Dolby HX Pro was introduced in the early 1980’s and provides high frequency headroom extension for analog tape recordings. Because it has the Dolby name and is included in most modern tape recording machines many people have erroneously assumed it is a noise reduction system, which it isn’t. It dynamically adjusts the record bias level to counteract a phenomenon known as “self biasing” that can take place when recording hi frequencies to analog tape. The result is greatly improved headroom, which can be used to record at higher levels thus improving the signal to noise ratio. HX Pro is found on all sorts of mid-level professional and consumer recording machines these days. It is a record only process (no decoding is required) so most machines have it “on” all the time with no way to turn it off. Since few, if any, negative side effects have ever been reported from using HX Pro most people are happy to use it on machines that are so equipped.