Circuit Notebook 82 - Oscilloscope Converter for Narrow Band Television

This circuit will allow a NBTV (Narrow Band Television) picture to be displayed on an oscilloscope.

The circuit has been produced for use with a Tektronix 2215 oscilloscope and should be suitable for use with other oscilloscopes which have following features:

The NBTV signal (which conforms to the NBTV Association Specification) [1] has 32 scanning lines, each line occupying 2.5 ms and a field rate of 12.5 fields per second. Line scanning is vertical, from bottom to top, starting bottom right. Field scanning is from right to left. Video modulation is positive-going with negative-going sync. Each picture line (except one) ends with a line sync pulse of around 100-250 us. The end of a field scan is defined by the absence of a line sync pulse, known as 'missing sync'.

Circuit description

The circuit operates from +5 V and -5 V supplies and is shown in Fig. 1. Consider first the scanning generators [2]. There are two scanning generators, IC3A with IC5 and C5 for the field scan and IC3B with IC6 and C6 for the line scan. The circuits are identical except for the values of C5 and C6. The op-amp IC5 has capacitor C5 connected between the output and the inverting input. A fixed positive current to the inverting input is defined by R9 and an equal and opposite negative charging current through C5 is provided by the op-amp output.

The result is an extremely linear negative-going output voltage that forms the scan output. The CMOS switch IC3A (part of a 4066 quad CMOS switch), produces the return scan or 'fly-back', when it is in the 'on' state, by rapidly discharging C5. When it is in the 'off' state it allows the scan to resume. The circuit is not free-running but is controlled by the incoming signal.

Now consider the input circuits. The incoming NBTV signal passes to the emitter-follower TR1. The signal is then coupled by C9 and DC restored by D1 which conducts on the bottom of each line sync pulse and clamps it to nominally -0.6 V. IC1 is used as a sync clipper. RV1 is set so that the sync is clipped at approximately half-way up the sync pulse. Clean sync pulses are fed to IC2A and IC2B the 4528 dual re-triggerable monostable. IC2A is triggered on the negative-going edge and IC2B on the positive-going edge.

IC2A is used as a missing pulse detector [3] [4] to generate a field reset pulse. The monostable period is set to approximately 3.5 ms. In this arrangement, the incoming line sync pulses keep retriggering the circuit and the monostable only completes its timing period when a line sync pulse is missing. Hence, it functions as a 'missing pulse' detector. During its timing period IC3A is 'off' and the field scan takes place. When the timing period ends IC3A is 'on' and the field fly-back occurs.

IC2B is triggered on the positive-going edge of the line sync pulse, causing IC3B to be held 'off' for 2.3 ms and the line scan to be generated. At the end of the timing period, IC3B is turned 'on' and the line fly-back takes place. It may seem strange that the monostable is used to define the scan period, rather than the fly-back, but this ensures that during the 'missing sync' line the output is held at 0 V.

The circuit, as shown, generates negative-going scanning signals having an amplitude of 2 V p-p. These can easily be inverted to suit other oscilloscope requirements by taking, for example, R9 and/or R10 to -5 V instead of +5 V.

The video signal is taken to the inverting amplifier IC4 to provide a negative-going video signal for the oscilloscope 'X' input. The value of R8 can be changed, if necessary, to alter the signal amplitude. Although IC1, IC4, IC5 and IC6 are shown as individual op-amps they could be replaced by a quad version.

In this circuit, a DC-DC converter NMA1205D (Farnell 330-887) is used to generate the +5 V and -5 V supplies from the more usual 12 V supply.

The results are shown in Fig.2. The rather squashed picture is caused by the oscilloscope controls not being correctly adjusted. The correct NBTV aspect ratio should be 3:2, vertical to horizontal.

The NBTV Association produce several CDs, for their members, containing NBTV pictures and test signals. Details from the NBTV web site, http://www.nbtv.wyenet.co.uk

References

[1] NBTV Standards, NBTV Handbook, Chris Lewis, Grant Dixon & Klass Robers NBTV Association. Page 56.

[2] Low-frequency Sweep Generator, IC Op-Amp Cookbook, Walter G. Jung ISBN 0-672-22453-4, page 485.

[3] Missing Pulse Detector, 555 & 556 Applications Data-Sheet, Signetics Corp.

[4] 4528 Dual Re-trig Monostable, CMOS Cookbook, Don Lancaster, ISBN 0-672-21398-2, page 250

Figures

Fig.1. Circuit of Oscilloscope Converter for NBTV

Fig.2. NBTV Picture