SYSTEM FOR ECHO CANCELLATION
COMPRISING AN IMPROVED GHOST
CANCELLATION REFERENCE SIGNAL
This application is a continuation of U.S. Ser. No. 5 08/699,277 filed on Aug. 19, 1996 which is a continuation of Ser. No. 07/949,284, Sep. 22, 1992, U.S. Pat. No. 5,568,202 which is itself a continuation of Ser. No. 07/831, 600, Feb. 5, 1992, U.S. Pat. No. 5,179,444, which is a continuation of Ser. No. 07/698,521, May 10, 1991, U.S. 10 Pat. No. 5,121,211, which is a continuation-in part of Ser. No. 07/693,737, Apr. 30, 1991, U.S. Pat. No. 5,111,298, which is a continuation-in part of Ser. No.07/595,112, Oct. 9, 1990, now abandoned.
BACKGROUND OF THE INVENTION
In 1979 the IEEE published an article which has become a basic reference in the field of television echo (or "ghost") elimination. The article is entitled "A Tutorial On Ghost Cancellation in television Systems" and was written by 20 Walter Ciciora, Gary Sgrignoli and William Thomas and it is incorporated by reference herein.
Although the Ciciora article described the fundamental principles, apparatus and algorithms applicable to ghost cancellation, the state of the art has only recently progressed 25 to the point of providing practical ways to implement and improve these basic concepts.
There are two main steps to the echo cancellation process. First the characteristics of the communications channel 3Q (which include the echo artifacts, if any) must be determined by the receiver. Once the channel characteristics are calculated, filters are used to implement the inverse channel characteristics to substantially eliminate the echos. The present invention relates to an apparatus and an improved 3J ghost cancellation reference signal, for identifying the characteristics of a communication channel.
Communication engineering continually must deal with the problem of restoring a signal which has been altered by the communication path over which the signal was trans- 40 mitted. Signal restoration often can be achieved if the communication path is fully characterized, at least as to those parameters which contribute to the signal alteration. Thus, a frequently essential component of the signal restoration problem is that of identifying the characteristics of the 45 communication path or channel.
A straight forward approach to the channel identification problem is to transmit a ghost cancellation reference signal (GCR) having a known characteristic, over the channel, and to receive the transmitted signal after it has passed through 50 the channel. The originally transmitted signal is compared with the received signal, and a model of the channel characteristics is developed based on the comparison.
The Japanese Broadcasting Technology Association (BTA) has adopted a GCR signal that is the time integral of 55 a windowed sin x/x pulse (sine) which is transmitted on line 18 of the vertical blanking interval (VBI) of a television signal. Although the BTA GCR signal has the necessary flat bandwidth in the frequency domain, its energy is relatively low. The BTA GCR signal therefore may be suboptimal 60 since its low energy limits its performance under high noise conditions. Additional processing time is needed to compensate for the noise present in the channel which increases the time it takes for the echo cancellation system to calculate the channel characteristics when conditions in the channel 65 change. The BTA GCR signal has a fixed time interval which cannot be changed without effecting its frequency spectrum
characteristics. This limits the possible applications for which the BTA GCR signal can be used. The time interval for an NTSC television system, for example, would be &52.5/«.
Other GCR signals have been proposed which have a higher energy level than the BTA signal. These signals, are cyclic in nature however, and therefore, not effective for detecting pre and post-echoes present in the channel.
SUMMARY OF THE INVENTION
The instant invention comprises a non-cyclic echo cancellation system which utilizes an improved, high energy non-cyclic GCR signal which provides the flat, wide frequency spectrum necessary to fully characterize the channel and which has a high energy level (amplitude) and a more even distribution of energy over a time interval. This time interval can be adjusted according to different system requirements while maintaining the necessary flat frequency response over the desired frequency range. The GCR signal of the invention can therefore be adapted for use in nonconventional television systems such as those providing high definition and enhanced definition television, as well as for other communication applications such as echo cancellation in telephony and microwave systems, for example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of one embodiment of an echo cancellation circuit comprising the invention;
FIG. 2 is a graph of the BTA GCR signal;
FIG. 3 is a graph of the frequency spectrum of the BTA GCR signal;
FIG. 4 is a graph of a first embodiment of a GCR signal comprising the invention;
FIG. 5 is a graph of the frequency spectrum of the GCR signal of FIG. 4;
FIG. 6 is a graph of a second embodiment of a GCR signal comprising the invention; and
FIG. 7 is a graph of the frequency spectrum of the GCR signal of FIG. 6.
DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS OF THE INVENTION
There are normally two main steps involved in cancelling echoes which occur during the transmission of television signals. First the characteristics of the communication channel (which include the echo artifacts, if any) must be determined at the receiver. From these characteristics, an inverse channel characteristic is derived in the form of a sequence of filter coefficients. These coefficients are then provided to filters which are used to implement the inverse channel processing, i.e. the echo cancellation.
A received video signal contains echoes which are comprised of superimposed copies of the originally transmitted signal, which have different delay times and amplitudes. The strongest signal component represents the originally transmitted or main signal component. Looking in the time domain, any echo component occurring before the main signal component is called a "pre-echo" and any copy occurring after the main signal component is called a "post-echo".
FIG. 1 describes an echo cancellation circuit which can be used to cancel both types of echoes. Such a circuit can be part of a television receiver 2 which receives a television signal comprising the GCR signal. The signal is received by