Digital headend

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Complete article

A digital headend (also headend or multiplex center) is a component of the systems for the distribution of digital radio signals. The term headend was adopted from the cable headend, which performs a comparable task in the distribution of analog broadcast signals. The digital headend receives the video, audio data (A / V data) and the additional data made available by the studio ( television studio , sound studio ), compresses and bundles them for further distribution to the radio receivers. The transmission to the radio receiver takes place via a satellite, terrestrial television transmitter (transmitter system), broadband cable or via the Internet.

Components and structure

A common digital headend essentially consists of:

• a switchable crossbar (also input matrix) for the distribution of audio and video data streams,
• one or more encoders to compress the video and audio data into a common broadcast standard ( MPEG-2 , MPEG-4 AVC ),
• a multiplexer for combining the data streams generated by the encoder usually into a transport stream ( MPEG-TS ) and
• Units for inserting additional data with a reference to the program

Integration in the production and distribution chain

The available program content (live, time-shifted, saved) is selected and played out over time by a playout . This program content is transmitted to the digital headend via a primary distribution with a high picture (resolution, color coding, frame rate) and sound quality. The transport stream generated in the digital headend is then transmitted via the secondary distribution, e.g. B transmitted to the television receivers on a terrestrial transmitter network.

Processing chain production and distribution of digital broadcasting

Input signals

The usual input signals can be divided into two classes:

• Uncompressed A / V data

This data is received via the standardized professional interfaces SDI , HD-SDI and 3G-SDI .

• Compressed A / V data

The usual interfaces for this are ASI , TS over IP and DVB-S or DVB-S2

processing

The processing essentially consists of two processing steps:

• The format conversion and data rate adaptation of the audio and video data. If necessary, the image size of the video data is scaled to a certain output size and usually encoded according to the MPEG-2 or MPEG-4 AVC standard . When the video data is compressed, the data rate is also adapted to the bandwidth of the output. Either a constant bit rate is assigned to each program or the programs share the available bandwidth variably ( variable bit rate ) in a statistical multiplex. The MPEG-1 Audio Layer 2 or MPEG-4 High Efficiency Advanced Audio Coding ( HE-AAC ) format with a constant bit rate is often used to compress the audio data .
• Combining (multiplexing) the compressed A / V streams and the additional data to form a transport stream . When merging, on the basis of a specified bandwidth, both the time conditions for an error-free display at the recipients and those for the respective transmission path are taken into account.

Output signals and formats

At the output, the digital headend delivers a transport stream that contains additional information that depends on the transmission path used to the television receiver.