Frequency Compression v. Frequency Transposition

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Answer

For the most part, we are talking about terminology differences: "Frequency Transposition" is a nontechnical, generic term to describe frequency lowering. Sonovation/AVR used this term in the early 1990's. The term Frequency Transposition was also used by others to describe instruments that used "Frequency Shifting" (another term that adds to the confusion). In an attempt to separate Sonovation/AVR's technology from the others, we started using the term Frequency Compression. Here are the main differences between "Frequency Shifting" and "Frequency Compression":

Frequency shifting is a technical term specifically relating to the use of a mixer (as in electric circuits not the kitchen appliance) to lower the signal by a fixed frequency value. The problem with shifting is that is does not reduce the bandwidth. It only shifts the signal down. This creates very strong distortions when the shifting frequency is greater than the signal frequency.

In comparison, Frequency Compression reduces both the frequency and the bandwidth by a preset ratio (factor). Because the spectrum is "squeezed" with Frequency Compression, operating in real-time requires a complex algorithm that maintains the critical information.

The graphics below demonstrate the differences.




Wendy Davis has been an audiologist for 15 years. She practiced as a clinical audiologist and educational audiologist for 6 years before joing Sonovation/AVR in 1994. Ms. Davis is involved in research and product development as well as training and clinical support for Sonovation/AVR. She can be reached at wedavis@avrsono.com.