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 21^st 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.    

Record Bias Self-Adjusting Cassette Tape Decks: Technological Tour De Force?

Given that this late 1970s ingenious innovation to further optimize the recording performance of the “lowly” compact cassette, did record bias self-adjusting cassette tape decks truly qualify as a technological tour de force? 

By: Ringo Bones 

Every serious cassette tape recordists during the late 1970s may don’t have yet the “technical vocabulary” to the problems besetting them arising from the Philips compact cassette performance and manufacturing limitations, it seems that every advances in technology aimed at extracting every bit of performance of the lowly compact cassette seems to be a fast track to nowhere. Even with the 4 types of broadly standardized cassette tape types, namely the: Type-I or Normal Bias 120 microsecond entry-level tape, Type-II or High-Bias 70-microsecond Chrome Position cassette tape, Type-III or Ferrichrome cassette tape purposely built for the “acoustically noisy” car environment and the Type-IV or Metal Position 70-microsecond metal particle cassette tape, which at the time, represented the pinnacle of cassette tape technology for those wanting open-reel tape performance in a compact cassette tape package. So “technological utopia” for the lowly cassette tape has finally been reached, right? 

When recording tape – whether open-reel or compact cassette types – is improved in any significant way – even if the change is something as ostensibly physical as a smoother coating surface, with the magnetic particles themselves left unchanged – the tape now behaves differently with real-world tape decks as an electromagnetic entity. Generally, the tape now requires – or at least would benefit from – a different level of recording-bias current, followed by the obligatory touch-up of the tape deck’s default record equalization curve. Unfortunately for the typical everyday cassette tape recording enthusiasts, these are adjustments that require good – and usually expensive – instruments and a certain amount of technical expertise to accomplish; Not to mention that these adjustments are also very time consuming to perform. 

Cassette tape recording enthusiasts back in the late 1970s more often than not experienced first hand the “rude awakening” of buying a handful of cassette tapes from their local retailer for the sole purpose of finding out which brand suit them best only to find out later that all of them require somewhat different levels of fine tuning of the recording bias and equalization of the cassette tape deck they currently own. Optimizing your cassette tape deck for each one so that you can evaluate noise levels and other important performance criteria for cassette tape recording will more often than not prove to be a project of Herculean proportions. Then there’s also the problem of cassette tape manufacturers’ batch variations. For the sake of example, TDK’s cassette tape production for June is not likely to be identical to the production batch made previously in March. Very serious cassette tape recordists at the time often buy in quantity from a single batch and resign themselves to readjusting their cassette tape decks when their supply stash of cassette tapes ran out and a new batch must be investigated. This is hardly a formula for progress thus the raison d’être for the self-adjusting record-bias cassette tape deck was born. 

The user-selectable tape type accommodation abilities of record bias self-adjusting cassette tape decks and the ease it can perform these “arduous” tasks has been widely hailed at the time as the solution for this very dilemma. Back then, they look as if they are the next step forward in the technological evolution of the few innovative cassette tape decks already incorporating two-tone oscillators that enables the user to achieve a reasonably flat frequency response with virtually any cassette tape out on the market when it comes to do-it-yourself field recording of music and other “real” acoustic events or just simply dubbing the recorded contents of other prerecorded cassette tapes being played from a separate dedicated deck. 

The most advanced of the first generation of these record bias self-adjusting cassette tape decks were equipped with 4-bit analog to digital converters to accurately sample the MOL of the test-tones being recorded on the cassette tape under evaluation and a 3-bit floating point microprocessor system to process the data in order to perform the necessary adjustments to dial in just the right amount of record bias to enable the tape deck to automatically “fine tune” the maximum MOL while minimizing total harmonic distortion and flat frequency response that the cassette tape under evaluation is truly capable of. 

Even though such analog-to-digital conversion and processing power seems lowly by today’s standards, it enabled those early generations of self-adjusting cassette tape decks to perform 96 steps of bias level adjustments and 16 steps of EQ and sensitivity adjustments – with 0.5 dB of resolution. With a good tape, these early decks were said to be able to maintain a recording frequency response tolerance – or flatness – of plus and minus 0.5 dB over much of the audio frequency range. This sort of performance will only do wonders to these tape decks’ ability to portray a fairly accurate imaging and sound-staging of the acoustic event in the space it is trying to record. 

Thanks to this record bias self-adjusting cassette tape decks, there will be no more tape jungle to flounder through for the serious cassette tape recordist. He or she will be able to buy an armload of different manufacturers’ cassette tapes, take them home, and in the course of a civilized, low-pressure evening decide for himself or herself what his or her brand is going to be, without the worry that a non-compatible cassette tape deck is making a good tape look (or sound) bad. By the same token, cassette tape manufacturers will finally be able to concentrate their full efforts on making a genuinely better cassette tape. It will not be just a somewhat better tape that has been compromised a little because it must match the factory-set performance and adjustment profile of earlier generations of cassette tape decks already in the hands of potential customers.