Banking on Dolby Volume's becoming as common as Dolby Digital surround sound, Freescale Semiconductor Inc. has built the technology into its entire line of Symphony digital signal processors. Dolby Volume turns up quiet programs, muffles loud advertisements and generally equalizes sound levels among different audio sources, such as DVD and TV.
“Dolby Volume is a technology that sells itself to consumers,” said Sujata Neidig, Symphony DSP product marketing manager at Freescale (Austin, Texas). “And Freescale is the first chip maker to implement Dolby Volume for multispeaker surround sound systems.”
Earlier this year, Cirrus Logic Inc. (Austin) introduced an audio DSP with Dolby Volume sound-leveling technology, but only for stereo digital TV applications. Freescale's DSPs implement both stereo DTV and full multispeaker surround sound systems for home theaters.
Freescale implemented the technology as firmware in the ROM of all the Symphony DSPs. That means OEMs can provide a switch for the consumer labeled “Dolby Volume.” The switch lets consumers set a reference level that “maintains itself when switching between sources and retains dynamic range while lowering [sound] levels,” Neidig said. The system matches the reference volume “moment by moment and whenever channels or sources are switched.”
Consumers' chief complaint about home theaters is uneven volume, whether it occurs when switching among sources or when a TV commercial blares much more loudly than the programming, said Neidig.
“The second problem is that if you are trying to watch a movie with surround sound while your spouse is asleep, then you are constantly having to grab the remote to turn up the volume during a quiet scene, then quickly turn it back down when an explosion or something else loud comes along in the next scene,” she said. “But with Dolby Volume, you just set a reference level that it maintains.”
In normal compression algorithms, leveling affects all frequencies equally, canceling out the surround sound effects by making ambient noises too soft. According to Neidig, Dolby's algorithm levels volumes without affecting surround sound effects.
“You still get all the ambient noises during quiet scenes, like the sound of crickets around a crackling campfire, plus the excitement of dynamic events like explosions, but without waking up everybody else in the house,” she said.
Dolby has not disclosed all the details of how its technology works, but Neidig said it avoids canceling out surround sound effects by constantly monitoring the volume levels and balances among the different frequency bands (highs, middles and lows). A car crash would surge mostly in the lows, allowing those alone to be muted by Dolby Volume and thus maintaining the clarity of higher-frequency sounds, such as breaking glass.
“You are getting the full dynamic range from the original mix, but Dolby Volume uses a psychoacoustic hearing model plus all the expertise Dolby already has for their surround sound encoding,” said Neidig. “Dolby Volume is a very DSP-intensive algorithm. There is a lot of processing done in the frequency domain, which lets us make use of both our 24-bit audio as well as our 48-bit double-precision mode.”
Neidig described Freescale's implementation as “very efficient, so there is still some performance left over for OEMs to do other processing with the DSP too.”
Other DSP vendors may soon announce Dolby Volume-enabled models, but Freescale notes that the entire Symphony line has the technology built in. Any Symphony DSP will run Dolby Volume from the ROM, but Freescale recommends newer, faster versions, such as its dual-core model, for high-end systems.