We now need convert the 7-bit latched data to an analogue signal. These are a number of 8-bit DAC I.C.'s suitable for the job, such as the DAC08 (used in the prototype), DAC0801 or MC1408. Since we are only producing 7-bit data then we connect the data into the upper 7 bits of the DAC, connecting the DAC's data bit 0 to 0V.
Whichever DAC is chosen, it is usually necessary to buffer the DAC's output. This job is often performed by a high speed op-amp. These op-amps can be troublesome particularly where poor wiring / grounding can cause problems with instability (for example "Vero Board" type of prototype P.C.B. is not well suited to high speed circuitry).
Instead it was found that simpler transistor circuitry provided good performance. The circuit shown opposite uses a single small-signal PNP transistor to both buffer the DAC signal and mix it with the composite synch pulses.
The composite video now needs to be connected to a modulator. As this may represent a low impeadance, an additional transistor emitter follower is used to buffer the composite video signal and provide a 75-80 ohm source impeadance.
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28th July 2002