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Frequency/Range

I have wireless over 700MHz - do I really have to stop using them after June 12, 2010?

Yes. No if's, and's, or but's about it. This is not new - everyone was supposed to vacate that band…

Yes. No if's, and's, or but's about it. This is not new - everyone was supposed to vacate that band LAST June in 2009. The FCC is getting serious about it now. The loss of this spectrum has been in the works since 1995. If you have gear in that range from Lectrosonics and it is under 5 years old, it may qualify for a frequency change program. It will not be free but it will be less costly than replacing all that gear - even with other brands who offer "rebates" or "trade-ins". 

Posted 3 months agoby LectroAdmin
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What is the 944-952 MHz band? Who can use it?

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

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.

Posted 3 months agoby LectroAdmin
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What are the advantages of 75 kHz deviation as allowed in the US versus the narrower 50 kHz (or less) deviation allowed in Europe and some other parts of the world?

Deviation is the measure of how far a frequency modulated RF carrier can change frequency in response to a signal…

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.

Posted 3 months agoby LectroAdmin
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What is a receiver's image frequency?

In general, wireless receivers have a local oscillator that mixes with the incoming RF signal to produce a lower frequency…

In general, wireless receivers have a local oscillator that mixes with the incoming RF signal to produce a lower frequency signal at the Intermediate Frequency (IF) that is then processed in the rest of the receiver. For instance, in a 180 MHz, CR187 receiver, the IF is at 21.4 MHz. This is much easier to filter and amplify than the transmitter's 180 MHz carrier. To produce this signal, the incoming 180 MHz is mixed with a signal at 158.6 MHz to produce a difference signal at 21.4 MHz.

The mixer can also mix a signal at 137.2 MHz to 21.4 MHz since the difference between the local oscillator at 158.6 and 137.2 MHz is also 21.4 MHz. Therefore there are two signals that can easily produce 21.4 MHz by mixing with 158.6 MHz: the desired 180 MHz and the "image" of 137.2 MHz. The mixer is equally sensitive to either signal and without a front end, a receiver is just as sensitive at the image frequency as it is at the desired frequency. The RF front end, which is tuned to pass 180 MHz but stop the image of 137.2 MHz prevents any response to the image. This is why the 187 series receivers have multiple helical resonators in the front end. The front end has to smack the image down 100 dB or more. This image frequency should be taken into consideration when doing frequency co-ordination, though with modern receivers, the image is almost completely rejected.

Posted 3 months agoby LectroAdmin
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Here's a reply to a problem with range on RAMPS. The test described can be done with any of our UHF receivers.

Vin,
I didn't get into the thread before, because I wanted to make some distance versus RF display measurements before I started mouthing off. What we wanted to determine was a setup such that a user could make a simple, repeatable measurement to check our equipment for proper operation. This "test" should show the proper operation (or not) of a Lectro transmitter and a Venue receiver. Here's the setup:

  • Two vertical right angle whip antennas, type A500RA, are on the back of the Venue receiver.
  • The Venue receiver is about 1 meter above the ground with no large metal objects close to the antennas other than a metal or plastic cart surface under the Venue and the metal case of the Venue itself.
  • The UM or SM transmitter is held by the metal case at arm's length away from the body with the antenna vertical.
  • Line of sight between the transmitter and the Venue.
  • All done outdoors.

We are not trying to duplicate a real use case here but we are trying to eliminate all variables such as body and clothing absorption (15 dB), antenna gain factors (0 to 5 dB), defective antenna amplifiers (30 dB), bad cabling (60 dB), reinforcement from room walls (6 dB), etc. Under our simplified but repeatable conditions, whether you have RF interference or not, the Venue RF display for a good system will be full scale at a separation of 100 meters. You may get dips due to multipath but moving a foot or more in any direction should get you out of the multipath. Again, the maximum reading is the correct one for this test since multipath will rarely increase the signal more than 6 dB but can decrease the signal by 30 dB.

We did this test in our parking lot with a clear line of sight between a UM400 100 mW transmitter and a block 26 Venue receiver. To double check the real world against theory we did a path loss calculation. Here's the path loss formula:
The formula for path loss between two 0dBd antennas given a separation D and a wavelength y with y=0.461 meters at 650 MHz is Path Loss in dB = 22 + 20 Log (D/y).

For 650 MHz at 100 meters separation the Path Loss = 69 dB. Since a 100 mW transmitter power level is is 20 dBm then the signal at the Venue antenna is 20 dBm - 69 dB = -49 dBm. Full scale on the Venue is 1000 uV or -47 dBm. (Remember 0 dBm at 50 Ohms is 0.224 Volts not .775 Volts as in a 600 Ohm audio system.) In any case, the -47 dBm at full scale is scarily close to the theoretical path loss result of -49 dBm. Ground bounce reflections can add 6 dB to the numbers and diversity antenna addition can add 3 dB. In any case, with some hand waving, the actual measurements seem very valid.

