THE STEREO TAPE DECK. SPECIAL FEATURES. ~ CLASSIC HI-FI EQUIPMENT

THE STEREO TAPE DECK. SPECIAL FEATURES.

A) Heads and number of tracks.

The width of the standard cassette tape is 3.8 mm instead of the 6.35 mm of the reel to reel tape or open reel tape; therefore, the recorded tape width is also smaller. As a direct consequence of the reduced track width, it is possible to deduce that the achievement of authentic high-fidelity decks to the cassette is a goal that has been widely achieved today and has always been a task that required time and enormous research effort.

One of the topics that has occupied the laboratories of the main manufacturers has been to find a material for the construction of magnetic heads, that works in satisfactory conditions in terms of wear and performance. Currently, generalizing, it is possible to summarize that 3 types of materials are used for the manufacture of magnetic heads for decks to cassettes: Ferrite, Permalloy and Sendust (Senalloy) Of the 3 materials, the one that offers a higher hardness index and wear resistance is the ferrite (650 on the Vickers scale) versus Sendust 500, Permalloy 200 hardened and 120 of Mullite Permalloy.

However, in magnetic terms the ferrite is more sensitive to saturation with high bias currents than the Permalloy or Sendust and its linearity with low signal levels (working condition of the reproduction head) is also lower. The greater magnetic linearity is achieved using Permalloy (thanks to its reduced coercivity and high permeability). Sendust provides the highest flux densities (required for pure metal tapes) with a level of wear similar to that of ferrite and a linearity very close to that of Permalloy. In decks to cassette it is also possible to find the configuration of two heads (erasing, recording-reproduction) or 3 heads (erasing, recording, playback)

B) Polarization and equalization.

In the cassette system standards accepted as standard by all manufacturers of stereo decks have been very closely linked to the appearance of new compositions of the magnetic layer of the tape. Every novelty in the market usually requires changes in the values of polarization and equalization, in order to obtain the best results in each case. Coercitivity and remanence acquire enormous importance as defining the performance that a magnetic tape can give.

In the 4 basic types of magnetic formulations for cassette tapes currently on the market, the proper polarization level for its correct operation is different. In general terms, it is possible to state that the better the performance of one type of cassette, the greater the amplitude of the polarization current necessary for optimum use of its possibilities.

Manufacturers often provide their devices with polarization-equalization selectors with different positions for each type of tape, some of which incorporate a small fine-tuning of polarization in each position to obtain maximum performance according to the brand of cassette used. For equalization there are standard values accepted by tape manufacturers and stereo decks.

In general it is possible to summarize that the effect of equalization on magnetic tapes to cassette is to reinforce, in reproduction, the response in treble, to avoid, as much as possible, the effects of losses in high frequency due to the reduced speed of passage of the Tape and others in the recording process. To implement this solution a fixed frequency is chosen from which all upper frequencies are enhanced.

In the case of the iron oxide strips the frequency is 1326.3 Hz or 120 μS (as it was already seen, it is preferable to indicate microseconds, instead of frequencies since the number of microseconds directly indicates the characteristics of the RC network necessary for Produce the desired response curve).

In contrast, chromium (Cro2) and pure metal oxide tapes require less reinforcement because of their better performance at high frequencies, the chosen frequency as the starting point for their increase is 2273.6 Hz or 70 μS. In most of the high frequency range the characteristic of 120 μS (normal equalization) is above 4.5 dB corresponding to 70 μS (Cro2 and metal equalization).

This means that while both curves produce flat response using the appropriate tape type, any residual noise picked up by the magnetic head will be increased by 4.5 dB with the equalization of 120 μS.

C) Speed of passage of the tape.

In the cassette format the standard speed of passage of the tape is 4.75 cm / s. Today there are many devices on the market capable of offering high frequency responses up to 20000 Hz. Nevertheless, the limitations imposed by such speed are important, and already exist in the market stereo decks to cassettes of 2 speeds 4.75 and 9.5 cm /seg capable of reproducing at 9.5 cm / seg frequencies up to 30000 Hz with an improvement of 4.5 dB in the signal-to-noise ratio.

The equalization time constants are reduced by the same percentage as the speed is increased. There is no reason to think that in the future no more devices with these characteristics will appear.

D) Motors. In stereo decks to cassette Hi-Fi is generally used single-engine traction systems for a long time. Currently the most widespread trend is to resort to the use of two motors, especially in order to achieve maximum constancy in the speed of tape passing. There are some appliances that incorporate up to three engines.

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