Digital Audio and Codecs II

For the transmission of TV sound to the customer, several procedures and audio codecs are available today.

Encoding formats for stereo TV (2.0)

Today there are several methods and audio codecs available to deliver audio for transmission: ISO MPEG 1 Layer 2 to Dolby AC-4.

In the case of SDTV and HDTV, stereo TV sound is often still transmitted in ISO-MPEG1 Layer 2 with data rates between 192 or 256 kbit/s. Smaller data rates (such as 96 or 128 kbit/s for stereo) should be used with caution to the expected quality degradation.

ISO-MPEG1 Layer 2 can be configured for mono or stereo and is also suitable for the transmission of surround sound in Dolby ProLogic II format (Lt / Rt with 4.0 or 5.1 surround) from a data rate of 192 kbit/s stereo. It is also possible to use Dolby Digital or Dolby Digital Plus in stereo mode (2.0) for the transmission of stereo sound. Dolby Digital Plus can also transfer the audio description in addition to the stereo main mix.

For some HDTV transmissions AAC and Dolby AC-3 or AC-4 are used, depending on the transmission network of the operator or country.

Matrix encoding formats for multi-channels (Lt / Rt)

Matrix encoding is used to deliver multiple audio channels within fewer audio transmitted audio paths, directly to the viewers receiver. Matrix coding works by combining audio tracks for transmission and then decoding them into an approximation of how the surround sound should be played back.

Broadcasters use digitally implemented phase matrixing and can transmit up to six audio channels (5.1) over each two-channel audio medium, which provides a good sound quality to data rate balance (starting at approximately 192 kbit / s with MPEG sound). The output signals of a surround sound track encoded in this way are referred to as Lt and Rt, which is for "Left Total" and "Right Total", i.e. a matrixed sum of up to 6 channels.

DTS Neural Surround and Dolby ProLogic II are available for professional applications for sending sound via contribution, as well as broadcasting to the consumer (distribution). DTS Neural Surround and Dolby ProLogic II are compatible with each other and both allow the transmission of 4.0, 5.0 and 5.1 as a Lt / Rt signal.

Matrix encoding formats in TV and home use

Discrete encoding formats for multichannel

If there are only two channels, but there are bit-transparent digital recording and contribution routes available, Dolby E can be used for coding in professional environments. Using Dolby metadata, it transmits 6-channels at 16-bits and 8 channels at 20-bits resolution over the transmission path and combined with timecode (VITC, LTC). Video Sync (Black & Burst) is essential for working with Dolby E,.

Dolby E (see the table below) adjusts itself by means of black burst as the video sync signals. This allow for exact frame duration of the video signal to be detected, so sound and pictures can be switched or cut in sync.

The prerequisite for working with Dolby E is an exact clock synchronisation of sound and images, and a complete bit-transparency of all transmission and recording media. Due to its high quality and robustness, Dolby E maintains its audio quality, even after 10 - 13 coding cycles, in terms of acoustics and manipulation in multiple ways. The metadata from Dolby E is passed on to Dolby Digital or DD + which are compatible with it.

For receiver or television decoding, Dolby Digital (AC-3) or Dolby Digital (E-AC-3), Plus is used worldwide. Unlike the older Dolby Digital, the Plus version offers far more scalable data rates up to 7.1 and Audio Description. For Next Generation Audio (NGA) and 3D audio, object-based codecs such as Dolby AC-4 and MPEG-H are available.

Discrete encoding formats in TV and home use

All the above formats use metadata to control the playback in the home – either on a television set or through a surround sound receiver. Care must be taken through configuration and testing of these settings to ensure that the viewer experience at home correctly decodes and plays back the audio tracks as designed.

Author: Karl M. Slavik, Arte Cast Vienna