US4220931A - Composite video automatic gain control amplifier - Google Patents

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The invention relates to pulse transmission systems, and in particular to the regeneration of signals transmitted via such systems which use correlative level coding for increasing the transmission capability of the system while maintaining the bandwidth to that normally employed in binary transmission systems.

Initially, correlative systems found use in data transmission systems. Most commonly, bit rates in the order of and bps were used, and transmission was either over metallic wire pairs or carrier-derived voice channels.

In such applications, equalization and regeneration of the correlative level-coded signal was not duobinary signaling wikifeet basic requirement. The use contemplated for the present invention is in the repeatered line of a time-multiplexed PCM system.

PCM systems, such as the Western Electric T1-type lines, have been employed in commercial telephone service since the early 's. Each such system provides 24 voice-grade telephone channels.

The system bit rate is 1. Primary use of such systems has been over cable pairs in the exchange plant. In transmitting a 1. Regenerative repeaters have been used, and such repeaters are examplified for prior-art systems by U.

In order to double the transmission capability of the cable pairs, prior-art systems, which have been duobinary signaling wikifeet developed, have multiplexed two channel PCM systems for transmission over an exchange cable pair in one direction duobinary signaling wikifeet transmission.

The bit rate is then increased to 3. This is required because of pulse stuffing, which is employed in the time-multiplexing process. In essence, the capacity of the system is doubled because 48 rather than 24 channels are available for transmission over the repeater span. Because of this, the bit rate over the line is also doubled.

For this reason, such conventional channel systems require a new repeater which will operate at the new bit rate and over the greater frequency band. Further, these conventional systems suffer from the problems duobinary signaling wikifeet substantial additional crosstalk coupling which increases with frequency at the rate of 4. Because of this and other factors, the number of channel systems that may be accommodated duobinary signaling wikifeet a single cable depends upon the pairs in the cable and whether screened cable is employed.

In certain applications, the use of two different cables, one for each direction of transmission, is the most desirable approach.

Thus, such channel conventional systems have limited applications and in many instances cannot be used to retrofit existing channel lines. In addition to the factors of crosstalk coupling loss, the cable loss is important with respect to the repeater spacing, i. The problems involved relate directly back to the higher bit rate employed for the transmission of the channel PCM group. Engineering considerations for a channel system, which employs a transmission frequency of about 3.

The subject GTE Duobinary signaling wikifeet publication is incorporated herein by reference. These problems were overcome by the present invention which provides for transmission of 48 voice-grade channels, provides an effective bit rate of 3. Thus, the crosstalk noise problems at higher frequencies are minimized.

Further, the invention provides a transmission technique which is a suitable channel retrofit for the channel lines. A regenerative repeater for modified duobinary signals includes fixed and variable duobinary signaling wikifeet to compensate for variations in line length and line characteristics, a clock recovery circuit, and a modified duobinary pulse-shaping and timing circuit.

It is well known that signals transmitted over a cable pair will suffer considerable degradation with respect to amplitude and delay distortion. The function of a regenerative repeater is to accept the degraded signal and to produce, at the output, a replica of the original signal as it was transmitted either from the duobinary signaling wikifeet station or the preceding repeater. A block diagram of the preferred embodiment of a regenerative repeater for modified duobinary signals duobinary signaling wikifeet shown in FIG.

For a description of the modified duobinary technique, reference should be made to U. Not as a limitation, but as a means of particularizing the description, the following discussion will only consider a repeater which is operating at a bit rate of 3. Also, as with the bipolar system, the power spectrum peaks at about kHz.

The spectral densities for bipolar and modified duobinary pulse trains are derived and compared below. The general expression for the spectral density of a random pulse train was derived by W. The expression for spectral density is: Let us assume a binary pulse sequence a n consisting of binary 1's and 0's represented by 1-volt pulses for binary "1" and no pulses for binary "0. In such a binary pulse train, the digits are independent, uncorrelated so that.

