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Improved Disc Mastering System

Posted by peahix on April 23rd, 2006

IMPROVED DISC MASTERING SYSTEM

The existing state of Photo-Optical disc masters provides for sustained modulation of extremely high audio quality. However, the “Joint” area of the concentric sound track is currently a random additive function of the superimposed wave forms which form the beginning and end of the track as it is closed into a circle. Being of random wave form, the joint itself will display geometric structure unique to the rest of the track; therefore, when reproduced in context, the joint will modulate the reproducer in a manner unlike any other portion of the sustained track. The audible result will be so-me type of an undesirable sound (or noise) at the joint.

An improved disc mastering system will produce “Joint” areas which will coincide in wave form to adjacent geometric structures, and will therefore be audibly undetectable.

INCREASED SOUND TRACK DIMENSIONS

The question is often asked, “Why don’t optical disc sound tracks sound as good as ordinary movie sound tracks in a motion picture theater?”

This question has no simple answer — there are a number of contributing factors (most of which deal with the playback device, not the recording itself). The problems must be attacked one by one.

One major problem area is the size of the track itself. Motion picture theaters generally project 35mm prints containing sound tracks which reach maximum (100% modulation) peak dimensions of 76 mils (.076 inches). Even 16mm prints (generally considered inadequate for commercial theater use) have 60 mil sound tracks.

Existing optical discs reach peak dimensions of 30 mils. This dimension should be increased (hopefully to 50 or 55 mils, depending on the total number of required tracks).

The resulting increase in audio output and effective photographic resolution would represent improved audio quality (especially at high frequencies), less distortion, and an improved ratio of signal to noise.

IMPROVED DISC PRODUCTION TECHNIQUE

Discs, in quantity, must obviously be secured by means of photographically reproducing the original master disc. Existing technology provides for a retention of approximately 60% to 70% of the master disc’s resolution.

Improved disc reproduction techniques shall provide for retention of approximately 85% to 90% of original photographic structure, resulting in more accurate reproduction of mid and high range frequencies and a corresponding improvement in the signal-to-noise ratio. Audible distortion (a direct result of inaccurate photographic reproduction) will be greatly reduced, especially if used in conjunction with larger tracks which will be less photographically critical.

PRE/POST EQUALIZATION CURVE

Existing photo-optical discs contain sound tracks which, for the sake of the overall system signal to noise ratio, are constantly maintained at a maximum modulation level. Recording pre-emphasis of high frequencies is therefore not practical.

However, if larger tracks were employed, such pre-emphasis could be used in conjunction with an overall attenuation of the entire modulated spectrum. By this method, a pre/post equalization curve could be established (something similar to the common “RIAA curve” as used throughout the Phonograph Record Industry).

The system would involve recording discs with intentional over emphasis of high frequencies, according to a specific predetermined response curve. Such discs would sound quite thin and “shrill” – if played back in a reproducer that provided “flat” response.

However, all reproducers would be manufactured with a built in “roll-off” toward high frequency response. A specific response curve — the inverse of that used during recording.

Thus, with the record and playback curves effectively cancelling each other, the net result to the program material would be nill. Flat signal response would therefore be restored; but only for the program material. All other modulation (specifically, film base noise) would fall subject to the reproduce de-emphasis curve only. Film noise would therefore be suppressed, and the ratio of signal to noise would be improved.

PHASE LOCK DRIVE

Phase locked drive systems are quite common. Both the advantages (functional) and disadvantages (cost) of such systems are well known.

However, I feel the application of a phase lock “loop” drive system to an optical disc playback device is highly deserving of serious consideration.

Such a system would not regulate its speed to the standard A.C. “wall voltage” 60 Hertz, but would rather rely on a pre-recorded tone on the disc as a hysteretic constant. The drive motor would “seek” correct velocity by monitoring the disc itself. The integrating amplifier which drives the motor should be capable of shifting slightly above or below the reference tone on the disc so as to slightly accelerate or decelerate the drive system, or even to effect a slight rapid variance up and down. Musically, this would produce sharp or flat deviations from true pitch, and real vibrato.

The musical advantages of such a system are obvious. However, the additional advantages in regard to production and service should not be overlooked. As a system, a phase locked drive would rely, for calibration, on the disc an easily controllable standard. Both assembly and service calibrations would, therefore, be greatly reduced.

DRAPED DRIVE SYSTEM

Existing photo-optical disc playback techniques involve scanning a disc which is assumed to be lying flat. The assumption is basically erroneous the rotating disc, being flexible, will continuously raise and lower itself in some regular form of cyclic repetition as it turns. Thus, the critical distance between the disc and the detector head will not remain constant. Two severe problems result:

1. Critical detector head placement (in relation to the disc surface) is not possible. Instead, the head must be placed at a maximum height in an attempt to avoid physical contact with the disc. Contact still often occurs, resulting in contaminated heads and scratched discs.

2. As the disc raises and lowers while turning, the critical scanning height at the detector head varies. This results in a proportionate (inverse) variance in resolution, which in turn effects high frequency response. The result is a cyclic, “once-around” sound which is highly audible. This phenomenon should NOT be confused with the disc’s “joint” area noise -these are two separate problems. However, they do compound each other in that the repetitive “cyclic” response deviations keep re-occurring with each revolution to form a prelude to the joint! Together, the two present a highly recognizable (and extremely undesirable) “pattern” to the sound as a whole.

The only solution to this problem is to stabilize the disc. Of course, it is not necessary to stabilize the entire surface of the disc; efforts need be concentrated only on that area which is being scanned.

