U.S. patent application number 14/823717 was filed with the patent office on 2016-06-30 for tagging digital television signals.
The applicant listed for this patent is TRILITHIC, INC.. Invention is credited to Terry W. Bush, Brett W. Emsley, Gregg Stephen Rodgers.
Application Number | 20160192025 14/823717 |
Document ID | / |
Family ID | 43309170 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160192025 |
Kind Code |
A1 |
Bush; Terry W. ; et
al. |
June 30, 2016 |
TAGGING DIGITAL TELEVISION SIGNALS
Abstract
A method and apparatus for detecting leakage of a digital CATV
channel signal from a CATV system. The digital CATV channel signal
is amplitude modulated with a tag signal having a frequency in the
range of about 3 Hz to about 35 Hz, and coupled to the CATV system.
At a point of leakage from the CATV system, the amplitude modulated
digital CATV channel signal can be detected using a leakage
detector capable of recovering the tag signal.
Inventors: |
Bush; Terry W.; (Greenwood,
IN) ; Rodgers; Gregg Stephen; (Noblesville, IN)
; Emsley; Brett W.; (Zionsville, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TRILITHIC, INC. |
Indianapolis |
IN |
US |
|
|
Family ID: |
43309170 |
Appl. No.: |
14/823717 |
Filed: |
August 11, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13376903 |
Dec 8, 2011 |
9106886 |
|
|
PCT/US2010/037458 |
Jun 4, 2010 |
|
|
|
14823717 |
|
|
|
|
61185677 |
Jun 10, 2009 |
|
|
|
Current U.S.
Class: |
725/118 |
Current CPC
Class: |
H04N 21/2383 20130101;
H04N 17/00 20130101; H04N 21/615 20130101; H04H 20/78 20130101;
H04N 21/6118 20130101 |
International
Class: |
H04N 21/61 20060101
H04N021/61; H04H 20/78 20060101 H04H020/78; H04N 21/2383 20060101
H04N021/2383 |
Claims
1-34. (canceled)
35. A CATV system comprising: a source of program material; a
digital modulator for modulating the program material into a
digital television signal; a channel modulator coupled to the
digital modulator for modulating the digital television signal to a
CATV channel frequency; an amplitude modulator coupled to the
channel modulator for amplitude modulating the CATV channel
frequency signal; and, a CATV plant coupled to the amplitude
modulator for distributing the amplitude modulated CATV channel
frequency signal.
36. The system of claim 35 wherein the amplitude modulator
modulates the cable channel frequency signal at a rate of between
about 3 Hz and about 35 Hz.
37. The system of claim 36 wherein the amplitude modulator
modulates the cable channel frequency signal at a rate of between
about 10 Hz and about 23 Hz.
38. The system of claim 35 wherein the amplitude modulator
modulates the cable channel frequency signal to a depth of between
about 0.5 dB and 5 dB.
39. The system of claim 38 wherein the amplitude modulator
modulates the cable channel frequency signal to a depth of between
about 1 dB and about 3 dB.
40-77. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U. S. C.
.sctn.119(e) of the Jun. 10, 2009 filing date of U.S. Ser. No.
61/185,677. The disclosure of U.S. Ser. No. 61/185,677 is hereby
incorporated herein by reference.
BACKGROUND
[0002] The tagging of analog CATV (CATV being sometimes referred to
hereinafter as cable) signals for leakage detection purposes is
known. There are, for example, the systems illustrated and
described in U.S. Pat. Nos. 5,608,428; 6,018,358; and, 6,804,826,
and references cited in these. The disclosures of these references
are hereby incorporated herein by reference. This listing is not
intended to be a representation that a complete search of all
relevant art has been made, or that no more pertinent art than that
listed exists, or that the listed art is material to patentability.
Nor should any such representation be inferred.
