PART FOUR - SYNC GENERATOR. "Much of a video signal is made by summing pulses together and ultimately addding this composite sync to the video generated from the mosaic of the iconoscope.

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Figure 4. Sync generator based upon the Fairchild 3262A

FYI: Fairchild [3262A Data Sheet] in PDF format

       Imagine a symphony orchestra for a moment. It is easy to see that the many musiscians are kept synchronized by the conductor and his baton. Well, the sync generator performs the identical function in our camera. That is to start the line and frame scans on time and in proper phase, blank the electron beam on and off at just the right moments. Provide a reference pulse that tells the video amp when to perform a 'clamp' or DC restoration function. And provide the stream of sync pulses that must be added to the iconoscope raw video to construct an entire 'composite' video waveform. R38, on the far right, is the tap off point that sends the CS (composite sync) signal to the the final video amplifier, where it will meet the video from the iconoscope for the first time. This control sets the amplitude (voltage) of the sync to precisely .286 volts or 40 IRE units. This output is especially filtered inside the 3262 so that it meets FCC rise and fall time specifications. We don't need a composite sync filter circuit in the final design.

       This sync generator is constructed around an obsolete part from the early 1980s. The Fairchild F3262 and F3262A were complete EIA and NTSC sync generators in a chip. We are only interested in F3262A, the color NTSC version. (Would it not be gnarly to add sepia overtone to the video from this camera?) The process starts with a 14.31818Mhz (910 times horizontal) oscillator. The four NAND gates, U5, balance the off/on ratio of the oscillator to exactly 50/50 and provide the clock in two opposing phases. By using this method, it gives the advantage that we treat the 14.318 clock as double frequency or 28.636Mhz, simply by choosing one phase or the other. The 3262 creates all of the common pulses used in twentieth century American television. These are:

Figure 5. Complet Fairchild 3262A based sync generator circuit

       In the photo above, from left to right, 14.31818MHz oscillator, 74LS00 quad NAND gate, Fairchild F3262A, and 74LS00 hex inverter. The oscillator is the organ grinder of this machine. Just like turning the crank on a victorian music box, the oscillator sets the tempo of the whole sync generating state machine. The 74LS00 conditions the clock pulses for the sync generator chip. The sync generator chip then pumps out all the other pulses we need. That last chip, far right, contains six logic inverters. From the sync chip, we get a pulse. Let's say that pulse is considered active when it is high. The next circuit needs the pulse to be low when active. Route the pulse through one of the logic inverters, and now we have the correct flavor (polarity) of the desired pulse. Page four of the F3262A Data Sheet shows the pulse diagrams. These meet broadcastable specs. No random sync for us, no sir!

Figure 6. Fairchild 3262A Sync generator Block Diagram

       As you can see above, the clock pulse is routed in from the left and starts driving various digital counters. The outputs of the counters are applied to decoders. When a decoder sees the count '909', for instance, it generates a pulse that could be routed back to the same counter's reset pin. So, when the top count of 909 is reached, the count is reset to zero. Zero to 909 is 910 clocks. This also happens to be the horizontal sync frequency. What a coincidence! The same process is applied to other decoders and counters resulting in a symphony of useful video pulses. Most importantly, all of these pulses are intimately related in time down to the 70nS (nano second) time period of the 14.31818MHz clock.