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Showing papers on "Signal published in 1985"


Journal ArticleDOI
TL;DR: The results show subtle differences between eukaryotic and prokaryotic sequences, but the general impression of signal sequences as being highly variable is reinforced.

2,053 citations


Journal ArticleDOI
TL;DR: In this paper, a method of constructing a single signal subspace for high-resolution estimation of the angles of arrival of multiple wide-band plane waves is presented, which relies on an approximately coherent combination of the spatial signal spaces of the temporally narrow-band decomposition of the received signal vector from an array of sensors.
Abstract: This paper presents a method of constructing a single signal subspace for high-resolution estimation of the angles of arrival of multiple wide-band plane waves. The technique relies on an approximately coherent combination of the spatial signal spaces of the temporally narrow-band decomposition of the received signal vector from an array of sensors. The algorithm is presented, and followed by statistical simulation examples. The performance of the technique is contrasted with other suggested methods and statistical bounds in terms of the determination of the correct number of sources (detection), bias, and variance of estimates of the angles.

1,067 citations


Journal ArticleDOI
TL;DR: This report describes how to extract true intensity measurements in the presence of noise in magnetic resonance imaging.
Abstract: Power spectrum or magnitude images are frequently presented in magnetic resonance imaging. In such images, measurement of signal intensity at low signal levels is compounded with the noise. This report describes how to extract true intensity measurements in the presence of noise.

1,057 citations


Journal ArticleDOI
S. Biyiksiz1
01 Mar 1985
TL;DR: This book by Elliott and Rao is a valuable contribution to the general areas of signal processing and communications and can be used for a graduate level course in perhaps two ways.
Abstract: There has been a great deal of material in the area of discrete-time transforms that has been published in recent years. This book does an excellent job of presenting important aspects of such material in a clear manner. The book has 11 chapters and a very useful appendix. Seven of these chapters are essentially devoted to the Fourier series/transform, discrete Fourier transform, fast Fourier transform (FFT), and applications of the FFT in the area of spectral estimation. Chapters 8 through 10 deal with many other discrete-time transforms and algorithms to compute them. Of these transforms, the KarhunenLoeve, the discrete cosine, and the Walsh-Hadamard transform are perhaps the most well-known. A lucid discussion of number theoretic transforms i5 presented in Chapter 11. This reviewer feels that the authors have done a fine job of compiling the pertinent material and presenting it in a concise and clear manner. There are a number of problems at the end of each chapter, an appreciable number of which are challenging. The authors have included a comprehensive set of references at the end of the book. In brief, this book is a valuable contribution to the general areas of signal processing and communications. It can be used for a graduate level course in perhaps two ways. One would be to cover the first seven chapters in great detail. The other would be to cover the whole book by focussing on different topics in a selective manner. This book by Elliott and Rao is extremely useful to researchers/engineers who are working in the areas of signal processing and communications. It i s also an excellent reference book, and hence a valuable addition to one’s library

843 citations


Book
01 Sep 1985
TL;DR: Fast algorithms for digital signal processing, Fast algorithms fordigital signal processing , and so on.
Abstract: Fast algorithms for digital signal processing , Fast algorithms for digital signal processing , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

797 citations


Journal ArticleDOI
TL;DR: The appearance of flowing fluid has been evaluated in several clinical situations using a flow phantom, computer simulation, and clinical magnetic resonance (MR) images.
Abstract: The appearance of flowing fluid has been evaluated in several clinical situations using a flow phantom, computer simulation, and clinical magnetic resonance (MR) images. Unsaturated protons just entering the imaging volume can emit a strong signal relative to the partially saturated adjacent tissue ("flow-related enhancement"). Slow laminar flow in veins can be distinguished on the basis of a stronger second echo due to rephasing effects ("even echo rephasing"). Synchronization of the cardiac cycle and the MR pulsing sequence produces increased signal in sections acquired during diastole ("diastolic pseudogating"). Intraluminal signal is shown to decrease as velocity is increased ("high velocity signal loss"). Onset of turbulence causes further loss of signal. Direction of flow oblique to the imaging plane can be predicted on the basis of decreased upstream and increased downstream signal.

