Handbook for Sound Engineers

1002 Chapter 26

26.3 Wide-Range VU Meters

Standard VU meters measure only the upper 23 dB of

the signal level. From the practical standpoint, this

limits the display to about 20 dB below the reference

level of the 0 indication.

This short range of operation limits its usefulness,

particularly when it is connected across a bridging bus

for monitoring program information. A wide-range

program-monitor meter, Fig. 26-8, displays the program

information over a 60 dB meter scale, spread from

57 dB to +3 dB. The large spread of program material

permits the very low-level signals to be observed as

well as the noise between program pauses. The

wide-range VU meter was not designed to replace the

conventional VU meter; however, its characteristics are

compatible with the VU meter. In addition, a dc output

is provided for connection to a linear tape recorder for

logging program levels over a range of 60 dB. The 0 dB

indication may be set to represent a reference level from

22 dBm to +18 dBm.

The basic component is a logarithmic amplifier, Fig.

26-9, with a nonlinear feedback circuit, a preamplifier, a

15 k: bridging input transformer, a reference-level

selector switch, and a sensitive indicating meter

movement.

26.4 Bar Graph VU and Spectrum Analyzers

The United Recording Electronics Industries (UREI)

Model 970 Vidigraf is a bar graph display generator that

operates any National Television System Committee

(NTSC) standard video monitor or (with an inexpen-

sive accessory) black-and-white television receiver. The

system provides both a VU level display and the

frequency-spectrum-level information. It is designed

primarily for multitrack recording studio applications.

However, its dc to 20 kHz input capability suggests its

use for a wide range of dc or ac analog voltage

measurements.

The 970 Vidigraf’s modular construction provides

users with complete flexibility to adapt the system to

their specific needs. A maximum of four 16-channel

input display modules may be installed for VU level,

automation control voltages, or frequency-spectrum

viewing. Each module may be individually switched to

the video generator in the single mode. In the dual

display mode, the screen is split vertically to accommo-

date the information from any two input modules simul-

taneously. Instantaneous identification of the input

channel sources and/or frequencies, as well as vertical

scaling indices are automatically provided by the built-in

Figure 26-7. Relationship of VU and dBm to power in watts
and voltage in a 600 : line.