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meeting reports
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11/14/06
Meeting Highlights
by Nick Kettman
David Clark was recently invited to speak to the Chicago Section on the topic
of subjective and objective evaluations of automotive audio systems.
Mr. Clark founded the company DLC Design in 1977, and is currently the
Director of Research at Alpine Electronics. His presentation included
discussion of two techniques he uses to assess audio system performance:
Listening Technology (LiT), which is a subjective method used to predict a
person's perception of the system, and Perceptual Transfer Function (PTF),
which is an objective method capable of linking subjective traits to
measurable data.
The two goals of LiT are prediction and analysis. Predicting the
performance of a system can assist in determining, for example, the long term
user satisfaction. Analysis of a system results in an absolute
numerical ranking which can be used for comparing different audio
systems. The LiT technique is based on comparisons between the system
under test and an agreed-upon reference which embodies what most people consider
"good sound." In most cases, the reference is the sound
produced by the playback system in the recording studio. By choosing
this reference, the goal of an automotive audio sysetm becomes the
reproduction of sound as the artist intended it to be heard. A complete
system evaluation is divided into smaller tasks which include multiple
performance categories, such as tonal balance, spatial reproduction,
distortion, clarity, and multiple listening modes, such as foreground,
background, maximum impact.
During the second half of his presentation, Mr. Clark discussed the virtues
of the PTF method. The PTF is treated as a black box, with music as its
input and listener opinion as its output. Unlike traditional, often
ambiguous links between perception and measurement, the attributes of PTF
provide a way to get directly at the things which people hear and describe
when evaluating a system. For example, the PTF method links the
perceptual attribute of "stage" (width, depth, balance, etc.) to
the measurable attribute of precedence (inter-aural time delay, interaural
level difference, etc.).
Mr. Clark concluded his presentation with a short demonstration of the PTF
method. Using a special foam "head," various measurements
were taken around the room to show the effects of inter-aural level
differences and time delays.
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