Autor: Karl M. Slavik

Audio Formats and Channel Configuration

There are many technical possibilities available today for sound on television.

The production of sound for television today offers numerous technical options: from mono audio for fast turnaround news reporting to 5.1 surround sound and 3D object audio for larger drama productions. It’s important to consider the technical advantages and disadvantages of these formats, when planning the production workflow, as well as how the sound might appear once delivered to a consumer set in stereo or 5.1. And of course, what is the impact of downmixing on the viewers enoyment of the programme.

Channel Format

The channel format describes how many audio channels will be delivered for the main mix of the programme audio (PGM). The usual configuration is either stereo 2.0 or 5.1 surround sound. The number BEFORE the point indicates the number of full-range audio channels (ie 20-20000 Hz), the number AFTER the point defines the number of band-limited low frequency effects (LFE) channels (usually 20-120 Hz). LFE is only used to playback extremely high energy surround sounds, such as explosions, earthquakes or the kick-off at football matches. For conventional concert and sports broadcasts, documentaries and entertainment shows the LFE channel is usually NOT required, but can be used with careful consideration (see Dolby 5.1 Channel Production Guidelines and Dolby Surround Mixing Manual).

Dolby-5.1-Channel-Production-Guidelines (external PDF)

Channel configuration

Audio channel configuration defines how the surround sound channels are arranged and will be used for playback in the room. This is set out in to ITU BS.775-1, 755-2 and now 755-3.  The ITU recommendation provides guidance on speaker positioning, up and down mixing as  well as used of LFE. It covers mono, stereo, and surround channel use and configuration. In addition to this, 3D audio processes can be used to enhance this monitoring set up with overhead mounted ceiling speakers, in order to realize a height axis (Z-axis) required for 3D sound mixes.

Interception circuit by ITU BS.775-1 and 775-2

If the monitoring environment cannot be delivered as pictured above – for example installation in a small OB environment – technical solutions and audio delays should be used to create an accurate sound stage for monitoring. The loudspeakers should be of the same design and reflect between 200 Hz and 10,000 Hz as linear as possible and with the same sound pressure level. The deviation to each other should not exceed ± 0.5 dB (ie a total of 1 dB).

The LFE channel is the subwoofer speaker and in surround sound equipment is used for bass management of low frequency effect for the other speakers. It should be noted that the LFE channel is always raised in monitoring to +10 dB - this increase between 20 and 120 Hz is called "in-band gain" and is applied in all home recievers and playback devices automatically.

In-band gain of the subwoofer to the LFE playback

All productions that are delivering Dolby Digital or Dolby Digital Plus are required to deliver metadata. Instead of individual, programme-specific metadata, many broadcasters require presets for the whole channel, or for specific genres. It’s important to check the delivery guidance for broadcaster specific requirements.

The metadata presets are issued to the supplier before the start of production and equipment that supports the generation of this metadata should be used, e.g. Dolby DP570 or Jünger Audio MAP for real-time programs and the Dolby Media Emulator DME as a software plug-in. It’s vital to check the impact of the metadata control on the final mix, as the audience will hear it. For more information see the article Dolby metadata and metadata presets.

Upmixing and downmixing

The use of up and down mixing is a normal part of television production and transmission. The use of panning of mono presenter microphones to create a stereo mix, the use of stereo signals for 5.1, with additional upmixing and panning.  In the case of stereo to 5.1, the two channels are distributed on the left, center, right and the two surround channels to create a realistic, balanced surround stage, whilst also maintaining the intelligibility of the audio. Hardware and software solutions exist to deliver an almost perfect LCR front signal with selectable surround Share (LS, RS) from two stereo channels.

Good upmixing can generate surprising 5.1 results, however this is not a replacement for a high quality real surround sound production. It is vital to check any 5.1 upmix with the correct metadata configuration on a receiver to ensure that the sound is correctly presented to the audience.

Down mixing often takes place automatically in the home equipment – for example when a DVD player is connected only in stereo to the TV, the Dolby Digital 5.1 received by the player only plays back in stereo. It’s vital to check the down mix settings, as extreme errors during production can lead to unpleasant phase cancellation or the reproduction of speech from the centre channel may suffer considerably.

[Illustration: Devices and software for upmixing from 2.0 to 5.1]

Author: Karl M. Slavik, Arte Cast Vienna

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