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
