Mejora continua y calidad en la Educación

1. What is the popular name of a portable tape recorder?

...

2. State one advantage and one disadvantage of digital audio tape over analog audio tape.

...

3. How can the same cassette be used to record new programmes?

...

4. In what respects is video tape different from audio tape?

...

32.4 Quality of Sound

We now know that sound is recorded either in analog form or digital form. The term format is also used to describe the number of channels or streams of sound used to record and playback. Two of the most common formats are: monaural (mono) and stereophonic (stereo). Earlier recordings were made in mono using single channel to record and playback sound. Broadcast on AM radio are also in mono. Stereo recording was introduced in the 1960s. It uses two channels for sound. In stereo recording, each channel has a different form of sound. The signals are sent to separate loudspeakers during playback. When played, the sound from these channels combines in air and gives the illusion of direction. A different percentage of each sound might appear at each loudspeaker, giving each sound source its own location between the two loudspeakers. The spatial quality of stereophonics are absent form monaural recording. Quadraphonic recording, which were popular in 1970s, used four channels. Each channel was different, and each of the four loudspeakers were placed in a separate corner of a room. This approach expanded on stereo and gives the listener a sense of sound coming from all directions. Quadraphonic recording is basic to sound systems used as home theaters now-a-days. Here we use more than four speakers to improve the quality of sound.

32.5 Types of Tape Recorders

A cassette tape recorder has the advantage that the tape is fully protected from dust and

Notes

Audio-Videography damage, and is easily loaded into the equipment with no threading whatsoever. It is convenient to send a cassette through the post and to store. Moreover, the recorder deck need not be so large as in a reel-to-reel machine.

But the cassette has a serious disadvantage in that the tape is very thin and does not allow a lot of handling. As such, a tape recorder is available in different formats.

(a) Reel-to-Reel: Reel-to-Reel machine uses a wider tape (6.25 mm or 1/4 in) and offers a variety of available tape speed upto 38.1 cm s^–1. This combination permits top quality recording and playback, resulting from an extended frequency range and better signal/noise ratio.

The capacity to take large reel (up to 27 cm in diameter) enable an uninterrupted playing time of 6 h 24min to be achieved using 1100 m triple play tape. The main advantage of a reel machine is that the tape is easily handled for editing, and this combined with the varied speeds makes the reel-to-reel format essential for the serious tape recording enthusiast who wants to make his own programmes. Its main disadvantage is its weight. Apart from one or two exceptions, these machines are large and heavy, because of the mechanical complexity and robustness required. Also the new machines are very expensive.

(b) Stereo or mono?

Most tape recorders now are in stereo formats having two separate recording and playback channels which allow the apparent sources of sound to be spread out into two channels, left hand and right hand. This is achieved by recording on two tracks of the tape simultaneously.

It must be remembered that stereo recording halves the playing time available with the machine in the mono mode. If one wishes to edit stereo recording by cutting the tape, then a full track machine is more suitable as there is no danger of cutting the recordings on the other two tracks. So the creative recordist needs a full track stereo reel recorder that can be used in either mono or stereo mode.

Mono cassette recorders play back stereo cassette tapes in a mono mode. But the same is not true for reel to reel machines. The stereo machine will replay mono tapes, but the reverse is not possible.

(c) Cartridge or cassette?

Both systems have advantages. The higher speed of the cartridge tape results, in better frequency response and less wow and flutter. However, cassette players have been so

Notes

Photography and Audio-Videography

well developed for hi-fi (high-fidelity) use that much of the improvement has filtered down to the in-car player, while cartridge player development has remained fairly static. Thus there is little difference in performance between the two. For example, the wow & flutter of a typical car cassette player is 0.35 percent while that of a cartridge player is 0.30 percent.

Track width of the cassette is greater, giving theoretical improvement in noise

& dropout (momentary breaks in sound due to irregularities in the tape or its coatings).

Cassette tape measures only (0.15 inch (3.81 millimeters)) in width Analogue tape carriers four tracks, two on each side.

Analogue cassette tape operators at 17

8 ips (4.8 centimeters per second) professional reel to reel tape, which is 6.2 mm (0.25 in) wide.

(d) Standard tape speed

It is apparent that the slower the motion, the greater is the amount of information or longer the program that can be stored on a tape of a given length. However, faster the tape motion, better is the high frequency response. The two common type of tape speeds are 3.75 inch and 7.5 inch of tape per second. For the highest quality audio work, a speed of 15 ips (inch per second) is used. For dictation and storing more information on a small tape, the tape recorder with is a speed of 7

18 ips is available. For accurate reproduction, a tape must be played back at exactly the same speed at which it was recorded.

(e) Compact Cassette

The compact cassette consists of two open spools contained within two plastic moldings which sometimes be separated by extracting several screws, to reveal the tape spools and other components.

The tape is about 3.81 mm wide, and on this four tracks can be laid in two stereo pairs, each track being about 0.6 mm wide. A 0.7 mm guard band of unrecorded tape separates the two stereo pairs as approximately half the width of reel audio tape and is generally

Fig. 32.9 : Cartridge or Cassette

Notes

Audio-Videography thinner, ranging from about 9mm to 18 µm (that is, 9 - 18 thousandths of 1 mm) depending on playing time. When one pair of tracks has been recorded or replayed, the cassette can be turned over so that the other pair can be recorded or replayed. There are two sides marked A and B. S 660 cassettes run for a total of 60 minutes (30 minutes per side) and a C90 get a total of 90 minutes (45 minutes per side). C120 cassette has two hours total running time, tape is rather thin being about 9 µm and nearly six times thinner than standard play reel-to-reel domestic tape. At about 12 µm thickness, the C90 cassette represent a good compromise.

Fig. 32.10 : Magnified view of compact cassette tape

It can be seen that this particular design of cassette employs five screws to secure the two molded half-sections. The tape is wound on to small hubs which, although sandwiched between two radically creased, graphite coated steps sheets (or shims), are free to rotate without undue binding and with just about the right degree of torque. The hubs have keyed holes which engage on the keyways of the drive shaves of the machine flanged; low friction rollers guide the tape to and from the hubs, while correct tracking of the tape across the head is achieved by six guide posts.

The coated side of tape passes several cutouts at the front of the cassette, and the main center cut out accommodates the record/replay head. (Some machines use a common head for both functions while more expensive ones use two separate heads.) In the record mode, the erase head makes contact with the tape through a smaller cutout at the left.

Large cutouts on either side accommodate the transport pressure roller, which work against the driving capstan so that the tape is “rolled” along between the capstan and pressure roller.

The head pole pieces make contact with the tape coating, and a close contact is ensured by a felt pad on a bronze circuit. The tape is permanently secured on the hubs by captive latches, and on this particular range of cassette a length of strong polyester leader absorbs the deceleration of foot spooling without stretching or breaking.

Notes

Photography and Audio-Videography