| United States Patent |
5,568,202
|
|
Koo
|
October 22, 1996
|
System for echo cancellation comprising an improved ghost cancellation
reference signal
Abstract
A system for improved echo cancellation for use in particular in television
receivers. The system features a superior ghost cancellation reference
signal which exhibits improved performance in noisy environments and which
exhibits the flat and wide bandwidth necessary for effective channel
characterization while exhibiting a higher and more evenly distributed
amplitude versus time characteristic than that provided by known,
non-cyclic ghost cancellation signals.
| Inventors:
|
Koo; David (Briarcliff Manor, NY)
|
| Assignee:
|
North American Philips Corporation (New York, NY)
|
| Appl. No.:
|
949284 |
| Filed:
|
September 22, 1992 |
| Current U.S. Class: |
348/611; 348/192; 348/614; 375/231; 375/350; 455/65; 455/67.14 |
| Intern'l Class: |
H04N 007/08 |
| Field of Search: |
358/187,139,167,905
375/13,19,350,231
455/65,67,67.1,67.4
348/611,614,192
|
Primary Examiner: Britton; Howard W.
Attorney, Agent or Firm: Marion; Michael E.
Parent Case Text
This application is a continuation of 07/831,600, filed Feb. 05, 1992, now
U.S. Pat. No. 5,179,444; which is a continuation of 07/698,521, filed May
10, 1991, now U.S. Pat. No. 5,121,211; which is a continuation-in-part of
07/693,737, filed Apr. 30, 1991, now U.S. Pat. No. 5,111,298; which is a
continuation-in-part of 07/595,112, filed Oct. 09, 1990, now U.S. Pat. No.
5,047,859.
Claims
I claim:
1. An electronic reference signal in a system for minimizing the effects of
ghosts occurring during the transmission and reception of a television
signal over a communications path, wherein said reference signal is
embodied in a processor readable memory, is non-cyclic, has a
substantially flat frequency response within the bandwidth of said
communications path and has a plurality of substantially uniform amplitude
peaks over a time interval, and wherein a replica of said reference signal
is transmitted as part of said television signal and is utilized by a
decoder to derive coefficients which are used with at least one filter to
remove said ghosts.
2. The reference signal of claim 1 further defined by the equation:
##EQU4##
where A, b and .OMEGA. are real numbers; and
where
W(.omega.) is a windowed window function.
3. The reference signal of claim 1 further defined by the equation:
##EQU5##
where
##EQU6##
where .alpha., A, C.sub.n, b and .OMEGA. are real numbers;
where
N is an integer; and
where
W(.omega.) is a windowed window function.
Description
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 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 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 (which include the echo
artifacts, if any) must-be determined at the receiver. Once the channel
characteristics are calculated, filters are used to implement the inverse
channel characteristics to substantially eliminate the echoes. The present
invention relates to an apparatus and an improved 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 transmitted. 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 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 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 a windowed sin x/x pulse (sinc) 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 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 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 .gtoreq.52.5 .mu.s.
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 non-conventional
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
(not shown) which receives a television signal comprising the GCR signal.
The signal is received by the tuner of the receiver and converted to
digital form using analog to digital converter 10. An IIR filter 20 is
used to cancel post-echoes and an FIR filter 15 is used to cancel
pre-echoes. Echo cancellation circuits of this type are described in more
detail in U.S. patent application Ser. No. 676,927 filed Mar. 28, 1991,
now U.S. Pat. No. 5,161,017, which is assigned to the assignee of the
instant application and which is specifically incorporated by reference
herein.
Video samples are received and input to A/D converter 10 and the GCR
signal, which is transmitted during the vertical blanking interval of the
television signal, is separated and fed to a buffer memory 30. The GCR
signal, which has been distorted according to the channel characteristics,
is sometimes sampled over a number of frames, and an average of these
samples is then fed to processor 25 which can be a microprocessor or a
digital signal processor. The processor comprises a memory which contains
a pre-processed and stored version of the GCR as transmitted, and the
contents of buffer 30 is compared to the stored version of the GCR in
processor 25 and from this comparison, the impulse response of the channel
can be modeled. This channel model is then used to compute a sequence of
coefficients for the filters which implement the inverse channel
characteristic of the channel to suppress the echoes which are present.
The parent application ('112) describes a method and apparatus for
restoring a received signal wherein no assumptions are made about the
communication channel characteristics other than that the signal and
channel interaction is described by linear system theory. Consequently,
the channel is characterized completely by its impulse response.
