Loudness Measurement and Control Strategies For The UK Broadcast Market

Manik Gupta

Author: Manik Gupta

Published: 13 May 2016

by Manik Gupta Issue 113 - May 2016

A common issue that today\'s broadcast ,cable, and satellite operators face is inconsistent loudness. Over the years, the loudness levels of some commercials have been deliberately increased to catch audiences\' attention. Ultimately, inconsistent loudness between regular programming and advertisements has discouraged continued viewing and resulted in consumer complaints.

To reduce these complaints, governments across the world have created loudness control regulations, such as the EBU\'s Loudness Recommendation EBU R128 in Europe. Broadcasters must comply with this legislation, or they could face penalties. In the United Kingdom, Ofcom is the government-approved regulatory agency that upholds these requirements.

Loudness Measurement Techniques and Specifications

Loudness is the perception of sound level, making it quite subjective and difficult to measure.
The first research on how the human ear hears different audio frequencies at different levels was conducted by Fletcher and Munson in 1933. The Fletcher-Munson curve(s) of equal loudness shows human ear perception against different frequencies. These contours have been utilized by multiple audio meters over the years. Voltage Unit (VU) and PPM are commonly used to measure audio level but often misused as devices for loudness measurement. Both were designed to measure the Sound Pressure Level rather than the perceived audio loudness. Sound engineers circumvent these meters by employing multiband compressors achieving heavy compression, thus making a softer sound louder and bringing down the peaks, making content hyper-compressed.
Considering the problems in audio level based approaches, the ITU (International Telecommunication Union) proposed an algorithm for measuring loudness objectively, which is into its fourth revision with the latest version, 1770-4. The industry hopes to agree on a common algorithm for loudness measurement with this update.
The loudness metering algorithm consists of a measurement of the energy equivalent average (Leq) with a weighting curve that forms the basis for matching an inherently subjective impression with an objective measurement. The weighting accounts for the acoustic effects of the head, low frequencies are de-emphasized, boosting high frequencies. In addition, gating concepts are applied to ensure that silence regions at the start and end of the program, or pauses between the dialogue, do not influence the overall program loudness. A technique for measuring multichannel audio is also included. Measured loudness is reported in LKFS (or LUFS) units, which stands for "Loudness, K-weighted, relative to Full Scale.\"

In addition to loudness, the ITU standard also defines true peak measurement for an audio signal. True peak measurement is ideally defined as a measure of maximum absolute sample value of an audio signal in a continuous time domain. The ITU standard describes one such approach where audio signal can be over-sampled by a factor of four for estimating the true peak.

Loudness Regulations in the UK

EBU R128 is the loudness recommendation widely used in Europe and followed in the UK. EBU R128 gives recommendations for program loudness, true peak level, and three other aspects of loudness:
- Momentary loudness measures sudden changes in audio levels, e.g., gun shots,
using a sliding time-window.
- Short-term loudness measures more
general changes in levels using a sliding window of 3 seconds.
- Loudness range is based on statistical distribution of measured loudness, taking
into account the difference between the softest and loudest part of the program.

An Effective Strategy for Audio Loudness Monitoring and Control

Because of these regulations, loudness monitoring is now a critical step in broadcast workflows. As may be expected, the tools and methodology for loudness monitoring in file-based content vs. live content are slightly different. File-based content allows multi-pass analysis of content offline, whereas that is not possible with live content.

Loudness control tools generally utilize metadata-based or audio normalization algorithms. Metadata-based control is primarily used with Dolby audio formats, where loudness-specific metadata such as dialnorm and dynamic range factors can be set per the user\'s requirements. On the other hand, audio normalization algorithms modify the original PCM samples to achieve a target loudness profile. Good normalization algorithms should ensure that the correction process doesn\'t lead to any kind of audio distortion. The correction cannot be a simple gain or attenuation process, as true peak and dynamic range are also vital parameters to be considered. Ignoring the dynamic range could lead to a situation where low audio levels become so low that audio is inaudible. Eventually, the viewer may increase the volume on the TV to hear the sound well, but would encounter the original loudness problem. Different approaches can be taken for short-form and long-form content to achieve better results.

The structural integrity of media files needs to be maintained during the correction process. After normalization of audio samples, audio will need to be re-encoded and rewrapped into the main media file. This step can potentially introduce encoding errors. If any media information is altered or lost during this step, it could adversely impact the content distribution chain.

Obviously, broadcasters want a system that can monitor all aspects of loudness to ensure compliance with EBU-R128 and a superior audio experience for viewers. After performing comprehensive checks, the system should have the option to correct loudness issues and provide a report to broadcasters for identifying trends and eliminating loudness issues in the future.

For a complete approach to audio loudnessmeasurement, broadcasters should also add a live content monitoring solution to their workflow. Loudness monitoring and control systems are now quite mature and have been successfully deployed by broadcasters worldwide.

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