This is a quick study guide I created for GCC managers when the company began reducing the use of Union projectionists, and managers in some areas were required to qualify for commercial licenses.








FILM... 1




















Lens Speed. 1

Lens Format 1



Keystoning. 1

Curved Screens. 1


MONO.. 1


THX.. 1











Substrate or Base

The film substrate is the base material, or backbone of the film.  Early films were produced on Cellulose Nitrate - a highly flammable product, also known as Nitrocellulose or gun cotton. 

With the development of better plastics, Cellulose acetate became the standard, and recently mylar and polyester have become the substrates of choice.


This is the layers of chemicals and dyes put on one side of the film substrate which actually holds the image and soundtrack. If you look carefully at the film, the emulsion side may appear to be duller.  There are several layers of chemicals placed on acetate film stock. There are three layers of dye - one for each of the primary colors - Magenta, Cyan and Yellow.  Gelatin or other binder is used to separate the layers and to help the chemicals adhere to the substrate.  These multiple layers are why a shallow scratch appears to be green (cutting through the Magenta layer), while a deeper scratch will appear to be yellow or white. When the film is running through the machine, the emulsion side is towards the lamphouse and away from the lens.


The frame is the actual image that will appear on screen.  The standard frame on a 35mm film is approximately 22mm wide and 19mm tall. 

Sprocket holes

There are 4 sprocket holes per frame on each edge of the film. 

Soundtrack, Optical

On a standard optical print the soundtrack consists of two clear lines going down the side of the film between the frames and the sprocket holes.  The tracks get wider and narrower, to vary the amount of light from the exciter lamp that can pass through the film.  The solar cell decodes this varying amount of light as sound.  On a stereo print, the light beams from the two soundtracks are read by two solar cells, then decoded by the stereo processor into 4 separate channels.

Soundtrack, Magnetic

On a 70mm film or 35mm magnetic print, the optical soundtrack is replaced by a strip of magnetic tape.  The sound is recorded onto the magnetic tape just as it is in a cassette tape player, on up to 6 separate tracks.  In early 70mm prints, the tracks were for:

Left  Center-Left  Center  Center-Right  Right        Surround

In 1977 the standard changed.  Center-Left and Center-Right were dropped to allow for a separate subwoofer channel.

Soundtrack, Digital

The soundtrack on a digital print varies depending on what TYPE of digital print it is. 

On a DTS Digital print, the Digital soundtrack is not actually on the film - it is on a separate CD, which is kept in sync with the film via digital information encoded between the sprocket holes on the film.  The soundtrack you see on the film is a standard analog soundtrack which the system can fall back to if for some reason the CD cannot be read.

On a Sony SDDS print, the 8 track digital soundtrack is encoded on both sides of the film.  If the soundtrack cannot be read on one side due to film damage, the scanner can read the opposite side.  Sony SDDS is the only digital soundtrack to allow for 8 channels:

Left  Center-Left  Center  Center-Right  Right  Subwoofer  Right Surround  Left Surround.

Dolby Digital soundtrack:

The digital soundtrack is encoded on the film between the sprocket holes.  Channels are:

Left Center Right Subwoofer Right Surround Left Surround

For Dolby EX films an extra Center Surround is encoded in the Right and Left Surround tracks


Cueing film varies greatly depending on the automation systems employed, and what the cue is intended to do.  In the days of reel to reel manual projection, the cue consisted of a small circle scratched into the emulsion side of the film, which told the projectionist to switch projectors.  The Reel-to-Reel Projectionist used these cues in conjunction with Academy leader - leader with frames counting down from 10 to 2.  By knowing how long it would take his projector to get up to speed, he could thread his second projector with a particular numbered frame in front of the lens, then start that projector when he saw the cue appear on screen.

Modern systems usually sense a piece of foil tape along the edge of the film or across the film depending on the system.  Where that tape is placed depends on what the tape is intended to signal the automation system to do and how long it takes the automation system to do it. 

The only thing that all systems share in common is the speed of the film - 90 ft per minute, 1 1/2 ft per second or 24 frames per second.  i.e. if it will take a particular automation system two seconds to dim the lights or perform another action, the cue will need to be placed 3 feet before the point in the film where you need that action to be performed.