We also use the same subscripts for the pulse shape G fwhich is rectangular with one volt amplitude across 1 ohm impedance duobinary signaling wikifeet binary "1" or absence of duobinary signaling wikifeet for binary "0. Here, for simplicity we duobinary signaling wikifeet an exact ratio between the two speeds of 2 to 1.

In practice, this ratio is EQU3. Hence, the error in spectral density in assuming a ratio of 2 instead of 2. Based on the above, the spectral densities of bipolar and modified duobinary are: EQU4 since the speeds and duty cycles of bipolar and modified duobinary differ by a factor of 2. The conversion factor S duobinary signaling wikifeet for bipolar and modified duobinary are, respectively:. Substituting 89and 10 into duobinary signaling wikifeet and 7it is clear that the only difference between W B f and W d f is the constant factor of 2 indicating that for pulses of the same amplitude in both cases, W D f has 3 dB more total power.

The first is item 1, representing the first function which is essentially analog and consists of fixed equalizers, adaptive equalizers, lowpass filter, control amplifier, and amplifier. The second function is clock extraction from the incoming modified duobinary signal, which is semi-analog in nature. The third and the last function is strictly digital and consists of threshold detection, sampling, and regeneration regenerator 20 of the modified duobinary signal for transmission.

The signal input 2 enters fixed delay equalizer-attentuator duobinary signaling wikifeet. The fixed delay equalizer is a well-known conventional all-pass network.

The attenuator is a conventional resistance network. The output of equalizer-attenuator 4 is followed by two stages of adaptive equalization which are represented as adaptive equalizer 6. The network of 6 will be described in more detail later.

The adaptive equalizer 6 equalizes both attenuation and phase since the cable is a minimum phase network. The adaptive equalizer 6 is followed by a fixed attenuation active equalizer which provides a gain reaching approximately 37 dB near the highest frequency duobinary signaling wikifeet the passband.

The fixed attenuation equalizer 8 is followed by lowpass filter 9, which sharply limits the bandwidth to approximately 1. The output of the lowpass filter 9 goes to amplifier 10, which is connected as an emitter follower, the output of which is distributed to three points as shown. The first is to the duobinary signaling wikifeet amplifier 14; the second is to the clock recovery circuit 22, and the third is to the regenerator Each of these circuits will be described in more detail hereinbelow.

The correction for variations in delay and loss characteristics of a transmission facility was investigated a number of years ago by H. Bode, duobinary signaling wikifeet his work resulted in U. The variable equalizers of Bode were designed to permit manual correction for these factors, which are duobinary signaling wikifeet by the minimum-phase characteristics of a cable.

In most configurations, the equalizer is controlled by a single variable element, which in most cases is a resistor. A typical circuit configuration of a Bode equalizer is shown in FIG.

The adaptive equalizer actually designed and used is shown in more detail in FIG. An advantage of using an active equalizer is that gain may be provided as duobinary signaling wikifeet as loss over the frequency range of interest. For the preferred embodiment, the fixed equalizer is centered so as to compensate for approximately 26 dB of attenuation at kHz between the transmitting terminal and the repeater input.

The adaptive equalizer either adds a loss frequency characteristic duobinary signaling wikifeet subtracts the desired loss frequency characteristic from that of the cable, depending upon whether the actual span length between the transmitting terminal and the repeater meets that for the average loss as determined by the 26 dB centered loss characteristic.

It provides a current duobinary signaling wikifeet to drive the passive network of the adaptive equalizer in FIG. The input impedance of duobinary signaling wikifeet same passive network, as seen from the point connected to the drain denoted by letter D at the FET 46, is made to vary with the frequency the same way duobinary signaling wikifeet the attenuation of the telephone cable. This input impedance is also controlled by the value of the terminating resistance, implemented by series-parallel connection of diodes 80, 82, 84, and 86 in FIG.

This circuit of the automatic equalizer in FIG. Thus, different values duobinary signaling wikifeet the terminating resistance correspond to different lengths of the telephone cable. The AC resistance of a diode depends on the value of the DC current flowing through it. By adjusting the value of the DC current flowing through the diodes 80, 82, 84, and 86, it is easy to compensate for attenuation of the length of a telephone cable within the desired range.

