circular depression on Eveready batteries of all types. This reduced the battery pressure and could cause a loss of contact if the SM's were shaken or dropped. This would cause the unit to shut down.

All of these are considered to be our design fault and come under the extended "we blew it" warranty and will be fixed at no charge or automatically upgraded if a unit comes in for other things. Also, these fixes have been in the units for some time. However, not all were done at once since we didn't find them all at once and they didn't fail on most units.

As far as battery life, the lower capacity "Advanced" lithium batteries run an SMd for 12.7 hours. We will have the "Ultimate" lithium numbers in a day or so but they should be around the 15 hour number.

 

Best Regards,
Larry Fisher
Lectrosonics