Introduction - Apex Digital Argos User Manual

Introduction.

APPLICATIONS OF THE ARGOS
The Apex ARGOS is a versatile apparatus which can be applied either as a :
• Sound level limiter or
• Sound leveler.
Applied as a sound level limiter, the ARGOS guarantees that a certain preset maximum
level won't be exceeded either to meet legal requirements or just to protect audio-equip-
ment against damage caused by excessive sound levels.
Applied as a sound leveler, the ARGOS will take care that f.e. spoken messages throughout back-
ground music can be clearly understood. For this application, the internal filter of the ARGOS has
to be switched from "A"-weighted to "C"-weighted by replacing jumper J4 which is set to "A"-
weighted by default.
PRINCIPLE OF OPERATION
Once installed within an audio chain, the ARGOS makes it impossible to exceed a preset Sound
Pressure Level (SPL). The ARGOS doesn't uncouple the audio signal when the SPL is exceeded
but adjusts the signal gradually and continuous in such a way that the maximum is not exceeded.
This adjustment occurs in a quasi inaudible way. The dynamics of the signal are unaffected.
There is an important difference between a normal limiter and a sound level limiter.
A normal limiter is used particularly to prevent distortion of the audio signal if sudden peaks in the
signal appear. Each electronic circuit, recording medium or radiotransmitter can only handle a
restricted dynamic range. If the input signals exceed a certain value, distortion is the result (over-
modulation or clipping).
On the other hand, the purpose of a sound level limiter is to limit the sound level produced by a
loudspeaker. However, the human ear has not the same sensitivity to all audio frequencies. The
human ear is more sensitive to the mid-range (300Hz-3kHz) frequencies and less sensitive to low
or high frequencies. This is the reason why most amplifiers have a loudness-switch to compensate
for this effect. In the ARGOS, this sensitivity-difference of the human ear is taken into account in
the measurement of the sound pressure level. An "A"-weighted filter in the measuring chain passes
the mid-frequencies and attenuates the low and high frequencies. This way the measered result
correlates with the sound pressure as experienced by the human ear.
The ARGOS measures the sound pressure level in exactly the same way as the calibrated sound
level meters used by official authorities to check if a sound pressure level meets the legal restric-
tions.
The ARGOS measures the sound pressure level by means of an omni-directional microphone
which has a flat frequency range from 20Hz- 20kHz. This signal is amplified and passed through
an "A"-weighted filter. The RMS (root mean square) value of this signal is calculated and passed
through a logarithmic amplifier .
This way a value is obtained which is linearly proportional to the sound pressure expressed in dBA.
This value is compared to the preset maximum SPL. If the measured value is lower than the maxi-
mum , the audio signal present on the line-input is passed to the output unaltered. However when
the measured value is higher than the threshold, the audio signal is attenuated gradually until the
measured sound pressure level is the same as the maximum admissible. The dynamics of the
audio signal are retained in contrast with a normal limiter that would affect the dynamic contents of
the music. Because of the integrating nature of the servo loop, sudden peaks in the sound level of
the input-source have no effect on the operation.
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