Wow is a relatively slow form of
flutter (pitch variation) that can affect
gramophone record
A phonograph record (also known as a gramophone record, especially in British English), or simply a record, is an analog sound storage medium in the form of a flat disc with an inscribed, modulated spiral groove. The groove usually starts ne ...
s and
tape recorder
An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present ...
s. For both, the collective expression wow and flutter is commonly used.
Gramophone records
When playing gramophone records, wow is a once-per-revolution
pitch variation which could result from warping of the record or from a pressing plate that was not precisely centered.
If the grooves are not centered exactly relative to the spindle hole, the linear velocity of the stylus, instead of dropping gradually as the groove spirals towards the center, varies every revolution to be too high (resulting in a higher pitch) when the stylus is further out, and too low when the stylus is further inwards (resulting in a lower pitch). The more eccentric the positioning, the greater the pitch variation.
The cause for "wow"-effects on a warped disc is basically the same; a variation in the linear velocity of the stylus relative to the disc. This can be by either radial warping (similar to the eccentric hole case) or by the disc warping out of its plane. This would mean that the stylus would have to travel faster over the warped section as it must displace both in and out of the plane of the disc.
Tape recorders
A similar problem can occur with tape recorders. The changes in frequency are caused by irregular tape motion during recording or playback. For example, a change in the angular velocity of the
capstan, or dragging of the tape within a reel or
audio cassette
The Compact Cassette or Musicassette (MC), also commonly called the tape cassette, cassette tape, audio cassette, or simply tape or cassette, is an analog magnetic tape recording format for audio recording and playback. Invented by Lou Otte ...
shell. The terms "wow and flutter" are often referred to together, flutter being a higher-rate version of wow.
Scrape flutter—a high frequency flutter of above 1000 Hz—can sometimes occur from the tape vibrating as it passes over a head, as a result of rapidly interacting stretch in the tape and
stiction
Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the words ''static'' and ''friction'', and is perhaps also influenced by the verb '' to stick''.
Any ...
at the head. It adds a roughness to the sound that is not typical of wow and flutter, and damping devices or heavy rollers are sometimes employed on professional tape machines to prevent it. Scrape flutter measurement requires special techniques, often using a 10 kHz tone.
A typical modern cassette recorder may have a wow and flutter specification of 0.08%.
Video recording
A similar problem can also occur in
videotape recordings, causing the picture to jitter slightly, or the top of the picture to wobble. This is known as
flagwaving.
Correction
Methods of
digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
have been developed that correct wow and flutter by tracking various spurious things on the tape or film, which can be re-purposed as timing references. A system developed by Plangent Processes substantially reduces wow and flutter of very high rates to extremely low levels, with a substantial improvement in quality. The software Capstan by
Celemony analyzes the already digitalized musical material and uses
varispeed playback to eliminate wow and flutter.
See also
*
Rumble measurement
A rumble is a continuous deep, resonant sound, such as the sound made by heavy vehicles or thunder. In the context of audio reproduction rumble refers to a low frequency sound from the bearings inside a turntable. This is most noticeable in ...
*
Phonograph
A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, or more recently a turntable, is a device for the mechanical and analogu ...
*
Wow and flutter measurement
Measurement of wow and flutter is carried out on audio tape machines, cassette recorders and players, and other analog recording and reproduction devices with rotary components (e.g. movie projectors, turntables (vinyl recording), etc.) This measu ...
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Audio storage
Sound recording