SUMMARY
[0003] According to an aspect of the disclosure, a method for
detecting leakage of a digital CATV channel signal from a CATV
system comprises amplitude modulating the digital CATV channel
signal with a tag signal having a frequency in the range of about 3
Hz to about 35 Hz, coupling the amplitude modulated digital CATV
channel signal to the CATV system, receiving the amplitude
modulated digital CATV channel signal leaking from the CATV system
using a leakage detector including a first bandpass filter having a
center frequency within the CATV channel signal bandwidth,
amplifying the bandpass filtered signal, analog-to digital (A/D)
converting the amplified signal, and processing the A/D converted
signal. Processing the A/D converted signal comprises mixing the
A/D converted signal with a mixing signal, lowpass filtering the
mixed signal to a cutoff frequency of about half the bandwidth of
the amplitude modulated digital CATV channel signal, detecting the
amplitude of the lowpass filtered signal, and recovering the tag
signal frequency from the detected amplitude of the lowpass
filtered signal.
[0004] Illustratively, amplitude modulating the digital CATV
channel signal with a tag signal having a frequency in the range of
about 3 Hz to about 35 Hz comprises amplitude modulating the
digital CATV channel signal with a tag signal having an amplitude
in the range of > about 0 dB to .ltoreq. about 5 dB.
[0005] Further illustratively, receiving the amplitude modulated
digital CATV channel signal leaking from the CATV system using a
leakage detector including a first bandpass filter having a center
frequency within the CATV channel signal bandwidth comprises
receiving the amplitude modulated digital CATV channel signal
leaking from the CATV system using a leakage detector including a
first bandpass filter having a bandwidth in the range of about 5
MHz to about 15 MHz.
[0006] Additionally illustratively, A/D converting the amplified
signal comprises A/D converting the amplified signal at a sampling
rate in the range of about 20 MHz to about 40 MHz.
[0007] Illustratively, processing the A/D converted signal
comprises processing the A/D converted signal with a processing
engine.
[0008] Further illustratively, processing the A/D converted signal
comprises processing the A/D converted signal with at least one of
a digital signal processor (DSP) and field-programmable gate array
(FPGA).
[0009] Additionally illustratively, mixing the A/D converted signal
with a mixing signal comprises mixing the A/D converted signal with
a complex mixing signal.
[0010] Further illustratively, lowpass filtering the mixed signal
comprises filtering the mixed signal in a filter having a cutoff
frequency of about half the bandwidth of the amplitude modulated
digital CATV channel signal.
[0011] Illustratively, detecting the amplitude of the lowpass
filtered signal comprises summing the squares of the real part and
the imaginary part of the lowpass filtered signal.
[0012] Further illustratively, recovering the tag signal frequency
from the detected amplitude of the lowpass filtered signal
comprises decimating the lowpass filtered signal.
[0013] Additionally illustratively, decimating the lowpass filtered
signal comprises decimating the lowpass filtered signal through
multiple decimation stages to a final sample rate below about 150
Hz.
[0014] Additionally illustratively, recovering the tag signal from
the decimated signal comprises bandpass filtering the lowpass
filtered signal.
[0015] Illustratively, bandpass filtering the lowpass filtered
signal comprises bandpass filtering the lowpass filtered signal
using a second bandpass filter with a bandwidth in the range of
about 0.5 Hz to about 1.5 Hz, and a center frequency equal to the
tag frequency to permit reliable detection of the tag frequency in
the presence of noise.
[0016] Illustratively, recovering the tag signal frequency from the
lowpass filtered signal comprises bandpass filtering the decimated
signal.
[0017] Further illustratively, recovering the tag signal frequency
from the detected amplitude of the lowpass filtered signal further
comprises amplitude detecting the bandpass filtered decimated
signal.
[0018] Additionally illustratively, amplitude detecting the
bandpass filtered decimated signal comprises determining the
absolute value of the bandpass filtered decimated signal and
lowpass filtering the absolute value of the bandpass filtered
decimated signal.
[0019] Further illustratively, the method comprises calibrating the
amplitude detected signal.
[0020] Further illustratively, the method comprises displaying the
magnitude of the leak.
