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Im Mai 1947 wurde das seit 1917 vorhandene "RADIO Journal" komplett zu "AUDIO Engineering" umgebaut.

Die Themen aus den Anfangsjahren des Radios hatten sich weitgehend überlebt und der Herausgeber hat 1947 (zwangsläufig) einen harten Schnitt gemacht. Das Hauptthema war von jetzt an als Mittelpunkt aller Themen der technische Aspekt der neuen Entwicklungen im UKW-Rundfunk und später auch im Fernsehen. Auch das Magnetbandgerät aus Deutschland - das Magnetophon - spielte eine immer größere Rolle und leitete einen welt- weiten Paradigmenwechsel in der Audio-Produktion ein.

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AUDIO ENGINEERING • JUNE, 1947
"Adapting Paper Tape Recorders For Broadcasting"

von RICHARD S. O'BRIEN

Describing the modifications required for greater utility in broadcasting service.
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THE INHERENT ADVANTAGES of magnetic recording on coated paper tape for broadcasting and other uses have been described previously. It seems likely that this new medium will be used for many spot recording applications. At present the technique is sufficiently advanced to be readily applied in many of these cases. In fact, the need for a recording process of this type has led to the immediate utilization by broadcasters and others of the first recording equipment commercially available in this country.

The Brush BK-401

This recorder, the Brush BK-401, was designed for home-recording use and as a result some of the refinements usually found in professional equipment were not included. A number of simple modifications have been evolved, however, to partly make up this lack without altering the basic recorder design. These modifications, together with several related maintenance and operating factors are discussed in this article. In addition, an automatic-control modification is described whereby the recorder and a radio receiver can he arranged to enable delayed-listening - a feature often desired for home or office installations.

The modified recorder

The modified recorder will prove quite serviceable for interim use pending availability of professional-grade equipment. Its use will enable operating personnel to become familiar with the medium. For example, certain problems of editing, splicing and general handling of the tape exist regardless of the type of recording machine. In general, the fidelity of the present recorder will be found surprisingly good.
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Modifications For Broadcast Applications

Magnetic tape-recording is an ideal medium for special events pickups, delayed broadcasts, interviews, etc.

Many such jobs can be handled readily with the present recorder. Of course, the unit must be equipped with input and output coupling circuits which allow it to be fitted into the broadcast station audio system. Also, replacement of the electron-ray volume indicator "eye" with a standard VI, and minor circuit refinements to reduce noise level and susceptibility to electrical interference are desirable. These "broadcast" modifications have been applied to the recorder shown in Fig. 1.

Various input and output circuits can be used depending on the intended use of the recorder. The gain of the built-in amplifier system is sufficient to handle input signals of microphone level (-50 dbm) or higher, while output levels up to +20 dbm may be obtained. Both input and output circuits, except in the case of the existing high-impedance microphone input, should provide isolation, should supply the proper impedance, and should be workable with either balanced or unbalanced external circuits. In general, this calls for transformer coupling.

Die Anbindung an die Studiotechnik

A typical coupling arrangement is shown in Fig. 2A. The input transformer primary is tapped to provide either bridging or matching impedances. With this input system, the recorder can be bridged across 600-ohm balanced or unbalanced circuits and will be fully modulated by signal levels from -29 to +10 dbm. A signal level as low as -43 dbm may be fed into the matched impedance coupling. The existing crystal microphone input is retained, and it should be noted that a microphone-to-grid cable-type transformer can be plugged into the microphone jack to accommodate velocity or dynamic microphones.

In the output circuit, a transformer having an impedance ratio of 100,000/250 ohms, in series with a 100,000 resistor, is bridged across the output transformer voice-coil winding. The arrangement provides an output which appears to have a source impedance of 250 ohms and results in an over-all loss of 56db. Thus, with an audio level of +10 dbm in the speaker circuit, the coupling circuit output would be -46 dbm - a level which can be applied directly to a microphone-input position of an audio system.

The recorder speaker volume can he independently attenuated by means of the L-pad control, once the recorder output level has been set. Other circuits may he used to provide low, medium or high-level outputs from the recorder. In one alternate output coupling, shown in Fig. 2B, the existing output transformer and speaker are replaced by a new plate-to-line transformer plugged into the amplifier output receptacle. With this circuit, levels up to +20 dbm can be fed directly to a transmission line. Lack of a loudspeaker is offset by provision for earphone and VI monitoring of the line level output.

