Video circuits

General video circuit topics AGC disciplines RF and fiber signals so they ain't misbehavin - you may need AGC to keep wayward RF, optical, and video signals within acceptable bounds Rate this link

Video mixing circuits A video mixer is a video production component that allows video signals from several different sources to be selected and combined. Basically mixing video sources is just summing the video materials from two video signals together. When mixing video signals from different sources, you need to ensure thatthose two video sources are in sync with each other.A good video mixer includes a frame synchronizer. This device delays one video source so that it starts at the same moment as a second source.Even more sophisticated is a frame synchronizer with time base corrector, or TBC, which not only synchronizes two pictures, but also cleans up their timing. This is important since VCRs, especially consumer models, are subject to some time base error, which causes wobbly lines and other picture flaws.A clear, steady picture requires an accurate, unchanging time base.A good commercial video mixer usually include a frame synchronizer / time base corrector, to insure that both sources are in perfect harmony and provide smooth, clean cuts between different sources.Some basic transitions can be performed without a video mixer by electronically-generating a source to match the timing of the moving video source. Such a signal is called genlocked because it is generated in a way that locks its timing to the video source. Genlocked images are usually solid color backgrounds, titles, or computer-generated graphics. Since the genlocked image is created in step with the moving video source, it is possible to perform fades, wipes, and other transitions between them. A genlocked system can fade from one scene to a solid color then fade from the color to the next scene. The term "genlock" is also used to refer to synchronizing a video signal to a house reference.When multiple pieces of video equipment are used together, there is a great advantage to having them all locked together so that every video frame from every source starts at the same instant. This is done using TBCs equipped with a reference, or sync, input. A standard reference video signal, often called house black or black burst, is routed to each reference input. This is how professional video studios are normally built. Using the GT4122 & GT4124 Video Mixer ICs - The GT4122 and GT4124 are broadcast quality monolithic integrated circuits specifically designed to linearly mix two video signals under the control of a third channel. Rate this link



Video Effects Unit - Digital AV mixers are available commercially, however these normally cost as much as a good video recorder, which is rather expensive for home use. The Video Effects Unit presented here has a more modest specification, with a more modest price tag! The unit enables the picture to be wiped to black at the end of a track, and then restored at the beginning of the next track. The recording video recorder would be paused once the picture is black, and the pause released just before fading in the next section. This circuit was originally published in Electronics in Action, December 1993 Rate this link

Other video related RF circuits Low-cost circuit biases VCOs for cable and TV - PLLs are useful in a variety of applications, most notably cable and TV tuners Rate this link

Video mixers and effects Video Effects Unit - unit provides various wipe patterns for use when video dubbing, wipes one signal to black, it does not provide mixing between two separate input signals Rate this link

Video overlay Video Clock Superimposer - This circuit superimposes the time of day in the bottom right-hand corner of an existing video signal, based on PIC16C711 Rate this link

Combining many signals to one cable In some video applications there is need to combine more than one signal to one cable. ?ne common application are surveillance cameras. Because remotely located video-surveillance cameras do not always have a ready source of power, it is convenient to run both the power and the video signal through one coax cable. One way to accomplish this task is to use an inductor to present a high impedance to the video and a low impedance to dc. The problem with this method is that the frequency spectrum of a monochrome video signal extends down to at least 30 Hz (and composite video even lower, down to 15 Hz). For example, a 0.4H inductor has an impedance of only 75 ohms at 30 Hz, which is approximately the minimum necessary impedance. Large inductors have their downsides (large series resitance, self-resonance usually below the 4-MHz), thus usually this kind of functionality is usually accomplished using active electronics (a gyrator, or synthetic inductor). Simple inductors pass the power and capacitors pass the signal only approach works well on higher frequency applications, for example antenna ampliifers powered through cable or in the satellite dish systems. Camera power and video use same coax cable - It is convenient to run both the power and the video signal through one coax cable but it needs some extra electronics on both ends of the coaxial cable. The circuitry at the monitor end of the coax cable supplies all the power to the coaxial cable with 24V through a gyrator circuit. The camera end uses a 12V fixed-voltage regulator to supply 12V to a CCD video camera. A driving transistor on camera end modulates the video on the 20V-dc line. Rate this link



Single Coax Carries Video, Power, and Channel-Select Signals - This is and interresting application note in pdf format. Rate this link