COMPUTER SYSTEMS ORGANIZATION - Structured Computer Organization - page 135

Figure 2-42. Typical details of the upstream and downstream channels in North
America. QAM-64 (Quadrature Amplitude Modulation) allows 6 bits/Hz but
works only at high frequencies. QPSK (Quadrature Phase Shift Keying) works at
low frequencies but allows only 2 bits/Hz.
used (the Diffie-Hellman algorithm) is beyond the scope of this topic. See Kauf-
man et al. (2002) for a discussion of it.
Finally, the modem has to log in and provide its unique identifier over the
secure channel. At this point the initialization is complete. The user can now log
in to the ISP and get to work.
There is much more to be said about cable modems. Some relevant references
are Adams and Dulchinos (2001), Donaldson and Jones (2001), and Dutta-Roy
(2001).
2.4.7 Digital Cameras
An increasingly popular use of computers is for digital photography, making
digital cameras a kind of computer peripheral. Let us briefly see how that works.
All cameras have a lens that forms an image of the subject in the back of the cam-
era. In a conventional camera, the back of the camera is lined with film, on which
a latent image is formed when light strikes it. The latent image can be made visi-
ble by the action of certain chemicals in the film developer. A digital camera
works the same way except that the film is replaced by a rectangular array of
CCDs ( Charge-Coupled Devices ) that are sensitive to light. (Some digital cam-
eras use CMOS, but we will concentrate on the more common CCDs here.)
When light strikes a CCD, it acquires an electrical charge. The more light, the
more charge. The charge can be read off by an analog-to-digital converter as an
integer from 0 to 255 (on low-end cameras) or from 0 to 4095 (on digital single-
lens-reflex cameras). The basic arrangement is shown in Fig. 2-43.
Each CCD produces a single value, independent of the color of light striking it.
To form color images, the CCDs are organized in groups of four elements. A
Bayer filter is placed on top of the CCD to allow only red light to strike one of the
four CCDs in each group, blue light to strike another one, and green light to strike