Details zur Elektronik

A standard VI may be coupled into the recorder as shown in Fig. 3. An audio signal is taken from the unby-passed cathode of the final stage in the "record" amplifier, to obtain an output signal isolated from the 30 kc r-f bias applied in the plate circuit of this stage and, incidentally, to make use of the low impedance transmission circuit thus provided. The signal is amplified and equalized to compensate for the prerecording low and high-frequency boost introduced in the recorder and transformer-coupled to a 600-ohm termination.

Either a standard VI panel or the built-in meter circuit may be plugged into the amplifier output. "Test" level at 1 kc requires an input to the VI amplifier of 0.2 volt; rms, and corresponds at the VI meter circuit input to a level of +10 dbm. "Program" level corresponds to a +4vu * (siehe etwas weiter unten) level at this point. The amplifier may be assembled on a small chassis and fitted into the recorder cabinet. Power may he applied to it from the vacated 6E5 socket, and if desired the audio input connection may be made via an octal adapter inserted under the final "record" amplifier, 6SJ7, tube.



* The term vu applies to standard VI meter indications on program material whereas dliiii refers to indications on steady test tone signals. The 6 db difference in 'test" and "program'' VI settings allows only a 6 db headroom for program peak excursions, this reduction from the usual 10 db resulting from a compromise of noise levei and distortion considerations.

Use of the VI enables more accurate control of recording level and leads to improved quality and uniformity of recordings.
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About the noise level

The inherent noise level of the tape recording process is low. However, because of the extremely low output from the pick-up coil and the proximity of copious sources of hum, several precautions are necessary to obtain full advantage of this feature.

In the Brush BK-401, the power chassis is usually oriented so the null in the power transformer field is in alignment with the pick-up coil. This orientation should be checked with the recorder running to make sure it is optimum.

A light-gauge magnetic shield placed around the front and exposed side of the take-up motor is often effective in further reducing the noise level. An adjustable heater circuit ground point (100-ohm potentiometer by-passed to ground) will provide a range of adjustment to buck out certain hum components. Separation of all low-level amplifier grounds from other ground returns and routing of this audio ground circuit independently to the First amplifier tube ground point is of further benefit.

In conjunction with this change, these same audio circuits should be rerun in shielded leads to reduce susceptibility to pick-up from strong, nearby r-f sources such as diathermy and vhf transmission.

Weitere Entstörmaßnahmen

It should be noted that additional pre-cuations may be required in installations where several recorders must be operated in close proximity. Thus, the power chassis orientation in a particula recorder must be arranged to produce minimum hum pickup in both that recorder and its neighbors.

Also, motor-circuit switch spark suppression is recommended so that adjacent machines can he run independently without clanger of introducing pops in the "on-air" circuit.

These various modifications can be installed in the existing recorder cabinet, Fig. 1 shows the placement of VI meter and switch and an earphone monitoring jack. The input circuit and connector can be mounted on a narrow vertical panel on the left side of the rear of the cabinet, as viewed from the tear.

The output connector and a speaker volume control can be mounted on a similar panel on the right side. In addition, the power input lead can be brought in through this panel to an extractor-post fuse mount and thence to (he recorder power chassis. This makes the fuse accessible and also clears the path for enclosure of the space between these two panels with a sheet of perforated-metal grille to protect the innards of the recorder. This grille can also be used to mount a ventilating fan which is recommended where the recorder is to he used for periods exceeding one hour.

An induction fan motor should be used to avoid commutator hash, and a type of motor in which the coils are completely enclosed by the field structure is essential to avoid hum fields.

A four-pole, 1600 rpm motor turning a 5" four-bladed fan will easily hold the recorder temperature rise to 30°F above ambient. The fan should be located as near the main recorder amplifier strip as possible.

A recorder which has been electrically and mechanically modified as discussed will be serviceable in many applications. However, a more thorough repackaging job should be considered where a more conveniently shaped and more rugged unit is needed for held use.

Modification For Automatic Operation

The modified Brush BK-401 recorder shown in Fig. 7 may be operated automatically, controlled by the built-in "Telechron radio timer". The recorder input may be supplied from any suitable audio source but the system shown is arranged especially to work with a radio receiver.

For automatic operation the timer can be preset within an 11 1/4 hour period preceding the intended recording time. Within this limit, any two fifteen-minute periods, either consecutive or separated, may he recorded; the thirty-
minute total recording time being set by the capacity of the tape reels.

