Digital audio for DTV
Feb 1, 2009 12:00 PM, By Aldo Cugnini
It is not clear exactly how many broadcasters are using associated services. There are also no ATSC receiver mandates for these services, so it is unclear how many receivers provide support. (ATSC A/53 Part 5 informatively asserts that “the capability to simultaneously decode one main service and one associated service may not exist in some receivers.” Readers are encouraged to refer to this standard, as well as ATSC A/52, for further details.) Therefore, neither transmissions nor receivers are currently required to carry or decode the associated audio services.
To get around this ambiguity, most stations will resort to a second CM program mix, labeled as a supplementary audio service, which is allowed by the ATSC standard. Although this is not bandwidth efficient, it does get around the problem of receiver compatibility, however, as multiple decoders can increase receiver costs.
Carrying audio metadata
Another area of growing interest is audio metadata. Although our discussion applies to all metadata in general, let's use one specific element as an example — dialnorm — because it's becoming increasingly used (or misused) in the audio chain. Dialnorm sets a consistent loudness level over time and across multiple services using AC-3 (Dolby Digital) encoding. We'll skip the background (as it's been covered here recently and extensively), but recall that the dialnorm value is used in the decoder to return the program level to a standard dialog loudness level. It's important to realize that not all sources and destinations use AC-3, and the ultimate recipient of the program is a home viewer/listener, who may or may not be receiving an AC-3 encoded program.
Today, a home TV viewer will receive broadcast programming from one of several means: OTA reception or retransmission by means of cable, satellite or telco. For OTA reception, all digital transmission in the United States is by ATSC, which will always have AC-3 audio, and therefore dialnorm will always be present. The critical issue here is whether it has been set properly by the network or local broadcaster.
When a local station receives programming from the network, it is usually by means of an uncompressed or near-lossless compressed feed. It is then up to the local broadcaster to set the correct dialnorm value when ATSC encoding is carried out. However, some local broadcasters take a precompressed ATSC feed from the network (or even one using Dolby E,which likewise carries metadata) and carry out program insertion (bit stream splicing) to insert local programming. In this case, the network-originated programs will already have the dialnorm value set, and this will be passed along to the viewers. However, the local insertion programming must also have the correct dialnorm setting, so the local production workflow should consider this.
Figure 2. Various sources and distribution channels may have different or absent metadata.
Click to enlarge
For cable, satellite and telco retransmission, a mixed mode operation is always present, as different sources may or may not be available with audio metadata. (See Figure 2.) Sources that already contain metadata can pass it directly to the audio decoder, as all AC-3 decoders will use this data properly. This is the ideal situation, as it will result in a closed system that is not susceptible to mishandling.
However, sources without metadata that will be delivered digitally must be encoded at the plant in either AC-3 or MPEG. The former will require a correct local setting of the dialnorm value. The latter has no equivalent, so the program audio level must be set manually. In this case (as well as that for an analog cable channel), a loudness processor must be used that sets the outgoing program level appropriately with respect to the digital channels, so that the audio decoder will have consistent levels across all programs.
Broadcasters and retransmission operators need to work together to manage loudness across all programs and services, and fully understand and agree on their specific loudness practices. Useful references include the CableLabs document “OpenCable Host Device 2.0 Core Functional Requirements” and the CEA document “EIA/CEA-CEB-11 NTSC/ATSC Loudness Matching.”
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to: aldo.cugnini@penton.com
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