You may remember the physical slide switch on the bottom, which switched the hum filter in and out. Tempo removed that switch many years ago and changed to using just the push-button and a microprocessor which also implemented the hum filter in place of the old charge-coupled device delay line; this allowed us to create a product that performed equally or better and reduced power consumption. Later, Tempo created the 200XP version of the product, which added a narrowband filter mode. This is ideal for use in environments where there may be excessive electrical noise such as switch-mode PSUs in equipment racks or near motor controllers, etc. The harmonics from these sources are often not directly related to local mains power frequency so the hum-rejection comb filter cannot reject them. Tempo added the narrowband mode to 200XP that rejects most of the interfering noise, allowing the user to concentrate on the pair-tracing tone