Telephone Hybrids

Audio Codecs



Audio Codecs are needed for high-quality Audio transmissions over different networks like IP, ISDN, 2-Mbit/s (E1) and X.21. Over IP and ISDN, both Leased Line connections as well as temporary dial-up connections can be used. There is a wide range of applications in which Audio Codecs are required, such as e.g. reporting, studio programme contribution and distribution as well as Studio-Transmitter-Links.

For installations in OB vans and reporting applications, it is often required to have com - pact and mobile systems. Especially for setups which are running 24/7, the systems need to be highly reliable. Very often it is required to have an automatic backup function for these systems.

Whereas ISDN and 2-Mbit/s were the main technologies used in the past, nowadays AoIP is the transmission standard. The European Broadcasting Union has defined a stand - ard for AoIP dial-up connections already in 2007. All important Audio Codec manufac - turers are supporting this standard (EBU Tech 3326) which ensures interoperability between Audio Codecs from different manufacturers. This is a great benefit compared to ISDN where no real standard has been created. Although it is planned in various countries that ISDN will be switched off in the future, some broadcasters are still using ISDN in parallel to IP or as backup. Depending on the application different coding algorithms are used.

The selection of the coding algorithm depends on the available bitrate, the desired quality and the accept - able delay. The EBU names the following Audio algorithms as mandatory to comply with the AoIP standard. G.711, G.722, ISO/MPEG Layer 2 and PCM (for stationary Audio Co - decs). Furthermore, MPEG4 AAC-LC, MPEG4 AAC-LD and apt-X are recommended as further algorithms.

For AoIP dial-up connections a SIP Server can be used. The Audio Codec registers at the SIP Server with a SIP account and a password. The SIP account corresponds to the telephone number under which the Audio Codec can be reached. If no SIP Server is used, the Audio Codec can be called only via its IP address. In Germany, the public broadcasters have installed one big SIP Server at the ARD Sternpunkt in Frankfurt where all users can register to communicate with each other.



Hardware Codecs

STL Audio Contribution System

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