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1947-48 Theatre Catalog, 6th Edition, Page 489 (473)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 489
Page 489

1947-48 Theatre Catalog, 6th Edition, Page 489

THE BATTERY VOLTOHMYST is a vacuum-tube voltmeter which operates without dependence on any external power source. This makes the RCA meter ideal in servicing where them is no power.

for adjusting one or two machines, and therefore separate sections of the first three targets have also been made available.

A new Altec theatre test film to be known as ED-35 transmission and test film has been developed by Altec with the aid of the Research Council of the Academy of Motion Picture Arts and Sciences. This new test film will be of laboratory precision and will be printed on safety stock and made up of a wide variety of constant frequency, buzz track and fiutter test sections.

The Altec engineering group, back in the old ERPI days, developed the first calibrated multi-frequency test film for theatre servicing more than 12 years ago. This was the first big advance in the science of theatre servicing. It marked the beginning of an era where sound in motion picture theatres was to be considered from a technological standpoint rather than merely a means of making Htalking pictures" just tftalk." Theatre service engineers were given a tool to actually measure the sound delivered to the audiences on a scientific rather than guesswork basis.

For years the only satisfactory method of producing high grade test film required the costly process of toe recording, which meant that every foot of film had to pass through the recorder. Printing methods would not produce a test film of a uniformly high degree of quality. After years of research and development on the part of Altccls New York engineering department, Altec's Hollywood laboratories and the Research Council, methods were developed f'or producing calibrated test film of steady uni 1947-48 THEATRE CATALOG

formity high quality through the controlled use of the printing process.


The Clarkstan Corporationls sweep frequency film, a Wayne R. Johnson development, provides a new method for making instantaneous qualitative and quaititative analyses of the performance of the entire studio recording systems, theatre sound reproduction systems and their individual components, such as microphones, transmission lines, power amplifiers, pre-amplifiers, light valves, and monitoring systems.

Before the developments of the sweep frequency film, frequency response measurements were made with the single frequency tone film. This method is both time-consuming and laborious. When adjustments were required on the equipment, a new frequency run was required after each adjustment. Now, all that is needed is a cathode-ray oscilloscope and the sweep frequency film for the instantaneous measurement of frequency response.

The sweep frequency signal is recorded on a high quality variable density recording head (single 0.200 inch). The repetition rate is sufficiently high to avoid objectionable fiicker results. The reasonate frequency of the recording head armature used is 11,000 cycles. This is well beyond the highest frequency recorded. No equalizer has been employed to compensate for slit losses of the playback heads or for processing losses.

If the sweep frequency film is used as a secondary frequency standard, equalization can be employed to provide a flat output. This equalization will vary with the slit width of the playback head.

The sweep frequency film is normally supplied in short lengths so that a continuous loop can be made, thereby eliminating rewinding.

For instantaneous and complete production testing of reproducers, filter networks, audio amplifiers, pre-amplifiers,

transformers, attenuators, tone control systems, and the like, each component may be quickly and thoroughly tested for all its over-all performance.

Harmonic distortion may be observed as a bright line, usually running irregularly through the middle of the pattern. The phase may change with the frequency and amplitude, this will cause the line to go either up or down if there is a phase shift. Critical observation can be made with an expanded sweep. The distortion frequency may be computed by observing the nearest marker pulse. This provides a qualitative analysis of the distortion generated by the playback system.

The sweep frequency is not limited to frequency response measurements. It may be used to observe a number of important parameters. Being logarithmic, the higher frequencies are swept at a faster rate. This shows the transient response performance of the playback system at the higher frequencies.

By using an expanded sweep of the oscilloscope, clear detail is obtained.

With high quality playback systems, the pattern will approximate the pattern which is obtained from the monitor oscilloscope of a primary standard sweep frequency generator. The playback system will become a secondary standard sweep frequency generator sufficiently accurate for most laboratory measurements.

Many playback systems use sharp cutoff filters to remove the higher frequencies because of grain noise and intermodulation. This may be checked for transient response by observing the 170 microsecond synchronizing impulse at the start of the sweep. The horizontal gain control of the oscilloscope may be advanced to produce an expanded sweep, at the same time adjusting the frequency adjustment control to bring the pattern into View. Playback systems adjusted to conform to the sweep frequency pattern will have a superior degree of quality.

THE OPTI-FOCUSSER is a completely self-contained amplifier which was designed tor the purpose of quickly aligning any motion-picture sound optical system. No connections other than the two simple clip leads are necessary in order to make the required adjustments. A loop of test sound track, supplied with the Opti-Focusser, is threaded and run through the soundhead during the period of the adjustment.
1947-48 Theatre Catalog, 6th Edition, Page 489