[0021] According to an aspect of the disclosure, apparatus for
detecting an approximately 3 Hz to approximately 35 Hz amplitude
modulated, digital signal leaking from a CATV system comprises a
digital leakage detector comprising a first bandpass filter having
a center frequency within the CATV channel signal bandwidth, an RF
amplifier coupled to the first bandpass filter, an
analog-to-digital (A/D) converter coupled to the RF amplifier; and,
a processing engine coupled to the A/D converter. The processing
engine includes a source of mixing signals, a mixer for mixing the
signals at an output port of the A/D converter with signals from
the source of mixing signals, a lowpass filter coupled to the
mixer, and a device for determining the magnitude of an output
signal from the lowpass filter and recovering the tag signal.
[0022] Illustratively, the apparatus for detecting an approximately
3 Hz to approximately 35 Hz amplitude modulated, digital signal
comprises an apparatus for detecting an approximately 3 Hz to
approximately 35 Hz amplitude modulated, digital signal modulated
to a depth in the range of > about 0 dB to .ltoreq. about 5
dB.
[0023] Further illustratively, the first bandpass filter has a
bandwidth in the range of about 5 MHz to about 15 MHz.
[0024] Additionally, the A/D converter has a sampling rate in the
range of about 20 MHz to about 40 MHz.
[0025] Illustratively, the processing engine comprises at least one
of a digital signal processor (DSP) and field-programmable gate
array (FPGA).
[0026] Further illustratively, the mixer produces a complex mixing
signal.
[0027] Illustratively, the lowpass filter comprises a lowpass
filter having a cutoff frequency of about half the bandwidth of the
amplitude modulated digital CATV channel signal.
[0028] Additionally illustratively, the device for determining the
magnitude of the output signal from the lowpass filter comprises a
device for summing the squares of the real part and the imaginary
part of an output signal from the lowpass filter.
[0029] Additionally illustratively, the device comprises a device
for decimating the magnitude of the lowpass filtered signal.
[0030] Illustratively, the device for decimating the lowpass
filtered signal comprises a device for decimating the lowpass
filtered signal through multiple decimation stages to a final
sample rate below about 150 Hz.
[0031] Further illustratively, the device for determining the
magnitude of an output signal from the lowpass filter comprises a
second bandpass filter with a bandwidth in the range of about 0.5
Hz to about 1.5 Hz, and a center frequency equal to the tag
frequency to permit reliable detection of the tag frequency in the
presence of noise.
[0032] Further illustratively, the device for recovering the tag
signal frequency from the lowpass filtered signal comprises a
second bandpass filter for filtering the decimated signal.
[0033] Additionally illustratively, the device for recovering the
tag signal frequency from the bandpass filtered signal comprises a
device for amplitude detecting the bandpass filtered decimated
signal.
[0034] Illustratively, the device for amplitude detecting the
bandpass filtered decimated signal comprises a device for
determining the absolute value of the bandpass filtered decimated
signal and for lowpass filtering the absolute value of the bandpass
filtered decimated signal.
[0035] Further illustratively, the device for recovering the tag
signal frequency from the lowpass filtered signal comprises a
device for calibrating the amplitude detected signal.
[0036] Further illustratively, the device for recovering the tag
signal frequency from the lowpass filtered signal comprises a
display for displaying the magnitude of the leak.
[0037] According to an aspect of the disclosure, a CATV system
comprises a source of program material, a digital modulator for
modulating the program material into a digital television signal, a
channel modulator coupled to the digital modulator for modulating
the digital television signal up to a CATV channel frequency, an
amplitude modulator coupled to the channel modulator for amplitude
modulating the CATV channel frequency signal, and a CATV plant
coupled to the amplitude modulator for distributing the amplitude
modulated CATV channel frequency signal.
[0038] Illustratively, the amplitude modulator modulates the cable
channel frequency signal at a rate of between about 3 Hz and about
35 Hz.
[0039] Further illustratively, the amplitude modulator modulates
the cable channel frequency signal at a rate of between about 10 Hz
and about 23 Hz.
[0040] Illustratively, the amplitude modulator modulates the cable
channel frequency signal to a depth of between about 0.5 dB and 5
dB.
[0041] Further illustratively, the amplitude modulator modulates
the cable channel frequency signal to a depth of between about 1 dB
and about 3 dB.
