Frequently Asked Questions - Wireless

FAQs - Wireless

The 195 and 200 series receivers had a dynamic noise reduction circuit that looked at RF level, audio level and audio frequency. Those three variables went through an analog multiplier and the result moved a variable high frequency filter up the frequency spectrum. Higher RF, higher audio level and higher audio frequency moved the high pass filter roll off, higher in frequency. The trick was to get the filter out of the way before the listerner could detect the roll off, so the attack time of the filter is less than 0.5 ms. In addition, for any significant high frequencies, the filter is 2 octaves above those high frequencies. There is a little different description in all the manuals for the receivers and is described as a trimode filter. 

The filter helped remove compander breathing and "halo" effects around the audio in weak audio conditions. Some designs accomplish this with substantial transmitter pre-emphasis and receiver de-emphasis, such as our 187, 190 and IFB series. This can lead to other problems at high audio levels with high frequencies, most easily shown with the dreaded key test. The down side to our filter method, is that at very low audio levels with very little high frequency content, the 200 system's high frequencies are rolled off. These audio levels are generally so low that they typically show up only when listening to room noise or microphone self noise. The 200 systems also made lavalier microphones sound quieter than they really were. The other drawback, was that it was possible to hear the filter working if the transmitter gain was turned way up in a moderately quiet room, so the self noise of the lavalier and the room noise was high enough to cause the filter to move up and down in response to the random high frequency noise. This was audible as a moderate roughness to the noise.

Just for the record, all the 195 and 200 series have had a basic flat response to 20 kHz or more. The system roll off at 20kHz is not what testers are hearing. ( I base this statement on the fact that some systems with highly regarded audio, roll off before 20 kHz and in one case before 16 kHz and I'm not aware of complaints that these systems are muffled.) At high audio frequencies, the tri-mode filter itself is out past 40 kHz. The reason for the roll off above 20 kHz in the basic system is to prevent supersonics from messing up the compander circuits.

The 400 series does not use a compander or pre-emphasis and so the tri-mode filter was left out. We figured this would help is in competitive comparisons at very low audio levels by making the system sound more open and capable of accurately reproducing small sounds and room noises. However, when some of the beta testers compared 200 and 400 series systems side by side, using a common microphone, they were bothered by the fact that the 200 sounded quieter. (Heart attacks at Lectrosonics.) Further trials by the testers, suggested by a very worried engineer, convinced them that the noise was indeed self noise in the microphone and not noise in the 400 system. This is not to imply that the 400's are as quiet as a wire.

Here came the suprise to the recovering engineers; fully half the beta testers wanted the noise reduction left in, in order to reduce the self noise of their favorite lavalier. Since we don't want to lose low level competitive comparisons because of of lack of "air" or transparency, we have compromised with a menu selectable tri-mode filter. Since users have been "hearing" this noise reduction system for 15 years in all our wideband systems, we have decided to implement it with the same parameters in the 400. The noise reduction is menu selectable at three "strengths".

The 400 series digital hybrid series transmiters and receivers will emulate other analog systems such as the Lectro IFB, 100, 195 and 200 series. Basically, we turn off the high level hybrid processing of the signal and just run the DSP as a dual band compander. We can also emulate any single band compander since that is a much simpler process but will choose those brands that have enough units in the field to make economic sense for us. At the moment we will only emulate two other brands and frankly we aren't going to heavily advertise the fact that we can do it. I don't want to get into a shoving match about whether the emulation is "correct" or not. Our dealers can simply make a recommendation of "try it and see". Interestingly, the single band emulation has most of the problems of the real unit in the areas of breathing, noise modulation and the dreaded "key test". The emulation gives only small improvements in these areas. We were hoping for much more since we could detect overload much more accurately but.... To be fair, the 200 series dual band emulation is no better or worse than a real 200 transmitter. One additional downside to the emulation is that you will have 1.5 ms of processing delay that you won't have with a real full analog system.

The emulation was created for three reasons: one is to sell a few more units by being compatible with a competitor's system, to have a reasonable upgrade path for users that have invested in 200 series systems and don't want to sell the farm to move to the 400 series and finally sometimes it is very handy on a movie set to be able to make a system imitate another when you are just one item short.

There are quite a few you can do with just your ears and some others that require a minimum of audio gear. I'll list some tests you can do, a simple explanation of what that test can show you and then a link to a longer explanation of how to do the test and how to interpret it. The best way to do the tests is simultaneously as a comparison in performance between several systems. It is easy to forget what a given system sounds like if there is a day or so between tests. Sometimes the differences are so dramatic you could remember them years later though.

