Hardware Reference
In-Depth Information
Heterogeneous Multiprocessors on a Chip
A completely different application area calling for single-chip multiprocessors
is embedded systems, especially in audiovisual consumer electronics, such as tele-
vision sets, DVD players, camcorders, game consoles, cell phones, and so on.
These systems have demanding performance requirements and tight constraints.
Although these devices look different, more and more of them are simply small
computers, with one or more CPUs, memories, I/O controllers, and some custom
I/O devices. A cell phone, for example, is merely a PC with a CPU, memory,
dwarf keyboard, microphone, loudspeaker, and a wireless network connection in a
small package.
Consider, as an example, a portable DVD player. The computer inside has to
handle the following functions:
1. Control of a cheap, unreliable servomechanism for head tracking.
2. Analog-to-digital conversion.
3. Error correction.
4. Decryption and digital rights management.
5. MPEG-2 video decompression.
6. Audio decompression.
7. Encoding the output for NTSC, PAL, or SECAM television sets.
This work must be done subject to stringent real-time, quality-of-service, energy,
heat-dissipation, size, weight, and price constraints.
CD, DVD, and Blu-ray disks contain a long spiral containing the information,
as illustrated in Fig. 2-25 (for a CD). In this section we will discuss DVDs since
they are still more common than Blu-ray disks, but Blu-ray disks are very similar
to DVDs except they use MPEG-4 instead of MPEG-2 for encoding. With all opti-
cal media, the read head must accurately track the spiral as the disk rotates. The
price is kept low by using a relatively simple mechanical design and tight control
over the head position in software. The signal coming off the head is an analog
signal, which must be converted to digital form before being processed. After it
has been digitized, heavy error correction is required because DVDs are pressed
and contain many errors, which must corrected in software. The video is com-
pressed using the MPEG-2 international standard, which requires complex
(Fourier-transform-like) computations for decompression. Audio is compressed
using a psycho-acoustic model, which also requires sophisticated calculations for
decompression. Finally, audio and video have to be rendered in a suitable form for
output to NTSC, PAL, or SECAM television sets, depending on the country to
which the DVD player is shipped. It should come as no surprise that doing all this
work in real time in software on a cheap general-purpose CPU is not possible.