Early motion picture theatres often used a “dynamotor” - a DC power generator connected directly to an AC motor. While noisy and wasteful of power, the dynamotors were practical in their day due to the low quality of early rectifier systems.  Early rectifiers consisted of stacked plates of copper coated with copper oxide, then in later years with selenium.  These rectifier stacks wasted  a high percentage of the current passing through them in the form of heat, and when overrun could release noxious gasses. 

The power from these early systems was fed to a pair of carbon rods.  The arc of electricity between the two rods would actually burn away the carbon, so the carbon arc lamphouses had to have small drive motors to feed the carbon rods towards the arc as they were gradually burned away.  The carbon arc also released carbon monoxide as the carbons burned, so the ventilation in these lamphouses was not just for cooling, but also to blow the poisonous gasses out of the booth.


A Xenon bulb consists of two Tungsten electrodes enclosed in a Quartz glass envelope.  The envelope is filled with Xenon gas at approx. 70 lb./sq. inch pressure when at room temperature.  During operation, the temperature in the bulb at the arc rises to 2000 degrees Centigrade, the pressure rises to 300 + lb./sq. inch, and the temperature at the bulb surface can be up to 600 degrees Centigrade.  The high temperature of the arc is the reason that quartz glass and tungsten electrodes are used - inferior glass, or stainless steel electrodes would melt at these temperatures.  Because of the high temperature and pressure, the quartz glass is under a tremendous strain when in operation.  Any defects or strain patterns in the quartz envelope can result in a catastrophic failure. (i.e. BULB EXPLOSION).  Strain can be induced in the quartz envelope during manufacturing, however the strain is normally removed by an annealing process and bulbs are inspected using a polariscope.  Strain patterns can also be induced by mechanical stress on the bulb during the installation in the lamphouse, or by chemical contamination of the glass, such as the oil in a human fingerprint.

Projection bulbs are filled with Xenon gas because Xenon gas produces a high temperature electric arc closest to the desired color of daylight (6000 Degrees Kelvin).  For comparison, older carbon arc projectors supplied an arc color of 5000 - 6000 Degrees Kelvin.

Within the bulb, the larger electrode is normally the anode - which will be connected to the positive terminal of the power supply during operation.  The more pointed electrode is the cathode, which will be connected to the negative terminal.  The reason for differences in shape is that as the bulb is operating, electrons flow from the negative cathode to the positive anode.  The negative electrode (cathode) is often doped with thorium to improve this flow. 


To maintain the arc during operation, the bulb needs a power supply capable of supplying from 22-30 volts at 50-100A (depending on the size of the bulb) of very stable DC power.  Any AC power or ripple passing through the rectifier can result in flicker on the screen and shorter bulb life.  A typical modern motion picture rectifier consists of the following components:

A three-phase transformer with moveable slug. 

Moving the slug in and out of the core varies the magnetic field and the current to the bulb.  As there is no electrical connection involved, it is safe to adjust the current (usually with a crank or wheel) while in operation.  Some early rectifiers adjusted current by making contact with different coils on the transformer and required that you shut down power to adjust current.  

6 heavy-duty solid state diodes. 

3 phase power: two diodes per phase. 

If a diode is open, then no current will pass through the diode 1/6th of the time.  This may cause a noticeable flicker in the light output.

Boost capacitors. 

These large capacitors filter and further smooth out the pulsating DC current coming from the diodes.  The boost capacitors also supply an extra surge of current during the initial start up of the bulb.  A typical bulb draws two to three times its normal current during the first 1/4 of a second that it lights.


As previously mentioned, a Xenon bulb requires 22-30 volts at high current during operation.  However, 30 volts will not jump the gap between the electrodes unassisted.  The gas in the gap must first be ionized to reduce the resistance and establish the arc.  To ionize the gas, the ignitor supplies high voltage and very low current to jump the gap.  A typical ignitor takes 115v or 220v and steps it up to 5000v.  A spark gap and doorknob capacitor boost the voltage again, to supply about 40,000v to the bulb. 


To get maximum life from a Xenon bulb, you should set the current level to approximately 85% of the bulb’s rated current when installing a new bulb.  As the bulb ages, you will need to boost the current 5-7% for every 500 hrs of operation.  

Horizontally mount bulbs can also benefit from rotating the bulb 180 degrees when the bulb reaches 1/2 of its warranted hours.  In time the bulb will develop a gray haze - deposits from vaporized metal from the electrodes.  When this haze develops on one side of the bulb only, it causes uneven heating of the quartz envelope, which can cause pre-mature failure.

