Circuit Notebook 89 - Audio Level On-screen Display

This is a development of the 'Flashing Cursor for ATV' in Circuit Notebook No. 87 and the 'Bar Display Audio Level Meter' in Circuit Notebook No. 88 which appeared in CQ-TV 212 and CQ-TV 213 respectively.

The on-screen display consists of a highlighted narrow vertical column at the left-hand side of the screen, the height of which indicates the level of the incoming audio signal. The display covers a range of over 24dB and has a fast rise and slow fall characteristic which can be altered by selecting component values.

Circuit description

A diagram indicating the sections of the circuit which affect the position and size of the display is shown in Fig. 1. The full circuit is shown in Fig. 2. IC1 is a sync separator, IC2 and IC3 are dual monostable ICs.

The audio input signal is fed to op-amp IC4, this amplifier has an adjustable gain of 0 to 40dB (x1 to x100) to bring the incoming signal to the required level. TR1 operates as an emitter-follower peak rectifier, conducting on the negative peaks of the incoming signal.

Capacitor C3 becomes rapidly charged to the peak value of the signal waveform causing a current to flow through R7 to the current mirror [1] formed by TR2 and TR3. This circuit has the property of causing a current to flow in the collector of TR3 which is equal to the current flowing through R7. In this way it converts the voltage across C3 into a current flowing 'down' from TR3 into IC2a. R8 is included to maintain a small 'bleed' current for the current mirror. The rise/decay time can be altered by changing the value of C3.

The charging current flowing into IC2a, together with C8 define the timing period of the monostable circuit. The greater the amplitude of the incoming audio, the shorter will be the timing period of IC2a. IC2a controls the top of the on-screen column thus making the height of the column proportional to the audio level.

Video processing

Moving on now to the video processing, the video input signal passes to the sync separator IC1 which provides line and field synchronising signals. The line signal triggers IC3a which generates a delay, set by RV3, to control the position of the display with respect to the left hand side of the screen.

The field signal from IC1 pin 3 triggers both IC2a and IC2b. As mentioned above, IC2a controls the position of the top of the display column. IC2b together with RV4 sets the position of the bottom of the column.

IC3b generates pulses which form the column display. Delayed line trigger signals from IC3a are fed continuously to IC3b, but it is held in a reset condition until the end of IC2a timing period (determined by the audio level) after which IC3 generates pulses to form the column display. This continues until the end of the delay period set by IC2b and RV4, at which point the triggering of IC3b is inhibited, thus setting the bottom of the column display.

The output from IC3b is a positive pulse of about 1.5us which is superimposed on the video waveform through D1 and R9. This pulse can be seen at the top of the line staircase waveform shown in Fig. 3.

A strip of masking tape was fixed to the monitor screen and marked in 3dB intervals, shown by the column height, as the input signal (1kHz sine-wave) was varied over a range of about 30dB, The result is shown in Fig. 4.

Power supply

A 12V supply is required for the 741 op-amp, although a more expensive op-amp with a 'rail-to-rail' output might just cope with a 5V supply. The signal voltage at the output of IC4 is about 3V p-p for full scale display. A 5V regulator, IC5, is included to feed the digital parts of the circuit.

Setting up

Set RV4 to maximum resistance so that initially the full length of the column is displayed. Set RV3 to position the column a suitable distance from the left hand side of the screen. With no audio input signal, adjust RV2 to set the top of the column near the bottom of the screen. Apply a suitable steady audio signal and adjust RV1 to set the top of the column near the top of the screen. Adjust RV4 to set the bottom of the column to a point a little below the zero signal position. The width of the column may be changed by changing the value of R14.

References

[1] Current mirror 'Microelectronics', Millman & Grabel, McGraw-Hill, p124.

Figures

Fig.1. Circuit components controlling the on-screen display

Fig.2. Circuit diagram

Fig.3. Video waveform with vertical column pulse added

Fig.4. Screen calibration of display