340 citations


Patent
03 Sep 1985
TL;DR: In this article, a digital receiver for GPS C/A-code signals is described, which provides reception and tracking a plurality of satellites simultaneously, using four separate receiver channels, and includes an analog front-end for selecting and frequency translating the received GPS signal.
Abstract: A digital receiver for GPS C/A-code signals is described. The GPS receiver of the present invention provides reception and tracking a plurality of satellites simultaneously, using four separate receiver channels. The GPS receiver of the present invention includes an analog front-end for selecting and frequency translating the received GPS signal. The GPS receiver further includes a highspeed digital signal processor for recovering the despread data of the GPS signal. The baseband signal is further processed by a general purpose digital signal processor for signal search, tracking, and data recovery operations, and a microprocessor provides overall receiver control, and interface with the operator of the GPS receiver.

338 citations


Patent
03 May 1985
TL;DR: The programmable logic array (PLA) as discussed by the authors is a programmable AND array with a plurality of memory cells arranged in addressable rows (40-45) and columns (32-38) and can be individually programmed to contain logic data.
Abstract: The programmable logic array device basically comprises a programmable AND array (Figures 5, 11) having a plurality of memory cells (30, 31) arranged in addressable rows (40-45) and columns (32-38) and which can be individually programmed to contain logic data; an input circuit (Fig. 9) for receiving an input signal and for developing a buffered signal corresponding thereto; a first row driver (Fig. 10) responsive to the buffered signal and operative to interrogate a particular row of the memory cells and to cause the AND array to output signals corresponding to the data contained therein; first sensing circuitry (Fig. 12) for sensing the signals output by the AND array and for developing corresponding data signals which are the logical OR of signals output by the AND array; first output terminal circuitry; and first switching circuitry (Fig. 14) responsive to a control signal and operative to couple the data signal either into the storage circuitry or to the output terminal circuitry (Fig. 16). The device has the advantages general of greater logic density and lower system power than standard family logic components.

323 citations


Journal ArticleDOI
01 Nov 1985
TL;DR: You must turn in your code as well as output files to generate a report that contains the code and ouput in a single readable format.
Abstract: You must turn in your code as well as output files. Please generate a report that contains the code and ouput in a single readable format. Getting Started ˆ You may want to download Irfanview image viewing software. It handles pretty much any image type, lets you convert, and provides batch processing. ˆ Download the sample images from the class website. The following question operates on the city.jpg image. (a) Perform image smoothing using a 7×7 averaging filter and a Gaussian filter with σ = 0.5 and 3. Compare the outputs. (b) Perform edge enhancement using the Sobel operator (Matlab's default parameters). Repeat using the Laplacian and Laplacian of Guassian operators. Compare the outputs 2. Frequency Domain Filtering The following question operates on the city.jpg image. (a) Find the Fourier transform of the image. Be sure to center the frequencies. (b) Perform image smoothing in the frequency domain using the filters defined in the previous problem. Compare the output images from the two methods (spatial and frequency) and the time for operation. (c) Perform edge enhancement using the filters defined in the previous problem. (d) Define a lowpass filter in the frequency domain with radius of 1/4 the height. Show the result. Repeat with a similar sized Guassian and compare the results. Give the σ parameter you used and show the output transform image. (e) Repeat with a rectangular filter with the same dimension as the ideal lowpass. Compare the results between the ideal filter and the rectangular approximation. 3. Canny Edge Detection (a) Give the convolution kernels for determining the gradient. You may examine the function gradient.m to help with the explanation. (It may be easiest to apply the gradient to an impulse and inspect the results. (b) Implement the simplified version of the Canny edge detector (single scale). The syntax of the function should be where E contains the detected edges, M the smoothed gradient magnitude, A contains the gradient angle, I is the input image, sig is the σ parameter for the smoothing filter, and tau= [τ h , τ l ] is the two element vector containing the hysteresis thresholds. See Algorithm 6.4 for non-maximal suppression and Algorithm 6.5 for hysteresis thresh-olding. (It may be more efficient to implement the hysteresis as edge tracking)