The same voltage causes current flow through the passive network consisting of inductor 78, resistors duobinary signaling wikifeet, 72, 76, capacitors 68, 74, and the diodes 88, 90, 92, and 94, constituting the second part of the adaptive equalizer. This second part of the adaptive equalizer is essentially identical in the design and operation to the first part preceding it in FIG. The main function of FET 64 is to provide a high input and high output impedance isolation between the two similar adaptive equalizer networks in FIG.

The variable resistance portion of the circuit consists of diodes 80, 82, 84, 86, 88, 90, 92, and The value of the variable resistance is changed by means of a control signal which is a DC output from control amplifier 14 along path The function of the control amplifier 14 is to convert the peak value of the analog duobinary waveform to a direct-current amplitude at the output of transistor A circuit for performing this function is shown in FIG.

Control amplifier transistors,and rectify the duobinary waveform applied at input The remaining elements operate to develop a direct-current output which is proportional to the peak value of the duobinary signal. It is readily apparent that the peak voltage of the duobinary signaling wikifeet signal is proportional to the attenuation of the cable at kHz. Consequently, the attenuation of the cable at kHz is, in effect, measured by the control amplifier, and this duobinary signaling wikifeet is converted to direct current at the output of transistor In turn, this current which is proportional to the cable attenuation is supplied to the eight diodes 80, 82, 84, 86, 88, 90, 92, and Change in the current through these diodes results in the change of the resistance of the diodes.

Typical control amplifier characteristics are shown in FIG. The AC resistance of the diodes ranges from 10 ohms duobinary signaling wikifeet ohms, corresponding to the necessary range of the variable element in the adaptive equalizer network.

A variable equalizer characteristic for various values of resistance for one section of an equalizer is shown in FIG. As noted hereinabove, the signal passes from the variable equalizer through path 7, fixed attenuator equalizer 8, lowpass filter 9, and amplifier Control amplifier 14 is directly connected from the emitter-follower amplifier 10 via path duobinary signaling wikifeet However, the outputs to the regenerator 20 and clock recovery 22 are obtained from a multi-winding transformer.

Referring now to FIG. Because of the three rectification stages plus amplification, the input to amplifier has a very strong component at the bit rate of 3.

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This invention relates to the transmission of signals by way of coaxial cables or the like, and is particularly directed to systems of this type wherein the cables may have variable lengths, and the signals are composite signals which include digital signals, such as synchronizing signals, and a video component at a frequency which may be quite different than that of the repetition rate of the digital signals.

The invention is especially concerned with the transmission of complex digital type signals, for example a complex digital signal of the type wherein the video signals are in the form of video pulses for a dot matrix type of display. It is frequently necessary to convey such signals from a signal source over a cable to be displayed on a cathode ray tube, and due to the wide variation of distances over which the signals must be transmitted, it is not feasible to separately design the transmission system for each application.

For example, in such a situation, the trasmission path, which is preferably a coaxial cable, may have lengths anywhere from about 25 feet to about 2, feet. Coaxial cable of course attenuate signals applied thereto, in dependence upon the lengths of the cables. The attenuation for low frequencies, however, is not the same as that for high frequencies.

For example, considering an RG62 coaxial cable, commonly used for digital transmission, the ratio of attenuation of the cable at kilohertz and 14 megahertz is about 1: These frequencies, incidentally, correspond to the synchronization signal pulse of a repetition rate and video signal frequency of a typical example of signal frequencies with which the present invention is concerned. It may of course be possible to provide compensating systems for manually controlling the gain of an amplifier in accordance with the length of a cable, while also adjusting a tuning circuit also in dependence upon the length of the cable.

Such adjustments in the installation of equipment are undesirable, however, since improper settings may be made or the adjustment may be overlooked entirely, to the detriment of the displayed image. In one system of this type, employing feedback control, as disclosed for example in U. Such a system is also not useful for transmission system adapted to transmit composite digital signal, since the average amplitude may vary as a function of attenuation of the signals, as well as due to variation of the energy content of the video signal itself.

