The theatre is putting on a new musical and they are changing from traditional rifle microphones to radio microphone system. The theatre has some concerns over doing this, they are: transmitter operation with regard to power supplies, inter-modulation problems and choice of transmitter frequency band and output power.
Please can you help me with this problem. I have a list of equipment below if it helps you.
15 SK 50 VHF pocket transmitters (5 additional SK for instruments)
5 MKE 2 clip-on mics
3 EM 1046 systems
1 Cadac J-type mixing console (72-inputs, 12 matrix and 24 matrix outputs)
41 loudspeaker system.
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There are four basic configurations of wireless microphone systems, related to the mobility of the transmitter and receiver components, as required for different applications. But we will focus on systems consisting of a portable transmitter and a stationary receiver. The transmitter is usually carried by the user, who is free to move about, while the receiver is located in a fixed position. The input source in this setup is normally a microphone or an electronic musical instrument. The receiver output is typically sent to a sound system, recording equipment, or a broadcast system. This is the configuration of the standard "wireless microphone" and is the arrangement most widely used in entertainment, public address, and broadcast applications.
Fixed receivers are offered in two basic external configurations: diversity and non-diversity. Non-diversity receivers are equipped with a single antenna while diversity receivers generally have two antennas. Both systems may offer otherwise similar outward features: units may be free standing or rack-mountable; outputs may include balanced/unbalanced microphone or line level as well as headphones; indicators for power and audio/radio signal level may be present; controls for power and permanently attached.
Though diversity receivers tend to include more features than non-diversity types, the choice of a diversity vs. a non-diversity receiver is usually dictated by performance and reliability considerations. Diversity receivers can significantly improve both qualities by minimizing the effect of variations in radio signal strength in a given reception area.
The net received signal strength at any location is the sum of the direct and reflected waves. These waves can reinforce or interfere with each other depending on their relative amplitude and phase. The result is substantial variation in average signal strength throughout an area. This creates the possibility of degradation or loss of the radio signal at certain points in space, even when the transmitter is at a relatively short distance from the receiver. Cancellation of the signal can occur when the direct and indirect waves are similar in amplitude and opposite in phase.
The audible effects of such signal strength variation range from a slight swishing sound ("noise-up"), to severe noises ("hits"), to complete loss of audio ("dropout").
Diversity refers to the general principle of using multiple (usually two) antennas to take advantage of the very low probability of simultaneous dropouts at two different antenna locations. "Different" means that the signals are substantially independent at each location. This is also sometimes called "space diversity", referring to the space between the antennas. In most cases, at least 1/4 wavelength separation between antennas is necessary for significant diversity effect, though increased benefit may be had by greater separation.
General rules concerning setup and use of receiver antennas:
First, maintain line-of-sight between the transmitter and receiver antennas as much as possible. Avoid metal objects, walls, and large numbers of people between the receiving antenna and its associated transmitter. Ideally, this means that receiving antennas should be in the same room as the transmitters and elevated above the audience or other obstructions.
Second, keep the receiver antenna at a reasonable distance to the transmitter. The maximum distance is not constant but is limited by transmitter power, intervening objects, interference, and receiver sensitivity. Closer is preferable to farther but a minimum distance of about 10 feet is recommended to avoid potential intermodulation products in the receiver. Ideally, it is better to have the antenna/receiver combination near the transmitter (and run a long audio cable) than to run a long antenna cable or to transmit over long distances.
Third, use the proper type of receiver antenna. A 1/4-wave antenna can be used if it is mounted directly to the receiver, to an antenna distribution device or to another panel which acts as a ground-plane. If the antenna is to be located at a distance from the receiver a 1/2-wave antenna is recommended. This type has somewhat increased sensitivity over the 1/4-wave and does not require a ground-plane. For installations requiring more distant antenna placement or in cases of strong interfering sources it may be necessary to use a directional (Yagi or log-periodic) antenna suitably aimed.
Fourth, select the correctly tuned receiver antenna(s). Most antennas have a finite bandwidth making them suitable for receivers operating within only a certain frequency band. When antenna distribution systems are used receivers should be grouped with antennas of the appropriate frequency band as much as possible. If the range of receiver frequencies spans two adjacent antenna bands, the longer (lower frequency) antennas should be used. If the range spans all three antenna bands (for VHF), one long antenna and one short antenna should be used (no middle length antenna). Telescoping antennas should be extended to their proper length.
Fifth, locate diversity receiver antennas a suitable distance apart. For diversity reception the minimum separation for significant benefit is 1/4 wavelength (about 17 inches for VHF). The effect improves somewhat up to a separation of about one wavelength. Diversity performance does not change substantially beyond this separation distance. However, in some large area applications overall coverage can be improved by further separation. In these cases one or both antennas may located to provide a shorter average distance to the transmitter(s) throughout the operating area.
Sixth, locate receiver antennas away from any suspected sources of interference. These include other receiver antennas as well as sources mentioned earlier such as digital equipment, AC power equipment, etc.