As long you have line of sight between the transmitter and Venue, it doesn't make any difference what kind of ground surface you are on. Interference, even at high levels will only increase your RF readings on the Venue scale. The European 50 mW units will shorten the 100 meter readings to 70 meters. The LM would be about 70 to 90 meters depending on the particular LM. The SMq, UM250 and UM450 250 mW units will increase the full scale range to 160 meters.

This test only checks the power level of the transmitter and the RF operation of the receiver; it does not address any added factors such as interference in the area or the rest of your setup. It does give you a starting point for diagnosing problems but it is only valid under the 5 conditions above. If you perform the above test and the results are good, then you can start adding antennas and cabling to the system. The remote antennas, cables, amplifiers, etc., should give at least the same distance results as this right angle whip test or there is something wrong. Before I get buried in replies that say full scale at 100 meters is impossible, reread the 5 strict requirements above; this is a very special test setup.

If you get the correct readings for RF level, then the next most probable cause of short range is interference. Then the Venue scanning function should find the problem.

Posted 3 months agoby LectroAdmin
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This is the same text as that engraved on the back of the UCR401 and UCR411a receivers.

FOR SPECTRUM ANALYZER
Press all 3 keys simultaneously to either enter or exit the spectrum analyzer. MENU key to stop, zoom or start scan. Zoom is indicated by < > icons. In zoom, since  most data is off the screen, the cursor is  centered and the data scrolls. Use the UP  and DOWN keys to scroll. To save cursor  frequency, press all three simultaneously  and then select "use new". To clear  spectrum, turn power OFF briefly.

FOR PILOT BYPASS
Step the menu key to the MAIN window.  Press the MENU and UP keys together for  b bypassed or p normal plot.

FOR THE 1 kHz TEST TONE
Step menu key to SETUP/EXIT window.  Step SEL UP key to SETUP/TONE  window. Press TONE (MENU) key. Press  TONE (UP) key. Step LVL (UP/DOWN)  keys to set tone level. Press MENU key to  stop and EXIT tone.

TO LOCK AND UNLOCK
Press and hold the MENU  key for 5 seconds.

TO RESET BATTERY TIMER
Press and hold MENU and DOWN key together  for one second.

Posted 3 months agoby LectroAdmin
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Can 100 mW transmitters be safely used in theatre applications or is 100 mW far too much power?

--Clearing up the Issue of Higher Transmitter RF Power and IM For many years, Lectrosonics has built wireless transmitters that…

--Clearing up the Issue of Higher Transmitter RF Power and IM

For many years, Lectrosonics has built wireless transmitters that are higher-powered than those from other vendors. In addition, when we say a typical power of 100mW, we don’t mean that we had one engineering sample reach that power with a westerly wind. We center our production on 100mW with small manufacturing variations both up and down. We have built 100mW units for what we feel are four good reasons:

  1. Increased range
  2. Fewer dropouts
  3. Reduction of interference
  4. Better signal-to-noise ratio


There have been discussions about problems with higher powered units like ours, but the only real negative of higher power when properly implemented is a slightly increased drain on transmitter batteries. Since most of the battery power is used supporting digital processing in the Lectrosonics’ transmitters, the increased RF power is only a minor consideration. Another way of saying it is, if you have made the choice of digital processing in the transmitter, you might as well have increased RF power too, since it doesn’t change the battery life that much. The new generation of rechargeable batteries can reduce battery costs for all wireless systems and battery usage isn’t quite as important a consideration as it was several years ago.

--Design Considerations

Careful design has removed the two other problems that are sometimes discussed when higher power is considered. The possible problem of increased intermodulation (IM) in the output stage of the transmitters at higher power has been solved by using an output isolator in the antenna circuit. This prevents two transmitters that are physically close together from creating IM products. This isolator is unique to Lectrosonics’ transmitters and is in all transmitters except the LM series. Possible IM in the receivers has likewise been solved, because Lectrosonics’ receivers have always had higher power RF stages for substantially better IM rejection than competing receivers, so the increased transmitter power is no problem whatsoever. 

The technology used in the Lectrosonics products is all fine and good but the real question revolves around what happens in actual use. What about a real world situation with lots of transmitters on a stage? We’ve been hearing stories that 100mW transmitters are absolutely unusable in a stage environment and that 100mW systems will wipe out the wireless operations of theatres over city blocks, if not the entire Eastern Seaboard.

