Method and apparatus for measuring television sound and picture

Claims

What I claim is:

1. In a tuning system for selectively extracting from a composite television signal the modulated picture signal on its carrier or the modulated sound signal on its carrier, the improvement of a circuit for indicating which of the two types of signal is being extracted comprising means for demodulating the extracted modulated signal to derive a signal representative of the envelope thereof, amplifying means tuned to one of the sync frequencies of a television signal for amplifying the signal derived by said demodulating means, and means for detecting the presence of a signal at the output of said amplifying means which exceeds a predetermined threshold level for indicating the presence of an extracted picture signal.

2. The improvement in accordance with claim 1 wherein said one sync frequency is the horizontal sync frequency.

3. The improvement in accordance with claim 2 further including means for providing a visual indication of the type of signal being extracted responsive to the operation of said detecting means.

4. The improvement in accordance with claim 1 further including means for providing a visual indication of the type of signal being extracted responsive to the operation of said detecting means.

5. The improvement in accordance with claim 1 wherein said tuning system includes means for processing the extracted modulated picture and sound signals at two different levels, and means for selecting the level in accordance with the operation of said detecting means.

6. In a signal strength meter for measuring the strength of each of two different types of information signal modulated on respective carriers in a composite signal, the signal strength meter including means for tuning to either carrier to extract a selected one of the two information signals and means for measuring the strength of the extracted information signal, the improvement comprising means for demodulating the extracted information signal, means for amplifying the demodulated information signal, said amplifying means being tuned to a frequency normally present in one of said information signals and normally absent from the other, and means responsive to the output of said amplifying means exceeding a predetermined threshold level for indicating the presence of said one information signal and responsive to the output of said amplifying means being below said threshold level for indicating the presence of said other information signal.

7. The improvement in accordance with claim 6 wherein the signal strength meter includes the same read-out means as part of said measuring means for representing the strength of either extracted information signal, and further including means for changing the response of the signal strength meter dependent upon the operation of said indicating means.

8. The improvement in accordance with claim 7 wherein said measuring means includes diode detector means, and said changing means is operative to adjust a bias current flowing through said diode detector means dependent upon the operation of said indicating means.

9. The improvement in accordance with claim 6 wherein said measuring means includes diode detector means, and further including means for adjusting a bias current flowing through said diode detector means dependent upon the operation of said indicating means.

10. The improvement in accordance with claim 9 wherein said composite signal is a television signal, and said two information signals are picture and sound signals.

11. The improvement in accordance with claim 10 wherein said frequency is a television sync frequency.

12. The improvement in accordance with claim 11 wherein said television sync frequency is the horizontal sync frequency.

13. The improvement in accordance with claim 6 wherein said composite signal is a television signal, and said two information signals are picture and sound signals.

14. The improvement in accordance with claim 13 wherein said frequency is a television sync frequency.

15. The improvement in accordance with claim 6 wherein the signal strength meter includes a read-out means as part of said measuring means for representing the strength of the extracted information signal, and further including means for providing a visual indication when the level of the extracted information signal is outside the accurate measurement range of said measuring means.

16. The improvement in accordance with claim 15 further including means for inhibiting the operation of said read-out means when the level of the extracted information signal is a predetermined distance away from the accurate measurement range of said measuring means, whereby as the level of the extracted information signal gets further away from the accurate measurement range a read-out is obtained together with a visual indication that it is inaccurate followed by the inhibition of the read-out.

17. The improvement in accordance with claim 6 further including means for introducing an offset in the output of said measuring means.

18. A signal strength meter for selectively measuring the picture signal level and the sound signal level in a television signal and displaying the results thereof on the same indicating device comprising means for extracting from the television signal a selected one of the modulated picture or sound signals, diode peak detecting means for operating on the extracted modulated signal to derive a signal proportional to the strength thereof for application to said indicating device, and means for controlling the bias current flowing through said diode peak detecting means at either a first level or a second level in accordance with the selected one of said modulated picture or sound signal.

19. A signal strength meter in accordance with claim 18 further including automatic gain control means for operating on the extracted modulated signal prior to the opertion on it by said diode peak detecting means, and wherein said indicating device indicates signal strength as a logarithmic function.

20. A signal strength meter in accordance with claim 19 wherein said indicating device is a combined analog panel meter and digital read-out.

