радуга, для желающих

The Bias Frequency
The bias frequency should be as high as possible for two
reasons. First, lower bias frequency causes the background
noise to increase: at 19 cm/s (7% in/s) tape speed, the use
of bias frequency of about 100 kHz (or more) reduces this
noise to nearly the minimum amount. Second, at high recorded frequencies, the harmonic distortion which is created
at high recording levels by the tape and recording amplifiers produces audible beats with the bias frequency and
these beats are recorded on the tape. A frequency-response
run at high levels may look like Fig. 3C. The response
above about 8 kHz is, in fact, a series of bias beats. This
4.75-cm/s (l%-in/s) recorder uses a 67-kHz bias frequency.
This problem may be especially troublesome when one
attempts to make tape recordings from an FJI-multiplex
tuner. Both 19- and 38-kHz signals are present in the multiplex unit and may get through to the tape recorder. If these
are of large magnitude, the bias beats will occur. Several
solutions are possible including better filtering of the multiplex carrier in the tuner and low-pass filtering in the tape
recorder input circuit. If the multiplexer is well-balanced,
so that only the 38-kHz is of concern, the choice of a 95-kHz
bias frequency will place the beats above the audible  freauencv ranpe.
If the bias wavefoim has even-order harmonic distortion,
a d.c. signal is remrded on the tape. This has the bad effect
of causing second-harmonic distortion as shown in Fig. 3D.
A tape noise is also added, as shown in Fig. 3E. The noise
consists of “cracks and pops”

http://www.mrltapes.com/mcknight_biasing.pdf

Вот, уже лучше!  Партагас менее обучаем.
А переть они будут везде и в первую очередь - на саму ленту. Суммарные гармоники не запишутся - а разностные - в аккурат попадут.

Есть такая вероятность.
Расширять спектр подмагничивания, видимо, не стоит.
Но вот надо ли бороться за 0,01% гармоник - это тоже сомнительно. На уровне ловли самых мелких блох из кучи крупных блох.

Нет ни одной  правильно сделанной деки . Везде есть компромиссы, обходные решения и прочие факторы понижения качества в итоге .
С другой стороны , такая дека была бы слишком дорогой .
А 99% юзеров , как мы знаем , были вполне рады убогости вроде 850 айвы .