Automatic gain control system for a high definition television signal receiver including an adaptive equalizer

DETAILED DESCRIPTION FIG.
1 illustrates a portion of a high definition television (HDTV) receiver for receiving and digitally processing an HDTV signal of the type shown in FIG.
2.
This signal will be described before proceeding with a discussion of the receiver shown in FIG.
1.
FIG.
2 illustrates the video frequency spectrum of a high definition television signal that is compatible with the 6 MHz bandwidth of a standard NTSC television signal channel, and that can be used as a simulcast signal.
The frequencies along the frequency scale of FIG.
2 (-1.
25 MHz to 4.
5 MHz) are referenced to the 0.
0 MHz frequency location of the RF picture carrier in a standard NTSC system.
The HDTV television signal is a data compressed signal divided into high and low priority information components.
In this example audio, synchronizing and low frequency video information components that are intended to be received with high reliability are assigned a high priority.
The synchronizing information may be in the nature of a training signal containing a unique signature or code to facilitate signal recovery and processing at a receiver, and illustratively may include field rate scanning information (e.
g.
, start of field markers).
Other less critical components, such as high frequency video information, are assigned a lower priority.
The high priority information is the information needed to create a viewable image, albeit a less than perfect image, and is conveyed with significantly more power than low priority information.
The high priority information exhibits a narrow bandwidth relative to the low priority information, and narrowband quadrature amplitude modulates a carrier with a 0.
96 MHz symbol rate.
The low priority information wideband quadrature amplitude modulates a carrier with a 3.
84 MHz symbol rate.
As shown by FIG.
2, this results in a 0.
96 MHz bandwidth measured between frequencies of 6 db attenuation for the high priority signal, and a 3.
84 MHz bandwidth for the standard priority signal.
A resulting composite signal is a form of a multiple QAM signal e