Seventh, mount receiver antennas away from metal objects. Ideally, antennas should be in the open or else perpendicular to metal structures such as racks, grids, metal studs, etc. They should be at least 1/4 wavelength from any parallel metal structure. All antennas in a multiple system setup should be at least 1/4 wavelength apart.
Eighth, orient receiver antennas properly. If transmitter antennas are generally vertical then receiver antennas should be approximately vertical as well. If transmitter antenna orientation is unpredictable then receiver antennas may be oriented up to 45 degrees from vertical. Yagi and log-periodic types should be oriented with their transverse elements vertical.
Ninth, use the proper antenna cable for remotely locating receiver antennas. A minimum length of the appropriate low-loss cable equipped with suitable connectors will give the best results. Refer to the chart presented earlier. Because of increasing losses at higher frequencies, UHF systems may require special cables.
Tenth, use an antenna distribution system when possible. This will minimize the overall number of antennas and may reduce interference problems with multiple receivers. For two receivers a passive splitter may be used. For three or more receivers active splitters are strongly recommended. Verify proper antenna tuning as mentioned above. Antenna amplifiers are not usually recommended for VHF systems but may be required for long cable UHF systems.
Most manufacturers recommend only alkaline type batteries for proper operation. Alkaline batteries have a much higher power capacity, more favorable discharge rate and longer storage life than other types of single-use batteries such as carbon-zinc. Alkaline types will operate up to 10 times longer than so-called "heavy duty" non-alkaline cells. They are also far less likely to cause corrosion problems if left in the unit. Consider bulk purchase of alkaline batteries to get the greatest economy: they have a shelf life of at least one year.
Use rechargeable batteries with extreme caution: their power capacity is much lower than the same size alkaline and their actual initial voltage is usually less. The conventional rechargeable battery uses a Ni-Cad (nickel-cadmium) cell. The voltage of an individual Ni-Cad cell is 1.2 volts rather than the 1.5 volts of an alkaline cell. This is a 20% lower starting voltage per cell.
Receivers for theatrical applications are not unique but they must be of high quality to allow multiple system use without interference. It is not unusual to use as many as 30 simultaneous wireless microphone systems in a professional musical theater production. This number can only be handled with systems operating in the UHF range. 10 to 15 systems is the practical limit at VHF frequencies. In addition, separate antennas and antenna distribution systems are necessary for any installation involving a large number of systems.
Every wireless microphone system transmits and receives on a specific radio frequency, called the operating frequency. Allocation and regulation of radio frequencies is supervised by specific government agencies in each country with the result that allowable (legal) frequencies and frequency bands differ from country to country. In addition to frequency, these agencies typically specify other aspects of the equipment itself, including: allowable transmitter power, maximum deviation (for FM), spurious emissions, etc. These specifications differ from one band to another and from one user to another within a given band. For this reason, it is not possible to select a specific frequency or even frequency band that is (legally) usable in all parts of the world. Furthermore, it is not possible to design a single type of wireless equipment that will satisfy the specifications of all or even most of these agencies around the globe.
The frequencies used for these systems may be grouped into four general bands or ranges: low-band VHF (49-108 MHz), high-band VHF (169-216 MHz), low-band UHF (450-806 MHz) and high-band UHF (900-952 MHz). VHF stands for "Very High Frequency", UHF stands for "Ultra High Frequency".
Except for assistive listening systems, however, low-band VHF is not recommended for serious applications. Due to the large number of primary and secondary users, and high levels of general radio frequency (RF) "noise", this band is prone to interference from many sources.
Next is the high-band VHF range, the most widely used for professional applications, and in which quality systems are available at a variety of prices.Unfortunately, the primary users in this band include many business band and government operations such as forestry, hydro-electric power stations, and the Coast Guard.
The primary physical characteristic of UHF radio waves is their much shorter wavelength (one-third to two-thirds of a meter). The most apparent consequence of this is the much shorter length of antennas for UHF wireless microphone systems. One less obvious consequence is reduced radio wave propagation both through the air and through other nonmetallic materials such as walls and human bodies, resulting in potentially less range for comparable radiated power. Another is the increased amount of radio wave reflections by smaller metal objects, resulting in comparatively more frequent and more severe interference due to multi-path (dropouts). However, diversity receivers are very effective in the UHF band, and the required antenna spacing is minimal.
The available radio spectrum for UHF wireless microphone system use is almost eight times greater than for high-band VHF. This allows for a much larger number of systems to be operated simultaneously. Like high-band VHF, licensing is still required in this UHF band. The required minimum 1/4 wave antenna size for UHF radio waves is 4-7 inches (only one-quarter to one-half that for VHF). Equipment is moderately expensive and diversity systems are strongly recommended, but high quality audio can be achieved along with a large number of simultaneous systems.
The process of wireless microphone system selection involves choosing first an appropriate radio frequency band and second the desired number of operating frequencies within that band.
