Introduction to the DC-1
Lexicon
Designed specifically to handle multichannel audio, AC-3 takes advantage of a psychoacoustic phenomenon known as auditory masking. It divides the audio spectrum of each channel into narrow frequency bands of different sizes optimized with respect to the frequency selectivity of human hearing. This makes it possible to sharply filter coding noise so that it is forced to stay very close in frequency to the frequency components of the audio signal being coded. By reducing or eliminating coding noise wher- ever there are no audio signals to mask it, the sound quality of the original signal can be subjectively preserved. The result is multichannel sound that is subjectively equal to 2 channel digital recordings, yet can be stored and transmitted efficiently.
AC-3 can process at least 20-bit dynamic range digital audio signals over a frequency range from 20Hz to 20kHz. The bass effects channel covers 20 to 120Hz. Data rates range from 32kb for a single mono channel to as high as 640 kb.
Dolby Digital source material has been released to date on many laser discs. These discs, while able to deliver two channels of PCM digital audio, are not capable of storing an additional six channels of digital audio information. The AC-3 bitstream is, therefore, converted into a frequency modulated signal (referred to as the AC-3 RF signal) and stored on the right channel analog track of AC-3 encoded laser discs. This signal must be demodulated, or turned back into a digital signal, before it can be decoded by the DC-1.
Several manufacturers provide equipment which will perform the neces- sary demodulation. Lexicon’s LDD-1 is designed specifically to provide external AC-3 RF demodulation and auto switching with minimal degrada- tion of performance due to RF interference.
Other AC-3 formats, such as DVD, output AC-3 code as a digital signal, and do not require demodulation.