Tuned filter

In signal processing, a tuned filter is a stage in the processing channel which accepts or rejects signals which are tuned for a specific type.

History

Historically, the concept of tuning was to maintain a specific musical scale. When choruses sing "in tune", the music reinforces itself with higher harmonics; aesthetically, the notes are more pleasing to the ear. But when signals are "out of tune", "dissonance" occurs; the effect is most noticeable for musical groups of small children, who have not been trained to sing in tune.

Radio

Tuning became important for the development of radio broadcasts. Stations learned to broadcast at a specific frequency, and to share the bandwidth of the frequency spectrum. Thus radio receivers had to have circuitry to tune to a specific carrier frequency. This was accomplished by creating electronic filters which could accept specific frequency ranges, or tuned filters.

Television and video

The specific techniques for audio broadcasting transferred naturally to television. However, as video techniques became more wide-spread, the types of tuning went beyond the simple radio and audio frequency ranges, to specific spatial discrimination of spatial frequencies and textures.

Cognitive tuning

David Hubel and Torsten Wiesel discovered that mammalian brains contained specific neural columns which were attuned to specific spatial frequencies, specific colors, specific shapes, specific motions (up and down, left and right, inward and outward, etc.). Thus the brain could be viewed as a collection of tuned filters.

Their discovery occurred accidentally. In 1962, Hubel and Wiesel were attempting to discover that triggered neurophysiological activity in a cat's brain. The cat they were testing showed no results until they displayed a "crack" to the cat, whereupon the cat's brain starting reacting to the stimulus. This began a long series of discoveries which continues to the present day. Hubel and Wiesel received the Nobel Prize for their discovery.

Primate brains like our own display even more arcane forms of tuning, which are occasionally discovered by fMRI or PET scans of impaired patients. These range from the recognition of faces, to the recognition of emotions, to other social, cultural, linguistic skills, etc.