Project Description
Figure 1 shows a high-level block diagram of a conventional analog television receiver. The analog signals from the studio camera and sound system are selected by the tuner from among all the ones available, recovered and separated by the demodulator and decoder, then amplified and output in a form you can sense. The entire information path is analog, although it may embody digital controls.
FIGURE 1: Analog television receiver
The analog signal format isn’t essential anywhere along the signal path except as light coming from the subject or sent to your eye. Cameras using charge coupled device (CCD) sensors output digital images natively, and LCD displays work better with digital inputs than analog ones. Television started in analog form because the available electronics technology could only support analog processing at reasonable cost.
Perhaps the strongest impetus towards digital television was satellite broadcast, because the economic viability of the satellite system is dominated by the number of distinct television channels it can support. Ignoring overlap at the channel edges, the 6 MHz channel required for U.S. analog television permits at most 12 channels in a standard 72 MHz transponder.Wider channel spacing to eliminate interference due to overlap reduces that number noticeably.
A 6 MHz spectrum segment can support 20 to 40 Mbps, however, depending on the waveform, so digital television offers the opportunity for more channels in the transponder so long as one channel fits into no more than 1.6 to 3.3 Mbps. The compression technologies devised by the Motion Picture Experts group (MPEG) solve that problem, reducing the approximately 160 Mbps raw video signal (720 ? 480 pixels per frame, 30 frames per second, and an assumed 16 bits per pixel) significantly:
MPEG-1 — 1.5 Mbps at VHS video tape quality
MPEG-2 — 3-10 Mbps at DVD quality
MPEG-4 — 4.8 Kbps to 4 Mbps at varying resolution and quality
The completely digital TV receiver in Figure 2 — including a digital display — looks a little different from the analog receiver in Figure 1. The tuner still selects a modulated waveform from the available spectrum, but the demodulator outputs error-corrected bit streams rather than analog waveforms. Those bit streams are decoded from the MPEG format to raw video and audio data.
FIGURE 2: Digital television receiver
In the digital receiver of Figure 2, there are many ways to transport the signal besides the lossy, inconvenient coaxial cable so common in conventional TV systems.