POTS codec

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A POTS codec is a device used in broadcast engineering to send high-fidelity digital audio over regular telephone lines. A hardware codec, implemented with digital signal processing, is used to compress the audio data enough to travel at a bitrate equivalent to a 56k modem. Thus, it is a modem able to use plain old telephone service (POTS) to send monophonic (and now stereophonic) audio.

An earlier version of the technology is the ISDN codec, which instead uses ISDN lines, and uses channel bonding for stereo.

[edit] Uses

The main use of a POTS or ISDN codec is for remote broadcasting by radio stations,

ISDN has the advantage of being natively digital, and a full 64 or 128kb/s when using both channels, which means less compression and potential compression artifacts. However, special lines must be run to a location, and must be ordered well in advance of the event so that there is plenty of time for installation. Because of this, ISDN codecs are generally only useful for recurring events such as weekly nightclubs, or major events which warrant the expense. ISDN is also no longer being installed in some areas, being supplanted by DSL, which does not guarantee quality of service though a virtual circuit as ISDN does.

Since POTS lines are almost universally available, the POTS codec can be set up nearly anywhere with little or no notice. Advances in compression have allowed the POTS codec, introduced in the 1990s, to overtake the ISDN codec in the 2000s.

Digital codecs are far superior to the old telephone hybrids and frequency extenders previously used to send analog audio over one or more lines. Telephone codecs also eliminate the need for remote pickup units, a wireless method of remote broadcasting. RPUs are still used in situations where no telephone line is available.

[edit] Functions

Both kinds of codec usually come in two types of units: rackmount for the studio and portable for the remote. Audio can be sent in either direction, and most can also pass low-speed non-audio data, allowing the remote DJ to control broadcast automation or other studio equipment via RS-232. Many have an automatic redial if the line should become disconnected. The remote unit usually has some basic mixer functions, while the studio unit usually has some kind of digital output.

Some codecs can use both ISDN and POTS, implementing codecs like G.722, MPEG AAC, aacPlus, and AAC-LD (low-delay). Some can even use GSM, and some have variable bitrate to compensate for poor connections. It is also possible to bond two POTS lines together for redundancy and fault tolerance, and better bandwidth. A MPEG-1 Layer II codec even supports audio over IP.