The number of hours you can typically expect from a Xenon bulb are as follows:



                1000                        1500-2400                                               3500

                1600                        1500-2400                                               3500

                2000                        2000-2400                                               3500

                2500                        1200-1800                                               3000

                3000                        1000-1800                                               3000

                4000                        1000-1200                                               1500



Caution must be used in removing and disposing of old Xenon bulbs.  Even when cold the bulb is under pressure and will “pop” if mishandled.  To eliminate the risk of injury, all Xenon bulbs should be deliberately destroyed before being placed in a trashcan or dumpster.  There are two safe methods recommended for disposal.

1) Wrap the old Xenon bulb in several layers of old newspaper.  After folding over the ends of the newspaper, smash the wrapped bulb with a hammer. 

2) Place the bulb back into the manufacturers safety cover and cardboard box - leave out any foam padding.  Put the boxed bulb in a plastic trashcan liner to capture any quartz dust that may escape the box.  Drop the package on end from a height of about 3 feet until you hear the glass break. 



Glass covered with a thin coating of silver.  Very good reflector, but breakable.  Silver coating can also sometimes flake off.


Highly polished steel.  Not quite as reflective as a silvered reflector, but can survive a Xenon bulb explosion. 


Any time light is “bent” by a mirror or lens different wavelengths of light bend at different rates - you get a prism effect.  A dichroic reflector (or lens) has a special coating to help reduce this effect and give you a more pure white light at the focal point.  A dichroic reflector will usually look amber in color.


Flicker can result from:

Current too low - This is the most usual cause.  If you see flicker, first try increasing current to the bulb.

Current too high - If the light on screen appears to be very bright and you see flicker, try reducing current to the bulb.

A bad diode in the rectifier  - An open diode will prevent its leg of power from the transformer from supplying current.  When a diode goes bad, you may find that the bulb is suddenly hard to start, and you may have to crank the current adjustment several turns to get the current level to the bulb up to the correct level.

Gray haze inside the bulb envelope

This is normal for a bulb with high hours.  If the haze is on one side only on a horizontal bulb, rotate the bulb to reduce uneven heating.  If you see this on a new bulb it can indicate contamination of the bulb during manufacturing, or excessive ripple in the power supply. 

Excessive ripple means that some AC voltage is leaking through a weak diode in the power supply.  In addition to the haze on the envelope, this usually causes excessive wear on the electrodes - particularly the cathode (-). 

If you do not see wear on the electrodes, install a new bulb and watch for the problem to repeat.  If the problem does not occur with the new bulb, contact the manufacturer about a warrantee return.

Blue haze inside the bulb envelope

This indicates that oxygen is leaking into the bulb through a bad seal.  This can be caused by poor manufacturing, or mis-handling of the bulb during installation.

Bulb failing to ignite

When the ignitor operates, you can usually hear a high pitched sound as the high voltage, high frequency spark jumps across the spark gap and between the electrodes in the bulb.  If you can hear this sound, but the bulb does not light, then either too little, or no current is getting to the bulb.  Most lamphouses have safety interlocks to prevent the bulb from lighting under unsafe conditions.  Is the lamphouse door closed tightly?  Is the exhaust fan drawing sufficient air? 

If safety interlocks are not preventing current from reaching the bulb, try increasing the current.  If you have to crank the adjustment several turns to get the bulb to light, you probably have a bad diode in the power supply.

If you do not hear the ignitor operate when you turn on the switch to light the bulb, you may have a bad ignitor, or ignitor relay.  Many systems have a manual ignition button to bypass the automation system and ignition relay.  If you press this button and the bulb lights, then the problem is in the automation system or ignitor relay.



The intermittent (also called Geneva Movement) is a package of gears which translate the steady speed of the projector motor into the start-stop movement needed to project a steady image on screen.  It is called the Geneva Movement because this special gearing arrangement was first used in Swiss watches.

The basic drive mechanism consists of a gear shaped like a Maltese Cross which is connected directly to the intermittent sprocket and a wheel with a drive pin.  As the wheel spins the pin engages the notches in the Maltese Cross and push it down one quarter turn.  For every full turn of the drive wheel, the film (driven by the intermittent sprocket) is standing still 3 times as long as it is moving, giving us a dwell ratio of 3:1.  While the AVERAGE speed of film traveling through the projector is 90 ft./minute, peak speed in the intermittent travel can reach nearly 900 ft./minute.

