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- Dr. Know's radio microphone help file |
| FAQs | Common problems | UK Frequencies | Reference Library | Radio mic hire | Your queries | Home |
Dr. Know is our resident Technical guru, responsible for the award-winning Reference Library, Technical Info, Data Sheets and the Audio Clinic. Visit the Audio Clinic by clicking here or write to dr.know@gbaudio.co.uk
STOP PRESS - Check out OFCOMs ill-considered plans to sell off the UK radio mic spectrum. Ofcom is considering re-allocating some or all of the radio mic spectrum, possibly within a 2 year time frame. If you currently use radio mics or in-ear monitors you must check this out! Please contact us to discuss the latest position if you are planning to purchase new wireless systems.
General notes
Wireless microphones are best considered as a system thus: The microphone capsule picks up the sound in the normal way. Then there is a battery powered radio transmitter pack (or built in to a handheld mic) which converts the audio into a radio signal, which transmits out in all directions. A receiver unit with antennas picks up the radio and converts it back into audio, which then feeds into a mixer channel, where it is controlled just like a normal microphone.
Wireless gives a great freedom of movement but is not without it's problems. The number of connections and processors added into the audio chain don't help reliability, let alone the unreliable and unpredictable things that can happen to low level radio transmission. Our rule is: if you are going to be performing in a fixed position or can manage to cope with a cabled mic then use a cabled mic. They virtually never fail. If you need unrestricted movement, then consider using a wireless system but please be aware of the potential pitfalls and try to minimise the risks by understanding how the system works and following good practices. top
Frequently asked Questions
What is the difference between Single
channel, Diversity and True Diversity?
Single channel receivers have one antenna and one receiver circuit. Diversity receivers
have two antennae and a circuit which selects the strongest signal. True Diversity
receivers have two totally independent RF receiver modules and a circuit which selects the
best channel for both signal strength and signal-to-noise ratio.
How many transmitters can I use with a
single receiver?
Only One. Two transmitters operating on
the same frequency will interfere with each other. It is possible to have both a handheld
and guitar transmitter on the same frequency, and use them with the same receiver, but not
simultaneously.
How far from the receiver can I place my
antenna?
As long as you use 50 Ohm fully
screened cable up to six metres, but the closer the better. It is possible, by employing
antenna amplifiers to run cables of up to ninety metres.
Can I mix systems from different
manufacturers?
Theoretically yes, but careful attention must be paid
to specifications, because different systems have different technical parameters
for equalisation and compression. Sometimes there are even compatibility issues
between different models from the same manufacturer. It would be prudent not to
assume cross compatibility between different manufacturers, unless tried and
tested
first.
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Is there a performance difference between
UHF and VHF?
No; there is little audible performance
difference but, with the high demand for a greater number of usable, interference free
frequencies, the UHF band offers more opportunities.
What is an LN Compander?
A means of preparing a signal with a
wide frequency range for transmission in a compressed form and expanding the signal back
to it's original state in the receiver, thereby improving the signal-to-noise ratio.
What is FM Modulation?
FM is the abbreviation for Frequency
Modulation, which is the method of transmitting information by modulating a carrier
frequency on a radio transmitter. The frequency of the carrier wave is varied in
accordance with the amplitude of the input signal, while the amplitude of the carrier
remains unchanged.
What is the difference between Squelch and
Mute?
Nothing; the output of a receiver is
muted by a squelch circuit when the received signal falls too low for reception. Some
units have preset mute levels, others have manual settings. It
should be set so that the receiver mutes when the transmitter is switched
off, or goes out of range, before the signal is swamped with noise or
interference. To achieve the optimum setting, place
the transmitter at a distance so that the receiver shows the carrier at
about half strength. It may help to remove the antenna from the transmitter.
Set the squelch control to maximum (ie muted) and then back it off until noise
starts to appear.
Then trim it back a tiny amount until it just mutes. If you are using multiple
systems, you will also need to check and adjust each squelch with all the other
transmitters switched on to make sure they don't interfere.
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Dr. Know is our resident Technical guru, responsible for the Reference Library, Technical Info, Data Sheets and the Audio Clinic. Visit the Audio Clinic by clicking here or write to dr.know@gbaudio.co.uk
Common Problems
Fragile connections between the very thin cable of lavalier mics and beltpacks are very vulnerable and liable to problems caused by pulling, twisting and straining when worn under costumes or during vigorous acting or aerobics.
Unwanted noises can be introduced in a number of ways: Bangs are commonly caused by intermittent battery contact, due to dirty or loose terminals. Clicks and scratchy noises are usually associated with fractured transmission antennae or poor earth continuity in the case of lavalier mics, due to fractured cable, dirty connectors or loose terminals.
Distorted sound is usually caused by the beltpack being overloaded by the microphone. Most beltpacks have a gain control to match the output of the microphone to the input sensitivity of the beltpack. For speech, a higher gain will be required but for singing or shouting, this will need to be attenuated for best results.
Dull sound can be caused by several things: the mic capsule poorly placed or trapped under clothing, the tiny mesh grille on top of the mic caked up with makeup or sweat. An unmatched transmitter and receiver can also cause this effect.
