Abstract:
A method for controlling dissipation in a video amplifier for a display signal. The method comprises the steps of setting a first current for establishing a first gain bandwidth product in the video amplifier. Generating a control signal in accordance with a slew rate of the display signal. Modifying the first current responsive to the control signal such that the video amplifier gain bandwidth product is controlled in accordance with the slew rate of said display signal.

Description:
[0001]    This invention relates to the field of video display technology and in particular to the dynamic control of power dissipation in accordance with the spectral content of the display signal.  
         BACKGROUND OF THE INVENTION  
         [0002]    In addition to the display of broadcast television programming, television receivers often provide a monitor capability for the display video or graphical material from other signal sources such as DVD players, computers and computer games. The bandwidth of these non-broadcast signals may vary from 3.5 MHz for a VCR derived video signal, to the region of 25 MHz for computer generated images. The power consumption of a video output amplifier in the TV monitor display increases with the bandwidth required to faithfully display the non-broadcast signal bandwidth and amplitude. Modern TV displays frequently display signals from different sources often with different content bandwidths. Typically an average power dissipation in a simple class-A video amplifier with a bandwidth in a range from 5 MHz to 20 MHz will correspondingly dissipate between 2W to 5W. However, in a high bandwidth active load amplifier this can easily exceed 5 Watts. Because a cathode ray tube receiver monitor contains three video output amplifiers these can represent a significant contribution to the total power consumption of the display.  
           [0003]    It is known to control an operating point of a video amplifier by means of feedback from a current path which undergoes a parameter change as a consequence of operation at increasing frequencies. The feedback signal can be developed across an emitter resistor in proportion to the current which increases at higher frequencies. The feedback signal is filtered and applied to the amplifier input to influence the operating point.  
           [0004]    To reduce levels of unintentional emissions it is known to analyze the spectral content of a display signal and generate an emission control signal in accordance with input signals likely to cause emissions. Such a control signal can be applied to reduce subjective video peaking effects produced by scanning velocity modulation (SVM), or to dynamically modulate video peaking circuitry to diminish emissions. It can be appreciated that analysis of the spectral content of the display signal can provide an accurate indication of likely emission candidates, with the remedy being the dynamic reduction of image enhancement. However such dynamic enhancement control is unlike the objectives herein which adaptively control amplifier bandwidth to be sufficient for the actual display signal present at the amplifier input.  
           [0005]    With increasing computer and video game usage plus 24 hour broadcast or cable programming availability, television receiver monitors can be operational for extended periods of time, hence it would be beneficial in terms of device reliability and energy consumption to reduce the power dissipated by the display.  
         SUMMARY OF THE INVENTION  
         [0006]    In an advantageous circuit arrangement the bandwidth of the video output stage is continuously adjusted in accordance with to the bandwidth requirement or spectral components present or expected in the input signal. As a result, power consumption is reduced when displaying video signals of lower resolution. In a further arrangement, the bandwidth can be adaptively switched between two bandwidth values, normal and wide, by a control signal. This adaptive bandwidth control can be generated by a micro-controller by extracting bandwidth-related data from the video signal or input signal selection.  
           [0007]    In an inventive method dissipation is controlled in a video amplifier for a display signal. The method comprises the steps of setting a first current for establishing a first gain bandwidth product in the video amplifier. Generating a control signal in accordance with a slew rate of the display signal. Modifying the first current responsive to the control signal such that the video amplifier gain bandwidth product is controlled in accordance with the slew rate of the display signal.  
           [0008]    In a further inventive arrangement a video amplifier for a cathode ray tube display comprises a video processing amplifier with an output drive amplifier coupled to video processing amplifier and supplying an amplified video signal from the video processing amplifier for display by the cathode ray tube. The output drive amplifier is coupled the video processing amplifier to form a negative feed back loop, and a feed forward open loop control signal generated by the video processing amplifier is applied to control a bandwidth of the output drive amplifier.  
           [0009]    In yet a further inventive arrangement the feed forward open loop control signal is generated in accordance with a slew rate of the video display signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram of a video amplifier including inventive arrangements.  
         [0011]    [0011]FIG. 2 is a schematic circuit including the inventive arrangements of FIG. 1.  
         [0012]    [0012]FIG. 3 is a schematic circuit showing an inventive slew rate detector for use with the inventive arrangements of FIG. 1.  
         [0013]    [0013]FIG. 4 depicts the frequency response of the inventive slew rate detector of FIG. 3.  
         [0014]    [0014]FIG. 5 shows current requirements and temperature variation of an a video amplifier controlled in accordance with the inventive arrangements of FIGS. 1, 2,  3  and  4 . 