It will also provide no solution to the problem, if one were to separate from the composite signal, signals only of the frequency of the synchronizing signals, by various filtering techniques, since the energy of signals at the synchronization signal frequency varies as a function of the video signal content, as well as a function of the synchronizing signal components themselves.

The difficulty of providing a suitable control voltage for a gain controlled amplifier is further evident from FIG. In each of these showings the synchronization pulses are negative going pulses at the left of the images, and the video signal pulses appear at the right. From these images, it is evident that the analog separation of synchronizing signals and video signal frequency pulses as mentioned in previous invention, to provide a control voltage that has a meaningful relation to cable length, could not be effected.

It is further evident that the attenuation of the synchronization signals and the video signal pulses has not been equal, in the two different cases.

The present invention is therefore directed to a system for overcoming the disadvantages of a known system, as above discussed.

Briefly stated, in accordance with the invention, an amplifing system is provided having an input stage with a control voltage terminal, and having a transfer function whose frequency response varies as a given function of a control voltage applied to the terminal. The amplifier further has a stage for deriving a control voltage for the input stage dependent upon the level of the synchronization pulses, but independently of the video frequency pulse content.

The development of the control voltage at the output of the level detector is preferably effected by gating the output of the amplifier to the level detector at times only at which the video frequency pulses themselves do not occur. This may be effected by the derivation of synchronizing signal pulses from the output of the amplifier, to control the application of signals to the signal level detector for the production of the control voltage.

The input stage of the amplifier, adapted to be connected to the output of a coaxial cable of variable length, may comprise a variable impedance in the form of an FET used as a voltage controlled resistor. This input stage may further comprise, for example, an active filter, whereby the FET impedance serves as the frequency determining element for the input stage of amplification. As a consequence, the gain of the input stage is varied as a function of the control voltage, and the frequency characteristics of the input stage are also varied as a function of the control voltage.

It is thereby apparent that the components employed for the FET amplifier, and the active filter, may be selected to match the attenuation ratio of the coaxial cable, so that the effects of the cable with respect to overall attenuation and differences of attenuation at different frequencies may be compensated, such that the amplifier output is independent of coaxial cable length.

In the consideration of the invention, it is important to realize that the amplifier is employed to reconstruct a triple level minus, zero, plus digital type signal, as opposed to an analog signal for which amplifiers of the prior art have been designed. The system in accordance with the present invention, provides feedback which ensures that synchronization pulses of determined amplitude occur at determined times, for example, during vertical retrace when the pulses are unaffected by video content and that the gain of the amplifier during the remainder of the time is based upon the gain occurring during the vertical retrace.

The control voltage is not dependent upon the video signal pulses themselves, but is employed by virtue of the variable transfer function of the input stage of the amplifier, to compensate for the complex transmission function of the coaxial cable, so that the composite output signal is produced faithfully independently of the length of the cable.

As a consequence, installation of video systems may readily be effected by inexperienced personnel, without the necessity for any adjustment with respect to length of the cable or the like.

This result is effected with a minimum number of components, and in an inexpensive manner. In order that the invention will be more clearly understood, it will now be disclosed in greater detail with reference to the accompanying drawings, wherein:. Referring now to the drawings, and more in particular to FIG. In other words, coaxial cables are known to attenuate signals applied thereto, and it is known that such attenuation is frequency dependent. The system of FIG.

In other words, the amplifing system 13 in accordance with the invention is adapted to compensate for the attenuation of signals of all frequencies of concern by the cable, even though the transmission characteristics of the cable differ at different frequencies. The invention is particularly concerned with the transmission of video signals, wherein the composite signals are of very wide frequency range.

For example, in one system, the video information signals may be generally in the 15 megahertz region, with synchronizing signal components being in the kilohertz frequency region.