21. A signal strength meter in accordance with claim 19 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known low strength, and third means for adjusting at least one of the bias current first and second levels to set the indication of said indicating device responsive to a measurement being taken of a picture signal of known strength.

22. A signal strength meter in accordance with claim 21 wherein the bias current flowing through said diode peak detecting means is set to be at the higher of the two levels when the strength of a picture signal is being measured.

23. A signal strength meter in accordance with claim 18 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known low strength, and third means for adjusting at least one of the bias current first and second levels to set the indication of said indicating device responsive to a measurement being taken of a picture signal of known strength.

24. A signal strength meter in accordance with claim 23 wherein the bias current flowing through said diode peak detecting means is set to be at the higher of the two levels when the strength of a picture signal is being measured.

25. A signal strength meter in accordance with claim 18 wherein said controlling means is manually operated.

26. A signal strength meter in accordance with claim 18 wherein said controlling means is automatic and includes means for demodulating the selected one of said picture or sound signal, amplifying means tuned to one of the sync frequencies of a television signal for amplifying the demodulated signal, and means for setting said bias current at a first level responsive to the signal at the output of said amplifying means exceeding a predetermined threshold value and for setting said bias current at a second level responsive to the signal at the output of said amplifying means being below said threshold value.

27. A signal strength meter in accordance with claim 26 further including automatic gain control means for operating on the extracted modulated signal prior to the operation on it by said diode peak detecting means, and wherein said indicating device indicates signal strength as a logarithmic function.

28. A signal strength meter in accordance with claim 26 wherein said indicating device is a combined analog panel meter and digital read-out.

29. A signal strength meter in accordance with claim 26 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known low strength, and third means for adjusting at least one of the bias current first and second levels to set the indication of said indicating device responsive to a measurement being taken of a picture signal of known strength.

30. A signal strength meter in accordance with claim 26 wherein the bias current flowing through said diode peak detecting means is set to be at the higher of the two levels when the strength of a picture signal is being measured.

31. A signal strength meter in accordance with claim 26 further including means for providing a visual indication when the level of the extracted signal is outside the accurate measurement range of the signal strength meter.

32. A signal strength meter in accordance with claim 31 further including means for inhibiting the operation of said indicating device when the level of the extracted signal is a predetermined distance away from the accurate measurement range of the signal strength meter, whereby as the level of the extracted signal gets further away from the accurate measurement range an indication of signal strength is obtained together with a visual indication that it is inaccurate followed by the inhibition of a signal strength indication.

33. A signal strength meter in accordance with claim 26 further including means for introducing an offset in the indication of said indicating device.

34. A signal strength meter in accordance with claim 18 further including means for providing a visual indication when the level of the extracted signal is outside the accurate measurement range of the signal strength meter.

35. A signal strength meter in accordance with claim 34 further including means for inhibiting the operation of said indicating device when the level of the extracted signal is a predetermined distance away from the accurate measurement range of the signal strength meter, whereby as the level of the extracted signal gets further away from the accurate measurement range an indication of signal strength is obtained together with a visual indication that it is inaccurate followed by the inhibition of a signal strength indication.

36. A signal strength meter in accordance with claim 18 further including means for inhibiting the operation of said indicating device when the level of the extracted signal is a predetermined distance away from the accurate measurement range of the signal strength meter.

37. A signal strength meter for selectively measuring the picture signal level and the sound signal level in a television signal and displaying the results thereof on the same indicating device comprising means for extracting from the television signal a selected one of the modulated picture or sound signals, detecting means for operating on the extracted modulated signal to derive a signal proportional to the strength thereof, means for driving said indicating device by said derived signal, and means for controlling the response characteristic of said detecting means in accordance with the selected one of said modulated picture or sound signal.

38. A signal strength meter in accordance with claim 37 further including automatic gain control means for operating on the extracted modulated signal prior to the operation on it by said detecting means, and wherein said indicating device indicates signal strength as a logarithmic function.

39. A signal strength meter in accordance with claim 37 wherein said indicating device is a combined analog panel meter and digital read-out.

40. A signal strength meter in accordance with claim 37 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known low strength, and third means for setting the indication of said indicating device responsive to a measurement being taken of a picture signal of known strength.

