(quote from web explaining bias)
If you take a tape, and coat it with magnetic material, you can record information on it by moving the tape past an electromagnet. This electromagnet is oriented so it's magnetic gap is completed by the magnetic coating on the tape. This leaves a pattern of magnetization on the tape. If you now run the tape that was just recorded over the electromagnet again, the magnetic field on the tape will induce a small voltage in the electromagnet. This signal is amplified, and should be a duplicate of the original signal. This sounds very simple to do, but in fact is not so easy. Let's examine why.

It was quickly discovered that you can't just apply an audio signal to the electromagnet (called a head) and expect good audio when you play the tape back. Indeed, it will be highly distorted. The reason is a property of magnetic materials called hysteresis. Hysteresis is the property of a magnet that keeps it magnetized. In order to magnetite a substance, or change the magnetization of an already magnetized substance, you have to apply a certain level of magnetic field before the change will occur. The solution to the hysteresis problem turned out to be the application of a high power-high frequency (5 or more times the highest frequency you will be recording. For audio, this should be 100 kHz or more.) signal to the head that has been amplitude modulated by the audio signal. The high frequency signal is called bias. The bias signal contains the energy needed to overcome hysteresis, and the audio modulation causes the the residual magnetism left behind to vary in step with the original signal. After hysteresis has been overcome, the amount of varying signal needed to leave a signal on the tape is very small. It is not unusual to apply 100 volts p-p of bias to a record head with just a couple hundred millivolts of audio riding on it!

(end quote)

With the wide variety of tape materials that are fed into these machines, detecting the proper bias to use would be important. It would seem that ET recording would be possible on the highest grade "standard" tapes, but the size of the "grain" of the tape would be the limiting factor, where "metal" tapes would record without problems and oxide tapes would have varying amounts of dropouts or snow.

SVHS tapes are still far less expensive than the first video tapes I bought (pre-1980). Those were $24 for a short beta tape.