Questions for Review
1. Soft magnetic material has essentially no coercivity or retentivity, so it will not retain the magnetic signals that have passed through it en route to magnetizing the cassette tape. This way the magnetization of the head follows the audio signal. Over time some head magnetization does build up, and a fast bias signal is used to "demagnetize" the heads.

2. The magnetic property of hysteresis, or the properties of retentivity and coercivity, are useful for recording audio signals on magnetic materials. The amplitude of an audio signal is proportional to the strength of magnetic field in the head, which is exposed to the tape. The tape responds by being magnetized, and it retains this magnetization as a record of the magnetic field in the head gap, i.e., as a record of the audio signal that is being recorded.

3. Higher tape speed means that the tape wavelength for a given audio frequency will be longer. The limitation in recording and reading a tape is in the head gap, which must be smaller than ½ wavelength of the shortest wavelength recorded. Tape at a faster speed will enable higher frequencies to be recorded and read by the tape head, because their wavelengths will be sufficiently larger than the head gap.

4. Tape saturation limits the dynamic range of a tape recording. As the amplitude of an audio signal builds, so does the degree of magnetization on the tape. There is a limit for how high the magnetization can be, and the amount of change in magnetization per change in audio amplitude decreases dramatically as this limit is approached. This essentially compresses the dynamic range available on the tape.

5. Both recording head and playback head utilize the principle embodied in Faraday's law, that a changing magnetic flux through a coil induces a voltage in the coil, or vice versa. As an audio signal moves through a coil around the recording head, it creates a changing magnetic field in the head gap which induces a magnetization on the tape. As the tape moves by a playback head, the magnetization on the tape is sensed by the head, creating a changing magnetic flux in the head. This flux oscillates through the playback head coil, causing an audio frequency voltage to develop across the coil, which is then amplified by the preamplifier, and so on.

6. The head gap limits performance in that any tape wavelength that is smaller than twice the head gap cannot be read effectively, a geometric limitation that translates into a limited frequency response in recording and playback.

7. According to Faraday's law, the nature of magnetic flux inducing voltage in a coil and vice versa is that the voltage is proportional to the rate of change of the magnetic flux through the coil, and not directly to the value of the magnetic flux itself. Thus, a high frequency signal creates a more rapidly changing flux, which results in a larger magnetization on the tape. The relative sensitivity of the head magnetic flux, and hence the tape magnetization, is that they are directly proportional to the signal frequency. In playback, then, a boost is given to lower frequencies to level out this effect. Equalization essentially involves playing the signal back through an amplifier whose gain is inversely proportional to frequency.

8. Soft magnetic materials have very low retentivity and coercivity, so that their magnetization can more or less follow the strength of an applied external magnetic field in real time, that is, with little or no hysteresis. Hard magnetic materials have significant retentivity and coercivity, so that an applied external field can create a magnetization that will remain after the field is reduced to zero. Hard materials are useful for magnetic storage, while soft materials are useful for recording and playback heads.

9. An advantage of vertical recording would be that additional layers of a magnetic record could be recorded, thus creating a higher storage density in the magnetic medium.

10. The dynamic range in recording audio on a tape is limited by the signal-to-noise and by saturation. At the low sound limit, tape hiss due to random alignment of domains that causes small, but non-zero magnetization in some regions, creates noise that limits the smallest audio signal that can be heard. At the higher amplitude end, saturation of the tape, a reflection of the fact that the largest magnetization is achieved when all domains are aligned, distorts the response of the tape at high levels and limits how large a signal can be recorded. Both effects conspire to limit the range of sound levels available in playback, creating a dynamic range that is significantly compressed when compared to CD sound, for example.

Exercises
1. B. synchronous motors have their speed locked to line frequency at 60 Hz.
2. B. erase, record, playback, from left to right.
3. A. a 3 kHz wave has a wavelength that is 10 times shorter than a 300 Hz wave.
4. B. Point X will become a south pole and Y a north pole.
5. B. a poor S/N results if the level is set very low.
6. D. if the level is set too high during recording, distortion results
7. B. f = speed/wavelength, wavelength = 2*gap= 2*1.875*10^-4 inch, f = 5 kHz
8. A. faster speeds generally produce better performance
9. A. the degree of alignment of magnetic domains
10. D. the purpose of a bias current on the recording head is to shake up the magnetic domains so that it is easier to magnetize the tape
11. A. recorded information on a magnetic tape is longitudinal
12. A. before playback equalization, the signal at 12 kHz is 6 times larger than that produced by a 2 kHz signal of the same amplitude
13. tape bias is typically at 100 kHz and it reduces tape distortion
14. E. magnetic tapes should have large retentivity and coercivity
15. E. wavelength = speed/frequency, so if both increase 4x, the wavelength is the same
16. A. without equalization, a cassette tape will have too much treble and too little bass
17. D. saturation limits the maximum amplitude of a signal recorded on tape
18. E. none of the above
19. C. wave X is 20x longer than Y, so its frequency is 20x smaller, and Y's signal will be 20x larger before equalization
20. B. for tapes with different hysteresis curves, the bias amplitude should be different
21. D. recording at a lower level on a tape deck gives a better frequency response
22. C. recording heads are electromagnets
23. E. erase frequencies are in the range 60-125 kHz
24. B. a dB meter responds well to the sudden peaks, or transients, in music
25. C. dolby C reduces tape his by about 20 dB