Telephone hybrid
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A telephone hybrid is a relatively simple electronic device used to connect telephone line to studio audio circuits. These are normally used in radio stations (and sometimes TV stations and broadcast networks) to connect callers into the airchain, so that conversations may be broadcast. A key benefit is to isolate the audio from the caller from that of the studio host so that the caller's audio can be played in the studio without feedback while still allowing the caller to hear the host over the telephone.
The main principle at work is impedance matching, although the hybrid must also block the 48 V direct current of the POTS telephone line from the normal audio side. Some inexpensive designs connect to the handset cord, with a button to activate either the handset or the hybrid. These only cost around US$100, need no power, and don't even need to block DC as none goes through the handset. More expensive versions can cost thousands of dollars or more, but can handle multiple lines, and may connect to a computer so that a producer can keep up with who is on which line, and what goes on the air next. Many also perform audio level compression, and even full digital signal processing to make the bandwidth-limited telephone audio sound more acceptable on-air.
Digital hybrids are even used for broadcasting over standard telephone systems, using a special unit with DSP audio data compression and decompression at each end. Audio bandwidths up to 15 kHz (full FM broadcast quality, and the range of most human hearing) can be achieved this way, along with slow auxiliary data that can remotely trigger relays on the mixing console or other automated equipment back at the station. Compression is often via MPEG, particularly now MPEG-4.
[edit] Modern technology
The audio quality of a telephone conversation is bad for AM & FM broadcasting, with a frequency response of about 300 – 3,000 Hz. Several technologies have been devised to solve this problem. Among them is the frequency extender that shifts the audio spectrum 250 Hz, reducing even further the high frequency response of the phone line. This system needs a remote encoder and a studio decoder. Frequency extender devices are now almost discontinued.
Digital systems, like Internet IP communications [1] or ISDN systems at the European market [2] are now in use for remote transmission between a journalist and the radio station.
When the radio station needs to call a person at the home or office telephone line for an interview, these systems don't work, and the only technology that works for telephone or cellular calls from a conventional telephone set is the Voice Quality Restoration (in short VQR). This technology, developed by Oscar Bonello, is based on the way the vocal cords create the sound. [3] Although the fundamental tones are lost, the harmonics above 300 Hz are in the audio spectrum and can be recognized, as Ernst Terhardt demonstrated [4] Then, using special algorithms this device can restore to a good audio quality the lost fundamental tones of the human voice. This new device is of the “one side” type. It does not need a special coding in the remote side; a simple conventional telephone set is accepted. A VQR phone hybrid is only needed at the radio studio. Some interesting audio samples can be heard at this reference: [5]
[edit] Notes
- ^ Please see AEQ device http://www.aeq.es/spn/cat/phoenix.pdf
- ^ See AEQ http://www.aeq.es/spn/pr_tle02d.htm
- ^ White paper: Scientific bases of VQR Technology, Oscar Bonello (Spanish), http://www.solidynepro.com/Documentos/TeoriaVQR.pdf
- ^ Algorithm for Extraction of pitch and pitch salience from complex tone signals, Terhardt-Stoll & Seewann, Journal of the Acoustical Society of America, 1982 page 679
- ^ You can hear audio samples of VQR system at: http://www.solidynepro.com/indexahtmlp_HA200ENG-v,m_3,p.htm
