Dolby digital (AC-3) encoding
Dolby Digital, also known as AC-3 (AC for audio compression) supports all channel formats up to 5.1. It uses a sampling rate of 48 kHz with an average of 16 bits allocated to each sample. Its compression technique dif- ferentiates between short transient signals from long continuous sounds. It gives prominence to the latter in the form of long sample blocks com- pared to the transient sounds. AC-3 uses frequency-transform technique similar to DCT employed in MPEG video encoding. It provides smoother encoding compared with MPEG which creates arbitrary boundaries by its sub-band technique. Bits of up to 24 bits per sample are allocated dynamically to compensate for different listening environment, e.g. theatre, home or auditorium. The bit rate may be variable, although a fixed rate is nor- mally used as is the case with MPEG audio encoding. Bit rates of 384 or 448 kbps are typical.
The Dolby Digital system is a high-quality digital sound system equipped with a channel dedicated to subwoofer output (LFE) in order to reproduce low frequency below 120 Hz in addition to the other five chan- nels. Unlike the conventional 2-track Dolby Pro Logic system, Dolby Digital is a 5.1 track system with each channel discretely and digitally processed from the beginning, so they are independent and contribute to excellent channel separation. All five channels are reproduced at full bandwidth of 3 Hz–20 kHz. With these features, sound may be accurately reproduced in a home theatre context with the intended positioning of sound image and feeling along with surround sound with presence and power comparable to the movie theatre experience.
Linear PCM
LPCM is uncompressed and thus a lossless digital audio which has been used in CD and most studio masters. Analogue audio is sampled at 48 or 96 kHz with 16, 20 or 24 bits per sample (Audio CD is limited to a sam- pling rate of 44.1 kHz). As a result of the absence of compression and the high rate of sampling and quantisation, the bit rate could be excessively high and for this reason it is limited to 6.144 Mbps. The equivalent bit rates for MPEG and Dolby Digital are 448 and 384 kbps. LPCM supports up to eight channels, however, due to the limit of 6.144 Mbps, for five or more channels the lower sampling rate of 48 kHz must be used as indicated in Table 20.6.
The bit rate required by the non-compressed LPCM is normally too high to be accommodated by a DVD program (which may include other
elements such as video and other audio configurations) within the limits imposed by DVD specification on the maximum bit rate available for the various elements, namely 9.8 Mbps. For this reason, meridian lossless pack- ing, MLP is used. MLP now named Dolby-HD compresses data, bit by bit, removing redundant data without any loss to quality. Dolby-HD achieves a compression ratio of 2:1 by using a combination of techniques namely lossless matrixing, lossless waveform prediction and entropy coding.
Multiple-language sound track may be included by dubbing each stream with a different language. At playback, a particular stream may thus be selected.