Drop out happens when wireless signals reflect off certain surfaces and therefore cancel out the direct signal on arrival at the antennae. Antennae should be positioned at least 1 metre from walls, floors or any other flat surface. Careful positioning of antennae to check for possible areas of drop out should be conducted before a performance. The closeness to antennae does not automatically guarantee operation. top
Poor RF signal strength may be caused by the transmission aerial being tangled or having dirty or broken contacts to the beltpack. Please don't just stuff it into a pocket. Try to dress the aerial in a straight line and preferably not on highly mobile places eg arms. Don't let the antenna contact bare skin as this has the effect of detuning the antenna - grounding RF through the skin - known as body absorption. Watch out for actors sweating; a sweaty shirt can act as a good conductor straight to the skin. One possible solution to this problem is to attach the transmitter to the belt with the antenna pointing downwards (giving clear instructions to the actor to take care when sitting down to avoid stretching or breaking the antenna). Incidentally, unavoidable Body Absorption is the reason why beltpack transmitters are allowed to have a more powerful output (typically 50mW in UK) than handheld devices (max 10mW in UK). Also check for low batteries. Only use alkaline batteries. Never use rechargeables as they only produce 80% of the normal voltage when fully charged and only last a very short time making them unreliable.
Very poor RF signal strength can be caused by metal getting in the way. In the theatre this is often a problem; steel girders, lighting bars, trussing, metal framed stage flats/platforms/risers, safety cages, suits of armour, even tin foil can reduce your signal strength. Remember that metalwork in the vicinity will act as an antenna and suck the transmitted signal away to ground. Sheet metal or bars or grille in between the transmitter and receiver will act as a Faraday Cage and stop radio from passing through.
Breakthrough can occur with cheap kits or when a large number of wireless systems are used together. Each system has it's own frequency but the actual bandwidth of the transmission is typically 30kHz i.e. 15kHz on either side of the centre frequency. Cheap units have a wider bandwidth which can spill and be picked up by other receivers on close frequencies, just like the breakthrough interference which occurs when two FM radio station frequencies are too close to each other. High quality transmitters have a tighter defined sideband and are therefore less likely to interfere with each other. top
Harmonic interference can also be picked up on other channels by receivers with poor input sections. Low level harmonics are generated and transmitted along with the original signal. These harmonics or ghost channels, although very weak, can sometimes interfere with other channels. Again, high quality receivers are better able to reject unwanted signals, whilst remaining locked on to the true signal.
Nervousness and anxiety have been cited as possible factors affecting radio mic performance. The reaction of electrical circuits to personal electromagnetic energy fields is a difficult phenomenon to understand, let alone measure scientifically in a performance scenario. However, I am certainly not alone in having witnessed situations where a wireless kit will work perfectly on Actor A but will drop out or give lower RF signal strength on Actor B without any obvious reason. Or Actor C who, after a perfect rehearsal, drops out or loses strength during the show. Indeed I have even heard anecdotal evidence of a certain actor for whom three (tested) industry standard lavalier mic capsules literally stopped working, dead. Changing to a different brand capsule made the same wireless system work, faultlessly. These cases are rare and the key is to address the proven scientific points (above) as far as possible. However, if every step has been taken to optimise performance then you may have to consider other methods of miking up a problem case, such as gun mics, plates or wired mics because there's nothing worse than a mic cutting in and out during a performance. top
Dr. Know is our resident Technical guru, responsible for the Reference Library, Technical Info, Data Sheets and the Audio Clinic. Visit the Audio Clinic by clicking here or write to dr.know@gbaudio.co.uk
UK frequency information
This summary users guide should help you understand the main choices
and limitations of radio microphone use in the UK. We have a range of systems available
for hire in most of the categories below, with licence cover if required. We have hand
held mics, lapel (tie tack) mics, headworn mics, instrument mics and line inputs.
Some other VHF and UHF frequencies are available for temporary use or for specific
geographic locations. You are welcome to call us for further information or visit the JFMG
website: http://www.jfmg.co.uk They administer
licences and site allocations. Reserved news and broadcast frequencies are not shown here.
To the best of our knowledge, this information is accurate at the present time
although it has recently (September 2006) come to light that Ofcom is
considering re-allocating some or all of the radio mic spectrum, possibly within
a 2 year time frame. Please contact us to discuss the latest position if you are
planning to purchase new wireless systems. No liability is accepted for any errors or
inaccuracies or changes which may occur or the consequences thereof.
| VHF deregulated frequencies. Anyone can use these frequencies for MPT1345/1311 type approved equipment. |
VHF regulated frequencies. MPT1350 type approved equipment may only be used at the licensed site. |
VHF regulated frequencies. MPT1350 type approved equipment may be licensed for use anywhere in the UK. |
| Free of charge | £75 per year
or £135 for two years for up to fifteen frequencies |
£75 per year
or £135 for two years for a block of fifteen frequencies |
173.800 MHz
* OK on it's own but may interfere with other units.
