Loudness management: Inconsistent audio leads to unhappy viewers.
Quality assurance is critical in order to be successful in a competitive broadcast market. One instance of poor-quality audio or video during an on-air broadcast can result in increased viewer complaints or viewers switching to another channel — and fewer viewers means less advertising revenue. Over the past several years, inconsistent loudness levels between advertisements and programming, in particular, have become a major source of contention for broadcast viewers, prompting regulatory bodies around the world to pass standards and legislation designed to mitigate audio loudness issues.
Advanced DTV loudness monitoring solutions promise to resolve these problems by remotely monitoring, measuring, recording and analyzing DTV streams for variations in audio loudness and ensuring that broadcasters remain compliant with audio loudness standards. This article will address the specific challenges that broadcasters face in monitoring and analyzing the audio loudness of their DTV streams according to industry specifications such as ITU-R Recommendation BS.1770, and provide effective strategies for addressing such issues.
The main challenge broadcasters must overcome in eliminating audio loudness issues is that they are receiving advertisements from a third party, whether that be a local car company, or a network, in the case of affiliate stations. Therefore, advertising content may be coming into the station with varying levels of audio. After receiving advertising (short-form) content from an outside source, broadcasters have to normalize it to ensure it possesses the same audio characteristics as the regular (long-form) programming. A broadcaster may handle hundreds of different pieces of short-form content a day, making this an extremely complicated task for station programmers to oversee.
The recommended audio loudness levels are set by standards bodies, such as the ITU-R and ATSC, and can be enforced by a geographical region’s legislative government; however, not all countries have passed legislation. The U.S. government’s Commercial Audio Loudness Mitigation (CALM) Act, passed in 2010, was the first piece of legislation designed to penalize broadcast stations and multichannel video programming distributors (MVPDs) not abiding by recommended audio loudness levels. Other regulatory bodies in Europe and the Asia-Pacific region are considering adopting similar pieces of legislation due to the increasing number of consumer complaints they receive regarding inconsistent audio levels.
Understanding dialnorm and ITU-R Recommendation BS.1770
Prior to ITU-R Recommendation BS.1770, broadcasters solely relied on the ATSC’s recommended Dolby dialnorm specification for audio loudness. Dialnorm is a metadata parameter that controls playback gain within a Dolby Digital (AC-3) audio compression system by measuring the average dialog level in the encoded audio of television programs. The dialnorm method derives the absolute value of a power sum of the A-weighted audio level in all digital channels to ensure consistency from program to program and channel to channel.
Although the dialnorm measurement proved to be effective at helping broadcasters stabilize dialog loudness levels, it did not take into account music loudness. This posed an issue, because if the short- or long-form program contained very quiet or loud spots — oftentimes caused by music — the dialnorm value would be thrown off. Therefore, in 2006 the International Telecommunications Union (ITU), a specialized agency of the United Nations, developed Recommendation BS.1770. The technique defines an algorithm for objectively measuring audio program loudness and true-peak audio level of broadcast signals referred to as LKFS, which stands for Loudness, K-weighted, relative to Full Scale, and includes a weighting curve with a smaller amount of low-frequency de-emphasis, as well as a peaking emphasis to compensate for the boost in loudness perception.
The algorithm consists of four stages:
“K” frequency weighting.
Mean square calculation for each channel>
Channel-weighted summation (surround channels have larger weights, and the LFE channel is excluded).
Gating of 400ms blocks (overlapping by 75 percent), where two thresholds are used: (the first at -70 LKFS; the second at -10dB relative to the level measured after application of the first threshold.)
ITU-R Recommendation BS.1770 has become the universally accepted audio loudness standard around the world, and the basis for legislation like the CALM Act. Relying on the standard, broadcasters can effectively equalize the audio loudness changes that frequently occur between commercial advertisements and regular programming to prevent or reduce viewer dissatisfaction and deliver a higher quality of on-air presentation.
By supporting a standard audio level value of -24 LKFS, as outlined in BS.1770, the broadcast industry has agreed to transmit all content at this loudness level and can thereby set all its transmission equipment to the same value. In a perfect world, advertisers and content creators would perform quality control upstream so that the correct loudness is delivered to broadcast stations without requiring additional modifications. However, this is not always the case. Impairments in the DTV transport stream can cause audio inconsistencies, making it imperative that broadcasters adopt an end-to-end DTV TS monitor and analyzer.
For more details about the ITU-R Recommendation BS.1770, broadcasters can read the standard in its entirety online at http://www.itu.int/rec/R-REC-BS.1770-0-200607-S/en.
Benefits of effective DTV loudness monitoring
Using a reliable DTV TS analysis system with advanced audio loudness monitoring functionality, broadcasters can easily maintain compliance with standards like ITU-R BS.1770 and dialnorm, as well as quickly identify and resolve potential audio loudness issues.
Sophisticated DTV loudness monitoring systems can provide the same level of analysis for mobile DTV as for terrestrial DTV, allowing broadcasters to comply with industry standards and protocols for their regular TV broadcast and mobile broadcast. This dramatically lowers the cost of monitoring audio loudness for new viewing platforms like smartphones and tablets.
Broadcast engineers can remotely measure, analyze and record loudness levels for multiple audio streams from any network location and support any modulation method, such as DVB-T’s COFDM in Europe. LKFS levels are automatically logged and can be adjusted based on the value predetermined by the broadcast station. Engineers are alerted if signals are above or below the recommended threshold of -24 LKFS to ensure they are complying with ITU-R BS.1770. If audio levels are more than several degrees above or below the recommended value, a station engineer can quickly make an adjustment and avoid audio inconsistencies within the on-air broadcast.
Using a DTV loudness monitor, broadcasters can also perform multiple instant spot checks of audio signals, as shown in Figure 1. Instant spot checks are currently performed in the United States to test a station’s compliance with the CALM Act, and may be necessary for broadcasters in other parts of the world as regulatory bodies adopt legislation modelled after CALM. Aside from helping broadcasters comply with legislation, they are extremely useful because they allow an operator to instantly determine how well its programming is adhering to BS.1770. DTV monitoring systems capable of monitoring multiple signals can specifically benefit a TV station operating several call letters, as they allow the operator to purchase one DTV monitoring system for its entire operation, dramatically reducing the cost of audio loudness monitoring.
Through an easy-to-use graphical user interface, broadcast engineers can interpret complex information and identify audio issues. After a violation occurs, the monitoring system generates a high-resolution video and audio thumbnail with a timestamp, as shown in Figure 2, so that engineers can determine precisely where in the programming the violation occurred, and whether it occurred in short- or long-form content. Customized reports can be generated on a daily basis and saved for several months, enabling stations to effectively resolve chronic or network-wide issues, such as those originating from a broadcast network headend outside of the local broadcaster’s control.
After an error has been detected, the DTV loudness monitoring system sends broadcast engineers a text or e-mail message with important information about the error so that they can quickly resolve it from any remote location. Errors are classified based on the severity of the error: An error is considered critical if it is capable of taking the transport stream or program off-air; moderate if it is capable of causing quality of service issues; or low if the programming is missing a component.
The end result
The end result of this monitoring is better-quality programming, which leads to increased viewer satisfaction. A reliable DTV loudness monitoring system is a necessity for broadcasters around the world who are committed to delivering the best possible quality of service for their viewers, as well as abiding by industry standards and regional legislation. By reducing the number of audio loudness-level issues within their on-air broadcast, stations can retain current viewers and increase advertising dollars, thereby maximizing their revenue.
—Ralph Bachofen is vice president of sales and head of marketing at Triveni Digital.