Variations on the Maltese Cross design are used in almost all intermittents.  If the cross has only three evenly spaced “wings”, the dwell ratio increases to 5:1.  With the frame standing still for a longer period of time, manufacturers can use narrower shutter blades, allowing 66% of the light from the lamphouse to pass (as opposed to 50% with a “standard” intermittent), giving you a more brilliant picture with a smaller bulb.  Other designs have used a three bladed shutter, which cuts the light back to 50% transmission, but reduces flicker.   The drawback to using a 5:1 dwell intermittent is that there is more strain on the film sprockets because while the AVERAGE speed of the film remains at the same 90 ft/minute, the film is only “on the move” 1/6th of the time rather than 1/4 of the time.  Now peak film speed can reach nearly 1300 ft/minute, and has to go from stopped, to full speed, then back down to a dead stop in 1/144 second.  This also puts more of a strain on the components in the intermittent.


The Trap is the portion of the projector which guides the film between the shutter and lens, mounted just above the intermittent.  The Trap contains lateral guide rollers, and/or a lateral guide plate to prevent the film from weaving from side to side during operation.  The Gate is the part of the projector which holds the film firmly in the Trap during operation.

If you see excessive jitter in the film, something in this part of the projector is usually the cause.

First, check the gate and trap for cleanliness.  Deposits of dirt anywhere in this part of the film path can cause problems.  If you find accumulations of residue that cannot be wiped off with a soft cloth, you can use soap and water to soften the deposits, and they can be scraped off with a stiff plastic, wooden, or brass scraper.  NEVER use a steel scraper, screwdriver or steel wool as these will damage the projector.

Next, check for free movement of any spring-loaded rollers and/or guide plates.  Clean and lubricate as needed. 

Check the gate film tension.  If the tension is loose, not only can it cause problems with jitter, but you may see problems with focus because the film is actually moving slightly to and away from the lens.  There are two methods to adjust tension:

The first technique requires a short length of film or leader, and a small spring scale.  After punching a hole in one end of the film, thread it through the gate, but do not engage any sprockets.  Hook the scale on the hole punched in the film, and pull the film through the gate.  Adjust the gate tension until it requires between 8 and 16 oz. of tension to pull the film through. 

The second technique is done during normal operation.  While the film is running, loosen the gate tension considerably.  Tighten the adjustment slowly until you hear a louder clattering noise from the intermittent, indicating that it is straining harder to pull the film through the gate.  Back off the tension slightly.


The beam of light from the lamphouse is round, while the image we will be projecting on screen is rectangular.  To block off the portion of the light beam we do not want to appear on screen, there is a metal plate with a rectangular hole in it mounted behind the Trap.  Because there are a variety of different screen sizes and distances to the screen, the aperture plate for each projector must be custom filed.  If you notice dark shadows along the edge of the screen, the aperture plate may need to be filed further.  Another problem associated with the aperture plate is fringing.  This is due to light bouncing from the edges of the opening filed in the plate.  Although the aperture plate is a fairly thin piece of metal, we are magnifying our 35mm film image by a large factor to fill a 20 or 30 foot wide screen.  There are two methods of controlling fringing:

First, thin the edges of the opening - file the back edge of the opening down so that the actual opening has a knife thin edge.

Second, eliminate light reflections from the edge - darken the filed edges with gun bluing, or high temperature paint.    


The shutter prevents light from reaching the film image until the film is at a standstill.  After the light has been allowed to pass through the image for a fraction of a second, the shutter blocks the light, then the film advances to the next frame.  To reduce flicker, shutters in most modern projectors have two blades, and the blades block the light while the film is moving, and once while the film is at a standstill.  The approximate timing at 24 frames per second works like this:

Shutter closed, film moving: 1/96 second

Film stopped, shutter open: 1/96 second

Shutter closed: 1/96 second

Film still stopped, shutter open: 1/96 second



Ghosting occurs when the shutter is open while the film is moving.  The shutter may be advanced - allowing light to the image just before the intermittent stops the film, or it may be retarded - not blocking the light before the intermittent starts moving the film to the next frame.

If ghosting appears to run DOWN the screen, the shutter is advanced.

If ghosting appears to run UP the screen, the shutter is retarded.

If ghosting appears at BOTH the top and bottom, this would indicate that the shutter blades are too narrow.


