When installing or
repairing video equipment or systems, a test pattern generator is a
must. However, for this type of work you don’t need an expensive colour pattern
generator with a myriad of options and settings. What is required is a simple
signal source that allows a go/no-go indication.
While the specifications for this project don’t put it in the
professional instrument class, it is light and rugged, can be carried in a
toolbox and has the distinct advantage of being cheap. It also uses common
components, is easy to build and should work first time. There is no setting up
to do and there are no controls to fiddle with.
The circuit is powered from a 9V DC supply, which would
normally be a 9V plugpack. It produces standard non-interlaced monochrome video
and audio signals (see specifications) that are compatible with just about all
TV sets that have a A/V inputs, VCRs and video monitors. Note, however, that
this device is not suitable for testing most computer
Fig.1: clock signals for the circuit are provided by a 4MHz oscillator based on IC1a and these are divided down by dual 4-bit binary counter IC2 to produce most of the video timing signals. Dual decade counters IC3 & IC4 further divide the 500kHz output from IC2 to produce the vertical sync and audio output signals.
Fig.1 shows the full circuit details of the Audio-Video
Generator. The circuit operation may not be obvious at first glance, mostly
because the way in which the video signals are generated is a bit more
complicated than normal. In addition, some circuit simplification and trickery
has been applied to reduce the component count and keep costs down.
Clock signals for the circuit are derived from a 4MHz crystal
oscillator formed around NOR gate IC1a. The 10MΩ resistor places the gate into linear
mode and feedback is accomplished with the 4MHz crystal (XTAL1) and the two 22pF
capacitors. Because we are using a crystal, the resulting clock signal is
accurate and stable.
Inverter stage IC1b buffers the oscillator output which then
clocks pin 1 of IC2, a 74HC393 dual 4-bit binary counter. In this case, IC2 has
been cascaded to form a single 8-bit divider. Three of its outputs (pins 8, 9
& 10) are used for video timing, while a fourth output at pin 5 (500kHz) is
fed to a divider circuit to derive a 500Hz audio signal. The signal at pin 5 is
also divided down to produce a 50Hz vertical sync signal.
Supply voltage 9-20V DC
Current drain 15mA @ 9V
Output connector RCA female
Output level 1V peak-to-peak into 75Ω; 2.4V peak-to-peak
Pattern 4-step greyscale
Horizontal sync 5μs negative sync every 64μs
Vertical sync 500μs negative sync every 20ms
Output connector RCA female
Output level 840mV RMS; 2.35V peak-to-peak unloaded
Output frequency 500Hz