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A soundhead is a device that reads sound-on-film soundtracks. This includes analog optical tracks, digital optical tracks, magnetic tracks, and DTS timecodes. On 35mm projectors, the optical soundhead is typically a separate component located below the projector head, known as a basement reader. Magnetic soundheads, DTS timecode readers, and some digital readers are located above the projector head, and are referred to as penthouse readers. Traditional analog readers can also be installed as penthouse readers if a digital delay is employed.
In addition to reading the soundtrack, many soundheads also serve an important mechanical function. Often the motor is connected to the optical soundhead, and the soundhead drives both the picture head and the takeup.
Methods of Stabilization
One of the most important factors in soundhead design is the method of stabilizing the film as the soundtrack is being read in order to ensure a constant speed, thus eliminating erratic changes in frequency, a problem described as wow and flutter. Early optical soundheads employed sound gates with a spring-loaded shoe to stabilize the film, but the standard method is to use a sound drum connected to a heavy flywheel. Some soundheads employ a Davis tight loop, in which idler rollers on tension arms are positioned before and after the reader. This method is employed in Century optical soundheads, many magnetic soundheads, and Dolby Digital penthouse readers. Other soundheads use a pinch roller that presses the film against the sound drum to stabilize it. Simplex, RCA, and Kinoton optical soundheads employ this method. While Simplex and RCA rollers make contact with the picture area, Kinotons use an O-ring on the inboard (non-soundtrack) film edge to press the film against the drum, reducing the risk of wear in the picture area.
In basement readers, holdback sprockets are often employed between the sound reader and the takeup to isolate the reader from any tension variations on the takeup. Although uncommon, spring-loaded idler sprockets are occasionally employed between the intermittent and the sound reader. DeVry XD and Simplex SP portable projectors use this design.
35mm optical soundheads originally scanned the soundtrack from the base side. Because the image of the track printed on the emulsion side was refracted through the transparent film base, this made it more difficult to read. Modern optical readers reverse that orientation, with the light source on the base side and the cell on the emulsion side, resulting in a higher quality scan. This method is called reverse scanning.
For 16mm film, the scan orientation has never been standardized because the emulsion orientation will vary depending on the number of generations between the projection print and the original camera negative. If the film is a positive print struck from a negative, the emulsion will face the lamp. If it’s a reversal print, the emulsion will face the lens. On some projectors, such as the Kinoton FP-16 and some models of Kodak Pageant, there is a lever that adjusts the focus of the sound reader from the lamp side to the lens side of the film to account for this variation. For 16mm projectors without this option, the soundhead calibration may be performed to find an average setting that will work for both emulsion orientations.
Optical soundheads originally used exciter lamps with incandescent filaments. These were eventually replaced by LEDs.
When the industry switched from silver soundtracks to cyan soundtracks in the 1990s, readers using white light were replaced with those using red light.
Sound readers typically run on DC power supplies, because AC power will produce a hum at the utility frequency (60Hz in North America, 50Hz in some other regions). Using a more powerful exciter lamp can mitigate this issue because the signal requires less amplification.
Impedance scratches are caused by film making contact with the sound drum when the drum is not yet up to speed, if the bearings on the sound drum shaft have failed, or if some other issue is preventing the drum from moving freely. Impedance scratches can also be caused by running a film without sufficient leader, or by stopping and starting a film in the middle of the reel. Kinoton’s ES line of studio projectors avoids this problem by using a magnetic clutch to decouple the flywheel when the film is not up to speed.
If the takeup tension is excessive, the holdback sprocket can stress or tear the perforations.