Paper 1: The Use Of Look-Ahead Limiters In Loudspeaker Driver Protection

The limiter in a loudspeaker processor is the last line of defence in protecting the speaker drivers
from damage, and as such it has a very specific and critical job to do. One of the chief modes of
loudspeaker failure is driver over-excursion, and unless the limiter is designed to act instantly in
response to sudden increases in level, it will allow through brief transients that can cause damage
through over-excursion. All dynamics processors take a finite amount of time to respond to a
change in input level, and unless additional steps are taken the result is that the input signal is
initially let through at anything up to its full level, until the gain element in the limiter can act in
response to the increase in signal level.

The DN9848 exploits the fact that digital signal processing works on a sample-by-sample basis (the
signal data samples are clocked through the unit at the sample rate of 48 kHz) and that there is a
small delay through each processing block, and literally ‘looks-ahead’ further back in the signal
chain to sample the data for the limiter side chain, so that the limiter can apply the required gain
reduction on an instantaneous sample-by-sample basis, so that the limiter never lets through any
dangerous transients.

In the example below a 10 kHz tone burst of 10 ms duration has been used as the input signal and
the output of both a DN9848 and a leading competitor are shown. Note the very large transient of
the competitor unit which does not have a look-ahead limiter.

Signal source: Audio Precision System One
Settings: Waveform: Burst - Normal.
Frequency: 10.0 kHz.
Burst: 10 ms.
Interval: 100 ms.
High Level: +10.0 dBu
Low level: -40.0 dBu
DN9848 settings:
HPF: 1.25kHz Lnk-Ril 24dB/Oct.
LPF: 20kHz Lnk-Ril 24dB/Oct.
Limiter Threshold:0.0dBu
Release: 50ms
Response: Hard Knee

Competitor settings:

HPF: 1.26kHz Lnk-Ril 24dB/Oct.
LPF: 22kHz Lnk-Ril 24dB/Oct.
Limiter Threshold: 0.0dBu
Attack & Release: Automatic

All other settings are default on both units.

As with all units that use sigma-delta ADC and DAC converters, there is a propagation delay from
input to output, 3.2 ms for the DN9848 and 2.1 ms for the competitor unit, the additional delay in
DN9848 is caused by the use of sigma-delta converters for both analogue-to-digital and digital-to
analogue conversion (the competitor unit uses a different method of digital-to-analogue
conversion), which allows the DN9848 to achieve its superior dynamic range.

In order to prevent driver failure, the competitor product’s limiter threshold needs to be reduced
such that the peak of the transient is at the same level as the threshold of the DN9848’s lookahead
limiter, with a major effect on efficiency of speaker systems, as the effect of reducing the
limiter threshold is to limit the amount of continuous output power available, which means more
amplifiers and more speaker cabinets to achieve the same SPL. In the example below the
competing unit’s limiter threshold has been reduced so that level of the transient peak matches the
threshold of the DN9848’s limiter:-

The limiter threshold of the competitor’s unit has been lowered to -10.0 dBu to avoid the risk of
driver damage from the initial transient, at the cost of greatly reducing the efficiency of the PA
system. The look-ahead capability of the DN9848’s limiters allows the thresholds to be set at the
levels required to protect the loudspeaker drivers, without the need to be concerned about
transients being passed by the limiters. This allows the PA system performance to be maximised
by safely exploiting the full operational range of the loudspeaker drivers.