  1. The Dreaded Key test. Some people complain that this test only shows how well a system reproduces keys. Though true, it also indicates how well a wireless system will handle sibilants. Doing poorly on this test will generally correspond to roughness or spitting in sibilant reproduction. This due to gross overload in the audio circuits due to large amounts of pre-emphasis in the transmitter. Frankly most listeners are not critical of sibilants since if a performer sounds an "s" all they are looking for is a corresponding hiss out the sound system. However, if you are a critical listener, once heard it is hard to ignore. (See Dreaded Key Test FAQ#034-WIRELESS)
  2. The Bump Test. This test will reveal the inherent signal to noise ratio of the wireless system and also how well the compandor handles low frequency audio signals. The “inherent signal to noise ratio” is the signal to noise ratio before companding. Poor results in this test will indicate a system that has what is commonly refered to as either "breathing" or a "halo" around the sound. (See Bump Test FAQ#035-WIRELESS)
  3. The Input Limiter Test. This test will check to see if the transmitter has an audio input limiter (most don't) and if it does have one, how well the limiter performs. A good limiter lets you operate closer to full modulation, reduces overload distortion and improves the noise and interference performance. Screaming into the microphone is not the best method of checking this feature. (See Input Limiter Test FAQ#036-WIRELESS)
  4. The Classic Walk Test. As the name implies, this is a test where one person takes a walk while talking into the transmitter,and the other person listens to the receiver output. The classic walk test is to see how far away you can get with the transmitter before dropouts are bad enough to make the system unusable. You can walk until a count of 8 to 10 dropouts occur, for example, and define that as the limit of the range. Or, walk until the dropouts or hiss buildup is objectionable according to your own assessment. (See Classic Walk Test FAQ#038-WIRELESS)
  5. The Short Range Walk Test. A “short range” walk test checks to see how well the receiver handles deep multi-path nulls that occur at a close operating range with a generally strong RF signal. This tests how well the squelch and the diversity system works. This test corresponds well with real world use where the Classic Walk Test is a test of range at distances that are rarely encountered. (See Short Range Walk Test FAQ#039-WIRELESS)
  6. The Hard Wired A-B Test. This requires a simple mixer two identical microphones, one connected with an audio cable and the other with a wireless system, to perform a listening test. Better than two mics would be to split one audio or mic signal so that one part goes through the wireless system and the other is direct. (See Hard Wired A-B Test FAQ#042-WIRELESS)

This simple test reveals how well a wireless mic system can handle high frequency audio transients and, in fact, the quality of the entire audio processing chain in the system. Set up the wireless system with a pair of headphones or a sound system at a fairly high level without feedback. It is best to be able to listen to the audio output of the receiver away from the acoustic sound that the keys themselves generate. Set the input gain on the transmitter for a normal level with an average speaking voice.

Gently shake the key ring loosely near the microphone so that the keys jingle and rattle. Shake the keys within a foot or so of the microphone, then move them gradually away from the microphone while you shake them until they are as much as 8 to 10 feet away from the mic. Listen to the audio that comes out of the receiver. Does it sound like car keys, or a bag of potato chips being crushed?
Next, have someone talk into the wireless system while the keys are shaken as in the previous paragraph. Listen for distortion of the talker’s voice while the keys rattle. Move the keys from a foot or so from the microphone and then away from the microphone to as much as 8 to 10 feet and listen to the effect on the talker’s voice.

This is a tough test for anything other than a hard-wired microphone. The results you hear will tell you, without argument, how well the input limiter, and compandor attack and decay times work in the design, and give you a clear idea of the audio quality you can expect from the system in real life. A loosely shaken set of metallic car keys on a key ring produces large quantities of high frequency transients. A wireless system that fails this test miserably, and a lot do, will also distort sibilants in the human voice. Often listeners don’t notice this high frequency transient distortion because sibilants don’t have a specific frequency but are more like random noise. Distorted random noise still sounds like noise. On a system that fails the key test, however, strong sibilants won’t have a clear, open quality but will instead have a muffled sound as if someone’s EVALUATING WIRELESS MICROPHONE SYSTEMS hand has been put between the mouth and the mic. The key test will warn you to listen closely for the effect. The key test will also reveal audio circuits that are upset by supersonics. The peak energy of jangling keys is actually around 30 kHz, well above human hearing. If the circuits in the transmitter don’t filter out the supersonics, the compandor will respond grossly. This is a valid test since sibilants in the human voice also contain supersonics. Supersonic overload will cause sibilants to sound ragged as the level is driven up and down by sounds you can’t hear.