41. A signal strength meter in accordance with claim 37 wherein said controlling means is automatic and includes means for demodulating the selected one of said picture or sound signal, amplifying means tuned to one of the sync frequencies of a television signal for amplifying the demodulated signal, and means for controlling a first response characteristic of said detecting means responsive to the signal at the output of said amplifying means exceeding a predetermined threshold value and for controlling a second response characteristic of said detecting means responsive to the signal at the output of said amplifying means being below said threshold value.

42. A signal strength meter in accordance with claim 41 further including automatic gain control means for operating on the extracted modulated signal prior to the operation on it by said detecting means, and wherein said indicating device indicates signal strength as a logarithmic function.

43. A signal strength meter in accordance with claim 42 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known low strength, and third means for setting the indication of said indicating device responsive to a measurement being taken of a picture signal of known strength.

44. A signal strength meter in accordance with claim 43 wherein said one sync frequency is the horizontal sync frequency.

45. A signal strength meter in accordance with claim 43 further including means for providing a visual indication when the level of the extracted signal is outside the accurate measurement range of the signal strength meter.

46. A signal strength meter in accordance with claim 45 further including means for inhibiting the operation of said indicating device when the level of the selected signal is a predetermined distance away from the accurate measurement range of the signal strength meter, whereby as the level of the selected signal gets further away from the accurate measurement range an indication of signal strength is obtained together with a visual indication that it is inaccurate followed by the inhibition of a signal strength indication.

47. A signal strength meter in accordance with claim 43 further including means for introducing an offset in the indication of said indicating device.

48. A signal strength meter in accordance with claim 37 further including means for providing a visual indication when the level of the extracted signal is outside the accurate measurement range of the signal strength meter.

49. A signal strength meter in accordance with claim 48 further including means for inhibiting the operation of said indicating device when the level of the selected signal is a predetermined distance away from the accurate measurement range of the signal strength meter, whereby as the level of the selected signal gets further away from the accurate measurement range an indication of signal strength is obtained together with a visual indication that it is inaccurate followed by the inhibition of a signal strength indication.

50. A signal strength meter in accordance with claim 37 further including means for introducing an offset in the indication of said indicating device.

51. A signal strength meter for selectively measuring the strength of each of two different types of information signal modulated on respective carriers in a composite signal and displaying the results thereof on the same indicating device comprising means for extracting from the composite signal a selected one of the two information signals, detecting means for operating on the extracted modulated signal to derive a signal proportional to the strength thereof, means for driving said indicating device by said derived signal, and means for controlling the response characteristic of the signal strength meter in accordance with the selected one of said two information signals.

52. A signal strength meter in accordance with claim 51 further including automatic gain control means for operating on the extracted modulated signal prior to the operation on it by said detecting means, and wherein said indicating device indicates signal strength as a logarithmic function.

53. A signal strength meter in accordance with claim 51 wherein said indicating device is a combined analog panel meter and digital read-out.

54. A signal strength meter in accordance with claim 51 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a first of said information signals of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a first of said information signals of known low strength, and third means for setting the indication of said indicating device responsive to a measurement being taken of the second of said information signals to known strength.

55. A signal strength meter in accordance with claim 51 wherein said controlling means is automatic and includes means for demodulating the selected one of said first and second information signals, amplifying means tuned to a frequency normally present in said first information signal and normally absent from said second information signal for amplifying the demodulated signal, and means for controlling a first response characteristic of the signal strength meter responsive to the signal at the output of said amplifying means exceeding a predetermined threshold value and for controlling a second response characteristic of the signal strength meter responsive to the signal at the output of said amplifying means being below said threshold value.

56. A signal strength meter in accordance with claim 55 further including automatic gain control means for operating on the extracted modulated signal prior to the operation on it by said detecting means, and wherein said indicating device indicates signal strength as a logarithmic function.

57. A signal strength meter in accordance with claim 56 further including first means for setting the indication of said indicating device responsive to a measurement being taken of a first of said information signals of known high strength and second means for setting the indication of said indicating device responsive to a measurement being taken of a first of said information signals of known low strength, and third means for setting the indication of said indicating device responsive to a measurement being taken of the second of said information signals of known strength.

58. A signal strength meter in accordance with claim 57 further including means for providing a visual indication when the level of the extracted signal is outside the accurate measurement range of the signal strength meter.

59. A signal strength meter in accordance with claim 58 further including means for inhibiting the operation of said indicating device when the level of the selected signal is a predetermined distance away from the accurate measurement range of the signal strength meter, whereby as the level of the selected signal gets further away from the accurate measurement range an indication of signal strength is obtained together with a visual indication that it is inaccurate followed by the inhibition of a signal strength indication.

