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Narrow Bandwidth Television Association: NBTV Sawtooth Test Generator

This document describes a sawtooth test signal generator circuit for narrow-bandwidth television (NBTV) that produces a 1 volt output signal with 31 line sync pulses conforming to the NBTV standard. The circuit uses a 7555 timer and LM317 regulator to generate a linear sawtooth waveform and a 4020 counter to produce sync pulses at a rate of one every 32 lines by differentiating and gating the falling edge from the counter. The sawtooth and sync pulses are then mixed to produce the full NBTV test signal.

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Sebastian Costa
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104 views2 pages

Narrow Bandwidth Television Association: NBTV Sawtooth Test Generator

This document describes a sawtooth test signal generator circuit for narrow-bandwidth television (NBTV) that produces a 1 volt output signal with 31 line sync pulses conforming to the NBTV standard. The circuit uses a 7555 timer and LM317 regulator to generate a linear sawtooth waveform and a 4020 counter to produce sync pulses at a rate of one every 32 lines by differentiating and gating the falling edge from the counter. The sawtooth and sync pulses are then mixed to produce the full NBTV test signal.

Uploaded by

Sebastian Costa
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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25/11/2014 Sawtooth

Narrow­bandwidth Television Association

[ Home ] [ Contents ] [ Previous chapter ] [ Next chapter ]

Chapter 13

NBTV Sawtooth Test Generator
To get the best from an NBTV signal it is important to use the full dynamic
range of the signal with no crushing at black level or peak white. To check
the linearity of a video path it is usual to use either a staircase or sawtooth
test signal. The circuit described is a free standing NBTV sawtooth generator
conforming to the standard of 1volt output level and complete with 31 line
sync pulses.

The 7555 timer and LM317 regulator form a linear sawtooth generator. Two
timing capacitors are employed, one setting the active line time which may
be trimmed by the pot, the second sets the line sync duration. The circuit is
un­usual in that the LM317 performs two roles. It acts as constant current
genera­tor to linearly charge the 0.22 mF capacitor, and also provides a
very low output impedance.
The 4020 counter is used as a divide­by­32 fed from line pulses. The

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25/11/2014 Sawtooth

grounded base transistor differentiates the falling edge from Q5 of the 4020
and gates out one sync pulse in 32, the output from the gate being non­
inverting. The remaining 31 sync pulses are mixed with an attenuated
sawtooth to form a standard 70%­30% 1 V NBTV signal and delivered at a
nominal 470 Ω impedance.
The circuit is sensitive to variations in the supply rail and the prototype was
driven from another LM317 set to a nominal 8.5 V allowing use of a 12V
supply. If a regulated supply is employed there is no need for this second
regulator.

[ Home ] [ Contents ] [ Previous chapter ] [ Next chapter ]

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