     
    
     DETAILED DESCRIPTION  
       [0015]    The block diagram shown in FIG. 1 depicts a cathode ray tube drive amplifier including various inventive arrangements wherein the amplifier bandwidth is adaptively controlled in accordance with the spectral content of the signal to be displayed. In a color cathode ray tube display blocks  100 ,  200 ,  250 ,  400  and  500  are present for each display color, however, microprocessor controller  400  can provide color specific control signals for each color channel. A color display signal, for example red green or blue from, from an exemplary color demodulator or external base band source is input to video processing block  100  for amplification and, for example, DC bias control. Control of the DC content or component of a display signal by means of an automatic kinescope bias or (AKB) feed back control loop is well known and serves to maintain a predetermined current at each cathode of a cathode ray tube display.  
         [0016]    Processing block  100  supplies a color display signal, for example red green or blue, to video output amplifier  200  and also supplies the amplified and DC controlled video signal to block  300  which generates a control signal Vg in accordance with the detected frequency content or slew-rate occurring within the video signal. The derivation of adaptive dynamic bandwidth control signal Vg will be described with reference to FIG. 3. The slew-rate related, adaptive bandwidth control signal Vg from block  300  is coupled to a video output block  200  which will be described with reference to FIG. 2.  
         [0017]    A further adaptive bandwidth control signal is developed within microcontroller block  400  which employs a microprocessor to deduce or infer information about the likely signal content from indicators such as the signal standard or type of input connector selected as an input source. In this way microcontroller  400  can provide a fixed or preset bandwidth control for video output amplifier  200 . In yet a further arrangement the temperature of the output transistors forming amplifier  200  can be used as a bandwidth control arbiter where a temperature excess causes a predetermined amplifier bandwidth reduction.  
         [0018]    The adaptive bandwidth controlled video signal from video output block  200  is coupled to an automatic kinescope bias or (AKB) sampling transistor Q  500  connected as an emitter follower. As is known, CRT cathode current from the collector of transistor Q  500  is sourced from video processor  100  and can be considered to form a negative back control loop for stabilizing the DC component of the display video and hence the brightness of the display image. An exemplary automatic kinescope bias control loop employs test or calibration lines, representing a dark gray or low level signal which are inserted into the vertical blanking interval of the display video signal. The CRT cathode current resulting from these calibration lines is compared with a desired current value and corrective adjustments applied to a DC component of the display video signal. In this way all three cathode currents are matched to a common value.  
         [0019]    The various adaptive control arrangements shown in FIG. 1 can be considered to represent not only a closed, negative feedback control loop for DC stability and automatic kinescope bias (AKB), but in addition open loop, feed forward controls for the dynamic control of output amplifier bandwidth. In this way the provision of unnecessary drive amplifier bandwidth is avoided and consequently power dissipation is significantly reduced.  
         [0020]    Video output amplifier  200  with adaptive bandwidth control is depicted in schematic form by FIG. 2. In the absence of dynamic bandwidth control the amplifier can be considered a common wide bandwidth, active load video output amplifier. An input signal from video processor  100  is coupled to the base of emitter follower transistor Q 1  via a gain trimming and feedback network formed by resistors R 1 , R 2 , R 3 , R 4  and R 5  and capacitor C 1 . The emitter of PNP transistor Q 1  is coupled to a +12 volt supply via resistor R 6  and drives the base of NPN amplifier transistor Q 3  via a parallel connected resistor R 7  and capacitor C 2  combination which provides frequency shaping. The base of transistor Q 3  is also connected to the base of PNP amplifier transistor Q 2  via a coupling capacitor C 3 .  
         [0021]    The signal at the base of transistor Q 3  generates a collector current Ic and the open loop gain of the amplifier is determined as shown below by current Ic,  
         [0022]    Gain open loop=−gm Rc, where Rc=transistor Q 3  load impedance, and substituting for gm, where gm=1/re, and re=25/Ic (mA),  
         [0023]    Gain open loop=−Rc. Ic/25,  
         [0024]    It is known that the gain bandwidth product or ƒT of a common emitter amplifier, for example transistor Q 3 , varies with collector current and follows a somewhat convex curve. For example, at a low collector current a lower ƒT results than can be obtained with a higher current. However, the provision of a transistor working point with a higher collector current thereby yielding a higher gain bandwidth product in expectation of wide bandwidth signal handling capability results in unnecessary transistor power dissipation. Thus the advantageous variation of collector current Ic in accordance with input signal slew rate permits the dynamic input signal regulation of open loop gain, gain bandwidth product and consequently amplifier bandwidth.  
