Dolby Tech Page

3.1 Analog

3.1.1 Frequency Response

The equalized A-chain frequency response, measured at the Lt and Rt pre-amplifier outputs, should be flat to within ⁺/-1 db from 30 Hz to 14 kHz. When a pink noise test film is used, and the pre-amplifier response measured in third-octaves, the output should be flat up to and including the 12.5 kHz band, and no more than 3dB down in the 16 kHz band.

Later versions of optical pre-amplifier boards installed in Dolby theatre equipment exhibit improved stability, and better linearity and phase response at high-frequencies. CP50 and CP200 units should ideally be updated with CN108 pre-amplifier cards of Revision C. All CP55 and CP65 unit pre-amplifiers have adequate performance. See Section 13B below.

A complete discussion of A-chain alignment can be found in each Dolby cinema processor manual.

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3.1.2 Slit

This bandwidth specification ideally requires a projector sound-head slit height of around 0.00075". A slit height of 0.00100" is unacceptable as impossibly large amounts of hf boost are needed to achieve a flat response above 10 kHz. A slit height of less than 0.00050" performs better at high frequencies, but the need for increased pre-amplifier gain may make the system more prone to spurious interference resulting from stray light landing on the solar cell.(l)

The pink noise on current Cat. No. 69 optical test film has a frequency response nearly flat up to 16 kHz, as shown in the calibration chart in Figure 3.1. The old black and white Cat. No. 69 test film has been replaced by two new test films. Cat. No. 69T (Dolby Tone) and Cat. No. 69P (pink noise) are both now printed on color stock, which represents release print behavior more accurately. The color stock fades, however, so the test films should be replaced after about six months of use.

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3.1.3 Exciter Lamp/Supply

The exciter lamp supply should have a regulated output.

An unregulated exciter lamp supply results in gain variations, bad decode tracking, and can cause audible hum.

A well-designed regulated supply will have a ripple level of less than 3%.

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3.1.4 Illumination Uniformity

The projector sound-head optical assembly should provide uniform illumination across the slit. When measured with a snake-track test film, output variations should not exceed ⁺/- 2dB from the average level.

Uneven illumination along the slit is frequently the cause of badquality optical sound playback. Level-dependent distortion and bad stereo imaging are the two most obvious results.

The most likely causes of uneven illumination are:

Dirty optics or slit
Misaligned optics
Dirty or carbonized exciter lamp
Insufficient voltage for exciter lamp

and less likely:

Uneven slit width resulting from bad machining

One method of checking illumination uniformity is to run a scanning beam uniformity test film (snake-track) loop (SMPTE Test Film No. P35-SB or equivalent), and to evaluate the summed Lt and Rt preamplifier outputs (or the center channel processor output with the unit set to mono, Format 01) with an AC milli-voltmeter, or preferably an oscilloscope.

A new test film is also available from Dolby Laboratories to check illumination uniformity quickly and simply. The Cat. No. 566 test film provides an instant visual display of any illumination problems and only requires the use of a real-time analyzer.

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3.1.5 Wow and Flutter

The projector should exhibit no audible wow or flutter.

Most contemporary first-class projectors have sound-heads designed and manufactured to a quality such that wow and flutter will not prove a problem. Badly maintained older projectors and some inferior contemporary designs, though, can have severe sound-head transport speed problems -- detectable speed variation is one of the few failings that can render a well-aligned A-chain playing an SR optical film audibly inferior to a l6-bit digital system.

It is generally accepted that flutter should be less than 0.15% DIN weighted to be inaudible. Measurement methods (2) are described in IEC 386, and a suitable test film is available from the SMPTE (No. P35-FL). A subjective test for wow and nutter is contained in the Jiffy test film from Dolby Laboratories, Cat. No. 251.

If cleaning and lubrication of the sound-head is carried out according to the manufacturer's recommendations, and the measured flutter is still unacceptable, a skilled mechanic should be consulted. In some cases, machining and balancing the flywheel and sound-drum, or the rebuilding of bearings, can reduce the problems to an acceptable level.

