ANSWER: Although the application wasn't a new concept, radio sound for drive-ins began catching-on with low power AM systems in the early 70s. Its attractiveness lied in the reduction or elimination of in-car speaker theft, speaker maintenance headaches and upkeep costs. Increased aural quality was an added benefit.

These AM "carrier-current" systems, as they are known, use the existing underground speaker wiring as a transmission "antenna". This allows reception to be conducted through what is known as the "induction" field of the radio wave; which is an area of electromagnetic energy existing around the wires carrying the radio signal. The strength of this signal decreases rapidly as the receiver is moved away from the source, making carrier-current systems ideal for a drive-in, where reception outside the theatre grounds is undesireable. It is important to remember that above-ground aerials are illegal in AM carrier-current applications.

Using an aerial antenna generates a "radiation" field, which carries the signal further. This type of emission can interfere with commercial broadcast stations and is therefore illegal.

The Federal Communications Commission (FCC) has deemed that FM systems are illegal for drive-in theatre use, however, many locations are getting by with it. Apparently, there is an FCC rule which permits low-power (under 50 milliwatts) FM transmitters to operate in the broadcast band without a license, and the theatres are courting this loophole. Either that, or the FCC doesn't consider drive-ins worth prosecuting.

Low-cost FM and hobby transmitters, many complete with stereo generators, are available in the classified sections of radio and electronics magazines. Prices range from $60 kits to $1,200 modulators designed for cable-tv use. Use them at your own risk!

At any rate, the majority of drive-in theatres now use some form of radio sound system.

QUESTION: How far can a drive-in's FM radio sound signal go? Is 50 milliwatts enough? What type of antenna would be needed?

ANSWER: Given that all the elements in the transmitting system are optimized (more on this below), FM coverage depends largely on antenna height and surrounding terrain. It's considered a 'line-of-sight' mode, which is easily attainable in most drive-ins; while AM depends more on output power as the limiting factor in its coverage capability. Consider that reception of an FM station may be lost after several miles distance, but then the same station may be picked-up again a hundred miles down the road! Conversely, you could solidly receive an FM broadcast station for over a hundred miles with no interruption...if the antenna were high enough and the terrain flat.

50 milliwatts isn't much, and it would be enough to cover the several-hundred-foot radius that would encircle a drive-in theatre...IF there were no other stations on the frequency (there's always one, somewhere). I've seen drive-ins run their FM transmitters around 20 watts or less. This gives plenty of "punch", to overcome any signal from out of town.

When planning radio sound system installations, remember some basic guidelines:

-Choice of FM transmitting frequency
(a) Stay several channels away from local stations.

(b) Ideally, pick a frequency that is 'sandwiched' between two distant stations that are moderately receiveable. For example, if you receive somewhat weak stations, say, on 93.1 and 93.5 MHz, choose 93.3 MHz as your frequency. This way, as soon as you drive away from the theatre, capture effect will cause the two bordering stations to mask your signal, as they fight for dominance in the discriminating circuit of the receiver.

-FM antenna system considerations
(a) It is of utmost importance to have the antenna system tuned to the frequency your transmitter is operating on. This is where most installations fall ridiculously short and result in complaints that the signal isn't "getting out". With a mis-matched antenna, part of the signal is reflected back into the transmitter, where it is wasted as heat. If the reflected power is great, it can destroy your transmitter.

(b) Orient antennas in a vertical plane, to match the orientation of receiving antennas.

(c) 1/4 wave, vertical antenna installation is the same as for an Amateur 2-meter FM rig. Complete instructions and parts can be found at your local Radio Shack store.

QUESTION: Can you see a movie on a drive-in screen during daylight? If not, what would it take to do so?

ANSWER: No, you can't. (The inevitable comparison to television often follows) "Well, then, how come I can see TV outdoors, huuuuh?"

You can see television outside (though not very well, if the sun is shining directly on the screen) because you're actually seeing a phosphor on the surface of the TV screen. In a way, it's like looking at a glowing ember.

A movie on a screen, in any theatre, is an image of an illuminated film frame. The light rays comprising the image are physically travelling through the atmosphere and falling upon a surface, which is subject to competition from ambient light. Ambient light from the sun is billions of times greater than the incident light produced by a projection system.

