Frequently Asked Questions - Wireless
There are a variety of professional lavaliere microphones that are well liked. Every user has their own opinion about which ones are the best for different situations. The problem is that these mics are radically different in their output levels, bias currents and in some cases the voltages that they will tolerate. In addition some are wired as three wire microphones (bias + audio+ ground) but others are two wire microphones with bias and audio on one lead plus a ground lead. Variations in output levels from different manufacturers can be more than 30 dB and bias currents can range from 20 uA to 800 uA. In the movie industry, the mic may be required to pick up a whisper in one scene and a scream in the next. It is no wonder that microphone and transmitter design is always a series of compromises. The input to the SM transmitter tries to overcome these compromises.
The bias voltage in the SM input is set by a servo loop that regulates the DC voltage at the microphone to a user selectable choice of 2 or 4 Volts. This is in contrast to the typical 5 Volts plus series resistor bias circuit that can result in a mic voltage that can vary from 1 Volt to almost 5 Volts. The lower voltage range can result in reduced headroom and the higher voltage can result in internal Zenering (overload) in some microphones. The SM input can handle mic bias loads from 1uA to 2000uA while still maintaining full bias voltage regulation. The servo loop also incorporates a filter that causes it to servo out frequencies below 20 Hz and rolls off the response of the lavaliere itself to wind noise, thumps and breath pops. These low frequency excursions are stopped right at the mic FET and then do not overload early audio stages in the transmitter.
At audio frequencies, the servo bias looks like an extremely high impedance resistor (constant current source) so that none of the output of the microphone is wasted in a 1k to 4k bias resistor. To prevent large voltage swings, the input to the first amplifier is a virtual ground input. This input is very low impedance so that the current developed by the mic FET is used entirely to drive the virtual ground input. Since the virtual ground input sees a high impedance source made of the mic FET's drain and the servo bias, the virtual ground input has very little loop gain noise. Since the mic's FET is operating into a virtual ground, there is very little voltage swing on the FET drain which reduces distortion on the FET compared to a conventional input.
The new input has the advantages of low noise since the noise is determined by the noise of the mic's FET and not by a bias resistor. It has the advantage of a well defined bias voltage that is not dependent on a compromise choice of transmitter bias resistors and mic current drain, i.e., two different manufacturers trying to guess what the other one is going to do. The input also has the advantage of very low voltage modulation on the FET drain reducing distortion. Finally, the input does not run out of voltage or current headroom since the bias voltage is well defined, DC current is supplied by the servo loop and AC current is "supplied" by the virtual ground amplifier. At minimum gain, the input will handle 240 uA of peak input current without engaging the limiter.
The most important advantage has to do with the limiter circuit that we have in all our transmitters since we can make it work better in the SM. Our standard limiter is a shunt circuit that shunts excess audio signal to ground when input levels are too high. In the past we have had to buffer this low impedance limiter circuit from the relatively high impedance input circuit for the mic bias supply. The amplifier that we had to have between the mic input and the shunt limiter was subject to overload at high input levels. Generally, the lavaliere mic overloaded before the buffer amp but not in all cases. Some high current mics could overload the buffer. The buffer amp also had to have unity gain so its output didn't overload and this meant this low gain amp added at least 3 dB of noise. With the new input circuit, the shunt limiter can be right at the input. No buffer amplifier is needed. This is because the virtual ground input circuit is very low impedance and is just what the shunt limiter is looking for. The advantage is that the limiter range is at least 30 dB no matter what the transmitter gain setting or input level from the lavaliere mic. There is no other transmitter that has anywhere near this limiting range for high input levels.
Some careful design went into this circuit and it is compatible with almost all of our previous mic wiring recommendations including line level inputs. Some microphones can benefit from a slightly different wiring scheme and that is noted in the SM manual. Old wiring, new wiring and compatible wiring is listed. About the only thing that doesn't work is the 40 dB attenuator wiring for very high signal level line level inputs. This can still be accomplished by putting a single 25k resistor in series with pin 5 of the TA5F input connector.
In the case of the Lectro transmitter, the input is unbalanced and you have to unbalance the mic output. (As far I know, the universal box is an innocent bystander here.) The problem is that the two different balanced systems require different wiring and what is right for one is very wrong for the other.
Fortunately, you can try one way and then the other and pick the one that gives the best results. By high and low, I am referring to plus and minus polarity from the mic. By best results, I mean loudest and clearest. Generally the differences will be dramatic.
For the fully floating balanced output, ground the shield at the TA5F pin 1, the low side wire (from XLR pin 3) at the TA5F pin 1 and the high wire (from XLR pin 2) to TA5F pin 3. This grounds the shield at the transmitter where the RF is the highest and ground references the mic low side at the transmitter.
