System and method for inserting and recovering an add-on data signal for transmission with a video signal

Claims

What is claimed is:

1. A system for transmitting a data signal with a video signal, comprising:

a first delay element receiving the data signal, the data signal being unrelated to the video signal, and producing a delayed data signal that is delayed a predetermined period of time from the data signal;

a first adder element adding the data signal and said delayed data signal to produce a first filtered signal having a plurality of spectral peaks with a spacing corresponding to spacing in an unused portion of the frequency spectrum of the video signal;

a modulator modulating a single unmodulated carder frequency with said first filtered signal to produce a modulated filtered signal, said carder frequency being selected to permit said first filtered signal to be inserted into said unused portion of the frequency spectrum of the video signal;

a second adder element adding said modulated filtered signal and the video signal to produce a modified video signal containing said modulated filtered signal with said first filtered signal inserted into said unused portion of the frequency spectrum of the video signal;

a transmitter transmitting said modified video signal;

a receiver receiving said transmitted modified video signal and processing said modified video signal to regenerate the video signal;

a separator circuit separating said modulated filtered signal from said transmitted modified video signal to reconstruct said first filtered signal; and

an inverse filter processing said reconstructed first filtered signal to recover the data signal, whereby the data signal is transmitted with the video signal and subsequently separated from the data signal in said receiver.

2. The system of claim 1 wherein said predetermined period is 1/60 second.

3. The system of claim 1 wherein said carrier frequency is selected to position said plurality of spectral peaks substantially in the center of said unused portion of the spectrum of the video signal.

4. The system of claim 1 wherein said carrier frequency is 3.579545 MHz.+-.k*60 Hz+30 Hz, where k is an integer having a value between 0 and 18,000.

5. The system of claim 1 wherein said carrier frequency is 3.579545 MHz.+-.30 Hz.

6. A system for transmitting a data signal with a video signal, comprising:

a first filter receiving the data signal and producing a filtered signal having a plurality of spectral peaks with spacing corresponding to spacing of an unused portion of the frequency spectrum of the video signal, said data signal being unrelated to the video signal;

a modulator element modulating a single unmodulated carrier frequency with said filtered signal to produce a modulated filtered signal, said carrier frequency being selected to modulate said filtered signal into said unused portion of the frequency spectrum of the video signal to produce a modified video signal containing said modulated filtered signal with said filtered signal inserted into said unused portion of the frequency spectrum of the video signal;

a signal separator processing, said modified video signal to recover said filtered signal; and

a second filter receiving said recovered filtered signal from said signal separator to recover the data signal from said recovered filtered signal, whereby the data signal is inserted into said unused portion of the frequency spectrum of the video signal and subsequently recovered from the video signal.

7. The system of claim 6 wherein said first filter is a comb filter.

8. The system of claim 7 wherein said comb filter has at least two taps.

9. The system of claim 7 wherein said comb filter comprises a delay circuit delaying the data signal by a predetermined period of time to produce a delayed data signal, and an adder element adding the data signal and said delayed data signal to produce said filtered signal.

10. The system of claim 9 wherein said delay circuit is a first-in-first-out buffer.

11. The system of claim 9 wherein said delay circuit is an analog delay line.

12. The system of claim 9 wherein said predetermined period of time is 1/60 second, such that said plurality of spectral peaks have a 60 Hertz spacing.

13. The system of claim 9 wherein said predetermined period of time corresponds the spectral spacing of said unused portion of the spectrum of the video signal.

14. The system of claim 6 wherein said first filter is a data buffer containing at least a portion of the data signal, said filtered signal being generated by continuously repeating the data signal contained within said data buffer at a predetermined rate.

15. The system of claim 14 wherein said data buffer stores 1/60 second of the data signal, and said predetermined rate is 60 Hertz, whereby said plurality of spectral peaks have a 60 Hertz spacing.

16. The system of claim 14 wherein said predetermined rate is selected such that said data buffer repeats the data signal contained within said data buffer every 1/60 second, whereby said plurality of spectral peaks have a 60 Hertz spacing.

17. A system for inserting a data signal into a video signal, comprising:

a filter receiving the data signal and producing a filtered signal having a plurality of spectral peaks with spacing corresponding to spacing in an unused portion of the frequency spectrum of the video signal, said data signal being uncorrelated to the video signal; and

a modulator element modulating a single unmodulated carrier frequency having a predetermined phase with said filtered signal to produce a modulated filtered signal, said carrier frequency being selected to modulate said filtered signal into said unused portion of the frequency spectrum of the video signal to produce a modified video signal containing said modulated filtered signal with said filtered signal inserted into said unused portion of the frequency spectrum of the video signal.