Toward this end, I recommend “draping” the disc over a common axis, as if across two pulleys (one being the drive pulley) which are in line with the center spindle. If supported by these two pulleys (placed at 12:00 o’clock and 6:00 o’clock as viewed from the top), the edges (3:00 o’clock and 9:00 o’clock) would drape naturally. Gravity could be “helped” a little by pulleys above the disc holding the edges down.

The flexible disc would continue to conform to this shape while turning, forming an immobile surface across the vertex of the arch. Even a crease in the disc would momentarily flatten as it crossed the apex.

But most important, the film passing over the two support pulleys and across the top of the arch would form a perfectly straight line when viewed laterally. The detector scanning system would be mounted directly over this line.

The advantages are numerous:

1. A stabilized optical scanning area, providing both a uniform resolution curve and a mandatory prerequisite to an improved optical scanning system.

2. No internal head wear;

3. No internal disc wear;

4. Greatly simplified head alignment, both in production and service.

FOCUSED OPTIC REPRODUCER

The development of a focused optical reproduction system for multi-channel disc playback represents an extremely ambitious task. Less optomistic (or more conservative) opinoins will claim it to be impossible; required hardware will be difficult to find, and in some cases will have to be created specifically for the project; and costs, though not necessarily astronomical, will certainly be a factor for consideration.

However, I am of the opinion that two evident factors should easily be sufficient to offset any anticipated problems:

1. Some years ago, during the conception of the photooptical disc, the above mentioned problems had to be overcome to an extent far outweighing their current status. Pessimism from the “Can’t be done . . . Impossible!” school of thought ran rampant. Drawing board concepts called for non-existant apparatus. Cost analysis looked unfeasible. But a small group of progressive “dreamers” carefully tempered their enthusiasm with reality, laid out a careful plan of attack, and went to work. The “impossible” became reality. What man hath done, man can do.

2. Commercial motion picture sound projectors essentially represent rather expensive devices for correctly playing back single sound tracks. My proposal involves building a relatively inexpensive device which will correctly play back many sound tracks. The task will obviously be difficult. However — it would be unfair to consider the problems without an equal consideration of the possible benefits: a focused optics system could potentially provide the ultimate in high fidelity distortion free photo-optical reproduction. Film noise, throretically, would be drastically reduced. Fingerprints and even most small scratches would not reproduce. Audio fidelity could be on a par with magnetic tape reproduction.

In short, a focused optical system could very easily represent the most dramatic improvement to photo-optical disc technology to date.

All photo-optical playback devices function on the basic principle of interupting (modulating) a narrow beam of light which is detected by a photo-electric transducer, the output of which is then amplified and presented as audible sound.

An extremely simplified (but optically correct) method of producing such a device might be considered as having three basic stages:

1. Illumination. This stage consists of a source of light (generally, an incandescent lamp filament), and a condensor system housing one or more lenses, sometimes operating in conjunction with a reflector.

The purpose of this initial stage is to provide ample illumination. The bulb alone is insufficient, since its light output quickly falls off at a ratio inversely proportionate to the square of the distance from the filament. A condensor lens will “absorb” the light striking its entire frontal surface, and converge all of it down to a small, intense beam (in the same manner in which a fire can be lit using dry leaves, a “magnifying glass”, and the sun’s rays).

2. Objective Stage. This is where the real optical “trickery” takes place. It is at this stage that the (now intensified) light is interupted (modulated) by the waveform. tracings of the film’s sound track.

The objective stage is the real heart of an optical playback system. It’s prerequisites are (I.) ample light (as provided by a condensor lens), and (2.) a stable film plane (which would be provided by a “draped” disc drive system).

The objective stage consists of a series of lenses and an optical mask or slit (NOT to be confused with the existing slit). In effect, the lenses would magnify the portion of the track being scanned and “read” it (through the mask) at larger than actual size. Thus, the sound track’s fine structure (high frequencies) which are currently too small to be “seen” by the existing detector system would be reproduced.

The objective stage (which operates something like a tiny projector) would then “pulse” the now modulated light onto the photo-electric transducer. But herein lies a critical difference from the existing detector head system: the objective stage would be constructed so as to project MODULATED LIGHT — NOT AN IMAGE OF THE FILM. The modulation contains mostly music. The film image contains mostly noise.

3. Photo-electric transduction stage. At this stage, a final collector lens would receive the now modulated light in fast “bursts” (corresponding to the track’s waveform), and converge it onto the face of a photo-electric cell.

The principles involved in this proposal work. I state them as being sound, functioning phenomenon just as emphatically as I deny any personal credit for having “invented” them. After all, people were building sound projectors with focused optical systems long before I was born (these methods go back to the early 1920′s)!

However, it’s going to take some work to figure out how to do it for a whole disc’s worth of tracks all at once — and for a reasonable price!

However, an R.&D. program to devise such a system offers two tremendous advantages:

1. By contrast, the existing system offers nowhere to go but up! We don’t have to convert our existing product into a high-precision optical instrument to get vastly improved results — the Law of Diminishing Returns is on our side! If we can build something tantamount to a $19.95 plastic lens Kodak Instamatic, it will sound much, much better than our existing “Lens-less Wonder” (which functions, quite literally, as a “pin-hole” camera).

2. The initial stages of an R.&.D. program will involve a small optical stage, a few lenses and pulleys, a disc, and a motor. Not very expensive. So for once, we can answer a lot of questions without spending very many dollars.

Needless to say, I am strongly in favor of commencing on this A.S.A.P.

- Mike LeDoux

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