For the balanced but ground referenced mic output (center tapped) everything is the same but the low side (XLR pin 3) is not connected to anything. If pin 3 were to be grounded, in this case, half the transformer winding is shorted to ground since the winding is grounded both at one end and the center. If it is an electronic output referenced to ground (Schoeps and some others) then that output is shorted and distortion will rise on the other output.
There are several ways of making a universal setup that will work with both types of mics. The first is to use a balanced to unbalanced transformer. The drawback here is that you need a very good transformer. The second way is to put 200 to 500 Ohm resistors in series with the low side signal (pin 3 XLR signal) and tie it to ground. This will not short a ground referenced output to hard ground but to a 200 to 500 Ohm load. A floating output will have one end of the signal referenced to ground through the same 200 to 500 Ohm resistor. The down side is that you have a resistor in the mic line and will lose a little signal but that usually isn't a problem.
After all of this was written, a customer sent in his wiring solution. Though it doesn't have the shield grounded at the transmitter, which bothers me a little, he has had good luck with the following wiring.
" I believe all's well now. I power the mics with a stand alone 48VDC power supply. I have two of them. A Neumann and Sennheiser. Both lost in effect about 10dB or so when the cable adapter to the input of the UM400 transmitter unbalanced the signal by grounding pin 3 to 1 as suggested in your wiring scheme for self powered mic level sources. Lifting pin 3 from ground brought the signal level back to normal. The wiring I am using now is this: The XLR pin 1 is tied to the shield but the shield is open at the transmitter TA5 end. Pin 2 (high) of the XLR goes to the transmitter input (TA5F pin 3) and XLR pin 3 (low) goes to pin 1 of the TA5F. The levels now are fine."
As long as the 48 Volt box ground doesn't get tied to the transmitter ground through something like a common power supply, this should work fine with either variety of mic, floating or ground referenced.
A similar problem in the UH400a transmitter was fixed in the following way.
(See FAQ#049-WIRELESS for UH400a fix)
We have a lanyard solution that is much stronger than the ball chain. It is a free part and is available by calling or emailing firstname.lastname@example.org and letting them know how many kits you need and what model of MM400 you have (MM400, MM400A or MM400B).
The VRS is the (S)tandard receiver module for the Venue receiver system. The VRT is the more complex (T)racking front end receiver module for the Venue system. There are no other differences between the modules in either the RF sections or audio sections. If you are familiar with our other receivers, the VRS front end is similar to the UCR201 and the VRT front end is similar to the UCR411. For an explanation of what a tracking front end is see the following FAQ.
What is a tracking front end? See FAQ#065-WIRELESS
This is cribbed from the UDR200C manual and it was written years ago but is still very true:
"A number of years ago, the problem posed to the design staff was to retain the RF reliability of the Lectrosonics’ fixed frequency designs but add the frequency flexibility of a frequency agile design. The universal (but not best) way to build frequency agile systems is to design a wide open front end that will pass any frequency within the tuning range of the system. This leads to compromised RF performance in the front end with the possibility of interference, forcing the user to switch frequencies in an attempt to sidestep the interference. This makes frequency agile receivers a self fulfilling system; you have to use the frequency agility to get away from the problems caused by the frequency agile design compromises. The problem of frequency agility is further compounded when you realize that frequency changes “on the fly” cannot be made on any type of wireless system. For example, if there is suddenly an interference problem with a system in use, on stage for instance, a frequency change cannot be made without interrupting the program. Basically, the show must go on. In multichannel applications, changing the frequency of one system will usually produce all kinds of new intermodulation problems with the other systems operating in the same location. Frequency agility is not the universal panacea for interference problems. It is only another tool and a limited tool at that. The first line of defense must be the system’s basic immunity to interference. That required a new look at frequency agile receiver design.
"FREQUENCY TRACKING FRONT-END
Our solution to the wide open front end problem was to design a selective front end that can be tuned to the frequency in use. Since we wanted this front end to be equivalent to our fixed frequency front ends, this was a daunting task. Lectrosonics has always used front ends with more sections and much more selectivity than any other wireless manufacturer. The final design consisted of a total of 12 transmission line resonators with variable capacitance applied to each resonator by a microprocessor. This allows each resonator to be individually tuned by the microprocessor for any user selected frequency in a 25 MHz band. This sophistication produced a front end that was as selective as fixed frequency designs, yet could cover the entire 25 MHz range.