60. A signal strength meter in accordance with claim 57 further including means for introducing an offset in the indication of said indicating device.

61. A signal strength meter in accordance with claim 51 further including means for providing a visual indication when the level of the extracted signal is outside the accurate measurement range of the signal strength meter.

62. A signal strength meter in accordance with claim 61 further including means for inhibiting the operation of said indicating device when the level of the selected signal is a predetermined distance away from the accurate measurement range of the signal strength meter, whereby as the level of the selected signal gets further away from the accurate measurement range an indication of signal strength is obtained together with a visual indication that it is inaccurate followed by the inhibition of a signal strength indication.

63. A signal strength meter in accordance with claim 51 further including means for introducing an offset in the indication of said indicating device.

64. In a tuning system for selectively extracting from a composite television signal the modulated picture signal on its carrier or the modulated sound signal on its carrier, a method for indicating which of the two types of signal is being extracted comprising the steps of demodulating the extracted modulated signal to derive a signal representative of the envelope thereof, detecting one of the sync frequencies of a television signal in the signal derived in said demodulating step, and responsive to the detection of said one sync frequency indicating the presence of an extracted modulated picture signal.

65. A method in accordance with claim 64 wherein said one sync frequency is the horizontal sync frequency.

66. A method in accordance with claim 64 wherein said tuning system includes means for processing the extracted modulated picture and sound signals at two different levels, and further including the step of selecting the level in accordance with the detection of said one sync frequency.

67. In a signal strength meter for measuring the strength of each of two different types of information signal modulated on respective carriers in a composite signal, the signal strength meter including means for tuning to either carrier to extract a selected one of the two information signals and means for measuring the strength of the extracted information signal, a method for indicating which of the two types of information signal is being extracted comprising the steps of demodulating the extracted information signal, examining the demodulated information signal for the presence of a frequency normally present in one of said information signals and normally absent from the other, and responsive to the detection of said frequency indicating the presence of said one information signal and responsive to the absence of the detection of said frequency indicating the presence of said other information signal.

68. A method in accordance with claim 67 wherein the signal strength meter includes the same read-out means for representing the strength of either extracted information signal, and further including the step of changing the response of the signal strength meter dependent upon the detection of said frequency.

69. A method in accordance with claim 67 wherein said composite signal is a television signal, and said two information signals are picture and sound signals.

70. A method in accordance with claim 69 wherein said frequency is a television sync frequency.

71. A method in accordance with claim 70 wherein said television sync frequency is the horizontal sync frequency.

72. A method for using a signal strength meter for selectively measuring the picture signal level and the sound signal level in a television signal and displaying the results thereof on the same indicating device comprising the steps of extracting from the television signal a selected one of the modulated picture or sound signals, operating on the extracted modulated signal to derive a signal proportional to the strength thereof for driving said indicating device, and controlling the response characteristic of the signal strength meter in accordance with the selected one of said modulated picture or sound signal.

73. A method in accordance with claim 72 wherein said indicating device is a combined analog panel meter and digital read-out.

74. A method in accordance with claim 72 further including the steps of setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known high strength and setting the indication of said indicating device responsive to a measurement being taken of a sound signal of known low strength, and setting the indication of said indicating device responsive to a measurement being taken of a picture signal of known strength.

75. A method in accordance with claim 74 wherein said controlling step is automatic and includes the substeps of demodulating the selected one of said picture or sound signal, detecting one of the sync frequencies of a television signal in the demodulated signal, and controlling a first response characteristic of the signal strength meter responsive to the level of said one sync frequency in the demodulated signal exceeding a predetermined threshold value and controlling a second response characteristic of the signal strength meter responsive to the level of said one sync frequency in the demodulated signal being below said threshold value.

76. A signal strength meter in accordance with claim 75 wherein said one sync frequency is the horizontal sync frequency.

77. A method in accordance with claim 72 wherein said controlling step is automatic and includes the sub-steps of demodulating the selected one of said picture or sound signal, detecting one of the sync frequencies of a television signal in the demodulated signal, and controlling a first response characteristic of the signal strength meter responsive to the level of said one sync frequency in the demodulated signal exceeding a predetermined threshold value and controlling a second response characteristic of the signal strength meter responsive to the level of said one sync frequency in the demodulated signal being below said threshold value.