285 citations


PatentDOI
TL;DR: An implantable electromagnetic middle-ear bone conduction hearing aid device is described in this paper, which provides variability to meet a variety of pathological entities of the hearing impaired user by using a sound processing component for converting sound into an analog electromagnetic signal and an output transmitter adapted to be placed supercutaneously on the skull of the user for transmitting the electromagnetic signal transcutaneously.
Abstract: An implantable electromagnetic middle-ear bone conduction hearing aid device is disclosed which provides variability to meet a variety of pathological entities of the hearing impaired user. The device utilizes a sound processing component for converting sound into an analog electromagnetic signal and has an output transmitter adapted to be placed supercutaneously on the skull of the user for transmitting the electromagnetic signal transcutaneously. A signal receiving and transmitting component is adapted to be implanted subcutaneously in the bone of the user outside of the middle ear for receiving the transcutaneous electromagnetic signal from the sound processing component and for transmitting the electromagnetic signal subcutaneously into the middle-ear of the user. A vibration generating component is adapted to be implanted in any of the small bones of the ossicular chain in the middle-ear for receiving the subcutaneous electromagnetic signal from the signal receiving and transmitting component and for vibrating the ossicular chain in response to such electromagnetic signal to stimulate normal functioning of the inner ear to create the perception of sound in the hearing impaired user.

234 citations



PatentDOI
Ali A. Abdelghani1
TL;DR: In this article, an ultrasonic generating system for coupling vibrating energy into a load, such as living tissue, has been described which employs a piezoelectric crystal having excitation electrodes on opposite faces thereof.
Abstract: An ultrasonic generating system for coupling vibrating energy into a load, such as living tissue, has been described which employs a piezoelectric crystal having excitation electrodes on opposite faces thereof A feedback electrode is disposed on one face of the crystal adjacent to and insulated from one of the excitation electrodes to provide a feedback signal between the feedback electrode and the other excitation electrode A power supply is provided for applying excitation energy to the excitation electrodes The feedback signal is utilized to slave the frequency of the power supply voltage to the resonant frequency of the crystal and to disable the power supply when the impedance of the load applied to the crystal rises above a predetermined level

Patent
Sergiu Silvian1
05 Apr 1985
TL;DR: In this article, the use of PSK (phase shift keying) as the phase modulation scheme for transmission and reception is also taught, where the analog signal is used as a clock in the phase encoding of the digital signal and thus provides a signal simultaneously carrying analog and digital information.
Abstract: In an implantable device, transmission and reception of digital information is accomplished as well as the transmission and reception of analog information. The analog signal is FM modulated for transmission and the digital signal is phase modulated for transmission. The FM modulated signal is used as a clock in the phase encoding of the digital signal and thus provides a signal simultaneously carrying analog and digital information. Means for reception of the analog and digital information is taught. The use of PSK (phase shift keying) as the phase modulation scheme for transmission and reception is also taught.

Patent
26 Feb 1985
TL;DR: In this article, a microprocessor controlled electrosurgical generator provides ground-isolated and mutually isolated monopolar r-f cutting signals for respective surgical pencils to permit plural surgeons to use the same generator while maintaining independent control over output power and mode selection.
Abstract: A microprocessor controlled electrosurgical generator provides plural ground-isolated and mutually isolated monopolar r-f cutting signals for respective surgical pencils to permit plural surgeons to use the same generator while maintaining independent control over output power and mode selection. A bipolar output signal is also isolated from ground and from the monopolar signals so that the removal of the dispersive patient return pad is not required when changing between monopolar and bipolar operation. Adjustment of the r-f cutting signal power level is effected by providing an adjustable d.c. bias level for an output amplifier. The d.c. bias level is derived from an a.c. control signal which is duty-cycle modulated in accordance with selected r-f power level settings. Digital phase control is employed to change the control signal duty cycle by subdividing the control signal cycle into precise count intervals and triggering a gate to pass the control signal through a rectifier to a filter for only the selected number of count intervals in each cycle as determined by a pre-settable counter. Negative feedback of the cutting signal to the input terminal of the output amplifier provides system stability during open circuit operation.


Patent
23 Oct 1985
TL;DR: In this paper, a high-frequency surgical apparatus with automatic control of the intensity of the electric arcs between the active electrode and the tissue to be out or to be coagulated is presented.
Abstract: 1. High-frequency surgical apparatus with automatic control of the intensity of the electric arcs between the active electrode and the tissue to be out or to be coagulated, wherein, for establishing the size of the arc, an electrical signal dependent thereon is derived via a filter and is fed to a rectifier (7), the output signal (E) from which is fed to a control amplifier (5), in which this output signal (E) is compared either with the set-point signal (S) of a set-point transmitter (15) for the cutting operation or with the set-point (K) of a set-point transmitter (8) for the coagulation operation, and wherein the control amplifier (5) produces a signal (C), proportional to the difference of the signals E minus S or E minus K respectively, which signal (C) is supplied to an amplitude modulator (3), which controls the amplitude of the output voltage (U) of the power amplifier (4), characterized in that the filter (6) is so constructed that it passes at least one frequency of the non-harmonic frequencies of the basic frequency (f1 ) of the high-frequency oscillator (2) generated by the electric arc (11) between active electrode (12) and tissue (9) in the output (13, 14) and simultaneously sufficiently damps the basic frequency (f1 ) and also it harmonic frequencies that they do not have any influence upon the control signal (C), the at least one frequency being determined from the electrical voltage (U) or from the electrical current (1) between active electrode and tissue.

