GENNUM. Using the GT4123 & GT4123A Video Mixer ICs APPLICATION NOTE DEVICE DESCRIPTION. is less than 0.5 volts, V CA

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1 GENNUM C O R P O R A T I O N Using the GT & GTA Video Mixer ICs by Ian Ridpath, Senior Applications Eng., Video & roadcast Group APPLICATION NOTE DEVICE DESCRIPTION The GT and GTA are the first dedicated single device, two input video mixer ICs available to the professional video and multimedia markets. The internal topology of the devices is shown in Figure. IN A IN CONTROL (V C ) AMP A XA V CA AMP X V C V C =. (V C.) AMP C AMP D V CA =. (V C.).V REF COMP When V C is less than. volts, V CA reduces and V C increases in proportion so that less of the Channel A signal and more of the Channel signal is transferred. Similarly, when V C is greater than. volts, the opposite occurs. The SPAN or control range is internally set so that a CONTROL voltage of volts completely cuts off Channel A and fully turns on Channel. Similarly, a CONTROL voltage of volt will fully turn on Channel A and completely turn Channel off. Figure shows the CONTROL transfer characteristics of the GT and GTA. GAIN (%) CH CHA Fig. Functional lock Diagram of the GT and GTA Each input signal is applied to a conventional differential amplifier (AMP A and AMP ). From the amplifiers, the signals are applied to analog multiplier circuits (XA and X) whose outputs are the product of the input signals and internally generated controlling voltages V CA and V C. These voltages are derived from a unity gain differential amplifier (AMP C) whose outputs (true and invert) are the difference between an externally applied CONTROL voltage (V C ) and an internal. volt reference voltage. In addition, the internal DC offset of. volts is applied to the controlling voltages. Therefore, V CA =.V (VC.V) and CONTROL VOLTAGE (V C ) Fig. Control Law There is a small dead band at either extreme of the CONTROL input. The amount of dead band is about mv and is shown in Figure. The CONTROL input can be preceded by an operational amplifier so biased as to overcome this dead band as well as level shift the control signal so that other than to volt ranges can be used. The bandwidth of the CONTROL input is sufficient to allow very fast keying and is in the order of MHz at d. The linear portion of the transfer characteristic has a linearity of % or better. V C =.V (VC.V) When the CONTROL input (V C ) equals. volts, V CA and V C =. volts and exactly % of each input signal passes to the output of the multiplier stages. 6

2 Referring again to Figure, the outputs from the multipliers are applied to an analog summing circuit ( ) whose output feeds a wideband amplifier and presents the mixed signals to the outside world. The noninverting inputs of each input amplifier are directly connected to the output. In this manner the closed loop gain is nearly unity providing wideband, stable operation. ecause the devices have only 8 pins and require virtually no external parts in order to function, they lend themselves to high density, multifunctional PC board layouts. Several devices can be used in parallel applications such as RG mixers and fourlayer keyers where close control law tracking is essential. The only difference between the GT and the GTA is the fact that the latter device can operate with nonequal power supplies. The negative supply can be as low as volts unlike the GT which can only operate down to ±9 volt supplies. TEST SETUP and MEASUREMENTS Figure shows a typical set up of a single device. (V) V (V) V The test circuit can be used to verify such parameters as frequency response, differential gain and differential phase, crossfade balance and channel to channel isolation (crosstalk). In the case of differential gain and differential phase, the output of the GT must be AC coupled to the amplifier. A capacitor of µf is suitable. Crossfade balance is measured by grounding both the V INA and V IN inputs and applying a volt peak to peak signal to the CONTROL input. This signal must have a.v DC offset. The frequency is then varied across the band of interest. If there was a perfect balance between both signal channels, then the output would remain at zero volts with only a small amount of noise present. Typically the output of the GT remains at least d below volt up to the colorburst frequency. Pin of the GTA is a FREQUENCY COMPENSATION pin and is used to tailor the frequency response of the IC. A 68 Ω resistor in series with a small trimmer capacitor of about to pf is all that is needed. Figure shows the effect on the frequency response of varying the COMPENSATING capacitor over its range.. V A 8 6 GT (GTA) 6d AMPLIFIER VIDEO GAIN (d) =pf =8pF. V. 68. =pf V C pf. 6 FREQUENCY (MHz) Fig. Effect of C COMP on the Frequency Response ( mv Overdrive) Fig.. GTA Test Set Up The amplifier shown is only necessary when driving low impedance loads such as coaxial cables. For some applications the inputs can be driven directly from Ω cables using suitable terminating resistors. For other applications the inputs can be preceded with clamp circuits such as the G synctip clamp or the G backporch clamp. oth of these devices are manufactured by GENNUM. In this case, the Ω input resistors are not necessary. The frequency response is also dependant upon the load capacitance that the PUT sees. In many applications the PUT will be feeding an opamp or another GTA. The load capacitance will normally be made up of stray capacitance and should not exceed pf. With proper adjustment of the COMPENSATION circuit, in conjunction with a nominal load capacitance, a full power bandwidth ( d) of at least MHz is possible. The small signal flat response (d) will be at least to MHz. 6

