Media Compression - Scalable Continuous Media Streaming Systems - page 26

Figure 2.1 A sequence of data samples in CD-quality digital audio

a natural extension of the PCM format but with a word length of up to 24 bits and sampling
frequency of up to 192 kHz. The medium is also capable of encoding surround sound with
up to 6 discrete channels of audio. The resultant data generation rate therefore can reach up
to 27 Mbps. Due to the extremely high data rate, the DVD audio standard supports the use of
a lossless compression algorithm called Meridian Lossless Packing (MLP) to reduce the data
rate by approximately a factor of 2.
Apart from the DVD audio standard, another industry group has introduced a second stan-
dard targeted to replace the CD format - Super Audio CD (SACD) [38]. Unlike CD and DVD
audio, SACD employs 1-bit delta sigma modulation at a sampling rate of 2.8 Mhz. Extensive
noise shaping is then employed to achieve very high signal-to-noise ratio and wide frequency
response performance. Again the standard supports surround sound of up to 6 channels, result-
ing in a raw data rate of up to 16.9 Mbps. The standard also includes a lossless compression
algorithm called Direct Stream Transfer (DST) to reduce the data rate by approximately a
factor of 2.
It is worth noting that both of the emergent audio formats employ only lossless compression
that does not discard any audio information in the compression process. This is clearly done
to preserve the highest audio quality, which is one of the major advantages of the new formats.
Nevertheless, the resultant data rate even at compressed format is still very substantial, and
thus whether lossy compression can be employed to reduce the bit-rate further and yet achieve
audio quality superior to the standard CD will be an interesting area of research.

2.1.2 Digital Video
In addition to audio, video is the other most commonly used continuous media. A digital video
sequence is divided into many video frames, each frame capturing one snapshot of the video
scene. The video frames are typically captured at periodic time intervals, such as 25 frames
per second (fps) in PAL-standard video and 29.9 fps in NTSC-standard video. In continuous
media system, however, there is more flexibility in the frame rate used, predominately due to the
often limited network/server bandwidth for delivering the video data. In these applications it is
common to use frame rates lower than the previous broadcast standards if network bandwidth
is limited. Clearly, the reduction in frame rate will generate fewer data but at the expense of
reduced visual quality (e.g., fast motion becomes jerky or jumpy).