Thursday, October 25, 2012

A Higher Reference Frequency For Selecting Cassette Tape Bias Level

A difference of 2 to 3 dB in MOL is already a big deal in the cassette tape recording world but are there any benefits of using a higher reference / test frequency in selecting the right recording bias level?

By: Ringo Bones

Even until well into the 21st Century, the reference frequency that is used by leading cassette tape and cassette deck manufacturers are either still 315 Hz or 1,000 Hz in order to set the MOL or maximum output level in choosing the proper bias current level when recording. But are there any benefits in choosing a much higher reference frequency when selecting the “optimum” recording bias level to obtain the chosen cassette tape’s MOL?

During a study conducted by their R n’ D engineers during the latter half of the 1970s, Hitachi – one of the leading Japanese cassette tape deck manufacturers of the time - determined that the bias level giving a peak output at 1,000 Hz is roughly 33 percent higher than that which gives a 5,000 Hz MOL for “Normal” ferric-oxide tapes, roughly 25 percent higher for chromium-dioxide tapes and 11 percent higher for ferrichrome tapes. Armed with such statistical information, Hitachi was able to create a system that was later used in their self-adjusting cassette tape decks that adjusted for MOL at 1,000 Hz while actually testing a tape at 5,000 Hz. Although a tape-type switch – i.e. Normal/Type-I, Chrome/Type-II, Ferrichrome and Metal/Type-IV – must still be used to make sure that the right correction factors are introduced.

This cassette tape recording “phenomena” discovered by Hitachi’s R n’ D engineers at the time became the “brilliant engineering solution” of its day when it comes to designing self-adjusting cassette tape decks that most enthusiasts can still afford because competing designers of self-adjusting cassette tape decks near the end of the 1970s have decided against incorporating distortion analyzers in their cassette tape deck designs – even their premium models – because the complexity and expense is deemed a bit too much for the market at that time. Instead they all aim to use various schemes to aim for maximum output level (MOL) at some specific frequency – usually around 1,000 Hz. But it looks like Hitachi’s method of using a higher frequency was deemed as a brilliant cost-effective engineering solution back then.