DMTH4
LECTROSONICS, INC.
12
0
0.02
0.04
0.06
0.08
0.1
0
10
x/x0 [dB]
time [sec]
Dynamic Compressor
Signal dynamics vary widely even in simple audio
conferencing applications, requiring different attack and
release timing for optimum performance. The DMTH4
provides a new DSP-based compressor that addresses
this issue very effectively, and at the same time simpli-
fies the setup. A true RMS level detector is used to
assess signal levels, so the values are accurate and
consistent regardless of the waveform of the signal.
Whether the audio signal is normal voice, loud bursts, a
sine wave for testing or random pink noise for adjusting
levels, the compressor will track the signal and apply
optimum attack and release times.
An RMS detector handles signal dynamics in much the
same way human ears and eyes handle abrupt tran-
sients. As a loud peak hits the ears, or a bright flash
of light hits the eyes, the muscles contract very quickly
to suppress overload. As the signal decays, muscle
tension reduces gradually as the senses recover. The
DMTH4 compressor effectively acts in the same manner.
A dynamic, variable attack time is applied by the DSP to
apply an optimal rate as the signal transient arrives. The
release time then follows a longer time constant as de-
fined in the system setup. Both attack and release times
are dynamically controlled to track the RMS value of the
signal level as it passes.
To simplify setup, a single time constant value is entered
to set the desired release time. There is no need to
specify separate attack and release times and try to pick
good average values. A value of about one half of the
desired release time is specified, and the attack time is
taken care of dynamically by the DSP.
In the example shown here, a value of 100 ms (the
default) is specified, which correlates to about 200 ms of
equivalent release time (a good value for most applica-
tions). The DSP controlled attack time varies to follow
an optimum curve as the signal level rises. At first, it is
the equivalent of about 2 ms, then as the signal nears
full level it slows to to about 100 ms. The release time is
also dynamically controlled and follows the defined rate
as set in the control panel. The overall envelope is very
transparent with no audible artifacts.
The RMS detector approach is used throughout the
DMTH4 for processes such as automatic mixing and
level metering in addition to the compressor action.
DSP controlled, variable attack time is based upon values
generated by an RMS detector to follow the ears natural
handling of signal transients.
T
RMS
= 100 ms
T
ATT
= 67 ms
T
ATT
= 40 ms
0
0.5
1
1.5
2
0
10
20
30
40
50
x/x0 [dB]
time [sec]
T
REL
= 200 ms
T
RMS
= 100 ms
The RMS detector dynamic release time set at 100 ms is
approximately the same as a fixed time constant of 200 ms.