In short it is servo on the bias as they explained. Tape bias is non audible high frequency recorded with music that reduces distortions of the tape. Bias also reduces upper bandwidth so usually some compromise is taken. Because high frequencies in music also work as bias, controlling total amount of them at given moment will keep distortions low while extending bandwidth. System that adjusts amount of bias to keep constant bias+musicHF is called servo and it is the best thing since sliced bread (I had Aiwa deck).
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