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Quantitative movement analysis

Krystian Nymoen discusses the advantages and disadvantages of different motion capture systems for empirical music research.
Quantitative movement analysis. In addition to qualitative analysis techniques such as the Labanotation and Laban Movement Analysis, there are also numerous methods of technologies for numerical recording or music-related motion, or what is often called motion capture. Very generally, we will talk about two main types of technologies. We have the camera-based systems and we have the sensor-based systems, and each of these have a few subcategories. First we are going to look a little bit more at the camera-based systems. The great thing about cameras is that they are an easy, fast, cheap, and not the least available solution to working systematically with the study of music-related motion.
Nowadays everyone has access to high quality video cameras even in their mobile phones, and the cost of professional quality video cameras is also within the reach for many. But in the world of motion capture, there are many other types of camera-based systems beyond the regular cameras. Here we often talk about two different types of variables that we need to take into account. The first one is whether we are going to use cameras that capture the same thing that we see– that is, regular light– or use cameras that record, for example, only the infrared light. Infrared cameras can, for example, be very useful if you want to study people dancing in a dark club or other types of dark environments.
The second we need to think about is whether we are using markers on the body or not. Markers can help identify people or specific body parts in a much more detailed way than a marker-less system can. Of course, putting markers on the body makes the whole setup much more complex. We’re later going to look at demonstrations of how both regular cameras and the more advanced camera-based motion capture system can be used to capture human motion. But first, let us talk a little bit about sensor-based motion capture. Sensor-based systems are built around sensors. And here, again, there are a number of different types of technologies such as acoustic sensors, mechanical sensors, magnetic sensors, inertial sensors, and electrical sensors.
Common to all of these is that they require to be put on the body of the people to be studied, which makes them less attractive for many musical applications. However, as the technologies are constantly getting smaller, faster, and more reliable, they are still very interesting to use also in a musical contexts. In this course we will demonstrate only one type of these systems– the inertial ones. But first, to conclude, there are a number of analytical approaches and technologies available for studying music-related motion, ranging from qualitative to quantitative, from cheap to expensive, from small to large, and from simple to advanced. It is impossible to give one answer to what type of method or technology to use.
They all have their strengths and their weaknesses. For that reason, it is important to decide on the right analysis method for the question at hand. And the, perhaps, most important factor that should be taken into account is whether to work in a controlled laboratory setting or in a more ecological setting such as a concert hall. This will, to a large extent, guide which methods and tools to use.
We can say that more specialised motion capture systems, like, for example, infrared marker-based systems or the electromagnetic systems, and various types of inertial sensors, often provide the highest recording speeds and also a very high spatial accuracy and precision. But such systems also come with several drawbacks. Price is one, although such systems have quickly become more affordable. More problematic is that the person being studied has to wear markers– sensors on the body– something that may be both alienating to the performer and obtrusive to the motion being performed. This is particularly problematic when using electromagnetic and mechanical systems with fairly large and heavy sensors and cables.
But also the lightweight reflective markers typically used in optical infrared systems are noticed by the performer, albeit to a lesser degree. The perhaps biggest challenge with the larger systems is the non-ecological setting they require, while small inertial systems, on the other hand, can be mobile, wireless, and more or less invisible, although the physical sensors need to be placed on the body. So if the aim is to study musicians in the real world concert situation, playing video recordings may be the only realistic solution.
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