[0042] According to an aspect of the disclosure, a method of
tagging a digital CATV channel for detection comprises providing a
source of program material, digitally modulating the program
material into a digital television signal, modulating the digital
television signal up to a CATV channel frequency, amplitude
modulating the CATV channel frequency signal, and coupling the
amplitude modulated CATV channel frequency signal to a CATV plant
for distribution.
[0043] Illustratively, amplitude modulating the CATV channel
frequency signal comprises amplitude modulating the CATV channel
frequency signal at a rate of between about 3 Hz and about 35
Hz.
[0044] Further illustratively, amplitude modulating the CATV
channel frequency signal comprises amplitude modulating the CATV
channel frequency signal at a rate of between about 10 Hz and about
23 Hz.
[0045] Illustratively, amplitude modulating the CATV channel
frequency signal comprises amplitude modulating the CATV channel
frequency signal to a depth of between about 0.5 dB and 5 dB.
[0046] Further illustratively, amplitude modulating the CATV
channel frequency signal comprises amplitude modulating the CATV
channel frequency signal to a depth of between about 1 dB and about
3 dB.
[0047] According to an aspect of the disclosure, a method for
detecting egress of a digital CATV channel signal from a CATV
system comprises amplitude modulating the digital CATV channel
signal with a tag signal having a frequency in the range of about 3
Hz to about 35 Hz, coupling the amplitude modulated digital CATV
channel signal to the CATV system, receiving the amplitude
modulated digital CATV channel signal leaking from the CATV system,
mixing the amplitude modulated digital CATV channel signal with a
local oscillator signal to produce an intermediate frequency (IF)
signal, filtering the IF signal to the approximate bandwidth of the
digital CATV channel signal, reducing the bandpass filtered signal
substantially to baseband, filtering the baseband signal to recover
the tag signal, and producing an indication of recovery of the tag
signal.
[0048] Illustratively, the method further comprises amplifying the
received amplitude modulated digital CATV channel signal leaking
from the CATV system prior to mixing the amplitude modulated
digital CATV channel signal with the local oscillator signal.
[0049] Illustratively, reducing the output of the IF filter to
baseband comprises amplitude modulation (AM) detecting the output
of the IF filter.
[0050] Further illustratively, the method comprises amplifying the
output of the IF filter before AM detecting the output of the IF
filter.
[0051] Illustratively, filtering the baseband signal to recover the
tag signal comprises filtering the baseband signal through a low
pass filter.
[0052] Illustratively, filtering the baseband signal to recover the
tag signal comprises filtering the low pass filtered baseband
signal through a band pass filter to recover the tag signal.
[0053] Illustratively, filtering the baseband signal through a low
pass filter comprises filtering the baseband signal through a low
pass filter substantially to remove a portion of the baseband
signal above the tag signal frequency.
[0054] Illustratively, filtering the baseband signal to recover the
tag signal further comprises filtering the baseband signal with
that portion of the baseband signal above the tag signal frequency
removed through a band pass filter to recover the tag signal.
[0055] Illustratively, filtering the baseband signal to recover the
tag signal comprises filtering the baseband signal substantially to
remove that portion of the baseband signal above the tag signal
frequency.
[0056] Illustratively, filtering the baseband signal to recover the
tag signal comprises filtering the baseband signal through a band
pass filter to recover the tag signal.
[0057] Illustratively, producing an indication of recovery of the
tag signal comprises peak detecting the output signal from filter
and displaying the level of the received tag signal.
[0058] According to an aspect of the disclosure, a leakage detector
is provided for detecting egress of a digital CATV channel signal
amplitude modulated with a tag signal having a frequency in the
range of about 3 Hz to about 35 Hz and coupled to a CATV system.
The leakage detector includes a local oscillator, a mixer coupled
to the tag signal-modulated digital CATV channel signal and to the
local oscillator for mixing the amplitude modulated digital CATV
channel signal with the local oscillator signal to produce an
intermediate frequency (IF) signal, a first band pass filter
coupled to the mixer for reducing the IF signal substantially to
baseband, a first low pass filter coupled to the first band pass
filter for filtering the baseband signal to recover the tag signal,
and a circuit coupled to the first low pass filter for producing an
indication of recovery of the tag signal.
