Report on AES San Francisco’s July 2003 meeting by Paul Howard
AES San Francisco’s July meeting was held at Dolby Laboratories in San Francisco.
Mike Babbitt gave a primer in broadcast digital audio. He is a broadcast field service engineer with Dolby.
Millions of households worldwide are equipped for home cinema. Demand is increasing for quality, multi-channel sound to go with the striking images provided by digital video.
Dolby manufactures a variety of electronic devices used for distribution and transmission of broadcast digital audio, including Dolby E technology for distribution, and Dolby D technology for transmission.
Dolby Digital was selected by the ATSC (Advanced Television Systems Committee) as the standard for audio broadcast over terrestrial digital television transmission.
According to ATSC standards, the maximum standard data rate for Dolby Digital encoded broadcast audio is 448 kBs (kilobits/second) the same as DVD-Video.
Significant variation in consumer playback equipment and listening environments exists in the home. Some consumers have invested in multi-channel sound systems with high accuracy, and large power-handling capacity. At the other end of the scale are portable televisions, with single speakers, a few centimeters in diameter. Some consumers have the luxury of acoustic isolation from neighbors, while others do not, and may want the option of dynamic range control.
The Dolby Digital data stream conveys wide dynamic range digital audio to all sorts of consumer listening environments, and playback equipment. This is achieved through metadata, a set of data encoded along with the audio into the Dolby Digital data stream, and decoded in the consumer’s home.
The "Three Ds of Metadata" are: dialogue normalization level, dynamic range control, and down-mixing. A standardized volume level centered around the level of speech within the program is set intuitively by the listener, and matched through the dialog normalizion parameter. This ensures consistency between programs and channels. Dynamic range control limits on the program’s dynamic range when desired by the consumer. Down-mixing provides the correct number of channels to match the consumer’s sound system.
Complete and unprocessed audio is always sent to the home in a Dolby Digital data stream. The consumer at home selects the amount of audio control based on his/her requirements.
The same audio/metadata stream is sent to all customers. One Dolby Digital data stream is used for all consumer playback environments. Metadata is sent by the audio engineer at the soundstage, and can provide an unbroken link between the content producer and the consumer at home.
Several industry leaders in broadcast audio attended AES San Francisco’s July meeting.
