When sound was first recorded on magnetic tape, it was discovered that extreme distortion would occur due to an intrinsic property of magnetic tape called hysteresis.

Did anyone at the time ever notice that the amount of distortion due to this varied according the amplitude and frequency content of the signal recorded? Whether they did or not, it seems strange that tape recorders made throughout the analog era all had fixed levels of bias. Variable bias in the form of Dobly Headroom extension (Dolby HX) did arrive in the early 1980s this being the beginning of the digital era.
Varying the added bias according to the amount of self bias in the signal increased the dynamic range and frequency response of the recording.

Studio recording equipment was already very good by the mid-1950s. For example the Miles Davis album 'Kind of Blue' was recorded in 1959 and the only thing giving away the age of the recording is a little tape hiss heard in the beginning of the first track. Had variable tape bias been available at the time and used in the original recording, the record level could have been higher and there would be less tape hiss in recordings like this.
 
Could it be down to sheer complexity ?
It's one thing optimising studio gear, its quite another hauling tape-decks to concerts. And how do you play such recordings any-where else ? I used to have a semi-portable reel-to-reel deck. (Don't ask !) As convenient as a brick-filled flight bag, it took five minutes for the valves to warm up evenly, was absolutely jammed with circuitry, motors etc.

Do you wonder domestic/mobile tape usage went via 8-track cartridge to Phillips' 'Compact Cassette' ??
 
What's so complicated about modulating bias? It's just a simple matter of detecting the amount of self-bias in the signal and adjusting the added bias accordingly. Bias is only needed when recording, not when playing back tape.

Before Dolby HX, there was a long period of debates about how much bias to add, so why did it take so long for anyone to figure out varying the tape bias?
 
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