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Correction of Wow and Flutter ArtifactsTheoretical Implications for Analog Signal Degradation
Machine speed instability in the motion picture and audio recording process is a well-known phenomenon. Two artifacts commonly known as “wow” and “flutter” can conspire to ruin a sound track. Heretofore, rejectable wow and flutter anomalies were unsolvable audio problems – absent finding an alternate “unflawed” source. These audio roadblocks have now been removed thanks to a unique technology called Clarity Audio Restoration (Clarity) by Plangent Processes. Clarity is a combination of proprietary DSP (Digital Signal Processing) and hardware for the playback of 35mm magnetic sound film and audiotape to correct wow and flutter due to machine speed instability.
It is well known that even “Rolls Royce” audio recorders and playback machines like Albrechts and Studers have a published wow and flutter specification. Wow refers to irregular cyclical motion, which creates variations in the pitch of a sound track (usually at a slow rate), while flutter is attributable to similar deviations in the transport at a higher rate of occurrence. Regardless of the quality of the equipment, all analog recordings suffer from these flaws. When gross errors occur, even the untrained ear can hear the problems associated with wow and flutter. More subtle errors manifest as masking phenomenon, and listener fatigue. Clarity is the first technology addressing these problems utilizing a novel method of “re-timing” the audio signal.
To re-time the audio signal, the special Clarity transfer equipment recovers signals in the ultrasonic region that can be found on the tape or film along with the audio of the original recording. By ascribing to these ultrasonic components the properties of a moving clock (think varying sample rate), and mathematically retiming these signals such that the clock is crystal-steady, the dsp now “knows” the speed fluctuations of the original machine occurring at the moment it made the recording. By inverting the error signal, and conforming the corresponding audio to it, Clarity’s dsp retimes the audio to a fixed and stable time base. The result: perfectly pitched audio with no wow or flutter. While this process effectively corrects gross errors, it also makes a significant contribution to the overall audio quality. In critical listening tests Clarity has demonstrated that it improves the transparency, depth and intelligibility of the overall sound track. This overall improvement in sound quality has advanced a new theory regarding analog audio degradation.
Conventional wisdom suggests that analog degradation is the result of two factors:
• The quality of the recording electronics
• The recording characteristics of the magnetic media and heads
As noted earlier, no analog recording mechanism is immune from these artifacts. What has not been studied extensively, until now, is the audibility of more “exotic” types of flutter, particularly those of higher frequency, and often with more of a noise-like, rather than cyclical, character. The latter problem is called “scrape” flutter. Other fast flutters are cyclical and manifest as intermodulation distortion. Thirty-five millimeter magnetic sound film is particularly vulnerable to another type of periodic fast flutter known as “sprocket-cogging.
Scrape flutter occurs as the audiotape or film is pulled across the recording head. The microscopically uneven surface of the oxide, sliding across the surface of the recording head, creates a “micro-stiction” noise modulation, which can be found in the 3kHz-5kHz range. This mid-range frequency is much higher than the traditional frequency range used to measure flutter characteristics, which rarely exceeded 200Hz. This higher frequency flutter can also induce a very fine mechanical vibration in the tape path that can also impact the purity of the audio signal.
Sprocket-cogging, as the name implies, is a function of the sprocket drive of 35mm film. With 4 perforations per frame and 24 frames per second a 96Hz (4 x 24), audio artifact can be found in magnetic sound film, not unlike “perf buzz” in optical sound. Though the sprocket-cogging is at a fairly low signal level, it creates an intermodulation distortion component that also affects the purity of the sound, and creates listener fatigue, or “ear glare”.
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