ZX81 Video Inverter V1


Here are two new ZX81Video Inverter circuits.


The first design is the ZX81 Video Inverter V1 circuit, which uses only

10 discrete components, is simple to build, draws less that 20mA and 

generates crisp white characters on a black screen.



This circuit is connected between the ZX81 ULA pin 16 video output signal

and the RCA jack of the modulator. I usually disconnect and discard the

little PCB inside the modulator to make room. The input pins of the

modulator are connected to the ZX81 motherboard solder pads for ground,

Vcc, TV channel select switch (0V/5V) and composite video. Depending on

The country the composite video signal may be attenuated.

We want to use the full signal from ULA pin 16 which is brought out to a

pad on the ZX PCB marked UK2.  This is a good point for connecting the

up ZX81Video Inverter input signal.


The ULA video signal is 1.8V p-p from white level to sync tip. The black level

is about 1.2V and the screen characters are black on white.  


Q1 detects the sync tip and turns off when the video level is below 0.6V. 

This causes Q2 to turn on and clamp the video output to ground. 

When the video input level is above 0.6V, Q1 turns on, Q2 turns off and the

output signal rises until D1+D2 clamp the output signal at 1.5V (white level).


When the video input level is above 1.5V, Q3 turns on and clamps the video

output to 0.8V (black level). Diode D3 prevent Q3 from saturating which

speeds up video switching times to makes the characters nice and sharp.  


The ZX Video Inverter V1 output is dc composite video, 1.5V p-p from white

level to sync tip with the black level at 0.8V and screen characters are white

on black. The inverted Video Out signal can be connected to a monitor input, 

with or without a 75 ohm termination, and the inverted Video Output is not 

harmed if accidentally shorted to ground.



ZX81 Video Inverter V2


The ZX Video Inverter V2 circuit uses 9 - 11 components including a 74HC86

XOR gate and features selectable Norm/Inverted B/W video characters.



The most interesting part of this design is the use of a reduced supply

voltage for the 74HC86 chip to lower the input threshold to about 1V.


A 100 ohm current limiting resistor (R1) plus a red LED provide a simple

shunt regulator to supply filtered 2V to pin 14 of the chip. The LED

can double as a ZX81 power on indication.


Only two of the four XOR gates are actually used for this circuit.


One gate is used as the sync separator and its output is either sync or

white level. The second gate separates the character data and its output is

either white or black level. These two output signals are mixed into a three

level composite video output signal with sync at 0V, black level at 1V and

white level at 1.4V.   


The video signal from ULA pin 16 is about 1.8V p-p. It is normally connected

to the TV modulator, attenuated by resistors and diodes on the ZX81 PCB to

make it compatible with the modulation level required for different countries.


One XOR gate is used as a non-inverting sync separator (pin 2 grounded).

The video signal from ULA pin 16 is connected directly to pin 1 of U1.


When the video signal on pin 1 is 0V (sync), this causes pin 3 to go low.

When the signal at pin 1 is most positive (1.8V/white level), pin 3 is high.

When pin 1 is at black level (1.2V), this is more positive that the threshold

of pin 1 and the sync separator output on pin 3 is high (data is ignored).


When pin 3 goes low, D2 clamps the base of the output transistor to 0.6V,

which results in 0V sync level at the video output.  Resistor R5 is optional

if the monitor input is internally terminated in 75 ohms. 


When the ULA pin 16 signal is above the threshold (>1V), the sync separator

output pin 3 goes high (2V) and this signal, through one side of R2 drives

the base of the transistor which drives the video output to white (~1.4V).


The ULA pin 16 video signal is also connected through D1 to pin 4 of the

second XOR gate.  This shifts the signal to 1.2V p-p white level to sync tip

with the black level at 0.6V.   Since the black level is now less positive

than the input threshold of pin 4, the output on pin 6 of the XOR gate will

switch accordingly when the video data is white or black. The output signal

on pin 6 is mixed with the sync signal from pin 3 the generate the

composite video output signal, using R2 to adjust ratio of sync to black

level to set the contrast. Adjust so that the contrast optimum and minimum

adjust is required for the monitor controls when switching from normal to

inverted video characters.  


The data XOR gate is also used to select the normal or inverted video mode

By switching the voltage on pin 5 to 0V or 2V.  The VHF/UHF channel select

switch, which has an output of 0V or 5V, can be connected to pin 5 using

series R4 to limit the input current as the 5V output exceeds the XOR gate

internal input protection diode clamping level. If a separate switch is added

to select between ground (0V) and 2V, then R4 may be eliminated.


There it is: two simple circuits to enhance you ZX81 computing experience.