Presetting of the system requires the user to go through all normal operating steps preliminary to switching over for automatic operation. A single switch takes care of all circuit changes in switching from Manual to Auto, the only remaining step being the setting of the tinier.

The modifications described above concerned with reduction of noise level and interference pick-up should be applied to the automatic home-recorder adaptation. In addition, a special control circuit is required to automatically start the recorder. Also, a suitable mean', of coupling the recorder and radio receiver audio systems together is required.

Weitere Modifikationen

sind alle übersprungen.

Maintenance

Absence of critical adjustments makes the tape recorder as simple to maintain as to operate. No unusual electrical maintenance problems are involved and the chief requirement is to clean off any accumulations of tape coating material in the recording and erase heads.
When this material rubs off as the tape passes the magnetic heads, it tends to form a cake over part of the face of the head.

The tape is then pushed away from the head, resulting in an increased air-gap length. When this occurs on the erase head, erase efficiency is decreased, allowing the previous recording to remain on the tape. When it happens on the play-record head, the signal level on playback is very low and high-frequency response is noticeably impaired.

The cake may be removed by dampening with a solvent such as carbon tetrachloride, acetone, or alcohol (taking care to keep the solvent away from the tape) ; rubbing with a soft cloth; and, if necessary, fine polishing with a strip of crocus-cloth cut to fit into the head slots.

The coating material also tends to collect in the reverse stopping-switch guide, through which the tape passes on rewind. If enough "mud" collects, the switch will he jammed so that as the tape end passes, a-c power is kept on the supply motor, allowing it to run indefinitely at high speed. Any accumulation in this guide should be cleaned out.

Entwicklungen sind im Gang

Developments are under way, it should be noted, which are expected to greatly reduce and possibly eliminate this rubbing-off tendency. However, with currently available tape, these cleaning operations should be done regularly-after every two to four hours of use.

After long use, the recorder may be unable to pull the tape on forward operation. This is caused by a smoothing or "greasing" of the drive-capstan cork surface.

When this occurs, the cork should be rotated against a cloth dampened with carbon tetrachloride and then against a strip of 6/0 or finer garnet paper (Schmirgelpapier). The sanding may be followed by a second wash to pick up the dust.

These few maintenance operations together with routine tube-checking, circuit inspection for overheated components, checking for noisy controls and switches will usually suffice to keep the recorder ready for use.

Tape Handling

As paper-tape magnetic recording becomes more widely used, techniques of, tape handling which will enable realization of the full capabilities of the medium will come in for attention.

An initial problem is that of splicing the tape for editing purposes. Although splices can be readily made with scissors and a roll of "scotch-tape", more convenient methods are desired for professional operations.

Figure 7 illustrates a first attempt to build a splicing unit that provides the accuracy and extra fingers a person finds useful. The assembly includes a modified film splicer, a trimmer, and a "scotch-tape" dispenser.

The tape is held, coated-side down, in a groove milled in the lower clamp jaw of the splicer by a felt-pad cemented to the upper clamp jaw. A special cutter blade is attached to each lower clamp jaw in such a way that both pieces of tape to be joined are cut on the same edge.

After both pieces have been cut they are held in proper alignment for application of a short piece of yi" wide "scotch-tape". A special guillotine trimmer is used to trim off the excess "scotch-tape". A trimmer which may be operated more easily is planned for a second splicing assembly.

Improvement and production of tools such as this will be needed as will the development of editing machines similar to the moviola used by the motion picture industry.

Conclusion

It is expected that improvements in handling- technique will parallel improvements in recorder performance. Paper-tape magnetic recording is in the broadcasting business to stay and has a growing field of application in this and other fields. It is felt that present equipment, properly used, is capable of handling many broadcasting assignments and t'hat its current use will enable the broadcaster to obtain valuable experience in the use of the medium.

The adaptations described in this paper were made under the general direction of Mr. Howard A. Chinn. CBS Chief Audio Engineer.

Nachtrag und Kommentar :

In den späteren Artikeln und Kommentaren der AES Members stand drinnen, dieser ganze "Bastelkram" vonund mit den "Recoder-Bastlern" von Rangertone und Brush kostete die Profis in den Sendern irre viel Zeit und die "Dinger" von diesen Heim- und Hobby-herstellern hätten nie richtig funktioniert. Erst die deutlich teueren AMPEX Studio-Maschinen mit dem von 3M entwickelten SCOTCH Kunststoffband waren im harten Studiobetrieb wirklich brauchbar. Das Papierband war nämlich (auch in USA) viel zu oft gerissen.

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