A Series Active Monitor Speakers
Date printed: 29.02.00 Service E4/39
4.6.5 MLS Advantages
The maximum-length-sequence (MLS) test signal is a pseudo-random noise
with special properties which allow analysis of response without the vari-
ability commonly associated with noise-based measurements. The analysis
technique provides a high degree of immunity from interfering noise, allow-
ing accurate measurements when the interference actually exceeds the test
signal level. The MLS signal more closely resembles program material than
does a sinewave and will therefore measure under conditions approximating
normal use.
The MLS technique has several significant advantages over either basic
impulse testing or the TDS technique. The principal advantage of MLS over
impulse testing is in signal-to-noise ratio improvement. Compared to the
TDS technique, MLS testing has a number of advantages. To obtain low-
frequency response data, TDS must sweep very slowly. MLS has the same
testing time whether evaluating only the anechoic portion (first arrival) or a
longer portion of the signal for accurate bass response. With MLS, one ac-
quisition and correlation produces an impulse response which may then be
evaluated over and over to look at anechoic response, response of any se-
lected reflection, integrated room response, etc. The TDS technique requires
that the generator-to-bandpass filter frequency offset (delay time) be reset to
equal the acoustical propagation delay for each acoustical path to be meas-
ured. Sweep speed is critical for the TDS technique, and an operator may
easily select a speed which produces erroneous data without knowing he is
making an error. Ambient acoustic noise tends to be greatest at low frequen-
cies due to heating and air conditioning systems, traffic noise, etc. To im-
prove signal-to-noise ratios under these typical conditions, MLS testing is
normally used with pink spectral shaping (high-frequency attenuation) of the
generated pseudo-random noise. A complementary filter in the analysis pro-
cess produces overall flat response.
To accomplish the equivalent function in the TDS technique would require
logarithmic sweeps and a variable-bandwidth filter, which is more complex
and not done in available TDS instruments. Additionally, pink noise is
similar in spectral distribution to voice and music, so the heating effect on
individual loudspeakers of a multi-way loudspeaker system with pink MLS
noise is similar to that which occurs during normal operation. The TDS
technique (or any swept-sine method) concentrates all the input energy into a
single loudspeaker driver at any single moment during the sweep, which can
change driver characteristics during the test.
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