Furthermore, the light passing through any projection system is doomed before it even leaves the projector, due to the mechanics of motion pictures. Consider this: the 4,000 watt light source in a typical drive-in projector has to travel first through a shutter, where 50% or more of it is lost immediately; then through an opening the size of a postage stamp to illuminate the film. Next, the light travels through several layers of glass (each with it's own loss). The few remaining rays are then spread out over an area, as expanse as 6,000 sq. feet. (the screen). A projected screen image cannot even remotely compete with the sun.

So, to summarize, the answer to your questions are:

a.-No, you can't see it in daylight.
b.(l) -It would take darkness. -OR-
(ll) -Something brighter than the sun? ; )

QUESTION: What about the so-called "daylight" screens, that were developed for drive-in operation in daytime?

ANSWER: You're referring to a containment screen, which was an array of concave, mirrored surfaces that were tuned to 'contain' a drive-in's picture within the area of the theatre. Viewers looking at the screen from outside only saw a blank screen. The screens were also touted to allow drive-in operation in daylight, which more accurately, meant the subdued light of early evening. Containment screens would allow a drive-in to get started as much as an hour or so earlier, however, it could not compete with direct sunlight. Only one or two of these screens were ever built, and then only as prototypes. They simply were too expensive to be practical.

QUESTION: What happens to the picture if it's raining? Is it affected?

ANSWER: On the screen, not by much. Watching from a projection booth, it looks fine, unless the rain is blowing hard, straight into the projector port. Then, water will collect on the glass (or the lens!) and blurr the picture. Water on the screen surface doesn't hurt either, but gives the picture a shimmery quality as the light is refracted by the rain droplets. Otherwise, viewing of the image is diminished to the same degree as would looking at any other object through a rain storm.

QUESTION: What companies make the top-notch movie projectors?

ANSWER: Different components make up a 35mm "projector". A basic system of equipment to reproduce sound motion pictures consists of:

-projector mechanism (or projector "head)
-sound reproducer mechanism (or sound "head")
-lamphouse and associated power equipment (the source of light)

The above is very simplified, but these are the core components of the machine proper. I have not included any sound amplification equipment.

The projector is mounted atop and driven by the soundhead (reproducer), which usually carries the system's drive motor. The soundhead is attached to either a floor pedestal or console, which also provides support for the lamphouse. The lamphouse is supported in such a way as to be aligned behind the film gate in the projector head.

There are different makes and models of each of these components, from a handful of manufacturers, and many makes are interchangeable with one another. However, some manufacturers have equipment that will only work with their own units, or have units that combine the projector and soundhead into one unit case.

Most American machines are built around distance and bolt-pattern standards established in the 1920s and 30s. So, a theatre can have any combination of equipments that make up a single "projector". The varieties are endless.

So, with this in mind, I answer your question with the following; my favorite high-performance combination, for a drive-in setup:

-Century S.A.W. projector head (single shutter, water-cooled aperture)(honorable mention: Simplex XL mechanism)

-Century R3 Reproducer fitted with Kelmar L.E.D. reverse analog reader (honorable mention: Simplex SH-1000 reproducer)

-XeTron XH-4000 Lamphouse and IREM power supply (4,000 watt Xenon light source)

QUESTION: I have been asked to set up a small Drive-In Theater (using 16mm equipment) at a collector's show. We have selected the site and plan to use a hill and mount a screen on the side of a large camper.

Am I crazy or do you think this will work?

ANSWER: No, you've not lost it, it can work fine. Your only limitations will be the size of the screen and how dark you can make the drive-in area.

I have seen "portable" drive-ins, just like the one you are planning, achieve nice results with standard off-the-shelf Bell & Howell 16mm projectors, using a regular 250 Watt, incandescent projector bulb. One portable drive-in I know of, operated fine with color film on just such a projector; with a lot of stray light from the road, too! I'm guessing their screen was 10 or 12 feet wide.

In B & H machines, however, do not get anything older than a 1500-series projector, and preferrably a 2500-series. The earlier technology won't carry you.

You can also rent portable 16mm projectors with a Marc 300 point-source arc lamp, which is much brighter and has a whiter color temperature, but is probably not necessary for your application.

Whichever way you go, the trick is to keep the screen as small as you can get by with, to have the brightest image possible. With the standard projector bulb, I wouldn't go larger than a 15 ft wide screen. Keep the screen's "back" towards the area with the most stray light (i.e., from street lights, passing cars, buildings, etc). This increases the apparent contrast and makes even the cruddiest images appear acceptable.

Sounds like a great idea, good luck. Be sure to take some pictures!