Frequently Asked Questions - Wireless
RAMPS is an acronym for "rec.arts.movies.production.sound". This is a news group for sound mixers in the film and video industry as well as others that have similar interests in field recording of sound. There are a number of very helpful pros that support the group and there is a lot for anyone to learn. You will need a news reader to access the group. You can use the link below to access RAMPS using Google Groups. Go to RAMPS group
If you are using alkaline batteries, you will have very short battery life. The MM400 and the SM transmitters use a single AA battery to reduce size and weight. Since they are 100 mW transmitters, the load on the single battery is unusually high, about 450 mAh. Alkaline batteries are not designed for that high a current and will last less than 2 hours at room temperature. In cold weather, the run time is much less and can be on the order of minutes.
Lithium batteries can provide higher currents and will run slightly more than 6 hours at room temperature. If run a few hours at a time, they can provide a maximum of 8 hours of life. However at freezing temperatures they have shorter run times. (See FAQ #087-WIRELESS)
NiMh batteries are now available with capacity ratings of 2500 mAh and will operate the MM400 or SM transmitters for slightly over 4 hours. The above times were made with very fresh batteries; as the ads say "Your mileage may vary".
With 15 minute chargers available from several large and reliable battery manufacturers, they are a very viable alternative. Some of these chargers (RayOVac and Eveready) will operate from a 12 VDC source. Finally, the NiMh batteries are not nearly as sensitive to low temperatures as other battery types.
A BIAST is used with the UFM50 antenna amplifier. It provides DC phantom power on the coaxial line to the UFM50 so a customer does not have to run a separate cable to power the UFM50. Some Lectrosonics receivers already have phantom power built in to the antenna connectors and don't need a bias T.
Using the bias T at the receiver with a CH-20 will put 12 VDC at up to 150 mA to power the UFM50. The internal polyfuse is a 300mA unit and should allow 150 mA under any temperature conditions. The UFM50 pulls 85 mA at 12 VDC. Good quality UHF cable, should allow several miles of length before resistance drops would reduce the available voltage. Since the amplifier can only compensate for 400 feet of Belden 9913, the lowest loss medium sized cable, this will never be a problem.
The bias T consists of a feed inductor to apply DC to the BNC that goes to UFM50 (the antenna side) and a blocking capacitor to keep DC off the receiver BNC side. The RF is connected directly from one BNC to the other with only the blocking cap in series. A locking power jack connects to the CH-20, the recommended power supply. A series diode and the polyfuse are in the circuit for protection against reverse voltage and shorts.
The Bias T is effective from 60MHz to 950 MHz with less than 1 dB of loss. From 150 MHz to 850 MHz the loss is less than .5 dB. The unit is labled for customer ease of operation.
See also FAQ #089-WIRELESS for wiring a COS-11 for more normal sound levels than the 114 dB SPL assumed in this FAQ #025.
We recommend a 2k to 4k source resistor. Wire the white lead to the resistor and the other end of the resistor to ground. The black drain lead is wired to the center pin. We do not recommend just wiring the source lead (white) to ground. Below is a long post to the RAMPS group that explains why:
After my post about the COS-11 wiring, I was asked by a dealer if the lower output COS-11 red dot wasn't a simpler solution than building a resistor into the connector to simulate a three wire hookup on a MM400, which is a two wire system. So we went back and made more measurements. Lectro isn't set up to make precision sound level measurements but we faked it fairly well. I used some B&W 602's (7" Kevlar cone) to produce a surprisingly low distortion 400 Hz audio signal (less than 0.3%) and a Radio Shack sound level meter for level measurements. The Radio Shack was calibrated to a B&K sound level meter and is actually pretty decent. As further proof of the pudding, some lavaliere mics that were spec'ed as overloading at 118 dB, indeed did overload at those levels. We placed the mic about 3 inches from the loudspeaker cone so we could easily get levels of 125 dB at the microphone.
With 114 dB into the mic and using a two wire mic setup (signal taken from the black wire or drain), we set a mixer level of 0 dBm for a COS-11 grey dot(?) with a shorted-to-ground source, and measured -8.8 dBm for the same mic with a 2.2k source resistor. A COS-11 red dot measured -9.1 dBm shorted to ground and -18 dBm with a 2.2k resistor. Distortion levels in the same sequence were grey dot shorted 9%, grey dot with 2.2k 1%, red dot shorted 2%, and red dot with 2.2k 0.44%. These results are pretty much what would be expected; the source resistor reduces gain and provides negative feedback to the FET, both of which reduce the distortion. In addition, the red dot is a lower gain mic and therefore has less distortion at high sound pressure levels. What is most interesting here is that a grey dot with a source resistor has less distortion than the red dot shorted at the same sound pressure level and at the same output level (gain).