Commonly, projectors have a single two bladed shutter, which allows about 50% of the light from the lamphouse to pass through the film and be projected on screen.  Some other systems that have been used by various manufacturers:

3 bladed shutter with a 5:1 intermittent.  Since images are flashing on screen at 72 times per second rather than 48 times per second, there is less perceived flicker.

Dual counter-revolving shutters. While one blade begins to block off the top of the picture, the blade of the other shutter is rising to block off the bottom of the picture.  Since the light is entirely blocked off when the two blades cross in the middle of the image rather than when one blade covers the entire frame, you effectively block off the light more quickly and can use a narrower blade, allowing a higher percentage of the light from the lamphouse to pass through the film.

Barrel shutter.  Rather than shutter blades, the shutter was a round barrel with matching holes front and back.  Light passes through the barrel only when it revolves to the point where both holes are lined up between the lamphouse and the film.  As the barrel revolves, the edge of one hole would begin to cut off the top of the picture, while the edge of the opposite hole would begin to cut off the light to the bottom of the picture, giving you the same effect as the dual shutter system. 


In all projectors the intermittent movement, shutter, and drive sprockets have to be running at exactly the same speed to project a steady image.  On some projector models this is handled by drive gears, while on others this is handled by a cogged drive belt.  If a tooth breaks on a gear or a belt, you may see film loops growing or shrinking or you may see ghosting.


Because early films were produced on highly flammable Nitrate stock, many projectors include fire traps.  These are closely spaced metal film rollers at the top and bottom of the machine.  Their purpose is to prevent fire from spreading from within the projector to the film magazine if the film were to catch fire.  A related component is the fire dowser.  The fire dowser is designed to prevent light from passing through the projector until the film is moving at a minimum safe speed.  This is to prevent the intense heat from the lamp from melting or igniting the film. 


Lens Speed

Lens “Speed” is a factor of how much light a lens collects, and gets on screen.  A higher lens speed more efficiently gets light to the screen.  In general, the higher the speed, the better.  However, a higher speed lens also has a shorter focal range, making it more difficult to get a sharp image.

Lens Format

A modern 35mm film image projected through a flat lens has an aspect ratio of 1.85:1.  The image is 1.85 times as wide as it is tall. 

A Cinemascope film has a compressed image on the film which is de-compressed using an Anamorphic adapter.  For a 35mm film, the aspect ratio of a scope image is 2.35:1.  Note that the anamorphic adapter is an adapter fastened onto the front of a standard lens.  The lens used to project a scope film in not normally the same lens as used to project the flat picture in the same auditorium - the selected focal length of the lens is probably different in order to get the desired image size on screen. 

In early films, the standard aspect ratio was 1.33:1. You may occasionally see this when older films are re-released.   



Exciter Lamp

The exciter lamp is a small, high intensity light bulb.  Light from the exciter lamp is focused by a lens into a fine beam of light which passes through the film soundtrack onto the solar cell.  Because of minor variations in manufacturing, the exciter lens must be refocused when an exciter bulb is replaced. 

To insure steady sound at the correct level, the exciter lamp has a regulated DC power supply which in most systems supplies it with 9VDC.  By comparison to other light bulbs, the exciter lamp has a very thick filament.  By using a thick filament at low voltage and high current, flicker is reduced. 

In recent years manufacturers have moved towards Light Emitting Diodes (LED’s) as replacements for tungsten filament exciter lamps.  LED’s are much more sensitive to voltage variations, so a more expensive, very well regulated power supply is required.  On the other hand, their life expectancy is much longer than that of a bulb and focus and light intensity do not vary as much with age.

Solar Cells

Solar cells on some old soundheads are made of cadmium sulfide (CdS).  Cadmium sulfide changes its electrical resistance depending on the amount of light falling on it.  A CdS cell has a glass covering over a yellow/green surface with a brown line zigzagging over the surface.

Solar cells on most modern soundheads are silicon solar cells.  A silicon solar cell generates a small electrical current, depending on the amount of light falling on it.  A silicon solar cell is dark blue or black. 

 The silicon cell responds more quickly to changes in light level, and will therefore give better frequency response.  Because of its better response characteristics, it can also be made smaller than the CdS cell. 

Forward/Reverse Scan

On a “Forward scan” system the light from the exciter lamp shines on the front, emulsion side of the film passes through the film to strike the solar cell. 

On a “Reverse scan” system the light from the exciter lamp shines on the back side of the film and passes through to a solar cell mounted in front of the sound track.