This test will reveal the inherent signal to noise ratio of the wireless system and how well the compandor handles low frequency audio signals. The “inherent signal to noise ratio” is the signal to noise ratio before companding. This test requires listening to the system in a very quiet environment with minimal background noise. Place the transmitter and microphone in a different room from the receiver, or use high isolation headphones to monitor the audio output of the receiver. In either case, there must be minimal background noise near the microphone. Background noise at a high enough level will negate the test.

Set up the system for normal voice levels, then place the transmitter and microphone on a table or counter. Make a fist with your hand and gently bump the table with the meaty part of you hand (not your knuckle). The idea is to generate a low level, low frequency “bump” near the microphone at just enough level to open the compandor on the wireless system. Try varying how hard you bump the table with your fist to find a low level that just opens the compandor and listen to the results. When you“bump” the table, listen for background noise that sounds like a “whoosh” or “swish” that accompanies the sound of the bump.

The idea is to listen to how much background noise is released through the wireless system when the “bump” occurs, and also to whether or not the “bump” heard through the wireless sounds the same as in real life.
This is an excellent test of the difference between a single-band compandor and a dual-band compandor with DNR filtering, as well as a test of the signal to noise ratio of the wireless system. With the transmitter gain set for a normal voice level during this test, the results you hear will be what the system will actually do in real use.

It is also interesting, although not a valid test, to set the transmitter gain at minimum, then turn the receiver output up to maximum, and do the bump test again. The only reason to do this is to help understand just how much noise is actually suppressed by the system in normal use, and to emphasize the importance of proper transmitter gain adjustment.

A wireless mic system design that uses a large amount of preemphasis/ de-emphasis as noise reduction will likely do fairly well in the “bump test,” however, it may also fail miserably in the previous “car key test.” (See Dreaded Key Test FAQ#034-WIRELESS)

In this test, you will need to make some loud noises at the microphone, but be able monitor the output of the receiver in a fairly quiet environment. It’s best done with two people. The purpose of this test is to listen to how well the transmitter input limiter can handle audio peaks well above the average level. 

Set up the wireless system for an average level so that the system indicates brief peaks at full modulation with a normal voice, with the microphone at a distance of 2 feet from the talker’s mouth. While the talker speaks at a constant level, bring the microphone closer and closer to their mouth. Make sure breath pops don’t get into the microphone when it gets close to the mouth by keeping the microphone to the side of their mouth. If the transmitter has a poor limiter, or no limiter at all, the signal will get louder and then begin to distort as the loudness increases. In a system with a good limiter, the sound will get louder up to the beginning of limiting, and then will remain at a fairly level volume even as the mic is moved closer to the mouth. The character of the sound may change due to the different distances as the mic is moved closer to the talker’s mouth, but the system should be able to handle the higher levels without distortion. 

You can also test a limiter by shouting into a microphone, but keep in mind that the character of the talker’s will change as they go from a speaking voice to a shout. This makes this method harder to judge. Some wireless system designs try to prevent overload by having low microphone gain available to the user. This compromise will result in a poor signal to noise ratio when the RF signal gets weak

Here's a long reply posted to the RAMPS news group. It has a variety of wirings depending on the output level you need.

To the Group:
To make the measurements on the B6 we used the same setup as in the previous post, "Lectrosonics MM400a and COS11 red dot wiring" (See COS-11 test setup in FAQ #025). We had available two B6 microphones and I assume (that word again) they are the standard 6 mV/Pa units since the overload point that we found was close to the specified values. One unit was one that Carl at Countryman kindly loaned us several years ago and the other was from a customer who was having difficulties matching to 400 series transmitters. We found several factors that could cause possible problems with the B6 and a UM400a. The bias resistor for the UM400 series is 4k. This is higher than what we have used on the UM200 by a factor of 4. We chose the higher value because of the improved noise performance of the 400 series. In an effort to increase input signal levels to get past self noise, we increased the bias resistor value between our pins 3 and 4 to 4k. We had also run into problems getting enough output out of some big name low current microphones that wanted to look into a 20k (!) load.