--Real World Numbers

Let’s first apply a little common sense and then run some numbers, first looking at general reception issues and then those specific to transmitters:

If intermodulation was really a consistent problem with 100mW transmitters then it would be only slightly less of a problem with 50mW transmitters, all things being equal. The only way to change from a supposedly “real” problem to a rare occurrence would be to make a radical change in transmitter power such as down to 5mW or less. A 3dB difference in power doesn’t mean much when typical signal levels on a stage are making 50dB swings as people move around. Just as an increase in power from 50mW to 100mW doesn’t solve all dropout problems and only extends effective range by 30%, reducing power from 100mW to 50mW won’t solve all IM and transmitter interference problems if there were any to begin with. In the same way, even a 3 to 1 change from 100mW to 30mW is insignificant when signal levels at the receiver can vary by 100,000 to 1, i.e., the previously mentioned 50dB.

To show why there shouldn’t be IM problems at either power level, let’s do the numbers. We will analyze the signals that would be present at a receiver from a 100mW transmitter in the worst possible situation and then in a real world situation. Even better, we will use published numbers from other manufacturers rather than our own measurements (even though they are about the same). We will do a third order analysis since it is accepted as being the worst case. Second and fourth order products are not a problem because they are totally removed by the receiver’s RF filters and fifth order and higher are at much lower levels than third order.

Posted 3 months agoby LectroAdmin
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What is frequency diversity and how is it used on the Venue receiver system?

Frequency diversity is the sending of the same information using two different frequencies of transmission or two different transmitters set…

Frequency diversity is the sending of the same information using two different frequencies of transmission or two different transmitters set to different frequencies. The idea is that the receiver(s) will choose which frequency has the better signal at any given moment and use that as its preferred signal. This is a method that is independent of spatial diversity (two spaced antennas). It is in fact possible to use both spatial and frequency diversity and gain benefits from both. It does require two transmitters and two microphones but it gives a substantial increase in redundancy and immunity to drop outs. See page 19 of the Venue receiver manual for more information.

Posted 3 months agoby LectroAdmin
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How much frequency separation do I need between transmitters and receivers in a bag system? The transmitters will be within 12" of the receivers.

This is a long posting made to the RAMPS news group. There is a clearer explanation here: see FAQ I…

This is a long posting made to the RAMPS news group. There is a clearer explanation here: see FAQ I am setting up a bag system. What should I consider as far as types of receivers, frequencies of operation and antenna setup?

Some weeks ago we made measurements of a simulated bag system to see what having a 100 mW transmitter 12 inches (30 cm) away from receivers would do to the receivers' sensitivity. If you read the previous post, you will remember I was somewhat surprised at the performance of the UCR210 receiver in this test. I thought the older, helical resonator front end, single frequency UCR195 would carry the day. In fact, the UCR210 performed as well and in some areas was a bit better. Some users on the group were wondering how the UCR201 would perform in a bag even though this was never our intended use for the 201. This time the numbers are more in line with what I would guess, since the 201 is definitely weaker in this test. The first set of numbers are for the UCR210 and the second set are for the UCR201. I'm not going to try to put them on the same line since I think various news readers will mangle the formatting. To see how the test was run, I included all the text of the previous test below the new numbers. Again, I'll put frequency separation in MHz, then dB of desensing, then resulting percentage of range with a percent and then a comma as a line separator.

Again, the explanation of the test setup and procedure are in the text after this posting.

(UCR210 from the previous test)

  • 0.5M 20dB 10%,
  • 1.0M 14dB 20%,
  • 1.5M 12dB 25%,
  • 3.0M 10dB 32%,
  • 4.0M 9.8dB 33%
  • 6.0M 3.6dB 66%,
  • 10.M 2.3dB 77%,
  • 20.M 1.8dB 81%

(UCR201 from a new test)

  • 0.5M 89dB 0%,
  • 1.0M 77dB 0%,
  • 1.5M 18dB 12%,
  • 3.0M 23dB 7%,
  • 4.0M 17dB 14%,
  • 6.0M 11dB 28%,
  • 10.M 6dB 50%,
  • 20.M 3dB 71%,
  • 30.M 2dB 80%

As can be seen from comparing the numbers, the UCR201 needs twice the frequency separation before the ranges are comparable. The 3 MHz number looks funny but that's what we measured. The 50% of normal range is reached by the UCR210 at 5 MHz of frequency difference while the UCR201 needs 10 MHz of separation. I would recommend separation of at least one of our blocks (25 MHz) between the 201 receivers and transmitters in the bag. On the other hand, the UCR210 can operate inside the same block with a little care. The difference is due primarily to the tracking front end in the 210 and secondarily due to the higher power level of the first RF transistor in the front end of the 210. Once the signal is past the front end, both receivers are essentially the same.