[0059] Further illustratively, the apparatus comprises an amplifier
for amplifying the received tag signal-modulated digital CATV
channel signal leaking from the CATV system. An output port of the
amplifier is coupled to an input port of the mixer.
[0060] Further illustratively, the apparatus comprises an amplitude
modulation (AM) detector coupled to the first band pass filter.
[0061] Further illustratively, the apparatus comprises an amplifier
coupled to the first bandpass filter before the AM detector.
[0062] Further illustratively, the apparatus comprises a second low
pass filter substantially to remove a portion of the baseband
signal above the tag signal frequency.
[0063] Further illustratively, the apparatus comprises a second
band pass filter coupled to the second low pass filter to recover
the tag signal.
[0064] Alternatively illustratively, the apparatus comprises a
second band pass filter coupled to the first band pass filter to
recover the tag signal.
[0065] Illustratively, the circuit for producing an indication of
recovery of the tag signal comprises a peak detecting and display
circuit coupled to the first low pass filter.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0066] The disclosure may best be understood by referring to the
following detailed description and accompanying drawings which
illustrate the disclosure. In the drawings:
[0067] FIG. 1 illustrates diagrammatically a test apparatus and
method useful in understanding the disclosure;
[0068] FIGS. 2-4 illustrate test results using different test
parameters with the apparatus and methods illustrated in FIG.
1;
[0069] FIG. 5 illustrates diagrammatically an apparatus and method
useful for generating certain signals according to the
disclosure;
[0070] FIG. 6 illustrates diagrammatically portions of an apparatus
according to the disclosure;
[0071] FIG. 7 illustrates diagrammatically apparatus and methods
useful for detecting a low frequency tag pursuant to the
disclosure; and,
[0072] FIG. 8 illustrates diagrammatically portions of an apparatus
according to the disclosure.
DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIMENTS
[0073] The following test was conducted to determine if a 256
quadrature amplitude modulated Annex B (hereinafter sometimes QAM)
digital channel can be tagged for leakage detection without
disrupting or otherwise affecting processing of the signal.
[0074] Referring particularly to FIG. 1, for the purposes of this
test, a broadcast digital channel was received by an antenna 20,
demodulated using a digital demodulator 22 such as, for example, a
Drake model DD860 digital demodulator, re-modulated into a QAM
signal using a digital modulator 24 such as, for example, a Drake
TMQAM Asi QAM modulator, and then modulated up to NTSC channel 15
at 129 MHz center frequency using a channel modulator 26 such as,
for example, a Scientific Atlanta model SA 6350 channel
modulator.
[0075] The signal was coupled from an output port 28 of the
modulator 26 to an input port 30 of a channel tagger instrument 32
of the type described in U.S. Pat. No. 5,608,428, a model CT-2 or
CT-3 channel tagger instrument available from Trilithic, Inc, 9710
Park Davis Drive, Indianapolis, Ind. 46235. An output port 34 of
the channel tagger instrument 32 was coupled through a 20 dB pad
(not shown) to an input port 36 of a splitter 38. One output port
40 of the splitter 38 was coupled to an input port 42 of a testing
instrument 44 of the general type described in published U.S.
patent application 2002/0019983 A1, a model 860 DSPi testing
instrument available from Trilithic, Inc. Another output port 46 of
the splitter 38 was coupled to an input port 48 of a second
splitter 50. A first output port 52 of splitter 50 was coupled to
an input port 54 of a spectrum analyzer 56 such as, for example, a
Trilithic model 8821 QR spectrum analyzer. Another output port 58
of splitter 50 was coupled through a 10 dB pad (not shown) to an
input port 60 of a digital (QAM)-ready television receiver 62 such
as, for example, a Samsung QAM TV.
[0076] The modulation error ratio (hereinafter sometimes MER) and
bit error rate (hereinafter sometimes BER) of the signal were
measured by the testing instrument 44 and the spectrum analyzer 56
at each of the tag frequencies from 10 Hz to 23 Hz at three
different depths (3 dB, 2 dB, 1 dB) of modulation. These results
are tabulated in Tables I (3 dB depth of modulation), II (2 dB
depth of modulation) and III (1 dB depth of modulation). The
television receiver was monitored during the test for any signs of
interference. The test setup is as illustrated in FIG. 1. FIGS. 2,
3 and 4 illustrate graphs of MER versus tag frequency for
modulation depths of 3 dB, 2 dB and 1 dB, respectively.
