The DTV standard has introduced its share of headaches, but it might actually eliminate one: the migraine viewers get from inconsistent audio levels. It is a scenario familiar to anyone who has wielded a remote: You change the channel, and the sudden bump in volume rattles your brain.
The audio portion of the digital-television transmission standard has a feature called Dialnorm, which brings the audio levels of all HDTV- and DTV-delivered channels within the same range, so viewers no longer have to adjust the volume as they channel-surf.
“Dialnorm allows for a wide variety of content to line up more closely in loudness to programs produced with a totally different philosophy,” says Dolby Broadcast Product Manager Jeff Riedmiller. Dolby's LM100 Broadcast Loudness Meter, which costs about $3,200, sits at the center of the Dialnorm process, automatically adjusting the incoming volume level so audio levels are consistent from channel to channel.
How It Works
In operation, the Dolby LM100 Meter measures the dialogue portions of programming and gives the broadcaster a Dialnorm value. That value is carried as a part of metadata in the Dolby Digital audio stream sent out with the DTV signal and decoded in the viewer's set-top box. If done properly, Dialnorm automatically sets the level of programming so the speech-audio level is consistent from channel to channel or program to commercial. The key to this process is that the actual, or measured, dialogue level for the program must agree with the transmitted Dialnorm value in the audio stream.
“The station or network can either choose a default Dialnorm setting and produce content that is pre-normalized to this setting [value], or individually supply the Dialnorm value for each piece of content or program,” says Riedmiller.
Dialnorm can benefit everyone from content creator to distributor. A broadcast network can use it to make sure sound levels are consistent from one show to the next, while the local station can ensure that content and commercials fall within the same range. And once the cable networks and satellite and cable operators are on board, each HDTV channel will have a similar audio level as well.
Dialnorm is also beneficial for feature films, many of which come into cable networks with audio levels that exceed the network's guidelines. Although some broadcast contracts stipulate that the network can't change the audio mix without the director's consent, Dialnorm guarantees that, even if the levels exceed guidelines, the movie's audio-track level will be corrected when it is delivered to viewers.
“It takes the subjectiveness out of measuring audio levels,” says Starz Manager, Post Production, Sean Richardson. “That average level for the dialogue becomes the foundation for the rest of the mix.”
Although most of the attention Dialnorm receives is on the broadcast side, it also comes in handy in the post-production process. Promos, interstitials and original content, for example, are often created in different editing suites—each with a different idea of what constitutes too loud or too soft. “Those situations are exactly why Dialnorm was developed,” says Tim Carroll, president and founder of audio company Linear Acoustic. “Three different studios can mix a program three different ways, and the programs will all be played out at the same level without sacrificing the dynamic range.”
Not Yet Embraced by All
In an ideal world, Dialnorm—which doesn't work with standard-definition analog or non-HD digital cable channels—would be embraced by everyone broadcasting a digital-TV signal. Yet some believe that scenario will remain out of reach for a while. “Dialnorm has to be used by everyone from Hollywood to the consumer for this to work,” says Carroll. “While it works fine today for someone like HBO or Starz, it's much harder for a terrestrial broadcast network to keep loudness consistent.”
A TV station, he explains, receives content from a variety of sources—the network, syndicators, commercials—all of which need metadata to help them meet Dialnorm levels. That requires a massive monitoring effort on the part of the station, a task many find too time-consuming. In addition, the system can be foiled by an advertiser who simply ignores the suggested volume range.
Now, says Carroll, it is up to everyone to get on board with the LM100 Loudness Meter, make a concerted effort to set proper audio levels and avoid the mass of complaints about noise disparities registered by viewers back in the '80s.
“If it isn't done properly,” Carroll says, “the same complaints will be heard again when the analog shutoff occurs and everyone is watching digital.”