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Protection > APP 3877

Aug 21, 2006

Keywords: ALC, AGC, notebook, portable, audio,
dvd, class D, amplifier, automatic, level. control

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APPLICATION NOTE 3877

ALC Improves Sound Quality While Protecting
Speakers

Abstract: A
problem for current notebook computers and portable DVD audio
is the limited dynamic range of available speakers. The
speakers are typically small due to size restrictions, and
therefore capable of producing only modest levels of sound
pressure over restricted frequency ranges. This application
note explains how automatic level control (ALC) can help
eliminate the audio problem posed by small speakers.

Attend
this brief webcast by Maxim on
TechOnline

Introduction
For an operating-system alert or similar
audio source,
the typical dynamic
range is sufficiently limited so that a user can turn up
the volume with no concern that the dynamic peaks will cause
distortion.
However, the limited dynamic range of notebook computer
speakers presents quite apparent problems for a DVD soundtrack
with a relatively wide dynamic range. The difference in volume
between dialog and sound effects can be significant,
compelling users to increase the volume to hear dialog and
reduce the volume to prevent clipping during loud
scenes.

Without adjusting the volume, the user must
compromise. You either pick a setting too high for the dynamic
peaks and live with the distortion, or pick a setting that is
too low for understanding the dialog. For notebook computers
with small speakers, this audio problem can render the
computer almost useless for watching DVDs.

Theory of Automatic Level Control
An amplifier with
automatic level control (ALC) can help eliminate the audio
volume problem posed by small speakers. The amplifier can not
increase its maximum output voltage without increasing the
supply voltage, and can not increase the power-handling
capability of a small speaker. The amplifier can, however,
make dynamic changes in the RMS
output voltage while playing audio. If peaks in the audio
waveform (above a predefined threshold) are reduced to a level
more closely matching the rest of the audio signal, you can
then increase the overall signal volume without clipping the
peaks. Known as compression or limiting, depending on the
compression ratio, this technique is commonly practiced
throughout the audio industry.

A small compression
ratio (2:1, for instance) reduces a 4dB increase at the input
to a 2dB increase at the output for signals above the
compression threshold. Larger compression ratios (20:1 or
greater) are referred to as limiting, because once the
threshold is reached, the output waveform remains at the same
amplitude regardless of increases at the input. Compression is
typically applied at the recording and mixing stages, but can
also be applied later in the audio stream.

As
implemented in an amplifier like the MAX9756, ALC acts
essentially like a limiter. It detects output signals above a
set threshold, and reduces gain to keep the output below that
threshold. The MAX9756's gain response above the threshold is
nearly flat, indicating a nearly infinite compression ratio
(Figure 1).

Figure
1. For low levels of volume, the amplifier operates as a
normal linear amplifier. For levels above the threshold, it
reduces gain to prevent the output from exceeding the
threshold. It limits this gain reduction to 6dB maximum, after
which the output voltage increases above the
threshold.

Automatic Level-Control Timing
The speed at which a
limiter reacts to changes in signal amplitude can dramatically
affect the sound. When the output signal surpasses its
threshold, the amplifier reduces gain at the rate specified by
the attack time. Gain is then held at a reduced level until
the signal amplitude decreases. Release time specifies the
rate at which the amplifier increases the gain, eventually
reaching its original value. For amplifiers like the MAX9756,
attack time is specified by the value of the capacitor
connecting the CT pin to ground. The following equation
derives attack time from that capacitor value:

Release
time is calculated as a ratio to the attack time, and is
adjusted by varying the voltage applied to the DR pin. One of
three ratios can be selected by applying VDD,
VBIAS, or GND to the DR pin. The MAX9756 adds a
fixed hold time of 50ms to all release-time values. No gain
adjustments are made during the hold time.

Figure
2 shows the effect of a large signal pulse occurring
during an otherwise small-amplitude signal. The amplitude of
the large pulse in the output waveform is clearly reduced as
the gain ramps down. The gain-reduction control voltage is the
voltage across the capacitor at CT, which sets the attack
time. That voltage is proportional to the gain reduction (in
dB) at a given time.

Figure
2. The full cycle of ALC can be seen by applying a brief large
signal. The hold time of a MAX9756 amplifier is set at 50ms,
and the attack and release times can be adjusted by external
components.

The length of attack and release times
should correspond to the source material and the desired
effect. For example, short time constants cause the ALC to
rapidly follow changing signal levels and thus protect against
even the shortest spikes in signal level. This action ensures
that large signals do not damage loudspeakers, but it can also
allow audible artifacts like 'pumping' and 'breathing' as the
gain rapidly adjusts to follow the signal dynamics.

For
a movie soundtrack with constantly changing signal levels,
longer time constants prevent artifacts and maximize the sound
quality. The gain in that case is held relatively fixed during
rapid changes in signal level, and adjusts only when a
long-term change gives the amplifier sufficient time to react.
Speaker protection remains intact, because the ALC still
reduces most of the signals that could cause
damage.

You can observe the effect of long vs. short
attack and release times by monitoring the gain-reduction
control voltage and the signal waveform (Figure 3). To
produce representative waveforms, the input signal is a
full-volume audio signal. Short attack and release times cause
frequent gain adjustments during a passage that, overall, has
relatively constant signal strength. Longer attack and release
times maintain a smooth gain response that prevents the
amplifier from overreacting; the overall signal level is thus
maintained and more of the signal dynamics
preserved.

Figure
3. Short attack and release times (a) cause frequent
gain changes that can degrade the sound. Longer attack and
release times (b) produce a smoother gain
response.

