A radio microphone system consists of:
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a microphone which could be any type of microphone but for many television programmes is a personal or lapel microphone; |
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a small FM transmitter carried by the programme participant (or it can be built into larger microphones) into which plugs the microphone; |
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a short aerial attached to the transmitter; |
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a receiver often mounted on the rear of the camera/recorder, powered by the camera’s power supply and designed to receive the same frequency signal as the transmitter; |
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an XLR connector cable from the radio receiver to an audio input on the camera usually at mic level (but check – some receiver outputs are at high level). |
Personal radio mics have been designed to be small and unobtrusive and their appearance is usually acceptable to be seen in-vision. It is good practice, when using several radio mics, to have the same type and clip for uniformity in vision and for ease of maintenance if spares are required. Although the most common application of a radio mic is as a lapel/personal type microphone, any microphone can be attached to a radio transmitter when it is difficult to lay out cable, for example, at a press conference when shooting from the back of a room. The transmitter’s range can be 50 metres or more depending on the site.
Choice of operating frequency
The power of the transmitter and its frequency are controlled by licence and legislation, which varies between different countries. In the UK the maximum power is 10 milliwatts which gives an effective radiated power of 2 milliwatts. This is to ensure that their use does not conflict with other users of radio communications. Location radio mics usually transmit on the VHF (138–250 MHz) band and in the UK, the use of frequencies between 173.8 MHz and 175 MHz does not have to be licensed. Each radio mic transmitter at the same location will need to operate at a different frequency with some separation, (minimum 0.2 MHz), otherwise they will interfere with each other. With the necessary 0.2 MHz channel separation, five common frequencies are 173.8, 174.1, 174.5, 174.8 and 175.0 MHz. An essential design requirement for radio mic transmitters is that they must be crystal controlled to ensure that their transmitting frequency does not drift outside their specified limits.
Design features
Most radio microphone transmitters/receivers are fitted with some or all of the following facilities:
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The ability to send a 1 kHz tone to check continuity of transmission strength and an input gain on the receiver to balance the use of various microphones with a signal strength indicator; |
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a ‘compander’ to compress the signal before transmission and expand the signal after reception to improve the signal-to-noise ratio and bandwidth. This is switchable at the receiver and transmitter and both must be either on or off; |
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avoid multipath reception (e.g. reflections from metal structures) ‘dead spots’ when the programme participant is moving, and signal cancellation by the interaction when a number of radio microphones are in operation, diversity systems are fitted. This involves having two receiving aerials and a circuit which switches to the strongest and least distorted signal path. |
Lining-up and rigging a radio microphone
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Check the condition of the transmitter battery and fit a good quality alkaline battery. Be aware that nickel cadmium rechargeable cells decay below operating voltage with very little notice. |
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The microphone needs to be clipped or taped at about 200 mm from the mouth about mid-chest on a lapel, tie or adjacent clothing. Personal microphones are condenser microphones but some are more prone to wind noise or rustling clothes than others. Choose a design that is suitable for location work. Omnidirectional personal microphones can be rigged pointing up or down. If the speaker is liable to look down and talk, breathing across the microphone (e.g. a cooking demonstration), point the capsule down. Use adhesive tape to secure the microphone when hidden under clothing to ensure that it cannot move and rub against the fabric. If they are to be fixed directly to the skin, use non-allergenic plaster or a sterilized strip. |
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Obtain a sound level check and adjust the gain on the transmitter using the peak level indicator on the transmitter. An LED is often fitted which flashes to indicate the onset of limiting. Set the gain with normal voice level so that limiting occurs and then reduce the gain so that only unexpected peaks will limit. |
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Rig the transmitter in a pouch, belt or pocket of the artiste so that it is visually unobtrusive and away from any metal objects carried by them. The aerial should either hang vertically down or be taped upwards if the participant is sitting. |
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The receiving aerial should also be upright and, if available, switch in zero level tone from the transmitter and set receiver level to zero level. |
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Adjust the input audio level on the camera to read -20 dB on the digital audio level meter on the display panel. |
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If time allows, ask the presenter/participant to talk and walk the anticipated area of movement. Check there is no interference to the signal path from large reflective surfaces. The receiver is fitted with a squelch circuit that mutes the output if there is insufficient signal strength. If this occurs try and identify the source of the problem, try a different frequency or modify the area of movement. It is better to prove the signal path in a rehearsal than discover problems on the recording, or worse, on transmission. |
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Problems are often experienced with man-made fabrics causing noise and static. If the capsule is rigged below certain types of dense fabric, sound quality and level will not be satisfactory. If the rustling of clothing is picked up, a windshield or small cage can be fitted to keep the fabric away from the microphone. |
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Avoid rigging personal microphones near necklaces or jewellery that could come into contact with the capsule if the artiste moves causing bangs and clicks. |
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Check for wind noise and use a windshield if necessary. As the capsule is usually quite close to the mouth and shielded by the body, the balance of background effects to voice is usually quite good although the balance is constant regardless of shot size and desirable sound perspective. |