Circuit Notebook 88 - A Bar Display Audio Level Meter

An audio level meter is usually fairly low down the priority list of equipment in the ATV shack. This is probably because most camcorders (used as the station camera) have a built-in automatic audio level circuit which provides a fairly constant level audio output. Once the audio sub-carrier deviation at the transmitter has been set, it can usually be left alone. However, if you use several different audio sources, VCR, DVD, separate microphone, etc. then an audio level meter is useful for setting deviation and avoiding reports of distortion or low sound.

Display Driver

The audio level meter to be described is based on the National Semiconductor Dot/Bar Display Driver I.C. It senses an analogue voltage level and drives 10 LEDs or other types of displays. There are three versions available, the LM3914, which provides a linear display proportional to input signal, the LM3915, which provides a logarithmic display and the LM3916 which provides a display equivalent to a 'VU' meter. Full details of the I.Cs are given in the National Semiconductor data sheet [1] which also includes lots of application information.

The LM3915 has been chosen for this level meter and each bar on the 10 bar display represents a change of 3dB in the input signal level. An application circuit is shown in Fig.1. It is possible to feed an audio signal directly into the LM3915 as it will respond to the instantaneous positive value of the waveform, but the results using this method are rather disappointing. Ideally, the input requires a d.c. voltage proportional to signal level, so the audio signal first needs to be rectified. The display driver can be configured by pin 9, to give either a single bar (pin 9 no connection) shown in Fig. 2, or a column of bars (pin 9 to +12V) as shown in Fig. 3. The bar display shown is intended for audio application and has the top three bars (8, 9, & 10) red and the others (1 to 7) green. The display is RS components 247-3107. Alternatively, any 10 bar display could be used.

Audio signal rectifier

There are plenty of rectifier circuits and 'absolute value' circuits which would be suitable, including those given in the data sheet, but most use op-amps which require both positive and negative supplies [2][3]. The intention was to operate everything from a single +12V supply and to avoid the use of a DC-DC converter for generating the dual polarity op-amp supplies. The complete circuit is shown in Fig. 4. The overall sensitivity for 'full scale' is -20dBm (77mV rms). At the input, R1, R2 & R3 provide an attenuator to allow -10 dBm and 0dBm to be accommodated. The audio signal is amplified by IC1 (741 or similar), the gain can be adjusted over a 20dB range by RV1, to allow calibration or setting up at some alternative or more convenient signal level. The output signal from IC1 is half-wave rectified by D1 and the peak value stored in C4. To compensate for the diode turn-on characteristic, a small positive bias voltage is provided by the voltage drop across D2 and the potentiometer RV2. In operation, RV2 sets the no signal condition to just below the illumination of bar 1 and RV1 the full scale with bar 10 illuminated. The relationship between signal level and illuminated bars is shown in Fig. 5. The accuracy is adequate down to about -30dB with slight errors below this.

The voltage across C4 is taken to the input of IC2. The 'attack', or rise-time of the display is quite short and the 'decay time' is about 0.3 seconds determined by R10 which is connected across C4.

On IC2, the display driver, R11 and R12 define both the current through each of the LEDs and the input sensitivity. With the values chosen, an input signal of +3.2 volts to pin 5 gives a full scale display (bar 10).

Operation

Connect the meter at the most appropriate place, for example, the audio input to the transmitter or the audio output of the receiver.

Calibrate the meter by setting RV2 so that under 'no signal' conditions, bar 1 is almost but not quite illuminated. Then, at the required level of signal, adjust RV1 for the appropriate bar to be illuminated. For example, you might set up your transmitter for full deviation and then adjust RV1 for illumination of Bar 10.

The circuit has only been constructed on a plug board so its susceptibility to RFI has not been checked. There are many web sites expressing strong opinions on average reading VU meters versus Peak Programme meters and how they should be calibrated. This level meter has been tested using a 1 kHz sine wave and is for general purpose amateur use.

References

[1] LM3915 Dot/Bar Display Driver National Semiconductor Application report www.national.com/pf/LM/LM3915.html

[2] Detector and rectifier circuits As above Figs 1, 2, & 3

[3] Precision rectifier circuits IC Op-Amp Cookbook, Walter G. Jung Howard W. Sams ISBN 0-672-22453-4 p.237

Figures

Fig.1. LM3915N Bar Display Driver

Fig.2. Bar display, single bar

Fig.3. Bar display, column of bars

Fig.4. Circuit diagram

Fig.5. Bar numbers versus signal level

Fig.6. Not included in the text. Close-up of Bar Display