18. The system of claim 17 wherein said filter is a comb filter.

19. The system of claim 18 wherein said comb filter contains at least two taps.

20. The system of claim 18 wherein said comb filter comprises a delay circuit delaying the data signal by a predetermined period of time to produce a delayed data signal, and an adder element adding the data signal and said delayed data signal to produce said filtered signal.

21. The system of claim 20 wherein said delay circuit is a first-in-first-out buffer.

22. The system of claim 20 wherein said delay circuit is an analog delay line.

23. The system of claim 20 wherein said predetermined period of time is 1/60 second, such that said plurality of spectral peaks have a 60 Hertz spacing.

24. The system of claim 17 wherein said filter is a data buffer containing at least a portion of the data signal, said filtered signal being generated by continuously repeating said portion of the data signal contained within said data buffer at a predetermined rate.

25. The system of claim 24 wherein said data buffer stores 1/60 second of the data signal, and said predetermined rate is 60 Hertz, whereby said plurality of spectral peaks have a 60 Hertz spacing.

26. The system of claim 24 wherein said predetermined rate is selected such that said data buffer repeats the data signal contained within said data buffer every 1/60 second, whereby said plurality of spectral peaks have a 60 Hertz spacing.

27. A system for recovering a data signal inserted into an unused portion of the frequency spectrum of an unrelated video signal, the data signal being filtered to generate a filtered signal having a plurality of spectral peaks with a spacing corresponding to spacing in an unused portion of the frequency spectrum of the video signal and modulated with a single unmodulated carrier frequency to generate a modulated data signal, the system comprising:

a signal separator separating the modulated data signal from the unused portion of the frequency spectrum of the video signal;

a demodulator coupled to said signal separator and demodulating the modulated data signal to recover the filtered signal, said demodulator having a single carrier frequency with a predetermined phase and a carrier frequency selected to correspond to the unused portion of the frequency spectrum of the video signal; and

a receiver filter receiving said filtered signal from said demodulator to recover the data signal from the filtered signal, said recovered data signal being uncorrelated to the video signal, whereby the data signal is inserted into the unused portion of the frequency spectrum of the video signal and subsequently recovered from the video signal.

28. The system of claim 27 wherein the data signal is filtered by a comb filter before being modulated, said receiver filter comprising an inverse comb filter to recover the data signal.

29. The system of claim 28 wherein said inverse comb filter comprises:

a delay element receiving said filtered signal from said demodulator and producing a delayed filtered signal that is delayed a predetermined period of time; and

a subtractor subtracting said delayed filtered signal from said filtered signal to recover the data signal, whereby the data signal is transmitted with the video signal and subsequently separated from the data signal.

30. The system of claim 29 where said predetermined period of time corresponds to the spectral spacing of the unused portion of the spectrum of the video signal.

31. The system of claim 29 wherein said predetermined period of time is 1/60 second.

32. A method for transmitting a data signal with a video signal, comprising the steps of:

filtering the data signal to produce a filtered signal having a plurality of spectral peaks with spacing corresponding to spacing of an unused portion of the frequency spectrum of the video signal, the data signal being independent of the video signal;

modulating a single unmodulated carrier frequency with said filtered signal to produce a modulated filtered signal, said carrier frequency being selected to modulate said filtered signal into said unused portion of the frequency spectrum of the video signal to produce a modified video signal containing said modulated filtered signal with said filtered signal inserted into said unused portion of the frequency spectrum of the video signal;

separating said filtered signal from said modified video signal; and

inverse filtering said filtered signal separated by said signal separator to recover the data signal from said filtered signal.

33. The method of claim 32 wherein said step of filtering uses a comb filter to produce said plurality of spectral peaks by delaying the data signal by a predetermined period of time corresponding to said plurality of spectral peaks to produce a delayed data signal, and adding said delayed data signal to the data signal.

34. The method of claim 32 wherein said step of filtering produces said plurality of spectral peaks having a 60 Hertz spacing.

35. The method of claim 32 wherein said wherein said step of filtering uses a data buffer containing at least a portion of the data signal, and said filtered signal is generated by continuously repeating said portion of the data signal contained within said data buffer at a predetermined rate.

36. The method of claim 35 wherein said predetermined rate is selected to correspond to the spectral spacing of said unused portion of the spectrum of the video signal.

37. The method of claim 35 wherein said predetermined rate is selected such that said data buffer repeats said portion of the data signal contained within said data buffer every 1/60 second, whereby said filtered signal contains substantially uniform spectral peaks with a 60 Hertz spacing.

