Telephone hybrid
A telephone hybrid (also called an interface device (ANG)) is an electronic device with which audio signals from a telephone connection can be fed to a standardized audio output for further processing. At the same time, an external audio signal can be fed to the other party at the other end of the telephone connection.
Distribution and areas of application
Telephone hybrids are mainly found in radio studios and television control rooms. Areas of application here are telephone interviews, quiz games and surveys. The procedure is usually as follows: The audio signal from the hybrid is fed to the transmitter mixer, while the total sent is fed to the telephone hybrid. If necessary, the conversation partner can be heard on the air and hears everything that is broadcast, including the moderator who wants to speak to him. The moderator can conduct the conversation without needing a telephone receiver . He speaks normally into the transmitter microphone and hears the conversation partner through his headphones or the sound system.
Technical design
From a technical point of view, a telephone hybrid is required to decouple the incoming and outgoing telephone signals as well as possible in order to avoid feedback . For this reason, a conventional telephone cannot usually be modified for the tasks of a telephone hybrid.
There are usually three different line interfaces that are used: analog a / b line interfaces (POTS), ISDN interfaces or Ethernet interfaces for Voice-over-IP technology. The devices can be controlled via software, some devices also have their own keypad and the complete equipment of a conventional telephone, while others can only work in the vicinity of a telephone system for the purpose of connecting an incoming / outgoing call.
Telephone hybrids are also available as "active" electronic assemblies, which have to be supplied separately with operating voltage. The "passive" and therefore cheaper devices are supplied with the required operating voltage via the speech circuit power supply of the supply . The function of passive devices can be considerably restricted if they are used e.g. B. be operated on telephone systems whose speech circuit power supply is insufficient - z. B. only for the operation of simple telephones - is designed.
Overall, the scope of performance, quality and control of the audio signals in active telephone hybrids are far superior to passive devices.
With passive devices, an analog and component-saving circuit structure can be found throughout. Active devices are characterized - regardless of predominantly analog or digital signal processing - by higher quality components and more complex circuit structure, as well as by more setting options for the individual electronic subcircuits of the hybrid.
Performance characteristics
Different performance features expand the use of the hybrid. Some devices have a call detector to mark incoming calls with an optical signal. The “incoming call” signal generated for this then enables further functions. Professional devices have an "auto-call" function that automatically seizes the line after an adjustable number of calls. This is useful in a self-drive studio as the presenter can concentrate fully on the broadcast.
Compressors and expander circuits are also used in the signal paths to compensate for any overloads and volume fluctuations in the transmission and reception paths. In order to ensure optimal adaptation to the characteristics of the telephone line, filter blocks are often used in both directions. These only allow frequencies in the range from 300 to 3400 Hz to pass. Therefore, a professional hybrid generally sounds better than adapter circuits with simple transformers.
The adjustment of the telephone line is done either automatically with the help of a sample & hold circuit - or manually with some trimmers. The transition attenuation of the hybrid circuit between the transmitted and received signals is optimized.
Professional broadcast systems use the special N − 1 circuit ("N" minus 1), also known as "clean feed" or "mix minus". Here the mixer provides two output signals that differ in the proportion of the telephone channel. The entire production signal from the transmitter is sent to the hybrid - only the telephone signal is missing here. (There are also applications in which only the signal from the speaker's microphone is sent to the hybrid. However, feeds from the current program - music / quiz noises ... - are not possible).
The N − 1 circuit avoids a feedback loop. Therefore, the professional telephone hybrids without "internal N-1" formation cannot be operated on simple mixing consoles ( semi-professional consoles without AUX paths) without difficulty. A loop is created that leads to a resonance point on the telephone line. If the assigned channel for the telephone line (by the hybrid) is postponed, the resulting feedback creates a whistle. There are also external N-1 boxes that generate the required signal through active subtraction. The circuits are often implemented with differential amplifiers, operational amplifiers being used.
The connection to an analog telephone line takes place either in parallel or as an additional device in the N socket of a TAE socket. Clamp connections are also used when the hybrid is operated in a permanent installation.
Individual evidence
- ↑ Telephone hybrids - AVT Audio Video Technologies GmbH. Retrieved June 29, 2017 (German).