These figures have been chosen since the circuit of FIG. Referring again to FIG. In the preferred embodiments of the invention, the impedance 14 is resistive in nature, although it will be apparent that other voltage variable impedances may be employed, as long as they are adaptable to properly affect the gain and frequency characteristics of a folloding amplifier.

The impedance 14 is connected to the input of an amplifier 15, serving as a filter, a feedback path 16 identified as a filter indicating this function of the amplifier. The output of the amplifier 15 may be applied, if necessary, to a further amplifier 17, which may also be connected as a filter, having a feedback path 18 identified as a filter to show this function.

The amplifier 17 may be an active filter. The output of the amplifier 17 may be connected directly, or by way of other amplifiers, filters or the like, if desired, to the output terminal 11, although it will be evident that variable transfer functions of such additional circuits may not be automatically compensated by the system of the invention.

In accordance with the invention, the output of the last stage of amplification, i. It will of course be apparent that other synchronization signals or pilot signals on the composite input may be employed to derive the control voltage. In this regard, it is necessary to render the level detector 19 operative to provide its output signal only at times corresponding to the occurrence of the synchronizing signals, i.

The synchronizing signal detector 21 may derive the synchronizing signal from the output of the amplifier 17, either directly or indirectly, in order to enable production of the gating signal.

The synchronizing signal detector 21 may, for example, be of the form generally employed in television receivers. The control voltage output of the level detector 19 is applied to control the impedance of the voltage controlled impedance In the system of FIG. The synchronizing signals are selected at the output of the amplifier 17, in order to provide a control voltage dependent upon the amplitude of the synchronizing signals.

In addition, since the transfer function of the filter is also dependent upon the source impedance, it is evident that the filter and impedance may be matched so that the transfer function of the filter additionally compensates for the different attenuation of other frequencies, such as frequencies of the video component. In this manner, in accordance with the invention, the control voltage for signals of one frequency may be employed to control the gain of signals of another frequency, so that the amplitude of the various signal components of the composite signal at the output terminal 11 will be substantially independent of the length of the cable A preferred embodiment of the invention is illustrated in greater detail in FIG.

The input amplifier, corresponding to the amplifier 15 of FIG. The signals from the transformer 25 to the amplifier are applied by way of the source-drain path of FET Q1, whereby Q1 and its associated circuit elements comprises a variable source impedance for the amplifier comprised of the transistors Q2 and Q3.

Typical values of components for the above described operating parameters are provided on the figure, and it is hence unnecessary to repeat these figures here. As above discussed, this porition of the circuit constitutes an active filter, fabricated in accordance with known design techniques for active filters. The characteristics of this portion of the circuit may be more clearly seen by reference to FIGS. In these figures, the frequencies f1 and f2 correspond to the design frequencies of kilohertz and 14 megahertz respectively.

A cable of 25 feet in length thus does not substantially attenuate the kilohertz signals, so that the control voltage derived from the synchronizing signals is at a level to effect a substantial reduction in the gain of the kilohertz signals, as shown in FIG.

The source impedance of the amplifier, in response to control voltages of this level, however, affects the transmission function of the amplifier so that the roll off frequency is substantially at kilohertz.

On the other hand, as shown in FIG. This increase, also due to the change in the source impedance value, also controls the transfer function of the amplifier, so that the roll off frequency is now in excess of 14 megahertz.

Since the source impedance of this amplifier is constant, the transfer function thereof will not vary as a function of the control voltage, as shown in FIGS. The output of the amplifier including transistor Q5 and Q6 may be connected to the output terminal 11, for example, by way of emitter follower Q7.

Referring now to FIGS. This amplifier is gated by the application of the output of the synchronization signal detector 21 to the inverting input of the amplifier The output of the amplifier 40 thus varies in response to the amplitude of synchronization signals only during the time of no video signal transmission for example, during vertical retrace.

These signals are applied by way of diode 42 to integrating capacitor 43 of relatively large value, so that the voltage at the control input of the voltage variable inpedance, i.

The control voltage circuit, including capacitor 43, has a charging time constant that is much smaller than the discharge time constant. The charging time constant is small to allow rapid response of the control voltage circuit to the synchronization pulses during the limited time the control voltage circuit is enabled on.