         [0025]    The upper transistor Q 2  is configured in a common emitter mode that is driven with the high frequency content of the video signal from emitter follower Q 1  via capacitor C 3 . The maximum output signal amplitude at the emitters of driver transistors Q 4  and Q 5  is proportional to the quiescent current of transistor Q 2 , which is set by biasing resistors R 9  and R 10 . The lower current limit is defined by the DC feedback current provided by resistor R 5 , the maximum current is set by the transistor parameters which specify the maximum permitted collector current or power dissipation. Advantageously, by changing the current in transistors Q 2  and Q 3  the overall bandwidth of the amplifier can be controlled. An inventive bandwidth control circuit, to be described with reference to FIG. 3, can change the quiescent current in transistor Q 2  and thus change the bandwidth of amplifier  200   
         [0026]    A control current for example Ic(Q 23 )/3 from the collector of transistor Q 23  is coupled to the base of transistor Q 2  via resistor R 14  and provides transistor Q 2  collector current with a control range of 6 mille Amperes (mA) to 15 mille Amperes (mA). As a consequence of dynamic current control and the DC loop gain of the video amplifier, the DC component present in the output CRT drive signal can vary. However, as mentioned previously, the negative feedback control provided by the automatic kinescope bias (AKB) control loop can adequately compensate for this source of display signal DC variation.  
         [0027]    The slew rate detector  300  is shown in the schematic drawing of FIG. 3 can be considered as a slew rate and a frequency detector. Since slew rate is proportional to the first derivative of a signal the detector must act as a high-pass filter. Video signals representing the individual color signals are AC coupled and differentiated by capacitors Cr, Cb, Cg, and summed in a low impedance at an emitter of a grounded base NPN transistor amplifier Q 21 . The base of transistor Q 21  is decoupled to ground by capacitor C 23  and biased by diode D 21  which is supplied from a +12 volt supply via resistor R 22 . Negative signal components of sufficient amplitude at the emitter of transistor Q 21  cause the transistor to conduct and form negative or rectified pulses, summed and amplified signal at the collector of amplifier Q 21 . This output signal is developed across load resistor R 23  which is connected in parallel with a notch filter formed by a series connected network of inductor L 21  and capacitor C 24  which resonate at about 6.5 MHz. Thus the rectified and filtered signal is supplied to the base of PNP transistor amplifier Q 22 .  
         [0028]    Whenever the signal at the base of transistor amplifier Q 22  is less than a certain level, transistor Q 22  becomes conductive causing capacitor C 25  to be charged towards the +12 volt supply. When transistor Q 22  is non-conductive capacitor C 25  is discharged via resistor R 26  and the input impedance of transistor Q 23  which define a discharge time constant of about 2 seconds. The discharge time is in part required to permit any display signal DC shifts to be corrected by the slow, field rate, action of the AKB loop. Furthermore the discharge time ensures that unnecessarily frequent bandwidth switching is avoided. The voltage across capacitor C 25  is applied to the base of transistor Q 23 , which with emitter resistor R 27 , form a current source that controls the bias of current source transistor Q 2  and the operating point of the video output amplifier shown in FIG. 2.  
         [0029]    As mentioned previously, microcontroller block  400  can generate a bandwidth determining control signal which is coupled to the base of switch transistor Q 24  which switches resistor R 28  to ground and changes the bias on transistor Q 22 . Thus transistor Q 22  can be forced to a predetermined current and hence the video amplifier  200  bandwidth can be preset in response to a bandwidth determination from microcontroller  400 . For example, with transistor Q 24  turned on transistor Q 22  causes the voltage across capacitor C 25  to rise thereby increasing bandwidth control current Ic(Q 23 ) which consequently causes the bandwidth amplifier  200  to be increased. Conversely, when microcontroller  400  holds transistor Q 24  off, negative signal components from transistor Q 21  are required to cause a rise in the voltage across capacitor  25  with the resulting increase in both current and bandwidth of amplifier  200 .  
         [0030]    The relationship between control voltage generated across capacitor C 25  and the corresponding effect on the bandwidth amplifier  200  is illustrated in FIG. 4. The beneficial consequences of adaptive bandwidth control is shown in the table of FIG. 5 which demonstrates temperature differences, measured at an ambient temperature of 25 degrees C., during operation with two different bandwidths and two different input signals. The advantageous use of a variable bandwidth video driver reduces the power consumption by 50% during low-bandwidth operation, saving a total of 6W and in addition the temperature of transistors Q 4 , Q 5  heatsinks are reduced by about 30%.