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MAX9756 Evaluation Kit
Step 3. Define the Attack Time
The attack time is the time it takes to reduce the gain
after the input signal has exceeded the threshold level.
The gain attenuation in attack is exponential and the
attack time is defined as one time constant. The time
constant of the attack is given by 15,000 x C CT sec-
onds (where C CT is the external timing capacitor).
Suggested attack time range is from 150μs to 50ms.
? Use a short attack time for the ALC to react quickly
to transient signals, such as snare drum beats
(music) or gun shots (DVD). Short time constants
will cause the ALC to rapidly follow changing sig-
nal levels and minimize the time when the signal is
large, but the gain has not yet been reduced. This
ensures that large signals will not damage loud-
speakers and will provide maximum speaker pro-
tection, which can result in audible artifacts such
as ‘ pumping ’ and ‘ breathing ’ as the gain is rapidly
adjusted to follow the dynamics of the signal.
? Use a longer attack time to allow the ALC to ignore
short-duration peaks and only reduce the gain
when a sustained increase in loudness occurs. For
movie soundtracks, where there is constantly
changing signal level, longer attack times are bet-
ter suited to maximize sound quality. In this case,
the gain is held relatively fixed for rapid changes in
signal level, and only adjusts when there is a long-
term change that gives the amplifier sufficient time
to react. This will maximize sound quality and still
provides speaker protection as the duration of
excessively large or clipped output signals is sig-
nificantly reduced in comparison to the uncom-
pressed output waveform. Avoid selecting the
attack time too long as it can result in some dam-
age to the loudspeaker under harsh conditions.
The MAX9756 EV kit provides capacitors C17, C18,
and C19 to set the attack time of the MAX9756 ALC cir-
cuitry. Jumpers JU10 and JU11 select the capacitance
at CT. See Table 6 for shunt positions.
Note that the attack time is related to the release time
by a ratio set by the DR pin. Understand this relation-
ship when selecting the capacitance of the external
timing capacitor (C CT ).
Step 4. Define the Attack to
Release Time Ratio
The release time is how long it takes for the gain to
return to its normal level after the input signal has fallen
below the threshold level and the 50ms hold time has
expired. Release time is defined as a release from 6dB
gain compression to 10% of the nominal gain setting
after the input signal has fallen below PREF threshold
and the 50ms hold time has expired. Release time is
linear in dB with time and is inversely proportional to
the magnitude of gain compression in dB. Release time
is adjustable between 99ms and 40.9s on this evalua-
tion kit. Customer optimized values of CT can extend
these limits further if desired.
Release and attack times are set by selecting the
capacitance value between CT and GND, and by set-
ting the logic state of DR (Table 9). DR is a tri-state
logic input that sets the attack-to-release time ratio.
? Use a small ratio to maximize the speed of the ALC,
providing best speaker protection.
? Use a large ratio to maximize the sound quality and
prevent repeated excursions above the threshold
from being independently adjusted by the ALC.
Table 9. Release to Attack Ratio
DR
V DD
V BIAS
GND
RELEASE/ATTACK RATIO
200
633
2000
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