2-wire systems
The 2 wire circuit was designed primarily for headset use. This format modulates many voice channels onto the same wire. This way the circuit only requires one live connection and one reference (earth) to support many users. Each circuit is called a loop. This has the advantage over the 4 wire circuit in that only one pair of wires needs to be run from the system PSU (Power Supply Unit) that are looped from one user to another; rather than the 4 wire circuits need to have each user individually connected to the circuit destination.
Audio signals are unbalanced and have 200ohm impedance levels to aid low interference.
Audio signals are unbalanced and have 200ohm impedance levels to aid low interference.
4-wire systems
The 4-wire system requires 4 wires to complete a 2-way communication circuit. Each direction of the circuit requires 2 wires over a balanced cable. A earth screen is often used to aid the blockage of interference in long cable runs. A 2 volt peak-to-peak signal level also helps with interference.
Four wire circuits are known as P-P (Point-to-point) circuits.
Four wire circuits are known as P-P (Point-to-point) circuits.
At each end of the circuit each user has a microphone powered with an amplifier and a talk button to connect the amplifier to the circuit. The button is a push to make, the connection is not made unless the button is pushed. This type of talkback is called 'switched talkback'.
This is often used when the commuications wants to be directed to a certain user, thus the user can only here what is required for them over a loudspeaker. Should both parties key at once then there will be an audio 'howl-round'. This is explained as one parties loudspeaker diectly feeding into their microphone and send over the wire to the other party where the process is repeated. A design of the talkback panel is that the loudspeaker is dimmed in volume when the button is pressed to make the circuit.
Open talkback is an open mic. There is no effect from a button and all users can here all the chat over the microphone at all time without control.
This is often used when the commuications wants to be directed to a certain user, thus the user can only here what is required for them over a loudspeaker. Should both parties key at once then there will be an audio 'howl-round'. This is explained as one parties loudspeaker diectly feeding into their microphone and send over the wire to the other party where the process is repeated. A design of the talkback panel is that the loudspeaker is dimmed in volume when the button is pressed to make the circuit.
Open talkback is an open mic. There is no effect from a button and all users can here all the chat over the microphone at all time without control.
ifb Circuits
IFB (Interruptible foldback) circuits are a 4 wire system variation often used for presenter talkback. A one way balanced circuit is used with a additional input circuit (standing feed) which is usually present unless the talk button is engaged when the foldback through circuit is interrupted and redirected to the operators voice.
This circuit can be used to feed programme sound to the presenter and to give the director/producer to over key or talk over the programme audio to feed instructions. This format alllows the presenter to always know what is happening without the direct conversation accessible to other crew.
Circuit converters
Due to 2-wire circuits carrying speech for both directions and 4-wire systems using 2 pairs they cannot be connected together without a signal converter.
This unit nulls any common components of the audio signal when comparing the 4-wire signal being sent to the 2-wire circuit with the signal being returned from the 2-wire circuit.
When the system is not properly balanced a ringing will be heard or in extreme cases howling will be heard. Digital systems use digital system algorithms to constantly analyse the lines adjusting the system dynamically.
This unit nulls any common components of the audio signal when comparing the 4-wire signal being sent to the 2-wire circuit with the signal being returned from the 2-wire circuit.
When the system is not properly balanced a ringing will be heard or in extreme cases howling will be heard. Digital systems use digital system algorithms to constantly analyse the lines adjusting the system dynamically.