Patent
12 Jun 1985
TL;DR: In this article, a system for processing articles selected for purchase and bearing distinct identification codes comprises, in one version, a code reader for generating an output signal indicative of such article identification code; a conveyor for receiving and transport of such articles; and an entrance sentry for defining an inlet to a secured zone extending along a portion of the conveyor and for generating output signals indicative of entry of the article into the secured zone and of a measurable characteristic of an article.
Abstract: A system for processing articles selected for purchase and bearing distinct identification codes comprises, in one version: (a) a code reader for generating an output signal indicative of such article identification code; (b) a conveyor for receipt and transport of such article; (c) an entrance sentry for defining an inlet to a secured zone extending along a portion of the conveyor and for generating an output signal indicative of entry of the article into the secured zone and of a measurable characteristic of the article; and (d) a controller for selective movement of the conveyor in respective article acceptance and article rejection senses. The controller is operable in several respects, namely, for storage, for each of a plurality of such articles, of a signal indicative of a predetermined value of the measurable article characteristic correlated with such article identification code, for response to the code reader output signal for comparison of such stored signal with the output signal of the sentry, and for operation of the conveyor selectively in response to the results of such comparison.

Patent
Jack Harriman Winters1
28 Nov 1985
TL;DR: In this article, a wireless PBX or LAN system is disclosed that allows multiple, simultaneous users per channel for communication between a base station (20) and a plurality of remote terminals (10,12,14,16) by a combination of adaptive signal processing arrays using optimum combining at the base station for interference suppression and separation of desired signals, adaptive power control at the remote terminals to keep interference at acceptable levels, and adaptive retransmission with time division in the direction from the BS to the remote users for communication there between including sending power control information to remote terminals.
Abstract: A wireless PBX or LAN system is disclosed that allows multiple, simultaneous users per channel for communication between a base station (20) and a plurality of remote terminals (10,12,14...16) by a combination of adaptive signal processing arrays using optimum combining at the base station for interference suppression and separation of desired signals, adaptive power control at the remote terminals to keep interference at acceptable levels, and adaptive retransmission with time division in the direction from the base station to the remote terminals for communication therebetween including sending power control information to the remote terminals. The adaptive signal processing arrays have a plurality of antennas (110) at the base station, permitting a plurality of users in any one channel. If after adaptation a user's signal cannot be received adequately, the user is dynamically reassigned to a different channel. Interference is minimized by keeping the signals transmitted from and received at the base station at the same power level. The received signals are kept at the same power level by extracting information about the power level of the signals received at the base station and by sending power control bits back to the corresponding remote terminal so that the signal level from the remote terminal may be adjusted either up or down until the power level of each of the signals received at the base station is at the same level. The transmitted signls are kept at the same power level by adaptive retransmission with time division.

PatentDOI
TL;DR: In this article, a suspension control system for automotive vehicles automatically adjusts the damping strength of variable shock absorbers or other dampers in accordance with road surface conditions as recognized by frequency analysis of a vehicle height or vibration sensor signal.
Abstract: A suspension control system for automotive vehicles automatically adjusts the damping strength of variable shock absorbers or other dampers in accordance with road surface conditions as recognized by frequency analysis of a vehicle height or vibration sensor signal. The sensor signal reflects vertical displacement of the vehicle body from the road surface and includes high-frequency components due solely to displacement of the wheels or unsprung mass relative to the road surface and low-frequency components due to displacement of the vehicle body or sprung mass. The sensor signal is filtered into these separate frequency bands, the amplitude of each of which is compared to a corresponding reference level. The results of comparison give an indication of the degree and scale of irregularities in the road surface; specifically, a high-amplitude low-frequency component indicates larger-scale bumps and dips capable of bouncing the vehicle whereas a strong high-frequency component reflects a rough-textured road surface, such as gravel. The comparison information is sent to a suspension system controller which causes actuation of the shock absorbers to a stiffer mode of operation when the low-frequency sensor signal components are relatively strong.