"HIGH CURRENT LOW NOISE AMPLIFIERS
The gain stages in the front end use some rather special transistors in a feedback regulated high current circuit that combine three parameters that are generally at odds with one another. These are: low noise, low gain and relatively high power. It is easy to understand the advantages of low noise and high power capability but why is low gain desirable? The answer is that in a receiver, low gain allows the front end to handle stronger RF signals without output overload, which is “increased headroom,” so to speak. The result of a design that takes all three of these parameters into consideration at once, is a low noise RF amplifier with a sensitivity rating equal or better than the best conventional design with a hundred times less susceptibility to intermodulation interference. Combining the high power gain stages with the tracking front end produces a receiver that is unusually immune to single and multiple interfering signals close to the operating frequency and in addition strongly rejects signals that are much farther away."
The wonderful thing is: THIS PROBLEM IS 100% FIELD RECOVERABLE!
It is easy to think that a unit that "won't power on" certainly isn't failing due to firmware, and even if it were, how can you upgrade if it "won't power on"? Well, actually, you can and I just did!
The way the Venue's power supply works, the micro always gets power, and it is in charge of turning the rest of the circuits on or off. If an upgrade attempt fails, it is possible that the program firmware won't work correctly, which can mean that the unit doesn't power on when the power button is pressed. Nonetheless, the micro has power.
The bootloader portion of the firmware is code protected at the factory, so that in theory, it will always be possible to recover from a botched upgrade. This case was no exception. I held down the two buttons to the left of the LCD and applied power, and the display lit up happily, displaying the word UPDATE. I was then able to load the correct firmware in from the PC, and the unit is now working!
So, word to the wise (and perhaps for the troubleshooting guide and FAQ list): a Venue that appears to have power supply problems may in fact just have bad firmware loaded. The way to check is to attempt a firmware update.
If you are using VRpanel.EXE to control and monitor one or more Venue frames (VRM), you can take advantage of the software's "nickname" feature. This feature allows a name of your choosing to be associated with each receiver channel or diversity pair. The new name can be viewed or changed in the "Set Up VR dialog" and is displayed in the "Main Window" next to the faux LCD corresponding to the applicable channel or diversity pair.
VR Dialog Screenshot
If no nicknames are assigned, the default names "Rx 1" through "Rx 6" are used for single receivers, and "Rx 1 & Rx 2" through "Rx 5 & Rx 6" are used for diversity pairs.
The original reply made 13 Jun 05 is below. It is long but does discuss why the problem exists. In short, the problem is due to a ground loop between the common ground between the audio cable shield, the power supply cable to the transmitter and the power supply ground to the mixer. On 15 Nov 06 we released a new product which solves the problem by isolating the ground to the transmitter. ISO9VOLT battery eliminator
This was an email from a customer:
We are unable to use the Lectrosonic 400 series wireless mics as camera links between our mixers and the Panasonic Varicams. While the units work fine between talent and mixer, there is a significant signal to noise problem when used from the mixer to the cameras. We have traced the problem to the use of the external power modules for the transmitters and/or the battery distribution box (Hawk Woods) we are using. Comparison with a 400 transmitter powered by a disposable 9 volt battery demonstrates the desired performance.
(Lectrosonic makes a unit called the battery eliminator which allows their transmitters to be externally powered and therefore eliminates the accidental loss of audio between mixer and camera due to an undetected battery run down. Our sound packages, mixer, receivers and transmitters are powered from a rechargeable NP 13 battery via a power distribution tap made by Hawk Woods with a variety of cables [4 pin to single and dual coaxial ] and the aforementioned battery eliminators.)
This s/n hiss is introduced at the transmitter and is related to a ground potential between the 400 transmitters and any other ground in the package. Dc voltage from 411 transmitters to ground of mixer measures 4.5mV. Resistance, which should be 0, measures 4 or 5 ohms. This results in audible hiss between our mixers and the cameras. Changing the power cables, audio cables or battery eliminators does not solve the problem. The more equipment added to the Hawk Woods power distribution, the louder the hiss becomes.
This is the same power scheme we have used with the rented Lectrosonic 200 series transmitters without any problem. We are shocked that this problem is unknown by Lectro and the wider sound community.
Our First reply:
The hiss problem is probably caused by a ground loop between the common battery feed to the mixer and transmitter and the audio ground to the transmitter. The switching power supply in the UM400 is noisier than the linear regulator in the UM200. This is invariably true of switching power supplies and is a trade off for their greatly improved efficiency. The reason the ground loop problem is showing up on the transmitter is that this is probably the lowest level audio in the system. When the UM400 is run from a 9 Volt internal battery, the ground loop is broken and every thing is normal.
One solution would be to star ground every thing with separate lines at the transmitter input since it is the most sensitive point in the system. The other would be to use an isolation transformer in the audio feed to the transmitter. Neither of these is very easy. The easiest solution is to use the balanced output of the 442 mixer to accomplish the same thing.