The B6 microphones that we measured here were pulling 750uA and 950uA at 3 Volts which is 2 to 3 times higher than the B6 spec sheet. At first I thought this might be the problem since this much current would pull the operating point well below 3 Volts.

I called Carl at Countryman and learned more about B6's than the average person should know. 

What Carl said specific to the B6 is that the ideal voltage at the B6 mic terminals is 1.5 Volts at which point the mic will draw 500uA. This was a lower voltage than I expected and changes what we would recommend for biasing. The 500 uA does differ some from the Countryman web site values but products always change the most right after the moment you publish "firm" specs. 

Countryman's original recommended UM400 wiring inside the TA5F, from their website was:

  • pin 1 shield
  • pin 4 white (center conductor)
  • pins 2 and 3 install a 2.8k resistor between them.
  • This will give slightly more than a 3 dB reduction in signal, compared to our Lectro wiring recommendation.


Carl now prefers another configuration, which is to ignore our internal resistors entirely and wire a 1.5 k resistor from pin 2 to pin 3 and and a 3.3 k resistor from pin 3 to the the hot lead of the B6. The reason for the new recommendation is to reduce the high B6's sensitivity and get it closer to other commonly used mics.With our bias impedance and the resistors this will drop the signal 6 dB total below our original Lectro wring and bias the mic at 2.1 Volts. So the wiring is:

  • pin 1 shield
  • pins 2 and 3 install a 1.5k resistor between them.
  • pin 3 3.3k resistor in series with the mic's white lead (center conductor)
  • i.e., a 3.3k resistor between pin 3 and the mic's hot lead (white).


Another wiring, for more attenuation, will change our 5 Volt bias to 1.7 Volts on the B6 and drop the level 14 dB below our original wiring is as follows:

  • pin 1 shield
  • pin 3 white (center conductor)
  • pins 2 and 3 install a 1.5k resistor between them.
  • pins 1 and 2 install a 1.8k resistor between them.


So here are three wirings which will drop input levels 3, 6 and 14 dB below the Lectro recommended wiring. I agree with Carl that the 6 dB wiring is the best all around. However, there is a bit more to the dynamics of the situation than just limiting and clipping levels. The 3 dB wiring will let the B6 drive the UM400 into 10 db of limiting (compression not distortion), even with the gain at a minimum. The 6 dB wiring will be 7 dB into limiting. The 14 dB wiring will not drive the transmitter into limiting before the mic itself clips. The downside is that input noise levels will come up by the same amounts which might be a problem in very quiet environments. 

The standard B6 is spec'd at a maximum input sound level of 118 dB. We measured gentle clipping at 114 dB on the high current mic and 117 dB on the lower current mic. These measurements are probably not as precise as Countryman's since they were made at higher voltage and current levels (more gain) but still are certainly comparable to Countryman's spec sheet. Considering that these are higher current mics than other electrets and at clipping, the mic is swinging the entire 500 uA bias supply, the std B6's are hot mics indeed. The gain reduction wiring above does nothing as far as increasing the sound level limit of the microphone itself to more than 118 dB spl. Therefore, the lower gain B6 (-10dB) version may be a good spare mic choice, certainly for loud situations since it would handle 128 dB Spl.

The long and short of it is that Carl wishes wireless mic manufacturers would standardize the input circuits and if not that, then at least not change the inputs willy-nilly. I agree with Carl and certainly we are guilty of changing the input values when we went from the 200 series to the 400 series. I would add that it would be great for the wireless manufacturers, if the all various mics had similar output levels and similar bias currents. What makes it tough, is that the bias currents between manufacturers vary by 15 to 1, the output levels by 25 dB or more and recommended loads from 1k to 20k.

Carl made a very interesting proposal which was to just provide a bias voltage (say 5 Volts or 3 Volts), a DC blocked audio input and a ground and let the mic manufacturers recommend the resistor values for the drain and/or source loads and build them into the mic connector. As Carl pointed out there is lots of room inside a Switchcraft TA5F connector. 

Best Regards,
Larry Fisher
Lectrosonics

The classic walk test is to see how far away you can get with the transmitter before dropouts are bad enough to make the system unusable. You can walk until a count of 8 to 10 dropouts occur, for example, and define that as the limit of the range. Or, walk until the dropouts or hiss buildup is objectionable according to your own assessment. When comparing two or more different wireless systems, it is very important to repeat the same exact path for each walk test, position the receivers and the transmitters on the body in the same location with the same interconnections, and apply the same criteria to define the limit of the range, or it will not be a valid comparison. Even if the maximum range of the system is well beyond what you would normally need, this test will demonstrate the sensitivity of the receiver and how well the system handles weak signal conditions in general.