Patent
04 Oct 1985
TL;DR: In this article, the authors proposed a monitoring system for automatically and continuously monitoring the well-being of a fetus, which includes one or more fetal cardiac sensors for detecting the fetal heart rate.
Abstract: Fetal monitors for automatically and continuously monitoring the well-being of a fetus. The preferably ambulatory monitors include one or more fetal cardiac sensors for detecting the fetal heart rate. The sensors can be acoustic, electrocardiographic or bioimpedence types. Interference sensors are included for selectively sensing the principle interference components contained in the fetal cardiac sensor signals. The preferred microprocessor based signal processing system converts the sensor signals into digital format and transforms the signals from time domain into frequency domain and back into time domain after appropriate processing. Adaptive cancellation of interfering signals and other signal enhancing operations are disclosed. Analytical subsystems continuously analyze the fetal heart rate information and information from a fetal movement sensor to perform a fetal non-stress test. Subsystems for other analytical tests are also disclosed. Alarms warn the mother when fetal behavior is outside of preprogrammed acceptable limits of such analytical tests so that remedial action may be initiated. The monitoring system also preferably includes a sensor belt for properly and conveniently wearing the sensors and a control unit containing the signal processing and related components.

Patent
06 May 1985
TL;DR: In this article, a GPS satellite receiver is disclosed in which the frequency of the received satellite signal is down converted in a single step by a balanced mixer driven by a half-frequency local oscillator.
Abstract: A GPS satellite receiver is disclosed in which the frequency of the received satellite signal is down converted in a single step by a balanced mixer driven by a half-frequency local oscillator and the down-converted satellite signal is converted from analog-to-digital form by a hard limiter.

Journal ArticleDOI
TL;DR: A new method for estimating the number and arrival times for overlapping signals with a priori known shape from noisy observations received by a sensor is presented, based on a recently developed eigenstructure technique for multitarget direction finding with passive antenna arrays and exploits the structure of the received signal covariance matrix.
Abstract: We present a new method for estimating the number and arrival times for overlapping signals with a priori known shape from noisy observations received by a sensor. The method is based on a recently developed eigenstructure technique for multitarget direction finding with passive antenna arrays and exploits the structure of the received signal covariance matrix. This problem is important in various applications such as radar and sonar data processing, geophysical/seismic exploration, and biomedical engineering. In many of these applications, a known signal is launched into a scattering medium and the returning response-in the form of delayed overlapping echos in noise-has to be processed to yield information on the nature and location of scatterers. The method presented also solves more general problems of signal detection and resolution.

Patent
07 Nov 1985
TL;DR: In this paper, an improved video phone which transmits a source image over a standard telephone line in conjunction with audio voice signals is disclosed, where the audio portion is disabled by coupling a capacitor across the telephone to effectively short the telephone.
Abstract: An improved video phone which transmits a source image over a standard telephone line in conjunction with audio voice signals is disclosed. The unique initialization protocol allows asynchronous transmission with its short format. The protocol format provides for a hardware preamble for synchronizing a receiver clock, a software preamble for identifying the video image and disabling the audio, automatic gain control adjustment data and amplitude calibration data. The audio portion is disabled by coupling a capacitor across the telephone to effectively short the telephone. The telephone is placed across the bridge circuit which is provided with switches to balance the bridge in both the telephone audio and the video modes. The transmitted as well as the received signal are passed through the same delay equalization circuit, so that half the delay equalization is done before transmission, with the remaining half being done after reception. The use of a multiplexer and demultiplexer allows the same circuit to be used. The digital signals are level shifted from 5 volts to 10 volts prior to being converted into analog, and then are level shifted down again to reduce the effects of DC offset. The digital and analog voltage supplies are coupled together with a resistor and diode, and a capacitor is coupled between the analog supply and ground to combine the reset and voltage supply functions. An RC circuit providing 180° of phase shift is coupled to the crystal oscillator to produce a clock which is locked into the series resonant frequency of the oscillator. The camera and the rest of the circuitry is turned on and off by a switch coupled to panel which can be slid in front of the camera, thus providing for the automatic protection of the camera lens when the system is switched off.

Journal ArticleDOI
TL;DR: In this article, the authors examined the effect of variations in impulse response functions measured in different rooms and found that the response functions changed drastically with the variation of these recording parameters, indicating that signal dereverberation by impulse response deconvolution has limited scope in most practical situations.