We ran the same tests with reduced transmitter power (30 mW equivalent) and found a one to one reduction in desensing. The 5 dB reduction in power (100 mW to 30 mW) reduced the desensing by 5 dB. So you could take the above numbers and reduce the desensing by 5 dB which would be equivalent to a 30 mW system at 12 inches or a 100 mW system at 18 inches (45 cm). Note that the levels drop faster than the square of the distance at very close distances. A 10 mW transmitter would improve the desensing numbers by 10 dB. The reason I mention 10 and 30 mW is those are the power levels of some other popular transmitters on the market. I don't think the matching receivers will be quite as tough as the UCR201 but with the lower transmitter power, the upshot is the 201 numbers probably will describe what other systems would do. The one exception, to the best of my knowledge, is the small Sennheiser receiver that does not have an RF amplifier but takes the antenna input directly to the mixer. It has a very good third order intercept but does not have the sensitivity of other Sennheiser receivers. It would be interesting to measure it and see if the resistance to desensing would counter balance the lack of an RF stage. I'm sure it would. A good universal rule of thumb is that 10 MHz of separation with reasonable quality systems will give you 50% of your range. You will need at least 5 MHz even with the UCR210.

[[[ Below is the text of the original measurements and a complete description of the setup and parameters. The new measurements above were made in the same manner. We tried to make sure we were comparing apples to apples.]]]


We did some interesting RF measurements on a simulated two way bag system to see how much the bag transmitters would affect the bag receivers' sensitivity. A two way bag system will at least consist of multiple receivers to receive audio signals from the talent, a portable mixer to mix the audio and one or more transmitters to retransmit mixed audio to the video cameras. The immediate question is "If the receivers and transmitters are on different frequencies why should the transmitter reduce the sensitivity of the receiver?" One obvious answer is that the RF front end of the receiver is not a perfect filter and can let strong, nearby frequencies pass through and overload the first amplifier. In addition, transmitters do not produce a single sharp frequency but have some noise 5 Mhz or more from the carrier. The levels are very low but bag systems have antennas that are very close together. In the same way, the local oscillator in the receiver produces some noise many MHz away from the desired frequency and acts the same as having noise in the transmitter. Instead of trying to calculate all this stuff it is simpler to just measure a simulated system.

We put a transmitter 12" (30cm) away from an antenna mounted on a power meter and measured an average signal of -5dBm (.5mW) from a transmitter with 20 dBm output (100 mW). This is a very strong signal to bleed into a receiver but will be very typical of a bag system with 12" of separation. We used this level for the interfering transmitter for all the sensitivity tests. We then checked the receiver sensitivity with the transmitter off and then on and measured the reduction in receiver sensitivity for different frequency offsets between the transmitter and receiver. So to simulate a bag system where the talent's transmitter is on 540 MHz and the bag is re-transmitting mixed audio to the camera on 550 MHz, we would inject a 550 MHz signal at -5dBm into a UCR210 receiver set at 540 MHz and see how much that affected the receiver's ability to pick up the desired 540 MHz signal. We attenuated a block 21 UM200C transmitter set at 550 MHz down to -5 dBm and combined it with a weak 540 MHz signal from a signal generator, set the receiver to 540 MHz and checked the sensitivity with the transmitter off and then on. With the transmitter off, the receiver had a normal sensitivity of -107 dBm for 30 dB SINAD.(Same as "signal to noise ratio" at these values) With the transmitter on, the sensitivity fell to -104.7 dBm for a decrease in sensitivity of 2.3 dB. Or the receiver was desensed by 2.3 dB. This means that in the real world, a 10 MHz offset in the two systems' frequencies with the antennas 12" apart, the usable range from the talent to the bag would have been reduced by 23%. This is pretty small and surprised me. I thought it would be much worse. (There is no reduction in the distance from the bag to the camera since the receiver at the camera is not near a transmitter.) At 0.5 MHz separation with the talent transmitter and bag receiver still at 540 MHz and the bag transmitter at 540.500 MHz, the desensing was much worse at 20 dB. This would reduce the talent to bag range to 10% of normal and is a good reason to never operate with only 0.5 MHz frequency separation. Here's some more values for a UM200 UCR210 system and I hope the news readers don't totally mangle the formatting. I'll put frequency, then dB of desensing, then resulting range with a percent and then a comma as a line separator. Something should get through.