The balanced input would be wired so that pin 1 of the 442 output XLR goes to the cable shield and pin 1 of the UM400. Pin 2 of the XLR goes to pin 5 of the transmitter (line level input). Pin 4 of the UM400 goes to pin 1 of UM400 to form the line level pad. Then pin 3 of the 442 XLR goes to pin 1 of the UM400 (along with the shield of the cable). This way the balanced output of the 442 is referenced to pin 1 (local ground) of the transmitter. This will require a 2 conductor plus shield cable of course.
We will duplicate the problem here if possible with our 442 mixer and then apply the "cure". We should get this done in the next few days and will let you know how well it works.
One other thing that could be adding to the noise problem is if the UM400 is not receiving a line level signal. If the output from the mixer is fed directly to pin three of the UM400, the sensitivity to ground loop noise will be 20 dB worse than if the signal is fed at line level to pin 5 with pin 4 tied to pin 1.
The SM input is a radically different input system compared to our previous microphone inputs. It is so superior to the old way of doing things that we will eventually introduce this input system on all our UHF transmitters. We realize this causes some confusion for our customers but the advantages are very real. The improvements are audible and make the transmitters easier to use and much harder to overload. It is no longer necessary on some mics to introduce pads to prevent overload of the input stage, divide the bias voltage down for some low voltage mics, or reduce the limiter range at minimum gain settings. For a more detailed technical discussion of the improvements in the SM servo input stage, see FAQ#061-WIRELESS. We have spent many, many hours trying to make the change from the old system to the new system as painless as possible.
For 90% of the microphones in common use, no changes are necessary to the wiring of the 5 pin connector. For some microphones the wiring can be simplified. For line level inputs, our custom musical instrument cables, adapter cables and so forth we have managed to keep the 5 pin wiring the same for old and new transmitters. You can find complete wiring diagrams for the SM transmitter on our web site. The exceptions to this compatibility are all three wire microphones (including our own M150) and a few odd wirings such as the 40 dB attenuator wiring for line level inputs. All this can be found on our web site under the Support tab, but I will list some of the more popular mics here after I discuss some of the headings on the diagrams on the web site.
The first section discusses what each pin of the 5 pin connector does. The most radical change is that pin 4 is now a voltage selector pin. You can skip this technical section if you just want to know how to wire your mic.
The next section is is boxed and labeled "Works with SM only". These wirings are specific to the SM transmitter and make wiring a Countryman B6 or E6 or a three wire microphone such as a COS-11 very quick and easy. However, these wirings won't work with older Lectro transmitters such as the UM400, UM200, etc. If you need the two wire Country B6 or any three wire mic to work with both older transmitters as well as with the SM go to the last section below labeled, "Compatible with SM and other Lectrosonics Transmitters".
Countryman B6 and E6 are shown in the first diagram of the section labeled "Compatible with SM and other Lectrosonics Transmitters". TheB6 and E6 are two wire mics but still need special wiring because they are unhappy if run from more than about 3 Volts. The added 1.5k and 3.3k resistors shown in the diagram make the microphones compatible with any Lectro transmitters. This wiring bypasses the servo section and runs the Countryman from the 5 Volt bias supply directly. If you can use the easy wiring above in the "Works with SM only" section for the B6, it gives a little better control of sub sonics and voltage drift with humidity; otherwise there is no difference in audio response. If you have a Countryman B6 or E6 already wired for attenuation for use with a UM200 or UM400, it should still work fine with the SM. font-family: 'Times New Roman'; font-size: medium; line-height: normal;"
Sanken Cos-11 microphones, the Lectrosonics M-150 and other three wire microphones to be used with the SM will all require new wiring. If the wiring is not changed, they will have much higher output than usual and extra distortion at high levels. The reason is that the source follower wiring used with the UM200 and UM400 series is not compatible with the SM virtual ground input. The second diagram in the "Compatible with SM and other Lectrosonics Transmitters" section shows a compatible wiring that will work with all 5 pin Lectro transmitters. This wiring converts the three wire microphone to a two wire system with no changes in audio quality. The microphone polarity will be reversed so you may want to enable the phase switch on the Lectrosonics receiver. This wiring is electrically equivalent to the easy wiring in the "Works with SM only" section above.
All two wire mics (except the Countryman B6 and E6 as described above) such as the MKE-2 and the Lectro M-152 will work with the SM with no changes. The two wire setup is shown in the third diagram in the "Compatible with SM and other Lectrosonics Transmitters" section.
The fourth diagram and fifth diagrams in the "Compatible with SM and other Lectrosonics Transmitters" for unbalanced and balanced line level inputs are the same as for previous transmitters.
The sixth diagram at the lower right for low z dynamic microphones is changed compared to previous transmitter wirings and has the addition of a jumper wire from pin 4 to pin 1. This tells the servo bias supply to shut down and set the pin 3 input voltage to 0 Volts. This additional jumper will reduce the mic output by less than a decibel when used with older transmitters.