38. A method for inserting a data signal into a video signal, comprising the steps of:

filtering the data signal to produce a filtered signal having a plurality of spectral peaks with spacing corresponding to spacing in an unused portion of the frequency spectrum of the video signal the data, signal being unrelated to the video signal; and

modulating a single unmodulated carrier frequency having a predetermined phase with said filtered signal to produce a modulated filtered signal, said carrier frequency being selected to modulate said filtered signal into said unused portion of the frequency spectrum of the video signal to produce a modified video signal containing said modulated filtered signal with said filtered signal inserted into said unused portion of the frequency spectrum of the video signal.

39. The method of claim 38 wherein said step of filtering uses a comb filter to produce said plurality of spectral peaks by delaying the data signal by a predetermined period of time corresponding to said plurality of spectral peaks to produce a delayed data signal, and adding said delayed data signal to the data signal.

40. The method of claim 38 wherein said wherein said step of filtering uses a data buffer containing at least a portion of the data signal, and said filtered signal is generated by continuously repeating said portion of the data signal contained within said data buffer at a predetermined rate.

41. The method of claim 40 wherein said predetermined rate is selected to correspond to the spectral spacing of said unused portion of the spectrum of the video signal.

42. The method of claim 40 wherein said predetermined rate is selected such that said data buffer repeats said portion of the data signal contained within said data buffer every 1/60 second, whereby said plurality of spectral peaks have a 60 Hertz spacing.

43. A method for recovering a data signal inserted into an unused portion of the frequency spectrum of an unrelated video signal, the data signal being filtered to generate a filtered signal having a plurality of spectral peaks with a spacing corresponding to spacing in an unused portion of the frequency spectrum of the video signal and modulated with a single unmodulated carrier frequency to generate a modulated data signal, the method comprising the steps of:

separating the modulated data signal from the unused portion of the frequency spectrum of the video signal;

demodulating the modulated data signal separated from the unused portion of the spectrum of the video signal to recover the filtered signal, said step of demodulating using a single carrier frequency having a predetermined phase with said carrier frequency selected to correspond to said unused portion of the frequency spectrum of the video signal; and

inverse filtering the filtered signal following said step of demodulating to recover the data signal from the filtered signal, said recovered data signal being uncorrelated to the video signal.

44. The method of claim 43 wherein the data signal is filtered by a comb filter before being modulated, said step of inverse filtering using an inverse comb filter to recover the data signal by delaying the filtered signal by a predetermined period of time corresponding to said plurality of spectral peaks to produce a delayed filtered signal, and adding said delayed filtered signal to said filtered signal to recover the data signal .

TECHNICAL FIELD

The present invention relates generally to video signal processing and more specifically to a system and method for inserting data into a standard video signal.

BACKGROUND OF THE INVENTION

The use of television is commonplace in the United States and throughout the world. Nearly every home in the United States has at least one television set. Many homes have cable television, which couples a large number of television channels to the home through a single coaxial cable. Other homes and businesses may have satellite receivers that are capable of receiving television signals from a number of satellites in stationary orbit around the earth.

Television signals are defined by the National Television Standards Committee (NTSC). Each television signal comprises a video signal and an audio signal. The NTSC signal, which evolved when only black and white (B/W) television was available has a baseband bandwidth of approximately 4.7 megahertz (MHz). The NTSC signal is modulated to a predetermined carrier frequency. For example, VHF channel 2 has a carrier frequency of 55.25 MHz. A small spacing in the frequency spectrum between adjacent channels prevents interference between channels. The bandwidth of the modulated signal is approximately 6.0 MHz. Other transmission systems, such as cable broadcasting, may use different frequencies for the television channels.

When color television was introduced, it was important that the color signals be added in a manner that did not interfere with the normal operation of B/W television signals. This was accomplished by introducing a chrominance signal modulated at a frequency that causes the chrominance signal for each line of the television signal to have an inverted phase with respect to the prior line. There are an odd number of lines in each television frame, with the result being that the chrominance signal for any given line is inverted in alternating frames of the television signal. The phase inversion causes the chrominance signal to cancel out temporally over the time of one frame, and spatially in the vertical axis over the space of two lines. The cancellation prevents the chrominance signal from erroneously being interpreted as part of the luminance signal. This effect, combined with the known persistence of vision in humans causes the chrominance signal to effectively cancel out in a B/W television so that it causes no noticeable interference. The NTSC signal has a modulated chrominance signal that overlaps the luminance signal in a portion of the frequency spectrum where the overlap causes minimal interference.