Before conducting these tests, the wireless mic system should be set up exactly the way it will be used. The microphone and transmitter must be in the exact postition on the talker’s body where they will be used, and the receiver must be connected to whatever equipment it will feed, with power and antennas connected and positioned as in actual use. Unless the wireless system is set up this way, the results of the walk tests will not be realistic. Do not remove antennas on the transmitter or receiver to try to simulate extreme operating range, as this will alter the way some receivers work, such as Lectrosonics models that use SmartSquelchTM and SmartDiversityTM circuitry.

If you have a frequency selectable system, try the walk test using at least 3 different frequencies since even tiny amounts of interference can radically change the results. If you are comparing two systems, try to select identical frequencies of operation thereby comparing apples to apples. If the receivers have scanning functions, check test frequencies that are free of interference As little as 1 uV of interference can reduce a good systems range by one half.

A “short range” walk test checks to see how well the receiver handles deep multi-path nulls that occur at a close operating range with a generally strong RF signal. This tests how well the squelch and the diversity system works. This test corresponds well with real world use where the Classic Walk Test is a test of range at distances that are rarely encountered. Do not remove the antennas on the transmitter or receiver to worsen the conditions, as this will negate the validity of the test.

Before conducting these tests, the wireless mic system should be set up exactly the way it will be used. The microphone and transmitter must be in the exact postition on the talker’s body where they will be used, and the receiver must be connected to whatever equipment it will feed, with power and antennas connected and positioned as in actual use. Unless the wireless system is set up this way, the results of the walk tests will not be realistic. Do not remove antennas on the transmitter or receiver to try to simulate extreme operating range, as this will alter the way some receivers work, such as Lectrosonics models that use SmartSquelchTM and SmartDiversityTM circuitry. 

If you have a frequency selectable system, try the walk test using at least 3 different frequencies since even tiny amounts of interference can radically change the results. If you are comparing two systems, try to select identical frequencies of operation thereby comparing apples to apples. If the receivers have scanning functions, check test frequencies that are free of interference As little as 1 uV of interference can reduce a good systems range by one half.

Find a location where multi-path reflections will be abundant, such as an area with lots of metal file cabinets or lockers, a medium to small metal building, a metal trailer, etc. Place the receiver antenna/s within a couple of feet or so of a metal surface to exaggerate multi-path cancellations at the antenna. The antennas on a diversity receiver need to be at least a 1/2 wavelength apart to achieve the maximum benefit of the diversity technique. If the receiver cannot be configured this way in actual use, then position the antennas as they will be used.

Walk around the area with the transmitter while speaking and try to find a location where a dropout or squelch (audio mute) occurs. Moving the transmitter around within a couple feet of a metal surface may help to generate a multi-path condition. The idea in this test is to see how prone the system is to producing dropouts, and to look for loud noise bursts that occur during a dropout if and when one does occur. An effective diversity system will make it difficult to find a dropout, which will tell you something about the effectiveness of the diversity circuitry. 

If and when a dropout does occur with a strong average RF level at the receiver, the receiver should simply mute the audio during the dropout and not allow any noise or noise burst to occur. An aggressive squelch system in the receiver is best in a close range situation, as it will eliminate noise bursts created by dropouts, however, it will also limit the maximum operating range as in the previous test. A less aggressive squelch allows maximum operating range, but will generally allow noise bursts to occur during dropouts at close range.

The two walk tests "Classic" and "Short Range" illustrate the dilemma of a conventional squelch system in having to choose between either close range or distant operating range, and also illustrates the benefit of an adaptive squelch system like the Lectrosonics SmartSquelchTM which automatically configures itself for close range or long distance operation as the system is being used. The tests are also a good proving ground for Lectrosonics SmartDiversityTM. 

After conducting both types of walk tests, you will have a good idea of what to expect in actual use. Some systems may provide excellent maximum range characteristics, but prove to be noisy in short range, multi-path conditions. Other systems may be great at the short range test, but be poor performers in the maximum range test. Of course, the ideal wireless system would do well in both tests.

The UH400TM has an extended low-frequency response (-3dB at 35Hz vs. -3dB at 70Hz) when compared to the UH400A. This allows for sound system measurement when testing low frequency system response.