** Only legal until December 2007. |
176.400 MHz 192.300 MHz
|
175.250 MHz * 191.900 MHz |
| UHF deregulated frequencies. Anyone can use these frequencies for EN300 220 type approved equipment. |
UHF regulated frequencies. MPT1350 type approved equipment may be licensed for use anywhere in the UK. |
| Free of charge | £75 per year
or £135 for two years for a block of fourteen frequencies |
| 863.100 MHz to 864.900 MHz |
854.900 MHz 855.275 MHz 855.900 MHz 856.175 MHz 856.575 MHz 857.625 MHz 857.950 MHz * 858.200 MHz 858.650 MHz 860.400 MHz 860.900 MHz 861.200 MHz 861.550 MHz * 861.750 MHz |
Your queries (selected email correspondence) |
Dr Know's Answers |
| Dear Dr Know. Help! I'm very worried about these rumours about my radio mics being made illegal. What will become of our school's end of term show if we can't use radio mics? J.M. | Don't panic. OFCOM are still considering how to allocate the current radio mic bands and the rest of the freed up radio spectrum following the UK's switch over to digital TV which starts in October 2007. We've put together a separate information page explaining more about the Digital Dividend Review (DDR) and its potential impact on all UK radio mic users - and what you can do about it. |
| Dear Dr., Thanks for a very
informative and helpful section on radio microphones. I have suffered
almost all of the problems you describe and tend to avoid using them
whenever possible, having been let down badly during performances, by
sudden outbursts of loud hissing noises, cutting off signal etc., even
after flawless rehearsals, with the same equipment, people and positions.
I would like to ask you for a bit of clarification on one point which you
did mention. Our kit consists of two hand held Audio Technica mics, 174.5
and 175.0. We also have two Audio Technica lapel mics, 174.1 and 175.0.
They all have their own antenna/receivers. Does
a "system" as you describe, mean the transmitter/receiver pair
or just one frequency? What I am trying to find out is if I can use the
pair of 175.0's at the same time.?
Another question I have is about the
positioning of of the receivers, which I have normally crowded round my
mixing desk. Do they interfere with each other? |
We refer to one system or one
kit as a complete mic/transmitter/receiver/antenna on the same frequency. You can only use one kit on each frequency ie you can only use one kit on 175.00MHz. Which model number are the AT kits? We have an AT reference chart which shows which frequencies will work together without interfering. We always try to persuade users to only use radio mics when no other alternative is viable, because they introduce an extra degree of unreliability to the overall system. A wire is much more reliable and predictable. If you have an antenna distribution
amplifier, you can stack all the receivers together and use only one
pair of aerials. |
| Question: As a general rule, do these type of radio microphones use FM or AM? I also (regretfully) bought a very cheap (made in China) radio mic for less than £20. I think it works on freq just above the FM band. The receiver is very noisy; in fact too noisy to use - so I've effectively wasted £20!!! Another question:- What is the significant difference between these "cheap" mics and the more expensive ones? Is it a question of poor IF design or PLL detector, companding circuitry, etc - or just plain all round cheapness to cut costs? J.B. | The
answer is FM. Please check above for legal UK frequencies. Only those shown may
be used in UK. Other frequencies are allocated (and licensed) to broadcasters, military,
police, taxis, etc. Unfortunately, I agree that you have probably wasted your £20 You wouldn't expect a £20 alarm clock to sound very good so why would a radio mic? The ancient rule of "You get what you pay for" applies here. Consider the mic capsule - bear in mind that a decent wired hand held or tie mic costs £100+ There are so many factors which affect the radio section eg: transmission signal strength, frequency stability, compression. The receiver must also have precise sensitivity, frequency stability and compression. Broadcast radio mics can cost up to £3000 - this reflects the quality and reliability of the radio link. |
| Dear
Dr. Know
We are an amateur theatre group and own two Sennheiser EM 1031-V VHF True diversity radio mics. These have six adjustable frequencies, four of which fall into the deregulated band and are free, but two fall into the regulated band. Unless we expand and purchase more we won't be using the two regulated frequencies. Do we need a license? Would be interested in your opinions. A C |
No, you do not need a license in the situation you describe. Frequencies 173.800, 174.100, 174.500 and 175.000 mHz are free to use anywhere in UK, subject to the equipment conforming to type approval MPT1345 or 1311. Your Sennheiser 1031V system does conform to this type. Your 176.400 and 177.000 mHz channels may be used at any fixed site venue which holds a license for those frequencies i.e. it is the site which requires the approval and not the equipment. You cannot get a license to use them at different locations. In order to add more channels in the future, you will have to use high band VHF or UHF units (see above for frequency choices and equipment specifications). These will require a license (except the newly de-regulated UHF band) but you would be able to use them almost anywhere in UK. Meantime, it is perfectly legal and OK to own this equipment so long as you do not transmit on 176.400 or 177.000 mHz |
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Related sites:
myradiomall.com is a leader in providing Motorola two way radios, accessories, batteries, and portable 2-way radios.
jfmg.co.uk UK radio mic licensing and radio frequency information.