Audio cassette tape can have a hard time handling very high and very low frequencies. In the low end, the lowest bass notes like the open E string can cause distortion if the level is too high. In the high frequencies sibilance (SSS sounds) can cause tape distortion so using a de-esser helps keep things under control. The de-esser works in the range of 5 to 10 kHz.

If you are already saturating the tape from excessive high frequencies, boosting your high frequencies will only cause more trouble.

The following methods might help to get more level, brilliance, and presence:

• Mixing for more presence in the upper midrange/lower HF range (from 2 kHz to 6kHz) to mask the tape hiss.

• Give more distance between the microphone and acoustic guitars and cymbals! (There was a time when overly brilliant acoustic guitars were in style and it caused a lot of problems going to cassette)

• Use a de-esser

• Use a a multi-band peak limiter with a built-in preemphasis before limiting, like the Behringer Combinator or Aphex Dominator, or a high frequency limiter.

• Use a multi-band compressor to apply compression only where needed like in the low frequencies below 100 Hz. We don't want the bass so loud that it is pushing the tape into distortion while the upper midrange is not present to mask the tape hiss.

• Use a high-pass filter to roll off very low frequencies

• Use a device that adds brilliance only to low-level signals. That is what Dolby B does in its encoding stage. The higher the input level, the less boost Dolby B gives. The Aphex Aural Exciter and similar devices work similarly by adding (hopefully-) low levels of HF harmonic distortion. This is tricky because you generally want to push the tape as loud as possible just to the cusp of distortion and saturation, but if you already have distortion in your mix there may be a compounding effect.

• You can reduce the recording bias to get a boost in high frequencies but you'll get increased distortion and will have to reduce the record level.

• Don't be adding 20 dB EQ boost at 20 kHz!

• very high frequency synth tones like 10 kHz at high record levels will distort on tape.

• Use better tape if available (there's not a lot of choices for new tape)

• Use high-bias tape in normal Type I shells with normal 120 microsecond EQ! To do this you need a recording bias selector on your cassette deck. Here are the buttons that permit recording with any bias and EQ and a Nakamichi cassette deck:

Many cassette decks combine the EQ and Bias functions into one button, or they use sensors inside the machine to detect what type of cassette shell is in the machine.

If you cannot choose the recording EQ on the machine you can adjust the audio input by applying a high-shelf EQ filter to cut 4.5 dB:

70 µS EQ for high bias tapes - good or bad?

In the early days of cassettes the tape hiss was quite bad. Chrome tapes had a much greater ability to handle high frequencies, so the tape engineers decided to change the EQ curve to give a 4.5 dB reduction in tape hiss instead of better high frequency performance. That was great for classical music but not much use for high energy rock and electronic music. With the advent of Dobly B which gave a 10dB reduction in hiss, the BASF engineers regretted the move to 70 us EQ.¹

¹ Terence O'Kelly, "EQ", The Inventor's Notebook, Technical bulletin #4 (BASF technical bulletin)