  • 0.5M 20dB 10%,
  • 1.0M 14dB 20%,
  • 1.5M 12dB 25%,
  • 3.0M 10dB 32%,
  • 4.0M 9.8dB 33%
  • 6.0M 3.6dB 66%,
  • 10.M 2.3dB 77%,
  • 20.M 1.8dB 81%

The above numbers are for a system with a tracking front end with relatively high power RF amplifiers both of which help. The relatively high power transmitter of course hurts. If you didn't need much range, bag to camera, you could attenuate the transmitter. The better solution would be to move the transmitter away from the receiver. On the other hand, 5 Mhz of frequency separation would leave you with 50% of your range and that would still be a gracious plenty in 99% of the cases. This is all a good reason to have the transmitters and receivers in a bag system separated by a frequency block (25 MHz). But if you have to operate in one TV channel, 6 MHz, off set the frequencies as much as reasonable.

I had postulated before we tried this experiment that the helical resonators in the fixed frequency UCR195 receiver might work better in a 2 way bag system than the higher power RF amps and wider tracking filters in the UCR210. Here's the numbers for a UCR195D at 536.250 MHz and a block 21 UM200C. The measurements were made the same as above,frequency, then dB of desensing, then resulting range and a comma.

  • 0.5M 30dB 3%,
  • 1.0M 28dB 4%,
  • 1.5M 26dB 5%,
  • 3.0M 25dB 8%,
  • 4.0M 7.8dB 41% 
  • 6.0M 4.4dB 60%,
  • 10.M 3.1dB 70%,
  • 20.M 2.8dB 72%

The numbers are much worse for separations less than 4 MHz. Basically the dual gate GasFet is wiped out in the UCR195D until the helical resonators can start filtering at 4 MHz off frequency. Note the rapid change in the numbers between 3 and 4 MHz. This is where the helicals really begin to filter. Overall, the higher power RF amps in the UCR210 carry the day, though the helicals are roughly equivalent at 4 MHz and higher. If I can say something in self defense here, that is why the newer Lectros are rightly accused of eating batteries; they burn it in the front ends.

Having been major wrong about the UCR195 and the helicals, I thought we'd test out my hypothesis that the "rock" transmitters (crystal controlled UM195) would have less noise off frequency than the synthesized transmitters such as the UM200. Here we used a UM195 on 536.250 and varied the frequency of a block 21 UCR210 to check sensitivity versus separation. Note the frequency offsets are a little different.

  • 1.5M 28dB 4%,
  • 2.0M 25dB 8%,
  • 4.0M 17dB 14%,
  • 6.0M 7.0dB 45%,
  • 10.M 4.2dB 62%

Notice that the range is much more affected by the UM195 transmitter. The receiver is the same UCR210 used in the first measurements yet the receiver is now much more affected. The only explanation is that the UM195 is noisier off frequency than the UM200C which is not what I would have expected. By the way, this "noise" would only show up under these bag conditions. Twelve inch separations from transmitters to receivers are not usual. We will measure some different UM195's but at the moment the synthesized radios seem to perform easily as well as the fixed frequency units in bag system conditions. Now where's the crow seasoning.

Posted 3 months agoby LectroAdmin
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Why are some Lectro transmitters only flat to 70 Hz?

Here's the total and long winded story on Lectro low end frequency response. Once upon a time, all the transmitters…

Here's the total and long winded story on Lectro low end frequency response. Once upon a time, all the transmitters were set up to be flat (1 dB down)to 50 Hz. Some UM195 users were having problems with low frequency rumble driving the bass compander and causing rumble modulated breathing in the system. We did 3 different fixes in the system, one of which was to roll off the input at 70 to 90 Hz. We picked this number cause that's what Vega was using and they were the Big Dogs at that time and we figured, correctly, that they knew what they were doing. This improved the operation of the systems and all seemed well except that Jerry Bruck (the Schoeps importer) wanted response down to 50 Hz. So for a while we did a Jerry Bruck modification that was basically back to the original input reponse. Jerry knew what he was doing and could handle the room rumble and wind noise in other ways. Then a lazy engineer (me) decided this was too much paperwork to track all his orders and redesigned the UM series transmitters to have a variable rolloff. That way Jerry could have his cake and we didn't have to build bake a special. You can hear the rolloff on voice if the rolloff is set to maximum (185 Hz). If you set it for 100 Hz or lower, it leaves voice alone.

The UH195 and UH200 plug on series were never rolled off since most pro microphones have a low end roll off anyway or have switchable filters. Or they have extended bass reponse and the user has selected the mic for that bass reponse. In any case, we left the UH's alone and they are flat to below 50 Hz.

Posted 3 months agoby LectroAdmin
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