ALC Threshold
The speaker amplifiers in notebook
computers often operate from a 5V supply. With an 8Ω speaker
driven in a bridge-tied
load (BTL)
configuration, the maximum available continuous power is
theoretically:

Because
of the 5V supply, any attempt to deliver more power to the
load produces a clipped waveform. Amplifiers like the MAX9756
let you adjust the gain-reduction threshold by selecting the
resistor connecting the PREF pin to ground. (The MAX9756
injects a constant current
of 12µA into the resistor.) You can calculate the value of
this resistor (for a 1.4W threshold in this case)
by:

You
can change the threshold to any desired level by adjusting the
value of RPREF. In some cases the sound system is
limited by the loudspeaker's power-handling capability. In
those instances, you can set the threshold considerably lower
than the amplifier's maximum output power and thus ensure that
the power-handling capability is not exceeded. If the
loudspeaker can handle the amplifier's full output power, then
setting the threshold just below the clipping level maximizes
sound quality and protects the loudspeaker from long-term
damage. Clipped waveforms not only sound bad, but they can
eventually cause permanent damage to a loudspeaker. The sharp
edges of a clipped waveform are difficult for the mechanical
elements of a loudspeaker to reproduce, and over time can
cause failure.

Figure 4 shows the effect of
setting the threshold just below the amplifier's maximum power
output. The input signal is a sinusoidal burst that alternates
between high and low levels. The output waveform clearly is
clipped during the attack time, but clipping ceases as soon as
the gain reduction is complete.

Figure
4. During this transition from small signal to large signal
the output waveform initially clips. As gain reduction ramps
up, the output waveform returns to the desired unclipped
state.

Reducing the Compression Ratio
Limiting, as provided
in the MAX9756, can be useful for protecting speakers and
preventing clipping, but it completely eliminates dynamic
changes while ALC is in effect. The infinite compression ratio
means that an increase in the input signal strength has no
effect on the output, thereby producing a lifeless and
monotone audio sound. If strict control of the output waveform
is not required, a lower compression ratio can help prevent
clipping while maintaining some dynamic content. Lower
compression ratios reduce the audio-signal dynamics rather
than completely eliminating them. The MAX9756 compression
ratio can be lowered by adding external circuitry, as shown in
Figure 5.

Figure
5. Adding an op amp like the MAX4400 and a resistor,
R2, to the external circuitry of the MAX9756
reduces the ALC compression ratio.

The MAX4400 op
amp buffers the MAX9756 CT output, thus ensuring that the
added circuitry does not affect the CT voltage and, hence, the
release time. The MAX4400 features a typical input impedance
of 1000GΩ, which eliminates leakage current that could
discharge the capacitor prematurely. The op amp output is fed
into PREF through R2. Thus, the parallel
combination of R1 and R2 determines the
new ALC threshold. The following equation determines the
threshold in this configuration:

Just
before the threshold is reached, the impedance seen by the
PREF pin is the parallel combination of R1 and
R2, because both are connected to GND at that time.
Thus, setting this parallel combination sets the threshold for
ALC in this configuration. The right side is the previous
formula (Equation 3) for calculating RPREF, and the
left-hand side is the equation for resistance
of the parallel combination of R1 and R2
(Equation 4).

The ratio of R2 to
R1 contributes to the new compression ratio. When
R2 is much larger than R1, the ALC has a
high compression ratio similar to that of the MAX9756's
standard hard-limiting configuration. When R2 is
less than R1, the ALC has a low compression ratio
and maintains most of the original dynamic range in an audio
signal. Therefore to achieve a useful compression ratio of
3:1, set R2 to 2.5 times the value of
R1. Figure 6 shows the effect on voltage
gain of the MAX9756 with standard limiting and the compression
ratio resulting from R2/R1 =
2.5.

Figure
6. Comparing the MAX9756 standard limiting with the
compression achieved with external circuitry shows a more
gradual adjustment of gain, and elimination of the horizontal
portion.

Improvements Offered by ALC
The improvements
contributed by ALC are significant (Figure 7). Plot (a)
shows the output waveform during a particularly loud passage
from a DVD movie without ALC, and plot (b) shows the output
waveform for the same input and the same volume setting with
ALC enabled.

Figure
7. Without ALC (a), the output is severely clipped due
to a strong input signal. For the same input waveform with ALC
enabled (b), clipping of the output waveform is
eliminated.

Users can turn up the volume to
acceptable levels for dialog and not worry about other
passages being too loud. The sound quality, moreover, is
better because clipping is less likely. In addition, ALC
extends the life of speakers (less clipping) and aids in
protecting low-power speakers. The maximum volume setting is
6dB higher than that of the same system without
ALC.

ALC can also be implemented digitally. ALC can
possibly be achieved with existing DSP hardware, which can
further improve sound quality and speaker protection by
performing sophisticated processing such as multiband
compression. Though convenient, ALC in the digital domain
burdens a DSP system by requiring more cycles
and potentially more power. Incorporating analog
ALC into the speaker amplifier, as demonstrated above, is an
excellent compromise for notebook computer and portable DVD
players, where battery life is at a premium and sound quality
traditionally suffers.

A similar article appeared in
the November, 2005 issue of Portable Design.

Related parts

MAX9756

2.3W Stereo Speaker
Amplifiers and DirectDrive Headphone Amplifiers
with Automatic Level Control

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Download,
PDF format (170kB)
APP 3877: Aug 21, 2006

AN3877, AN 3877, APP3877,
Appnote3877, Appnote 3877

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