78. A signal strength meter in accordance with claim 77 wherein said one sync frequency is the horizontal sync frequency.

79. A method for using a signal strength meter for selectively measuring the strength of each of two different types of information signal modulated on respective carriers in a composite signal and displaying the results thereof on the same indicating device comprising the steps of extracting from the composite signal a selected one of the two information signals, operating on the extracted modulated signal to derive a signal proportional to the strength thereof for driving said indicating device, and controlling the response characteristic of the signal strength meter in accordance with the selected one of said two information signals.

80. A method in accordance with claim 79 wherein said indicating device is a combined analog panel meter and digital read-out.

81. A method in accordance with claim 79 further including the steps of setting the indication of said indicating device responsive to a measurement being taken of a first of said information signals of known high strength and setting the indication of said indicating device responsive to a measurement being taken of a first of said information signals of known low strength, and setting the indication of said indicating device responsive to a measurement being taken of the second of said information signals of known strength.

82. A method in accordance with claim 81 wherein said controlling step is automatic and includes the sub-steps of demodulating the selected one of said first and second information signals, detecting in the demodulated signal a frequency normally present in said first information signal and normally absent from said second information signal, and controlling a first response characteristic of the signal strength meter responsive to the level of said frequency in the demodulated signal exceeding a predetermined threshold value and controlling a second response characteristic of the signal strength meter responsive to the level of said frequency in the demodulated signal being below said threshold value.

83. A method in accordance with claim 79 wherein said controlling step is automatic and includes the sub-steps of demodulating the selected one of said first and second information signals, detecting in the demodulated signal a frequency normally present in said first information signal and normally absent from said second information signal, and controlling a first response characteristic of the signal strength meter responsive to the level of said frequency in the demodulated signal exceeding a predetermined threshold value and controlling a second response characteristic of the signal strength meter responsive to the level of said frequency in the demodulated signal being below said threshold value.

This invention relates to an apparatus and method for measuring television sound and picture signals, and more particularly to the use of a single detector and display which provide accurate readings of the strengths of both types of signal.

A conventional television signal consists of sound and picture components. The sound component is a frequency-modulated signal of constant amplitude. The picture component is a combination pulsed and amplitude-modulated signal. In many applications, for example, when installing a cable TV system, it is necessary to determine the individual strengths or amplitudes of the two signal components. For this purpose, a signal level meter (also referred to as a tunable radio frequency voltmeter, a signal strength meter, and a field strength meter) is usually employed. The instrument includes a tunable input section for deriving an intermediate frequency signal (typically, at 42.5 MHz) on which there is modulated the sound signal or the picture signal of a selected channel. The instrument may include a single peak detector for measuring the amplitude of the selected sound or picture signal, and a single display for registering the result.

The basic problem with conventional signal level meters is that the peak detector and display respond differently to sound and picture signals of equal strengths. Measurements of at least one of the two signals are not accurate. Other problems with conventional signal level meters relate to the manner in which they are used. For example, although a calibrated dial may be set so that the sound signal or the picture signal of a particular channel will be processed, there is no way for the technician to be certain that it is indeed the selected type of signal whose strength is being measured. Similarly, it has been found that many technicians consider readings as accurate when in fact an under-range or over-range condition exists, that is, the signal strength is too weak or too strong for an accurate measurement to be made. Most conventional signal level meters employ an analog meter display, and such a display by its very nature does not always provide the desired degree of resolution.

It is a general object of my invention to provide a signal level meter which overcomes the aforesaid problems which are inherent in conventional instruments.

A conventional instrument may employ a peak detector for measuring the peak of the selected sound or picture signal. By suitably calibrating the meter, the actual results may be displayed in the form of RMS values, if that is desired. But the initial stage in the measurement circuit is a peak detector whose function is to generate a voltage which is proportional to the peak of the sound or picture signal being processed. The sound signal is a continuous-wave signal of constant amplitude. The picture signal, on the other hand, has a varying amplitude whose peaks correspond to the sync pulses. If a standard peak detector is calibrated to provide an accurate measurement of a continuous-wave signal, it will not generally provide an accurate measurement of the peaks of a picture signal, for reasons well understood in the art and to be described below. It is for this reason that a conventional signal level meter will usually provide a lower reading for a picture signal than it will for a sound signal of the same strength (peak amplitude), the discrepancy increasing toward the lower end of the meter scale.