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3.2 Digital

It is important to note that factors such as mechanical alignment, illumination uniformity, and print cleanliness which affect sound quality of analog playback in a more-or-less linear way, may have no immediate audible effect in digital playback, but serve to reduce the margin for error when defects such as scratches or dirt are encountered.

Figure 3.2 shows, in symbolic form, the effects of misalignment, uniformity of illumination, wear, and cleanliness on the reproduction of analog and digital prints.

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3.2.1 Alignment

Digital soundheads manufactured by Dolby Laboratories are factory aligned, and no on-site adjustments to the soundhead should be required. Verification of correct alignment is described in the DA10 digital film sound processor installer's manual. Mechanical alignment of the Soundhead with respect to the projector film path may be verified by threading a length of film through the Soundhead and observing equal tension on each edge.

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3.2.2 Wow and Flutter

The elimination of flutter components at frequencies greater than 84 Hz which are not present in the master recording is inherent in the reproduction of sound by the digital processor. This is because the output sample rate is held constant for the duration of a perforation. Lower frequency flutter and wow from mechanical inperfections are attenuated by digital filtering of the sample rate generator control function. Steps in sample rate are limited to approximately 0.005%, a factor of around 30 below the audible minimum. A much higher slew rate" of about 6.4% / second (about I semitone / second) is permitted during projector ramp-up to speed in order to avoid getting out of synchronization with the picture.

Although digital processors from Dolby Laboratories are designed to track projector speed variations with a range from -7% at 24 fps to +7% at 25 fps, best performance will be obtained by maintaining projector speed as closely as possible to the intended speed as recorded.

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3.2.3 Illumination Uniformity

The illumination source for Dolby Digital Soundheads is a tungstenhalogen projector bulb. Uniformity of illumination may be verified by observing the video signal at the DA10 on a 25MHz oscilliscope, triggered by any Dolby digital signal; the process is fully described in the DA10 and DA20 Installion manuals. Illumination should be uniform over the width of the perforation area to within ⁺/-0.5 Volt.

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3.2.4 Exciter Lamp / Supply

The digital soundhead exciter lamp supply should be a DC regulated power supply set for 11 Vdc, supplying approx. 6 Amps for each lamp and fan. Polarity must be observed, not for the lamp, but to ensure that fan rotation is correct. In an emergency, any DC supply with adequate capacity and regulation of 3% or better will serve.

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3.2.5 Soundhead and Surround Delay

Proper adjustment of Soundhead Delay and Surround Delay are critical to setup of the DigitalProcessor. Complete procedures for delay adjustments are contained in the DA10 and DA20 Installer's Manuals. It should be noted that the optimum surround delay setting for a digital processor is different from that for surround playback of an analog surround track. The requirement for playback of a digital surround channel is to achieve coherent arrivals of screen and surround sounds over the optimum seating area. For an analog sound-track, slightly greater delay is required, so as to achieve masking (3) of any crosstalk from front channels to the surrounds.

Figure 3.3 provides a table of approximate delays for digital surround delays. Aproximately 15mSec should be added to these numbers for analog surround delay settings. In a small theatre, the height of the surround speakers should be taken into account, bearing in mind that the table is based on the difference in path lengths at the optimum seating area to the screen channels and the closest surround loudspeakers.

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1 Most projector manufacturers offer retro-fit slit lens assemblies with a slit height of 0.00050 or 0.00075". In addition, slit-lens assemblies for a variety of projectors are manufactured by Sankor Ltd., distributed by Marble Company Incorporated, P.O. Box 160080, 421 Hart Lane, Nashville, TN 37216, Tel: 615-227-7772, 800-729-5905, Fax: 615-22&1301.

2 A suitable meter for measuring wow and flutter is available from Leader instruments, Japan-Model Number LFlM-39A

3 For a discussion of how acoustic masking is used to suppress crosstalk in 4:2:4 matrix systems, see "Multi-Channel Audio and Surround Sound in the Movie Theatre and in the flome," Dolby Pub. No. S88/8292

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