Modern Telephone Interfaces

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Trends in Technology, Aug 2010

Telephone communications have advanced at starlight speed from the early experimentation days of Alexander Graham Bell, Thomas Edison and Jimmy Smith. Who was Jimmy Smith, you ask? He and his buddy Pete were two of the many who discovered they could talk to each other at a distance by tying a piece of string to two cans and then communicating with each other over the taut string.

Long gone are the days when live operators sat in front of a telephone switchboard all day patching in telephone lines and, in many cases, dialing the requested phone number. Those operators were some of the first telephone interfaces. Now, all kinds of things come to mind when one hears the phrase telephone interface. In place of cans, string and patch cables, we now have terms that include hybrid, conferencing, VoIP, PBX, SIP-trunking, T1/E1, ISDN, BRI, traditional telephone service, DSL, Wi-Fi, acoustic echo cancellation, digital dynamic EQ, adjustable smart leveler, symmetrical wide-range AGC, noise gating and pitch shifting - all of which have become available through the advancement of microelectronics. The telephone interface vocabulary section of our minds has become so invaded that it is difficult to determine what's what, which unit accomplishes a given list of specifically required tasks and if its purchase would fit into an allocated budget. I hope to provide a guide through this telephone interface maze.

Basics defined

A telephone interface provides for a connection between a telephone device (wired or wireless) and a station's audio equipment. There are several manufacturers of telephone interfaces, and their applications are widely varied. The mention of a specific manufacturer's name or model of equipment is not meant to be an endorsement of product. Rather, it is to simply inform and educate the reader. Companies that manufacture telephone interfaces include AEQ, Comrex, JK Audio, Telos and many others.

The simplest and often easiest to use telephone interfaces are those intended for single-line use - either with a standard single-line telephone or a mobile phone device. Like all telephone interfaces, the single-line units allow the user to send audio to or receive audio from the connected telephone. The device has internal hybrid nulling that provides maximum isolation of sound between the caller and the in-studio host. The isolation, often known as acoustic echo cancellation, reduces or totally cancels the echo, the empty barrel sound that is sometimes heard on broadcasts that do not use proper telephone connectivity. Most interfaces of this type also have built-in automatic gain control (AGC) circuitry that works to keep the caller's volume at a consistent level. Additional audio control features found in some models include caller audio compression and limiting. Caller ducking, a feature whereby the caller's voice level is reduced when the announcer is talking, is another feature found on some models and offers the announcer improved control over the caller's volume.

Many interfaces incorporate some form of digital signal processing (DSP) for improved sound quality and ease of setup. Most will likely provide for analog and AES/EBU audio input and output connections. All telephone interfaces require some type of mix-minus circuit capable of providing caller audio from the device to the studio console or recording equipment. They also send the announcer audio to the caller without the caller being able to hear their voice. Otherwise, the caller would hear his own voice being sent back when speaking and become confused. Basic single-line telephone interfaces require an externally provided mix-minus circuit, which is typically generated in the studio console. Sophisticated interfaces may create the required mix-minus circuitry internally. Telephone interfaces are often used for remote broadcasts, news feeds or talk shows whether broadcast over radio or the Internet.

Climbing up a rung on the telephone interface ladder, we find portable single-line phone interfaces that include an audio mixer. Many units of this type can provide up to 15kHz mono audio over a standard telephone line when using a mating unit at the studio - and in some cases, nearly equal audio quality over certain mobile phone connections. Program and cue back audio can typically be sent over the same line. Devices of this type will often automatically scale their audio quality dependent on the performance and reliability of the connected line. For stations that require remote broadcast equipment for use at a sports or local business venue, interfaces of this type typically provide for the connection of one or more professional microphones, auxiliary audio inputs, a headphone jack for monitoring and, often, auxiliary contacts for remote synchronization of devices. Advanced features may include a built-in audio limiter and an audio output jack for recording or for connection to a PA system. Stations that require field audio mixing capability and seek a step up in audio quality will find advantages with interfaces that accept ISDN modules providing for the use of MP3 or G.722 technology.

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