The CGS408 is a utility module providing the user with a 10-LED VU Meter useful for monitoring signals within the system. It has 2 operational display modes selected by an internal jumper:-
- in BAR mode all the LEDs below the current 'display point' remain illuminated forming a 'vertical bar' with the top of the bar indicating the signal level,
- in DOT mode only the 'display point' LED is illuminated.
The CGS408 has 2 'meter' modes:-
- VU Meter, and
- Peak Program Meter
VU Meter
The audio level meter most frequently encountered is the VU meter. Its characteristics are defined as the ANSI specification C165. Some of the most important specifications for an AC meters are its dynamic characteristics. These define how the meter responds to transients and how fast the reading decays. The VU Meter is a relatively slow full-wave averaging type, specified to reach 99% deflection in 300mS and overshoot by 1 to 1.5%. In engineering terms this means a slightly underdamped second order response with a resonant frequency of 2.1Hz and a Q of 0.62. The CGS408 VU mode meets these criteria.
Peak Program Meter
The VU meter, originally intended for signals sent via telephone lines, has shortcomings when used in high fidelity systems. Due to its slow response time, a VU meter will not accurately display transients that can drive an amplifier into clipping. The fast-attach peak program meter (PPM), which does not have this problem, is becoming increasingly popular.
While several European organizations have specifications for peak program meters, the German DIN specification 45406 is becoming a de facto standard. Rather than respond instantaneously to peak, however, PPM specifications requires a finite "integration time" so that only peaks wide enough to be audible are displayed. DIN 45406 calls for a response of 1dB down from steady-state for a 10mS tone burst and 4dB down for a 3mS tone burst. These requirements are consistent with the other frequently encountered specification of 2dB for a 5mS burst and are met by an attack time constant of 1.7mS