Operating Frequencies: Intermodulation
A single wireless microphone system can theoretically be used on any operating frequency. When a second system is added it must be on a different operating frequency in order to be used at the same time as the first. This limitation arises from the nature of radio receivers: they cannot properly demodulate more than one signal on the same frequency. In other words, it is not possible for a receiver to "mix" the signals from multiple transmitters. If one signal is substantially stronger than the others it will "capture" the receiver and block out the other signals. If the signals are of comparable strength none of them will be received clearly.
If the wireless microphone systems must be on different frequencies, how "different" should they be? The limiting characteristic of the receiver in this regard is its "selectivity" or its ability to differentiate between adjacent frequencies. The greater the selectivity the closer together the operating frequencies can be. Most manufacturers recommend a minimum frequency difference of 400 kHz (0.4 MHz) between any two systems
When a third system is added to the group it is no longer sufficient only that the frequencies be at least 400 kHz apart. In order to choose a frequency for a third system to be compatible with the first two it is necessary to consider potential interactions between the operating frequencies. The most important type of interaction is called intermodulation (IM), and it arises when signals are applied to non-linear circuits.
A characteristic of a non-linear circuit is that its output contains "new" signals in addition to the original signals that were applied to the circuit. These additional signals are called IM products and are produced within the components themselves. They consist of sums and differences of the original signals, multiples of the original signals, and sums and differences of the multiples. Non-linear circuits are inherent to the design of wireless components and include the output stages of transmitters and the input stages of receivers.
IM can also occur when transmitters are operated very close to receivers. In this case IM products are generated in the receiver input stage which can interfere with the desired signal or be detected by the receiver if the desired signal (transmitter) is not present.
The simplest IM products that can occur between any two operating frequencies (f1 and f2) are the sum of the two frequencies and the difference between the two frequencies:
f1 + f2
f1 - f2
If we choose f1 = 200 MHz and f2 = 195 MHz, then:
f1 + f2 = 200 + 195 = 395 MHz
f1 - f2 = 200 - 195 = 5 MHz
These IM products are sufficiently far away from the original frequencies that they will generally not cause problems to a third wireless microphone system in the original frequency band.
However, as mentioned earlier, the other products of non-linear circuits are multiples of the original (fundamental) frequency. That is, application of a single frequency to a non-linear circuit will produce additional products at double, triple, quadruple, etc. the original frequency. Fortunately, the strength of these products decreases rapidly as the multiplier increases. The practical result is that only the products at two times and three times the original frequency are significant. Since these products then combine with the original frequencies, the following additional products can occur:
(2 x f1)
(2 x f2)
(3 x f1)
(3 x f2)
(2 x f1) ± f2
(2 x f2) ± f1
(2 x f1) ± (2 x f2)
(3 x f1) ± f2
(3 x f2) ± f1
(3 x f1) ± (2 x f2)
(3 x f2) ± (2 x f1)
(3 x f1) ± (3 x f2)
The "order" or type of IM product is identified by the particular combination of frequencies that created it. The order of an IM product is the sum of the multipliers (coefficients) of the frequencies in the expressions above.
The complete group of possible frequencies (original frequencies, intermodulation products and combinations) that can exist when two systems (at 200 MHz and 195 MHz for this example) are operated simultaneously is thus:
Product Order Frequency Significant
2 x f1
2 x f2
f1 + f2
f1 - f2
3 x f1
3 x f2
(2 x f1) + f2
(2 x f1) - f2
(2 x f2) + f1
(2 x f2) - f1
(3 x f1) + f2
(3 x f1) - f2
(3 x f2) + f1
(3 x f2) - f1
(2 x f1) + (2 x f2)
(2 x f1) - (2 x f2)
(3 x f1) + (2 x f2)
(3 x f1) - (2 x f2)
(3 x f2) + (2 x f1)
(3 x f2) - (2 x f1)
(3 x f1) + (3 x f2)
(3 x f1) - (3 x f2) 1
Though this list of calculated frequency combinations is lengthy, it can be seen that only the IM products at 185, 190, 205 and 210 MHz are in the same general band as the two original operating frequencies. These products will not cause compatibility problems between the two original systems but can interfere with other systems that may be added in this band. In this example, the operating frequency of a third system should be chosen to avoid these four IM frequencies.
In general, only odd-order IM products are considered because even-order products typically fall well away from the original frequencies. Furthermore, though higher odd-order IM products may also fall near the original frequencies, only 3rd order and 5th order IM products are strong enough to be of concern.
In order to avoid potential IM problems most manufacturers recommend a minimum margin of 250 kHz (0.25 MHz) between any 3rd order IM product and any operating frequency. This further restricts available frequency choices as the number of simultaneous systems increases. It should be apparent from this discussion that the prediction of potential compatibility problems due to IM products is best left to computer programs. The complexity increases exponentially for additional systems: a group of 10 wireless microphone systems involves thousands of calculations.