Prosound News - April 2008 - (Page 18)

ADVERTORIAL Controlling Loudness in Digital Television ORIGINAL LOUDNESS NORMALIZED OUTPUT Example of dialnorm operation showing original vs. normalized levels for a range of programming. Note that headroom above average dialogue level remains constant after normalization Introduction As the transition from analog to digital television broadcast nears its cutoff date in less than a year, broadcasters and content creators are dealing with many issues relating to both audio and video. Regarding audio, no topic is getting more attention than the issue of level matching between channels and between program and commercial segments. The level at issue is the perceived loudness, which is what viewers hear, rather than the electrical levels that professionals are used to dealing with. There are many examples of networks and cable operators who have successfully addressed this issue, but there remains confusion among many others as to the cause of level mismatches and the options for managing them. Dolby Digital (AC-3), the standard audio format for most digital television broadcasting in North America, includes specific capabilities designed to manage the difference in loudness between programs and channels. Loudness Matters Although standard reference levels for alignment have their place, and monitoring peak levels is necessary in order to avoid overload, neither one of these has much to do with how loud a program seems to a viewer. Instead, research has shown that the average level of dialogue, measured with a weighting curve such as Leq(A), or the recently standardized ITU-R Rec. BS.1770, provides the best way of objectively measuring the way the human ear perceives differences in loudness between signals. In other words, if the average level of dialogue in two programs is the same, they will be perceived as having the same loudness, even if there is significant variation in other program elements such as short-term special effects or background music. So, in order to keep viewers from reaching for the remote to adjust the volume, the average dialog level must be kept consistent. For this reason, the Dolby Digital system uses an indication of the average dialogue level, known as dialnorm, to control the reproduced level of decoded signals. signal is not changed in the encoding process. The dialnorm value is not applied until the final stages of the decoder, where the output level is normalized to a target of -31 dBFS. The normalization occurs as an attenuation of the signal level based on the dialnorm value. For example, if the dialnorm value is -24, then the signal will be attenuated by 7dB in order to equal the desired -31 dBFS. If dialnorm is set at -31, then no attenuation takes place, because the signal is already at the desired level. A crucial principle to understand about this operation is that the original dynamic range of the audio signal is unchanged. Unlike single-ended processors typically used in analog broadcasting, or automatic gain controls in TV sets that apply a constantly varying level adjustment, dialnorm is designed to operate on a program-by-program basis. Instead of the viewer adjusting the volume in between every program, commercial or channel change, the level is adjusted automatically by the Dolby Digital decoder, according to the dialnorm value. Clearly, this process can only work properly if the dialnorm value is accurate. If not, then several undesirable results may result. First, the amount of attenuation will be incorrect, so the normalized outputs will not match the target loudness level. In addition, an incorrect dialnorm value can cause degradation of the audio quality, because other aspects of the Dolby Digital system, such as Dynamic Range Control, also depend on dialnorm to work correctly. In fact, large discrepancies between the actual level and the dialnorm value can cause a serious loss of dynamic range, as well as even more severe effects when the program is reproduced through systems with restricted range, such as TVs with built-in speakers. The use of accurate dialnorm values is so important that the FCC has mandated it. The ATSC Standard for Digital Television (A/53) states, “The value of the dialnorm parameter in the AC-3 elementary bit stream shall indicate the level of average spoken dialogue within the encoded audio program ” The Dolby Media Meter is a new software product that makes it easy to measure dialogue level within the Digidesign® ProTools® environment or as a standalone application on Mac® or Windows®. It’s also available as a plug-in for the Minnetonka AudioTools™ AWE. The Media Meter also includes Dialogue Intelligence™ and is especially convenient when working in file-based postproduction or authoring environments for VOD, packaged media or broadcast. More and more programming is stored on servers and then played out as files. This allows the opportunity for much more to be done to assure optimal results while still maintaining the integrity of the original program. In the filebased domain, an entire program can be analyzed and its loudness corrected in faster-than real-time, without affecting the original dynamic range. For example, the Dolby DP600 Program Optimizer can automatically measure loudness, correct the dialnorm parameter and recalculate all other parameters that depend on the dialnorm value. The DP600 also features Dialogue Intelligence™ and offers a flexible platform with many other capabilities, including encoding, decoding and transcoding between popular audio formats. Setting the Dialnorm Value Once the dialnorm value is determined, its value must be applied to the Dolby Digital encoder. This can be done in a number of ways, but in general, the closer to the production process that the metadata can be authored, the better. If dialnorm (and other metadata) is created upstream from the Dolby Digital encoder, it can be carried through the distribution path as part of a Dolby E stream or as separate data embedded within the video signal. In either case, the metadata is ultimately applied to the Dolby Digital encoder as an external signal. Dialnorm, like all other metadata parameters, can also be set directly through the Dolby Digital encoder interface. Measuring Dialogue Level Metadata and the Dialnorm Parameter Dialnorm is just one of many pieces of information that travel with the encoded audio in the Dolby Digital signal, known collectively as metadata. These parameters describe various aspects of the audio signal and control the encoder and decoder behavior. Dialnorm is one of the most important and most misunderstood metadata parameters of all. A complete description of all Dolby Digital metadata parameters and their applications can be found in A Guide to Dolby Metadata, which is available at the Dolby Laboratories web site: http://www.dolby.com/tvaudio. Several tools are available that make it relatively easy to measure dialogue levels. Traditionally, television programming has been stored and distributed in linear fashion, whether originating as a live event or played out from tape. In either case, the measurement of such signals must be done in real time. Tools like the Dolby LM100 Broadcast Loudness Meter can easily measure a real-time program and compute an average dialogue level, which is then used as the dialnorm value for the program. The LM100 also incorporates a patented process known as Dialogue Intelligence™ which detects speech and, when desired, will limit measurements to those areas where it is the primary signal component. In production or postproduction environments, the Dolby DP570 Multichannel Audio Tool is an economical way to combine loudness measurement with other important metadata emulation and monitoring capabilities. Conclusion One of the major consumer benefits provided by today’s digital television services is the availability of enhanced audio quality and capabilities, similar to those available on DVD. More and more viewers are watching in high definition with 5.1-channel (or more) surround systems that go a long way toward reproducing the cinema experience or the feeling of being present at a live sporting event, but the majority of viewers are still watching and listening on systems with more limited capabilities. The Dolby Digital system has been designed to provide all viewers with the highest quality audio possible, whether they are listening in 5.1 or mono, on a home-theater system or built-in TV speakers. By simply making sure that dialnorm and a couple of other parameters are accurately set, a broadcaster or content provider can ensure the best possible audio experience for all their viewers. Dialnorm Operation The dialnorm parameter has a range of values from -1 to -31dB. The value represents the average dialogue level in relation to full-scale digital (0dBFS). Although this value is present in the encoded bitstream, the actual level of the http://www.dolby.com/tvaudio

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Prosound News - April 2008

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