We now increased the sound pressure level to 124 dB and ran the same sequence. Distortion levels were grey dot shorted (unusable)%, grey dot with 2.2k is 1.75%, red dot shorted is 8%, and red dot with 2.2k, is 0.7%. Again, these results are pretty much what would be expected; again the source resistor reduces gain and provides negative feedback to the FET, both of which reduce the distortion. Again,the red dot is a lower gain mic and therefore has less distortion at high sound pressure levels. This is a repeat of the previous findings and once again a grey dot with a source resistor has less distortion than the red dot shorted at the same sound pressure level and at the same output level (gain).
These numbers jive fairly well with info from Sanken's site. So my recommended hookup for a MM400 is to use a 2k or 4k source resistor with either of the Sanken microphones. The MM400a transmitter will be driven about 6 dB into compression when set for minimum gain before the microphone starts to clip and this will be at sound pressure levels of 120 dB and 130 dB respectively. Without the source resistor, the levels will be more than 10 dB lower. For the UM200 or UM400 transmitters, always wire the microphones as three wire microphones which is shield to pin 1, black to pin 2, white to pin 3, and pin 4 grounded back to pin 1.
To reduce the gain of the standard B6 4 dB, put a 4k resistor in place of the 10k across the B6 leads in the connector, i.e., from the center pin of the MM400 connector to ground. Three dB is due to a reduced load and a dB or so due to reduced current. This will get the max output of the B6 close to the max non limiting input of the MM400a. If this is still too hot and you are uncomfortable with running the MM400a at minimum gain in loud situations, then I recommend the reduced gain B6 which is down 10 dB from the standard unit.
The big change was the way we indicated the onset of limiting in the transmitter. The LED was changed to green to warn our users that this was a different system than in the past. The previous red LED on the B version came on just before limiting and users were setting the gain too low on the transmitter and having noise artifacts from both the compander and RF link. On the C version, the green LED is turned on by the same matched FET as is used in the limiter itself so the LED comes on when you are in limiting and not before. Too many users were being "scared" by the previous limit LED even though the limiter is pretty smooth and IMHO, does little damage to the signal.
In scientific tests performed on anybody we could drag into engineering, we found that the gain on the C version is set about 10 dB higher by the average user than on the B version. What is really important though, is the questions and complaints from end users about low level noise, artifacts, etc. have dropped off by a factor of ten. In reality, if the gains are set the same on the B and C versions they will perform the same.
The biggest unseen change to the C transmitters is the addition of a circular isolator or circulator to the output stage of the transmitter. This is a magnetically polarized non linear ferrite device that has three ports, any of which can be used as input or output. What's black magic about this device, is power applied to port A goes to port B, but power applied to port B goes only to port C, and power applied to port C goes only to port A. What this does for a transmitter is this: the output stage is connected to port A of the circulator and the power is delivered to port B which is connected to the antenna. The transmitter acts just like a regular transmitter so far. However, if the antenna tied to port B picks up power from another transmitter such as a two way radio or more commonly another wireless , this power doesn't get back into the output stage that is connected to port A but gets transferred to port C. Port C is tied to a 50 Ohm resistor and the incoming RF is simply dissipated as heat. The circulator reduces intermodulation between transmitters by 30 to 40 dB. Intermodulation between transmitters is probably as common as intermodulation in receiver front ends and some interference attributed to the receiver may be really due to transmitter intermod.
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.
The Venue receiver will do both antenna and frequency diversity simultaneously. The reduction in drop outs and noise ups should be just as dramatic as the reduction going from single antenna reception to regular diversity reception. Definitely use two antennas. An antenna port is a terrible thing to waste.
Here's what I think I know about antenna orientation. If you are outdoors with the transmitter in the normal position with the transmitter antenna vertical, then you will get the best antenna strength ON THE AVERAGE with the dipole antenna arms also vertical. Inside where you have lots of reflections and the antenna signals are coming from all over, then vertical or horizontal polarizations are about the same. So vertical polarization is still, the safest orientation. There are two exceptions to this. If the transmitter antenna is horizontal, then the dipole arms should also be horizontal. Or if you can't separate the two dipole antennas by ideally at least a 1/2 wavelength (8 inches) to get good space diversity, then you can go to polarization diversity by rotating one antenna 90 degrees to the other. My advice is, if you can get the antennas 8 inches apart or more, then go with the vertical polarization.