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Audio FOR HD: Common problems, simple cures

Sep 1, 2008 12:00 PM, By Michel Proulx

Stop multichannel audio variance and inconsistency dead in its tracks

However, it is still not practical for many facilities. This is because broadcasters must ensure that all the audio processing used in their facility passes and adjusts audio metadata if the audio content is modified. Unfortunately, the reality is that many HDTV facilities use a large number of devices that were designed and deployed before SMPTE 2020 existed, making it impossible to ensure metadata survival throughout the chain.

Static metadata

At the other end of the range of possibilities, this model involves producing all content to a known dialog level and setting the corresponding dialnorm value statically at the Dolby Digital (AC-3) emission encoder. This requires close collaboration with external content providers to make sure all the content provided is mixed to the target loudness level. For non-live content delivered on tape or file, it is possible to measure the loudness of the entire program and reprocess the audio to make sure it meets the target dialog level. But for live content, or content that is delivered in real-time as a stream, it is not possible to reliably perform this task. This is because the dialog level is a function of the entire program or segment, and the entire program or segment must be received before the dialnorm value can be accurately known.

Loudness measurement and on-the-fly audio level control

The third possibility is to ignore incoming metadata and add a device at the end of the chain, which measures the program loudness and either sets the dialnorm value accordingly or processes the audio to meet the target dialnorm value. (See Figure 3.) This scheme is roughly equivalent to an automatic gain control (AGC) on the output of a facility, using loudness as the control signal.

However, because loudness is a measurement that is supposed to be integrated over a long period of time, ideally the entire duration of the program or segment, this method can be problematic. For example, in a quiet scene, the loudness level and even the dialog loudness level could be temporarily low. If this low level is used as a cue to increase the audio level, and if the quiet scene is followed by a louder scene, then the audio loudness increase will be amplified. This will likely result in an abnormal, undesirable rise in the level of audio.

Hence, applying the loudness control too quickly will create a pumping of the audio levels. All of this goes against the original intent of the dialog normalization and dynamic range control of the Dolby Digital format.

Double-checking with loudness monitoring

Ultimately, the best solution to loudness problems will depend on the individual facility and especially on the broadcaster's level of control regarding incoming content, particularly live or streamed content. In all three of these solutions, it is recommended that audio loudness is continuously measured and compared to the outgoing metadata value, be it static or dynamic. (See Figure 4.)

There have long been dedicated devices for measuring loudness, although they add to the complexity of the channel chain. Hence, some broadcast equipment manufacturers now integrate loudness measurement inside other devices that are commonly present in the output chain. These products provide continuous measurement of audio loudness and extraction of the dialnorm metadata value. The measured loudness and the extracted dialnorm value are compared against each other and against configured operational targets. Then an alarm or warning is reported if the values diverge too much for a specified time duration, thereby offering improved audio control.

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