I have found that this problem can be overcome simply by changing the bias current through the peak detector in accordance with whether a sound signal or a picture signal is being measured. By increasing the bias current, the response of the peak detector to an A.C. signal increases. Thus by providing for a larger bias current when a picture signal is being processed, it is possible for the same peak detector and display to provide accurate readings for both sound and picture signals over broad ranges of signal strengths.

Preferably, the bias current is changed automatically to insure that the operator will not inadvertently fail to place a sound/picture selector switch in the proper position. In the illustrative embodiment of my invention, the instrument "looks for" horizontal sync pulses in the IF signal. If these pulses are detected, it is an indication that a picture signal is being processed and the system automatically applies the larger of the two bias currents to the peak detector circuit. In the absence of the horizontal sync pulses, the system assumes that a sound signal is being processed and the smaller of the two bias currents is applied to the detector circuit. (In a less expensive embodiment of the invention, a selector switch may be provided instead of the automatic bias control.)

While a digital read-out would appear to be highly desirable for a signal level meter, just as it is for most other instruments, in actual practice there is an important reason why such read-outs have not been incorporated in signal level meters. With the use of a single detector there has just been no way in the prior art to avoid erroneous readings by as much as 2 dB. The same is true with prior art analog meter displays, but the average technician is not as aware of an erroneous analog meter reading as he is of an erroneous digital meter reading. The average technician, when applying a signal of known strength to a signal level meter and when viewing an analog meter error of up to 2 dB, usually assumes that such an error is simply "inherent" in any analog meter instrument. But when the same error appears in a digital display, he thinks that there is something wrong with the instrument (which indeed there is when the same peak detector and display are used for both sound and picture readings). It is partially for this reason that digital read-outs have not been practical for use in signal level meters. It is the elimination of the error in accordance with my invention that overcomes the psychological problem associated with the use of a digital read-out.

In addition to providing a digital display, I also provide the conventional analog display, despite the fact that it might appear that the analog display is not necessary.

When selecting a particular channel for test, and more particularly the sound or picture signal of that channel, a dial is rotated to a particular position. But this initial setting of the dial is only a coarse adjustment. The dial must be further adjusted slightly in order to provide a maximum reading. If the input section of the instrument is not perfectly tuned so that a maximum reading results, the reading is necessarily affected by improper tuning of the instrument and is erroneous. As the dial is fine-tuned, the digital read-out changes; in order to determine that the instrument has been tuned properly, the dial is turned until a maximum reading results. But it is difficult and time-consuming to obtain a peak reading in this manner, especially when the two least significant digits of the digital display may be constantly changing. It is for this reason that I also provide the analog meter display. The calibration dial is turned in a direction which causes the analog meter pointer to increase in deflection. Turning of the dial continues until the pointer deflection just starts to decrease. At this point, it is known that the input section is tuned almost perfectly. The digital display can then be observed. In a three-digit display, the last digit will usually be in error by only one or two units. The dial may be turned slightly in the proper direction to achieve a maximum reading. This is a relatively simple step inasmuch as it usually requires an examination of only a single digit position. Thus the analog meter display in conjunction with the digital meter display allow very rapid fine tuning, a feature which is of considerable practical importance when it is realized that the technician may have to examine the sound and picture signal strengths of numerous channels in succession. Furthermore, the two read-outs are superimposed on each other so that the technician need not transfer his gaze from one read-out to the other, thus further speeding up the fine-tuning process.

To allow the technician to verify that he is measuring a sound signal or a picture signal, I also provide a light indicator which is illuminated whenever a picture signal is being processed. This indicator is also controlled by the detection of horizontal sync pulses.

As a further aid to the operator, the instrument includes an audio amplifier and speaker, the input to which is the demodulated sound or picture signal being measured. When a picture signal is being processed, a buzzing sound is produced from the vertical sync pulses which occur at a 60-Hz rate. When a sound signal is being detected, the sound signal itself is heard. Finally, when the input section is tuned to neither a sound signal nor a picture signal, all that is heard is background noise.

The system is also provided with various light indicators for indicating under-range and over-range conditions, and the sign of the digital reading .