Proceedings ArticleDOI
26 Apr 1985
TL;DR: The noise statistics are incorporated to reveal that certain processing of the Wigner distribution (WD) signal representation can lead to an optimal, and often easy to compute, detection scheme.
Abstract: A variety of methods have been proposed for the detection of a signal, with unknown signal parameters, in a noisy environment. In this paper, the noise statistics are incorporated to reveal that certain processing of the Wigner distribution (WD) signal representation can lead to an optimal, and often easy to compute, detection scheme. For the special case of linear FM signals in complex white Gaussian noise, it is shown that the optimal detector is equivalent to integrating the WD along the line of instantaneous frequency. If the position and sweep rate of the linear chirp are unknown, then a Generalized Likelihood Ratio Test (GLRT) leads one to integrate the WD along all possible lines in the time-frequency plane and choose the largest integrated value for comparison to a threshold. Simulation examples of the WD detection scheme are given to demonstrate the utility of the proposed method. Finally, some comments concerning the detection of the general phase modulated signal are offered.

Journal ArticleDOI
01 Oct 1985
TL;DR: In this paper, an approximate digital signal processing technique is presented which can assist in the early detection of local defects such as fatigue cracks in gears by enhancing the changes which these defects produce in the signal average of the vibration of the gear.
Abstract: An approximate digital signal processing technique is presented which can assist in the early detection of local defects such as fatigue cracks in gears by enhancing the changes which these defects produce in the signal average of the vibration of the gear. The technique is demonstrated by the identification of an early fatigue crack in a helicopter gear. The importance of phase modulation in the detection of defects is indicated.

Patent
29 Nov 1985
TL;DR: In this article, a television signal receiving apparatus with a program schedule display function is provided, which includes a tuner, a TV signal processing circuit connected to the tuner and a switch connected to a digital signal processing (DSP) circuit for selectively generating a local program schedule signal based on the Standard Time selected by the switch.
Abstract: A television signal receiving apparatus is provided having a program schedule display function. The apparatus includes a tuner, a television signal processing circuit connected to the tuner, a digital signal processing circuit connected to the tuner for generating a program schedule display signal based on program schedule data obtained through the tuner, and a switch connected to the digital signal processing circuit for selectively generating a local program schedule display signal based on the Standard Time selected by the switch.

Patent
10 May 1985
TL;DR: In this paper, a position determining system for providing electrical signals representative of the positions of a light source movable within a defined area is presented, which can be used for data entry or for purposes of object location or surveying.
Abstract: A position determining system for providing electrical signals representative of the positions of a light source movable within a defined area. The system may be implemented for purposes of data entry or for purposes of object location or surveying. When embodied in a data entry tablet, the invention provides signals representative of the positions of a stylus movable within the area of the tablet and comprising first and second photodetectors spaced along a base line by a predetermined distance, and means associated with each photodetector and operative to provide a movable narrow field of view for each photodetector. Means are provided for monitoring the angular position of each of the fields of view with respect to the base line and for providing a signal indication of the respective angular positions. A stylus having a light emitter at the tip thereof is movable within the area of the tablet and provides light detachable by each of the photodetectors. Each of the photodetectors provides a respective signal when the field of view of the corresponding photodetector receives light from the emitter. Means are provided which are operative in response to the signal indication of the angular position of each of the movable means at which light from the emitter is respectively detected to provide a signal representing the position of the emitter within the area of the tablet.

Patent
17 Apr 1985
TL;DR: In this article, a video signal is modified so that a television receiver will still provide a normal color picture from the modified video signal while a videotape recording of the recorded video signal produces generally unacceptable pictures.
Abstract: A video signal is modified so that a television receiver will still provide a normal color picture from the modified video signal while a videotape recording of the modified video signal produces generally unacceptable pictures. This invention relies on the fact that typical videocassette recorder automatic gain control systems cannot distinguish between the normal sync pulses (including equalizing or broad pulses) of a conventional video signal and added pseudo-sync pulses. Pseudo-sync pulses are defined here as any other pulses which extend down to a normal sync tip level and which have a duration of at least 0.5 microseconds. A plurality of such pseudo-sync pulses is added to the conventional video signal during the vertical blanking interval, and each of such pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a false measurement of video level which causes an improper recording of the video signal. The result is unacceptable picture quality during playback. This approach utilizes positive-going pulses which occur at a time not ordinarily used for black-level restoration in typical television receivers, and therefore does not give rise to black level depression in such typical television sets.