The discharge time constant is very high so that the control voltage generated can be retained until the next time the circuit is enabled on during vertical retrace. Thus, in the circuit illustrated in FIG. Such a long time constant is of course quite adequate from the standpoint of the objective of the present invention, as above discussed, since the transfer function control is primarily to compensate for differences in cable length, and cable lengths are only infrequently varied.

The invention does, however, have the advantage that this factor may no longer be considered in the interconnection of a cable between systems. While the invention has been disclosed and described with reference to a single embodiment, it will be apparent that variations and modifications may be made therein, and it is intended in the following claims to cover each such variation and modification as follows within the true spirit and scope of the invention. An amplifier for video signals transmitted by way of coaxial cable includes synchronizing signal separating means for gating synchronizing signals to a level detecting circuit, in order to produce a control voltage.

The control voltage controls the gain of an input stage of the amplifier to maintain the level of the synchronizing signals transmitted by the cable, while compensating for different attenuation which may be effected at video frequencies signal frequencies by cables of different length. What is claimed is: An amplifier for composite input signals having a periodically occurring digital component at a given repetition rate and video signals with frequency components substantially different from said repetition rate, said amplifier having an input stage for receiving said input signals, an output stage, means coupled to said output stage for providing a control voltage that is dependent upon the amplitude of said digital component and is independent of the amplitude of said video signals, said input stage comprising an amplifier with a control voltage terminal coupled to receive said control voltage and having a gain and frequency response that varies as a given function of a control voltage applied to said terminal throughout the frequency range of said video signals and digital component.

The amplifier of claim 1 wherein said control voltage providing means comprises gate control means, and a high discharge time constant circuit connected to the output of said gate control means.

The amplifier of claim 2 wherein said high time constant is much greater than one CRT refresh time. The amplifier of claim 1 wherein said means providing a control voltage comprises means coupled to said upward stage for selecting said digital component from the output of said output stage, and means deriving said control voltage from said digital component. An amplifier for composite input signals having a periodically occurring digital component at a given repetition rate and video signals with frequency components substantially different from said repetition rate, said amplifier having an input stage for receiving said input signals, an output stage, means coupled to said output stage for providing a control voltage that is dependent upon the amplitude of said digital component and is independent of the amplitude of said video signals, said input stage comprising an amplifier with a control voltage terminal and having a gain and frequency response that varies as a given function of a controlled voltage applied to said terminal, said input stage comprising an input terminal, an active filter, and a voltage variable impedance connected between said input terminal and said active filter.

The amplifier of claim 5 wherein said voltage variable impedance comprises an FET having a source-drain path connected between said input terminal and said active filter, said control voltage terminal being coupled to the gate of said FET. The amplifier of claim 6 wherein said active filter comprises a transistor amplifier having a feedback impedance.

The amplifier of claim 8 wherein said input stage comprises an active filter. A transmission system for composite video signals having synchronization signal portions and video portions occurring at different times, said system comprising a coaxial cable, an amplifier connected to the output end of said cable and having an input stabe with a control voltage terminal, means coupled to the output of said amplifier for deriving a control voltage responsive only to the amplitude of said synchronization signal, and means applying said control voltage to said input stage, the transfer function of said input stage being responsive to said control voltage whereby the overall level of synchronization signals and video portion is independent of the length of said cable.

The system of claim 10 wherein said cable has a length between 25 and 2, feet. The system of claim 10 wherein said coaxial cable has a transfer function wherein the attentuation with length varies as a function of frequency, and said transfer function of said input stage is varible as a function of said control voltage to compensate for the attentuation of coaxial cable at all ranges of frequency components of said composite video signal.

The transmission system of claim 10 wherein said input stage comprises a voltage controlled impedance. The system of claim 14 wherein said gate control means comprises an operational amplifier. US USA en DE DEC2 en CA CAA en An amplifier with an automatic gain control system which is especially suitable for composite len provide video.