3 APPLICATION CIRCUIT A PC board which uses two GT mixers, two G backporch clamps and a GX, x video switch is available from GENNUM. This board can be used to gain experience in using these devices in a simple Effects Generator application. Figure shows the circuit of this board which contains the video signal paths but does not include the control circuitry needed to perform the various functions such as a KEY,, RAMP or FADE. These control signals are generated from other circuitry using standard logic gates, monostable multivibrators and comparators. The two video signals (VIDEO and VIDEO ) are applied to the G backporch clamps via µf capacitors C and C. These inputs must be suitably terminated with Ω resistors. The VIDEO signal must also be applied to a sync separator circuit in order to produce a negative going backporch pulse used by the Gs. The clamped output from the first G feeds the V INA input (pin ) of the first GT mixer. It is also used to drive a comparator in order to produce a luminance key signal. The V IN input (pin ) of this mixer is fed from the second G. oth of these clamped video signals are applied to inputs of the GX, x video switch. y linearly varying the voltage on pin of the GT, a smooth A/ mix can be achieved and a resultant signal appears at the output (pin ). The voltage on pin can also be controlled by a chroma or luminance key signal or by an appropriate horizontal or vertical wipe waveform. The only restriction on the control signal is that the peak to peak amplitude should be volt centred around. volts. KEY COMMON VIDEO R C pf C GA 8 GTA C pf R 68 R8 R ADDRESS A A SELECTION MIX IN A IN GX MIX/FADE 8 R R V VIDEO VIDEO ACK PORCH C R C pf 8 G CONTROL R8 GT CONTROL (FADE) R 68 C9 pf R9 C C6 C9 C C V A PIN PIN A PIN 8 PIN 8 GX PIN C C8 C C C A PIN PIN A PIN 6 PIN 6 GX PIN 8 All resistors in ohms, all capacitor V NOTE: ALL DECOUPLING CAPACITOR VALUES ARE µf Fig. Video Mixer oard Circuit Diagram The mixed video signal from the first GT is also applied to the x video switch (GX) as well as to the V INA input (pin ) of a second GT. The V IN input (pin ) of this second GT is fed from the output of the second G. This set up allows the mixing of the already mixed video signals with the VIDEO signal itself. For example, when the control voltage on pin of this GT is zero volts, % of VIDEO is present at the output. When the control voltage is one volt, % of the already keyed, wiped or ramped signal appears at the output. y linearly ramping this voltage between zero and one volt, a clean FADE function can be achieved. This new signal is also applied to the x video switch. The VIDEO PUT from the board can thus be switched to VIDEO alone, VIDEO alone, VIDEO () or lastly, VIDEO (). The inputs are selected by applying binary codes from to on the address pins and. 6

4 Figure 6 shows one possible method of generating HORIZONTAL and VERTICAL waveforms which can be applied to the first GT. The HORIZONTAL and VERTICAL circuit consists of two dualretriggerable monostable multivibrators and four NAND gates. The horizontal monostable (H) is triggered from a composite sync signal generated by the sync separator. The start and stop timing is controlled by two potentiometers R and R. The NAND gate ( A) combines the output pulses to provide the HORIZONTAL waveform. In a similar manner, a second monostable (V) is triggered from the vertical sync. The start and stop timing is again controlled by two potentiometers (R and R). A second NAND gate () combines the pulses to produce the VERTICAL waveform. STOP C C V R8 EXT R. R EXT /C EXT k 68k CLR Q H COMPOSITE SYNC 6 START C6 C EXT V R R. R EXT /C EXT 9 k 68k CLR Q H VERTICAL SYNC V V R k R 68k R 68k C C STOP C EXT R EXT /C EXT CLR Q V 6 START C EXT R EXT /C EXT 9 CLR Q V 9 8 HORIZONTAL TO A PIN A A.S.S V 9 8 VERTICAL TO A PIN Fig. 6 Horizontal and Vertical Wipe Circuit Figure shows one method of generating the KEY and FADE control signals. The circuit consists of a high speed comparator (LM) whose output is gated through the 8 AND gate to produce the KEY signal. The HORIZONTAL and VERTICAL signals from the circuit in Figure 6, are combined and gated using the NAND and 8 AND circuits. The ramp generator is triggered by the vertical sync. In order to DC restore the ramp, a G DC Restorer is used. This device is available from GENNUM. All the controlling signals are individually adjusted to have a range of zero to one volt and are selected by a way MODE SELECT SWITCH. The FADE control is a simple potentiometer connected to a regulated supply. In order to produce a linear RAMP function, a discrete ramp generator is made up of transistors Q through Q. 6

5 VERTICAL SYNC Q RAMP R9 N9 LEVEL R8 k ADJUST k HORIZONTAL 6 VERTICAL A A NORMAL ACK PORCH 8 INVERT NORMAL/INVERT SWITCH C9 KEY COMMON 8 6 (CLAMPED VIDEO ) LM V R8 k Q N9 C R M R C R k KEY LEVEL D 6V Q N9 R C. V KEY LEVEL V R k Q N9 Fig. Key and Fade Generator Circuit C8. C. V LEVEL V R k k D G C9. V DC RESTORER MODE SELECT SWITCH D CONTROL (FADE) to PIN V V CONTROL TO A PIN V FROM VIDEO GX PIN ACK PORCH EL PIN G PIN 8 R8 GC VIDEO 6 R EL pf _ C pf V EL PIN G PIN R R8 R 8 GAIN (Set for a Gain of ) Fig. 8 Output Clamp and Driver Circuit In order to drive a coaxial cable, a suitable cable driver circuit must be used. Figure 8 shows an EL amplifier being fed from a G backporch clamp. The input of the clamp comes from the GX, x video switch on the video mixer board. The combination of the video mixer board, the, KEY, FADE and RAMP circuits and the output driver circuit produce a simple Effects Generator that can be used to evaluate the GT video mixer IC. In this way, the resulting VIDEO PUT is accurately clamped to the black level. Gennum Corporation assumes no responsibility for the use of any circuits described herein and makes no representations that they are free from patent infringement. Copyright September 99 Gennum Corporation. All rights reserved. Printed in Canada. 6