[0077] The depths of modulation were verified using the spectrum
analyzer 56. The tag frequency was verified using a GoldStar (now
LG) Electronics model FC-2130 frequency counter. The level at the
television receiver 62 was padded down to be roughly 1 dBmV using
the not illustrated 10 dB pad between ports 58 and 60 on splitter
50 and receiver 62, respectively.
[0078] The results of the tests follow. The BER was observed and
recorded, but no errors were noted at any time during the test
using either the testing instrument 44 or the spectrum analyzer 56.
Therefore the BER is not illustrated.
TABLE-US-00001 TABLE I 3 dB Depth of Modulation instrument 44
analyzer 56 Tag Frequency MER MER 10 40 41 11 40 41 12 39 41 13 40
41 14 39 41 15 38 41 16 38 41 17 38 41 18 38 41 19 39 41 20 40 41
21 39 41 22 39 41 23 38 41
TABLE-US-00002 TABLE II 2 dB Depth of Modulation instrument 44
analyzer 56 Tag Frequency MER MER 10 39 41 11 40 41 12 40 41 13 40
41 14 40 41 15 40 41 16 40 41 17 40 41 18 40 41 19 40 41 20 39 41
21 38 41 22 39 41 23 39 41
TABLE-US-00003 TABLE III 1 dB Depth of Modulation instrument 44
analyzer 56 Tag Frequency MER MER 10 41 41 11 40 40 12 40 40 13 40
40 14 40 40 15 40 40 16 40 39 17 39 38 18 38 38 19 38 40 20 40 40
21 38 39 22 38 40 23 39 39
[0079] Additional observations noted while the above-described
tests were being performed follow.
[0080] The signal levels would fluctuate slightly due to the
channel tagger instrument 32. This was verified by turning the
channel tagger instrument 32 off while testing.
[0081] The MER readings would fluctuate slightly due to the channel
tagger instrument 32. Due to this, all recorded levels were rounded
to the nearest single digit.
[0082] When observing the 256 QAM symbol constellation with the
spectrum analyzer 56, the spectrum analyzer 56 would lose signal
lock after a few seconds and would not regain signal lock. However,
the testing instrument 44's symbol constellation display remained
stable with the tagged signal.
[0083] Due to the unsettled symbol constellation issue with the
spectrum analyzer 56, the readings were taken while the unit was in
long term BER display mode. In this mode the spectrum analyzer 56
did not lose signal lock, and the readings were consistent and
stable.
[0084] The television receiver 62 appeared to be unaffected by the
tagged signal. The television receiver 62 only displayed momentary
pixelation when the channel tagger instrument 32 was turned on and
off.
[0085] From this testing, it was concluded that the channel tagger
instrument 32 can successfully tag a digital channel, in this
testing, a QAM channel, with little or no impact. The MER overall
was roughly 1 to 2 dB lower than without the tag, but remained at
acceptable levels. The BER readings consistently showed no errors.
There appeared to be no difference in results regardless of what
specific tag frequency was used. All tag frequencies in the 10
Hz-23 Hz range tested gave similar readings.
[0086] Turning now to FIGS. 5-7, a digital signal to be used in a
leak detection scheme is first amplitude modulated with a 3 dB, 20
Hz tag, as illustrated in FIG. 5. This tagged, digital signal is
coupled to the CATV (hereinafter sometimes cable) system 90,
illustratively in the same manner as any other TV or cable modem
signal, that is, by modulating it onto a CATV system carrier and
coupling the thus-modulated carrier to the CATV system's downstream
path. The above-described tests have established that the 3 dB 20
Hz tag does not interfere with subscribers' reception and
demodulation of the thus-tagged digital signal. When the tagged,
digital signal encounters, and is radiated through, a leak in the
cable system 90, the low-amplitude leakage signal is broadcast into
the air, and received by, for example, a leakage detector 92 of the
general type described below.