The sequence of channel impulse response function samples thus obtained by
processing the test signal, serves to provide the correct sequence of
coefficients to filters 15 and 20. After the filter coefficients are fed
to the filters, the complete television signal is processed through these
filters whereby the echo components are substantially reduced. The output
of the IIR filter 20 is then fed to a digital-to-analog converter (D/A) 35
and presented as a video output signal to the video display of the
receiver.
The GCR signal is used to characterize the frequency or impulse response of
the channel (including the transmitter and receiver, as well as the actual
transmission path). It is therefore required that the frequency spectrum
of the GCR signal used in a conventional television system must be
low-pass and contained entirely in the 4.3 MHz. pass band of the NTSC
signal. In addition, it must be as flat as possible in that band. If there
is a null in the spectrum of the GCR signal, where the spectrum is near
zero over some frequency interval, then the channel will not be adequately
characterized over that interval. Even if there is a rolloff in the
spectrum, where the spectrum is significantly less then the peak value,
then in the presence of noise, the accuracy of the channel
characterization will suffer for the frequencies where the rolloff occurs.
FIG. 2 is a graph, in the time domain, of the BTA GCR signal. This is the
signal as transmitted and as stored in ROM 10 and/or a processed version
of which is stored in ROM 10. Before this signal is transmitted, it is
integrated and inserted in the blanking interval of each field of the
television signal. When viewed in the frequency domain (FIG. 3) it is
clear that the BTA GCR signal provides the necessary bandwidth
characteristics for effective channel characterization. When viewed in the
time domain (FIG. 2) however, it can be seen that the amplitude versus
time characteristic of the BTA GCR signal has relatively little energy.
This results in poor performance in a noisy environment.
The invention comprises an improved, non-cyclic GCR signal which features
higher energy and improved energy distribution with respect to both time
and frequency, and at the same time, the wide and flat bandwidth necessary
for effective channel characterization.
FIG. 4 is a graph of a first embodiment of the improved GCR signal. This
GCR signal has a higher energy level with peaks more evenly distributed in
time than the BTA GCR signal shown in FIG. 2.
As shown in the frequency spectrum, FIG. 5, this GCR signal has a very flat
spectrum in the band of interest and a higher energy level with respect to
frequency (about eight times higher) than the BTA GCR signal shown in FIG.
3.
The equation which describes this embodiment of the GCR signal is:
##EQU1##
where
##EQU2##
where .alpha., A, C.sub.n, b and .OMEGA. are real parameters.
where
N is an integer parameter.
where
W(.omega.) is a windowed window function.
A Hamming or Hanning window is used in this example however other windows
can also be used.
The parameters are in the range of from 0 to 10.sup.12. For this example
N=7, C.sub.1 =60, C.sub.3 =10, C.sub.5 =3, C.sub.7 =-1, C.sub.even =0, A=1
.OMEGA.=4.2.times.2.times.10.sup.6 Rad (where .OMEGA. equals the end of
the frequency band of interest, which in this case is 0 to 4.2 MHz.).
FIG. 6 is a graph of a second embodiment of the improved GCR signal. This
GCR signal has a higher energy level with peaks more evenly distributed in
time than the BTA GCR signal shown in FIG. 2.
As shown in the frequency spectrum, FIG. 7, this GCR signal has a very flat
spectrum in the band of interest and a higher energy level with respect to
frequency (about ten times higher) than the BTA GCR signal shown in FIG.
3.
The equation which describes this embodiment of the GCR signal is:
##EQU3##
where A, b and .OMEGA. are real parameters; and
where
W(.omega.) is a windowed window function.
A Hamming or Hanning window is used in this example however other windows
can also be used.
The parameters are in the range of from 0 to 10.sup.12. For this example
A=1, b=0.0004 and .OMEGA.=4.2.times.2.times.10.sup.6 Rad (where .OMEGA.
equals the end of the frequency band of interest, which in this case is 0
to 4.2 MHz.).
The foregoing disclosure and description of the invention is illustrative
and explanatory thereof and various changes in the methods and apparatus
indicated herein may be made within the scope of the appended claims
without departing from the spirit of the invention.
* * * * *
Prosecution History of Koo Patent
The Board of Patent Appeals and Interferences affirmed the examiner’s rejection of two-hump signal claims as being non-statutory under Section 101. Koo appealed to the Federal Circuit, which remanded the case to the PTO to permit Koo to amend his claims to incorporate the signal into a computer-readable memory.