The frequency spectrum of the NTSC signal is shown in FIG. 1A. As can be seen in FIG. 1A, the video signal comprises a luminance signal 2 and a chrominance signal 4. The luminance signal 2 provides the signal intensity for both B/W and color television signals. The luminance signal 2 has spectral peaks 6 every 15.75 kilohertz (kHz), which corresponds to the horizontal frequency in the television. The amplitude of the luminance spectral peaks 6 decreases up to 4.2 MHz. The video signal is suppressed above 4.2 MHz to permit the insertion of an audio signal 5 in the spectrum for the particular video channel. The audio signal 5 is modulated with a 4.5 MHz carrier.

The chrominance signal 4 is introduced beginning at about 2 MHz in the spectrum. The chrominance signal 4 has chrominance spectral peaks 8, which are also spaced 15.75 MHz apart in the frequency spectrum. The chrominance signal is modulated at a frequency of 3.579545 MHz (an odd multiple of half the line scan frequency) to cause the chrominance signal peaks to interlace with the luminance peaks, as shown in FIG. 1B, which illustrates a magnified portion of the spectrum of FIG. 1A.

As seen in FIG. 1B, the luminance spectral peaks 6 and the chrominance spectral peaks 8 are spaced apart by 7.875 kHz. Although FIG. 1B, shows the frequency spectrum with no overlap, there is some degree of overlap in these signals due to the non-periodicity of the signals with respect to the line scan frequency.

The NTSC signal has temporal characteristics as well as the frequency characteristics described above. A single video frame comprises 525 video lines that are displayed in two interlaced video fields. Each video field is displayed with a vertical display rate of approximately 60 Hz (59.94 Hz) so that a video frame (with two interlaced video fields) is displayed at a vertical display rate of approximately 30 Hz (29.97 Hz). As seen in FIG. 2, there are two luminance peaks L1 and L2, spaced apart in the frequency spectrum by 30 Hz. The chrominance signal 4 is inserted between alternating pairs of luminance peaks 6. If one selects an arbitrary luminance peak L1 as a reference luminance peak, it is readily seen that the chrominance signal 4 has a spectral peak C 15 Hz above the reference luminance line peak L1. A second luminance peak L2 is spaced 15 Hz above the chrominance peak C (and 30 Hz above the reference luminance peak L1 ). The luminance peak L1 appears again 60 Hz above the reference luminance peak L1. Thus, the pattern repeats every 60 Hz. It should be noted that there is no signal in the spectrum 45 Hz from the reference luminance peak L1. As described in the prior art, that spectral "hole" in the spectrum is currently unused, and could carry additional information. The frequency spectrum of the NTSC signal with additional information signal D added is shown in FIG. 3. Note that the additional information signal is added to an unused portion of the spectrum that, in an ideal case, will cause no interference with the normal video signal processing.

The use of this spectral hole is described in U.S. Pat. No. 4,660,072, which is incorporated herein by reference. The patent describes a technique for adding an additional luminance signal to a standard video signal by inserting the additional luminance signal into the unused portion of the spectrum. The system disclosed in the patent modulates a high frequency luminance signal with a 3.579545 MHz carrier that abruptly switches phase every field of the NTSC signal (60 Hz). The carrier signal is thus modulated by a 30 Hz square wave that has alternating phases of the carrier signal.

The selected carrier frequency and alternating phases cause the additional luminance signal to cancel out temporally and spatially in the same manner as the chrominance signal. The additional luminance signal ideally averages to zero, but in reality the signal averages to zero only if it is unchanging over time. Thus, the additional luminance signal will completely cancel only if it is unchanging. In signal processing terms, only common mode signals are completely canceled. Differential signals do not cancel each other out and will remain in the NTSC signal as a residual signal that may cause interference with the luminance signal. The amount of residual signal depends on the bandwidth of the additional luminance signal and the correlation of the additional luminance signal with the NTSC standard luminance signal. The greater the bandwidth of the additional luminance signal, the greater the amount of additional luminance signal that will feed through and become visible to the television viewer (in the form of interference). In addition, the less correlation between the additional luminance signal and the NTSC standard luminance signal, the greater the amount of additional luminance signal that will feed through and become visible to the television viewer in the form of interference.