The accuracy of sound reproduction is not affected by the direction of the beam of light as much as it is by the actual components in the system.  On older Reverse scan Century sound heads, the solar cell was a large cadmium sulfide(CdS) cell.  On newer Century Forward scan systems, the solar cell is a small silicon cell.  


A typical motion picture screen consists of a large perforated sheet of heavy plastic mounted on a steel frame with springs or elastic material.  The perforations are to allow the sound from the speakers to pass through to the audience.  The springs (or elastic) are to keep the screen taut and flat as the plastic stretches and shrinks with changes in temperature. 

The surface of the plastic can be coated with different materials depending on the theatre’s needs.  In all cases, the standard that we must meet is 16 foot-lamberts of light reflected towards the audience at the middle of the screen.

A Matte screen has a flat white coating that reflects light equally in all directions. 

A Gain screen has fine glass beads that reflect a larger percentage of the light straight back.  This gives you brighter scenes than a Matte screen using the same lamphouse, so you have “gained” extra light.   

A Silver screen has a silver coating that reflects almost all of the light straight back towards the projector.  Silver screens were popular during the last revival of 3-D films because 3-D films require more light to get the same brightness in a scene.  The disadvantage of a silver screen is that it reflects very little light off to the sides.  If you are watching a film from the side seats of a wide auditorium the scene appears to be dark.

Screens are always cleaned with a soft camel hair brush to avoid damaging the light reflective coating on the surface. 


In an ideal situation, every part of the screen should be exactly the same distance from the projector lens.  If one side of the screen is farther from the lens, the image on that side will be larger.  If the bottom of a screen is farther from the lens (typical of an old 3-4 story theatre with a projection booth above the balcony) then a square projected on the screen would be wider at the bottom, or keystone shaped. 

Keystoning is often also a problem with large auditoriums that have been split into smaller auditoriums.  If the projector is not relocated, then projector will be close to the new wall.

The only way to correct keystoning is to tilt the screen from side-to-side or top-to-bottom until it is perpendicular to the lens, or re-locate the projector.  In theory, you might be able to use mirrors to bring the light beam from the projector perpendicular to the screen, but in practice it does not work very well.  Any vibration in the mirror will show on the screen.

Curved Screens

With a “normal” flat screen the edges of the screen are always farther from the lens than the center of the screen.  This problem is particularly pronounced when an auditorium is wide and shallow.  Some theatres have installed curved screens to correct this problem. 



Often when discussing sound systems, technicians will refer to the “A chain” or the “B chain”. 

The “A chain” consists of the parts of the sound system that first processes the sound.  This would include the solar cell, pre-amplifiers, and sound processors.

The “B chain” consists of the parts of the sound system that carry the processed sound to the auditorium.  This includes the power amplifiers, crossover networks, stage speakers, surround speakers and subwoofers. 


One channel, one speaker. 


On a Dolby Stereo encoded print, there are two soundtracks side by side.  These two soundtracks have a total of 4 channels of information encoded on them.  One track is the Right channel.  One track is the Left channel.  The Center channel is encoded on BOTH tracks.  The surround channel is encoded as the DIFFERENCE between the two tracks. 

The Dolby processor also is designed to reduce noise.  During the recording process the frequency bands where noise is usually picked up during reproduction are boosted.  Then during playback the same bands are cut back to their normal level, cutting the undesirable noise at the same time.


THX is not a sound system per se but a standard for sound quality set by Lucasfilms.  To be certified for THX, an auditorium must meet standards of background noise and reproduce sound with little distortion over a wide frequency range as well as meeting desired sound levels.  A film “presented in THX” is not recorded with a special soundtrack.  It is usually a standard Dolby Stereo encoded optical print.


In essence, a digital system is designed to eliminate all noise from the movie sound track.  With an analog system, the soundhead reads a varying beam of light or varying magnetic field to reproduce the sound.  Scratches, dirt on the soundtrack or anything that varies the beam or field will be reproduced through the sound system along with the soundtrack.

With a digital soundtrack, the sound is recorded on the film or on a separate CD Rom disk as a series of digital numbers.  The numbers are fed to a computer which converts it to sound.  If there is a scratch on the soundtrack the computer does not see a number for that fraction of a second, and is programmed to handle the glitch, rather than send a “crackle” or “pop” to the speakers as an analog system would. 