Further objects, features and advantages of the invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 depicts a conventional peak-type detector;

FIG. 2 depicts a TV sound signal and the response of a conventional peak detector thereto;

FIG. 3 depicts a TV picture signal and the response of a conventional peak detector thereto;

FIG. 4 is a plot of the picture signal pulse-peak reading errors of typical commercially available signal level meters;

FIG. 5 is a block diagram of the illustrative embodiment of the invention;

FIGS. 6 and 7, with FIG. 6 being placed to the left of FIG. 7, are a detailed schematic of the illustrative embodiment of the invention, but without showing the circuitry for forming the digital display;

FIG. 8 depicts the response of the instrument of FIGS. 6 and 7 to TV sound and picture signals without the detector-bias-change feature of the invention;

FIG. 9 depicts the respective responses of the instrument to the same two signals with the detector-bias-change feature;

FIG. 10 is a plot based on the TV picture signal curves of FIGS. 8 and 9 and illustrates the improvement which results with the detector-bias-change feature;

FIG. 11 shows the different scales which are obtained for the instrument of FIGS. 6 and 7, with and without the detector-bias-change feature;

FIG. 12 depicts a system similar to that of FIGS. 6 and 7, but one in which the detector bias changes are manually controlled rather than automatic, and in which some additional features of the system of FIGS. 6 and 7 are omitted;

FIG. 13 depicts a preferred form of the display of the overall instrument of the invention;

FIGS. 14 and 15, with FIG. 14 being placed to the left of FIG. 15, depict a circuit, for operating with the circuits of FIGS. 6 and 7, for generating the digital display;

FIGS. 16 and 17, with FIG. 16 being placed to the left of FIG. 17, depict the detailed circuitry in block 65 of FIG. 15; and

FIG. 18 depicts the circuit of an "offset" feature which may be added to the circuit of FIG. 14.

Each TV channel is occupied by a picture, or visual information signal, and a separate sound, or audio, information signal. These two signals are widely separated within the TV channel. The TV channel is 6 MHz wide, and the picture and sound carrier frequencies are 4.5 MHz apart. Since it is desired to measure these signals separately, a typical signal strength meter must have a passband which is much narrower than 4.5 MHz so that both signals cannot be received simultaneously. Typical bandwidths fall between 0.1 and 0.7 MHz; in the illustrative embodiment of the invention, a bandwith of 0.5 MHz is used.

The picture signal is a combination pulsed and amplitude-modulated signal. At 63.5 microsecond intervals, a 4.5 microsecond sync pulse initiates the sweep of a horizontal line on the picture tube. During the remaining 59 microseconds, amplitude-modulated picture and other information is transmitted on the picture signal. Also, at intervals of about 1/60 of a second, a series of vertical sync pulses is transmitted to initiate the start of a new picture field. During the horizontal and vertical sync pulses, and only during these pulses, the RF (radio frequency) signal rises to its maximum amplitude. At nearly all other times, and especially during the amplitude modulation of picture intensity information, the RF signal voltage remains below 75% of the maximum amplitude. The percentage of time during which the RF signal voltage rises above 75% and reaches 100% amplitude is 8%.

It is the level of this 100% RF signal amplitude which the TV cable installer technician desires to determine when measuring the strength of a TV picture signal. Since the 100% RF signal voltage amplitude is constant, an ideal tunable radio frequency (TRF) voltmeter will indicate the level of picture signal pulse peaks as a constant reading regardless of the level of the picture intensity signal which appears during the interval between horizontal sync pulses. (For a white line, the picture signal is 15% of maximum RF signal voltage; for a black line, the picture signal is about 70% of maximum RF signal voltage).

The TV sound signal is a frequency-modulated (FM) signal. The amplitude of the signal is constant, and its frequency is varied a very small percentage in accordance with the modulating signal. Since the amplitude is constant, its absolute level is readily determined by means of a conventional semiconductor diode peak-type detector. The detector "sees" only a continuous wave (CW) signal, the modulation having no influence on the detector output.

Nearly all of the TV-TRF voltmeters in use today use the same detector for both the picture and sound signal level measurements. The detectors are of the peak type, and the meters are calibrated for CW signals only.

The basic peak detector, shown in FIG. 1, operates as follows. Diode 20 permits current to flow in only one direction, the current charging capacitor 21. The capacitor charges to a potential equal to that of the positive peak of the input. The capacitor slowly discharges through resistor 22 and meter 23, but each peak of the input CW signal renews the peak charge on the capacitor. The meter defle