The selection of a 30 Hz square wave as a modulation source creates additional problems not solved by the system described in the U.S. Pat. No. 4,660,072. Because an ideal square wave contains an infinite number of odd harmonics, the additional luminance signal is modulated not only at 30 Hz, but at all odd harmonics of these two signals as well. The modulation by many multiple frequencies increases the possibility that the additional luminance signal will overlap in the frequency domain with the video signal. The overlap with the video signal may not present a significant problem in the application described in the patent because the additional luminance signal is highly correlated with the NTSC standard luminance signal, so the interference may not be noticed by the viewer.

However, if the additional information signal added to the standard video signal is unrelated to the video signal, the approach disclosed in U.S. Pat. No. 4,660,072 may be unsuitable because the interference with the video signal may be intolerable. Furthermore, there may be unacceptable interference for the additional information signal itself. To avoid interference, it is necessary to reduce the bandwidth of the additional information signal. There is theoretically a 1.8 MHz bandwidth available in the unused portion of the chrominance spectrum. Because standard modulation creates two sidebands, the actual data bandwidth is limited to 0.9 MHz. The modulation technique proposed in U.S. Pat. No. 4,660,072 causes an unacceptable spectral spreading of the additional information signal that can cause interference with normal television operation.

Therefore, it can be appreciated that there is a significant need for a system and method for introducing an additional information signal into a video signal without the undesirable effects of signal interference or reducing bandwidth to avoid interference.

SUMMARY OF THE INVENTION

The invention is embodied in a system for inserting a data signal into a video signal. The system comprises a first filter which receives the data signal and produces a filtered signal having filter characteristics that permit the insertion of the filtered signal into an unused portion of the spectrum of the video signal. Modulator elements modulate a carrier frequency with the filtered signal to produce a modulated filtered signal. The carrier frequency is selected to permit the insertion of the filtered signal into the unused portion of the spectrum of the video signal to produce a modified video signal containing the modulated filtered signal with the filtered signal inserted into the unused portion of the spectrum of the video signal. A signal separator in a receiver portion separates the filtered signal from the modified video signal and a second filter receives the filtered signal from the signal separator and recovers the data signal from the filtered signal.

In one embodiment, the first filter is a comb filter with at least two taps. The comb filter comprises a delay circuit which delays the data signal by a predetermined period of time and an adder to add the data signal with the delayed data signal to produce the filtered signal. The delay circuit may be a first-in, first-out buffer, or an analog delay line. The predetermined period of time used by the delay circuit is 1/60th of a second such that the filtered signal contains substantially uniform spectral peaks with the 60 Hz spacing.

In an alternative embodiment, the first filter is a data buffer containing at least a portion of the data signal with the filter signal being generated by continuously replaying the data signal contained within the data buffer at a predetermined rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts the spectrum of a standard NTSC signal according to the prior art.

FIG. 1B is an enlarged view of a portion of the NTSC signal in FIG. 1A.

FIG. 2 depicts individual spectral lines in the NTSC signal of FIG. 1A.

FIG. 3 depicts the introduction of an additional information signal into the NTSC signal of FIG. 2 according to the prior art.

FIG. 4 is a functional block diagram of the system of the present invention.

FIG. 5 illustrates a data buffer filter implementation of the system of FIG. 4.

FIG. 6 illustrates an alternative comb filter implementation of the system of FIG. 4.

FIG. 7A depicts a typical waveform input to the comb filter of FIG. 6.

FIG. 7B depicts the waveform of FIG. 7A after passing through the delay line of the comb filter of FIG. 6.

FIG. 7C depicts the filtered output from the comb filter of FIG. 6.

FIG. 7D depicts the spectrum of the comb filter of FIG. 6.

FIG. 7E depicts the spectrum of an NTSC signal with the insertion of the filtered output signal of FIG. 7C.

FIG. 8 illustrates an example of an inverse comb filter used by the system of FIG. 4 to reconstruct the data signal.

DETAILED DESCRIPTION OF THE INVENTION

The present invention resides in a system and method for introducing an additional information signal into an NTSC signal without a reduction in bandwidth. The additional information signal may be an analog data signal or a digital data signal. Whichever form the additional information signal may take, it will be referred to herein as a data signal.

As previously discussed, the technique disclosed in U.S. Pat. No. 4,660,072 modulates the incoming data signal with the 3.579545 MHz carrier signal that switches the phase of the carrier signal at a 30 Hz rate. The method described therein requires that data be frame periodic so that the inserted data signal does not become visible to the viewer in the form of interference. That is, the inserted data signal must repeat itself each frame, but with opposite phase so that the data cancels out, making the inserted data signal invisible to the viewer. Unfortunately, this means that the effective bandwidth is reduced to one-half the theoretical bandwidth because the data is repe