In Broadcast June 2012 - 32

WWW.INBROADCAST.COM | ISSUE 9 - JUNE 2012

InSOUND

The Loudness Control Dilemma
By Oliver Masciarotte, Minnetonka Audio Software, Inc Fundamentally, loudness is a subjective psychoacoustic phenomenon “using your ears” that correlates sound pressure level to perceived amplitude…
Several years ago, the EU attempted to bring some level of consistency to perceived “loudness” across television programmes and interstitials. Recently, the US has passed legislation mandating the control of perceived loudness. Loudness as a concept is perceptual, but for the broadcaster it is now a quantiﬁable value, one that strips away subjectivity. In the past, most audio that accompanied motion pictures was encoded as Dolby Digital which is, to this day, the default standard of audio quality for surround distribution. In addition to the 5.1 audio essence, AC-3 carries metadata, metadata that commands the consumer’s decoder to alter dynamic range and amplitude based on the consumer’s preferences. That metadata, while laying the infrastructure foundation for today’s loudness measurement and control, has also caused some confusion. Although dialogue has been the historical “anchor element” used as the loudness benchmark for measurement, neither the ITU’s BS.1770 or the EBU’s R 128 speciﬁcation mentions anything about particular frequencies in a loudness measurement. This will most likely result in additional revisions to the current measurement techniques and standards.

Oliver Masciarotte

A Solution
Minnetonka Audio’s AudioTools Loudness Control Server (ALSC) provides multivariate loudness measurement, along with enhanced workﬂow features such as support for common broadcast audio and video formats plus full metadata validation and correction. ALCS is a complement to pre-set hardware loudness control choices, carefully conforming loudness for the majority of content. Because ALCS can decode or extract audio essence from Dolby E, MXF and QuickTime, it is also able to measure, correct and reconﬁgure almost any standard audio payload prior to re-encoding or wrapper insertion. This addresses the need to identify whether the source audio is stereo or 5.1 in order to switch an encoder into the appropriate format. ALCS can distinguish linear PCM from Dolby E, can determine whether the content is stereo or surround, measure and correct it accordingly, and re-encode it while changing the coding mode and channel ordering if necessary. Being an audio-speciﬁc software platform that uses standard SOAP and XML messaging protocols, ALCS can be updated as rules and regulations change over time.

Sound Trust
In the ﬁrst method every step in the production chain is going to deliver content that’s been properly adjusted for target loudness and has valid dialnorm metadata. That’s possible when an outlet has end-toend control of all inputs, yet even then there’s often a fair amount of manual adjustment involved to conform all contributions to a -24 LKFS standard.

The Third Path
Since a compliant programme loudness algorithm must process an entire show before it can process it and send it on its way, an off-line or ﬁle-based processor can measure and process at faster than real-time speeds. For true compliance, without the loss of ﬁdelity and dynamic range inherent in compression and limiting, the dvanced Television Systems Committee (ATSC) recommends just such a filebased scaling device. That is, “…a device used to apply an overall gain correction to audio content stored as ﬁles”. File-based workﬂows provide several advantages over real-time operation, not the least of which is that the creators’ artistic dynamic range intent can be preserved without placing an undue burden on the audio engineers in post. Along for the ride is higher ﬁdelity, due to the absence of multiple and/ or drastic dynamic range reduction and peak limiting. You have to make sure that any programme that somehow escaped all prior quality control stages is still dealt with appropriately.

Loudness Consideration
Legacy notions of loudness measurement and control were a solid basis for current best practices. However, simplistic notions, such as subjective evaluation, manual adjustment of dynamic range, and short-term averaged measurements, are no longer applicable to today’s demands on content creators to stay both compliant and productive. As a result, a multi-pronged approach is needed, with ﬁlebased automation working as a preprocessor to attend to the majority of content, and real-time dynamics hardware as a last line of defence. With an integrated, multi-faceted approach to loudness control, content providers can satisfy the needs of regulators and the tastes of consumers while creating the highest fidelity programming possible. ■
InBroadcast Abridged Version ©2012 Minnetonka Audio Software, Inc

Dynamic Processing
The second approach employs a real-time dynamic range processor to match the measured loudness of non-conformant input to the target value. Unfortunately this is the exact opposite of what was sold to the public as a beneﬁt of digital television. While there is no reason why a carefully designed dynamic processor could not produce compliant output while preserving quality, the hindrance is the requirement of real-time operation, which forces the programme into rigid amplitude and dynamic range envelopes by manipulating compression, peak limiting and crest factor.

Requirements
The ATSC’s A/85 2011 lists three methods to effectively control programme-to-interstitial loudness: a) ensure that all content meets the target loudness and that the dialnorm value matches this value; b) employ a real-time loudness processing device to match the dialogue level of nonconformant content to the target value; c) employ a ﬁle-based scaling device to match dialogue level of non-conformant content to the target value.

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In Broadcast June 2012

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