[0087] FIG. 6 illustrates an embodiment of a digital leakage
detector 92. First, the incoming signal from system 90 is
band-limited using a bandpass filter 100 having a center frequency
of, for example, 133 MHz, with a bandwidth of, for example, 10 MHz.
The band-limited signal at the output port 102 of bandpass filter
100 is coupled to, and amplified by, an RF amplifier 104. The thus
band-limited and amplified signal at the output port 106 of
amplifier 104 is digitized by an A/D converter 108 with a sampling
rate of, for example, 25 MHz. The digitized samples at the output
port 110 of A/D converter 108 are coupled to a processing engine
112, such as a digital signal processor (hereinafter sometimes DSP)
or field-programmable gate array (hereinafter sometimes FPGA),
where further detection occurs.
[0088] FIG. 7 illustrates an example of a type of algorithm which
may be used by the processing engine 112 to detect the 20 Hz tag on
the received leaking tagged digital cable signal. First, the
digitized samples are complex mixed down to baseband using an
e.sup.j2.pi.Fc.sup./Fs source 114 and a mixer 116, where e is the
base of the natural logarithms (.about.2.718), j=sqrt(-1), F.sub.c
is the aliased center frequency of the bandpass filter 100, about 8
MHz in this example, and F.sub.s is the sampling rate of the A/D
converter 108, about 25 megasamples/sec. in this example. Complex
mixing is employed because it does not produce mixing images and,
when used in conjunction with amplitude detection, is not sensitive
to frequency errors. The complex signal at the output port 118 of
mixer 116 is lowpass filtered by a filter 120 with a cutoff
frequency of about half the bandwidth of the tagged signal of
interest, about 3 MHz in the case of an ITU J.83 Annex B digital
cable channel. The magnitude of the signal is computed 121 by
summing the square of the real part and the square of the imaginary
part of the complex signal.
[0089] The 20 Hz tag component next needs to be recovered from the
signal at the output of 121. This can be accomplished several ways,
but in this example algorithm is accomplished by a combination of
lowpass filtering, via the illustrated lowpass filter in decimator
122, and decimation 122, to achieve a final sample rate of around
100 Hz. This decimation permits creation of a reasonable length
bandpass filter 124 with a bandwidth of 1 Hz, and a center
frequency equal to the tag frequency, in this example, 20 Hz, which
is required to reliably detect the received leakage's 20 Hz tag in
the presence of noise. Using a reasonable length for a decimation
filter 122, the operation to decimate from the A/D converter 108's
sample rate of 25 MHz to about 100 Hz can be accomplished in 5
steps, each one decimating the sampling rate by a factor of 12.
After the bandpass filter 124, the signal is amplitude detected by
taking the absolute value of the signal at 126 and then lowpass
filtering at 128. The output of this stage is calibrated at 130 and
the strength of the leak is displayed at 132 for the user.
[0090] FIG. 8 illustrates another system for detecting the tag
signal. In FIG. 8, the tagged digital cable channel signal is
detected using a superheterodyne receiver 150. A 133 MHz single
conversion receiver 150 is described. However, it should be
understood that the input frequency may be any frequency used to
carry a cable TV channel and that additional conversions and/or a
combination of digital and analog receiving techniques are
possible.
[0091] The received 133 MHz leakage signal is amplified 152 and
supplied to a mixer 154 where it is mixed with a 169 MHz signal
supplied by a local oscillator 156 to produce an intermediate
frequency signal of 36 MHz. The 36 MHz IF signal is bandpass
filtered 160 to select the approximate bandwidth of an ITU J.83
Annex B digital channel (.about.6 MHz). The output of the IF filter
160 is amplified 164 and coupled to an AM detector 168 which
reduces the digital signal to baseband, providing a signal with the
20 Hz tag component. The detector 168 output is passed through a 35
Hz low pass filter 172 to remove most of the baseband signal above
the tag frequency of 20 Hz. The signal is then coupled to a narrow
band 20 Hz band pass filter 176 which may be, for example, a
switched capacitor filter. The output signal from filter 176 is
then peak detected and coupled to a display circuit 182 that scales
and displays the level of the received 20 Hz tag signal.
* * * * *