By eliminating the noise, the soundtrack range can be expanded.  Very quiet noises like whispers and a dog barking in the distance can’t be recorded on an analog soundtrack because the noise created by a little dust on the soundtrack can be “louder” than the desirable sound.  If the volume is set loud enough to hear the quiet noises, then the dust and scratch noises are loud enough to disturb the audience. 


A projectionist should have a basic understanding of electronics in order to handle simple wiring and repairs.  Following are some very basic bits of information on electronics.  Even basic training in electronics is beyond the scope of what I can cover here.  I recommend that anyone training to be a booth technician get a beginning book on electronics.  The following books can be found at Radio Shack: 


This is a very basic book on electronics.  It covers uses very simple language and does not attempt to cover anything beyond the basics.


This book covers the basics and goes on to slightly more complex concerns.  It is designed as an instructional manual, with a quiz at the end of each chapter to test your comprehension.



When working with simple electronic circuits, such as wiring speakers the voltage present in a circuit (E) is equal to the total current (I) times resistance in ohms (R). If you know any two of these values in a circuit, you can always calculate the third.  For example, if you have 12 volts passing through an 8 ohm resistance, the total current can be represented by I=E/R or I=12v/8ohms or 1.5A.


Power (in Watts)=Voltage(E) times Current(I)

In the same circuit as above, 1.5A times 12 volts = 18Watts.


When speakers are wired together they can be viewed as simple resistors in a circuit.  When resistors are wired in series with another, the total resistance in the network equals the sum of all resistors in the network:

Rtotal = R1 + R2 +R3 +R4 .......

If resistors are wired in parallel, the total resistance in the network equals the inverse of the sum of the inverse of all resistors in the network:

1/Rtotal= 1/R1 + 1/R2 + 1/R3......

If you have four 8 ohm surround speakers that will be wired together, the total resistance of the network would be 32 ohms if wired in series (8+8+8+8), and 2 ohms if wired in parallel (1/(1/8+1/8+1/8+1/8)).

Now suppose we have a typical amplifier designed to drive 8 ohm speakers.  With the resistance of the network too high (32 ohms) or too low (2 ohms) you will see distortion.   To correct this problem and let the amplifier “see” a load of  8 ohms, we set up a series AND parallel network.  Connect two of the speakers in series to get a small network with a total resistance of 16 ohms. (Rtotal = 8 + 8).  Repeat the process with the second pair of speakers and you now have two small networks, each with a total resistance of 16 ohms.  Now wire these two networks together in parallel.  Rtotal = 1/(1/16 + 1/16) which equals 8 ohms.    


A capacitor consist of two metal plates charged with opposite polarity, separated by an insulator.  In an electrical circuit, the plates serve as a “reservoir” for positive and negative charges.  In DC circuits, they are used to filter power supplies.  When wired in parallel to the power source, if the current flow drops for a fraction of a second, the charge from the capacitor “makes up the difference” and keeps the current flowing smoothly.  If wired in series to a power source, the capacitor would allow positive and negative charges to build on its plates until it reached it’s capacity, then no longer allow any current flow.

In AC circuits, capacitors will allow current to pass through, depending on their frequency.  A small capacitor will pass high frequency signals, but tend to block lower frequency signals. 



An induction coil consists of a coil of wire wrapped around a hollow tube or a metal core.  When a current is passed through a coil a magnetic field is formed.  Magnetic fields resist change.  In an AC circuit - an induction coil will act to block rapid changes (i.e.: high frequencies). 


By combining the effects of an inductor and a capacitor, we get a simple speaker crossover network.  A small capacitor is wired in series with the high frequency horn - blocking low frequency signals, but allowing high frequency signals to pass.  An induction coil is wired in series with the low frequency speaker, allowing slow changing low frequency signals to pass, but blocking out the high frequencies.



Transistors of all types consist of silicon or germanium impregnated with positive or negative ions.  By arranging this “P” and “N” type pieces of silicon in different ways we get transistors, Triacs, FET’s and other assorted electronic components.  In a simple NPN transistor, the source of current or “emitter” is N, the base material is P and the “collector” which collects current for the load is N.  When there is a negative charge applied to the base, no current can flow from the emitter to the collector.  The transistor is “off”.  When a small positive charge is applied to the base, current flows from the emitter to the collector.  The transistor is “on”.  This effect is used to allow a small change in current to control a larger current flow - amplifying a signal.