You are here: Home Page » Audio » Audio FOR HD: Common problems, simple cures

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

Time and time again multichannel audio, rather than video, causes the most problems when transitioning to HD. Audio is a common cause for both audience complaints and technical difficulties at television facilities. Two of the most common complaints from HD viewers are:

audio level variance between channels, and variation from segment to segment; and
inconsistent 5.1 presence, with surround sound appearing and disappearing between channels and between program segments.

Let's look at the causes of these issues and then review several solutions.

Managing audio levels

Figure 1. Dialnorm metadata value travels throughout the broadcast chain and enables the home receiver to compensate for changes in audio operating levels.
Click to enlarge

A promising feature of the Dolby Digital (AC-3) delivery system is the built-in loudness control mechanism. This feature, commonly known as dialog normalization, was created to allow broadcasters to operate at different loudness levels, with the differences in loudness managed in the home by the Dolby Digital (AC-3) decoder. The general idea was that all program content would include a metadata parameter called dialnorm, indicating the nominal level of the dialog in the program. The dialnorm value is presented to the AC-3 encoder, which then sends it to the home decoder. There, it is received and applied to the decoded audio signal in order to modify the level of the program. As the viewer changes from channel to channel, or as the broadcaster switches from segment to segment, the associated dialnorm value is used by the audio decoder to dynamically adjust the overall audio level so that consistent loudness is maintained. (See Figure 1.)

Figure 2.Dialnorm metadata are often wrong, dropped or not properly updated, resulting in incorrect levels at the home decoder/receiver output.
Click to enlarge

However, it seems that instead of helping to iron out interchannel and intersegment loudness issues, the dialog normalization feature has often made things worse. This is generally the result of inconsistencies in the dialnorm setting during program creation or when video is received and processed by a facility. This may happen when the content is first created, if the value is not set properly, or during incoming feed processing or ingest, when audio metadata may be removed by equipment or the wrong value applied. Similarly, during production or in master control, the dialnorm values may not be set properly (left at default), or the audio may be modified but the dialnorm values are not properly readjusted. The net effect of these different possibilities is that the audio levels heard by the viewer are often adjusted using incorrect dialnorm values and will therefore be unstable and inappropriate. (See Figure 2.)

The three most common solutions to this audio loudness problem are dynamic metadata, static metadata, and loudness measurement and on-the-fly audio level control.

Dynamic metadata

This solution relies on a combination of good production practices. It makes sure that all content created or ingested has the proper dialnorm value, as well as good equipment design, which ensures that metadata is maintained throughout the facility and is delivered to the emission encoder. This model has recently become more effective due to the soon to be ratified SMPTE 2020 standard, which specifies a way of transporting Dolby metadata in the ancillary space of HD-SDI and SD-SDI video.