Overview of magnetic recording.
Magnetic recording is a fairly new technology, with the first machines appearing just before WW2. The first machines recorded signals on a thin iron wire; wire recorders are actually still used today as data recorders on aircraft. (The 'black box'.) But, magnetic tape came into use around the end of the war, and gave better results. Germany was a big pioneer in this area.
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!
The bias is critical
The amount of bias required for a good analog recording is very critical. It varies from brand to brand of tape, and even from tape to tape in a lot. This is why some modern cassette decks have a front panel bias control. Bias affects record level, frequency response and distortion in a relationship too complex to discuss here.
The other factor that affects the amount of information that can be recorded on a tape is wavelength. The shortest wavelength that can be recorded on a tape is one that is slightly more than half the width of the gap in the head electromagnet. Although magnetic particle size and coating thickness also affect the shortest wavelength that can be recorded, it can all but be ignored for purposes of this discussion. So, there are two things that can be done to increase the highest frequency (shortest wavelength) that can be recorded on a tape: 1.) Speed up the tape so the short wavelengths are made 'longer' on the tape, and 2.) Make the head gap smaller.
The maximum tape speed is limited by mechanical considerations. It is difficult to transport tape smoothly past a head at high speeds. It also causes the amount of tape needed for a given recording time to increase. For audio, the practical maximum speed is 30 inches per second (ips).
Physics limits the narrowness of the head gap. The smaller the gap, the larger the energy needed to leave the same amount of magnetization on the tape. There is a practical minimum, but it is not a serious problem for audio, where tape speed is a bigger problem. It is, however, a problem for video.
Another problem with analog magnetic recording is that the output of the head increases with frequency at a rate of 6 dB an octave until limited by the head gap width. This is because a rapidly changing magnetic field induces more energy into a head than a slowly-changing one. This limits the low-frequency response to a certain minimum value. To compensate for this effect, equalization is required on playback (And a little on record) to flatten out the frequency response.
But, you may ask, why does it seem high frequency response is the most important parameter for an audio recorder, if the head output increases with frequency? The answer is, head gap wear, dirt, misalignment, and overall losses have a much more profound effect on highs than on lows. This is true even though the high frequency signal output at the head is many decibels higher than the low frequency response.
The last consideration for magnetic recording is head-to-tape contact. The head must be in intimate contact with the tape in order for everything to work right. The output signal from the head drops at a dramatic 1 dB for every 2.2 microns of separation from the tape. (At 9MHz, a video RF frequency.) Even a small gap between the tape and head is intolerable. This is why the heads have to be kept clean in any magnetic recorder!