Wow (recording)
Encyclopedia
Wow is a relatively slow form of flutter (pitch variation) which can affect both gramophone record
s and tape recorder
s. In the latter, the collective expression wow and flutter is commonly used.
variation which could result from warping of the record or from a spindle hole that was not precisely centered.
If the hole is not centered exactly, 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). Obviously, the more eccentric the positioning of the spindle hole, 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.
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 at the head. It adds a roughness to the sound that is not typical of wow & 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%.
, causing the picture to jitter slightly, or the top of the picture to wobble. This is known as flagwaving
.
have been developed that correct wow and flutter by tracking various spuriae 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.
Gramophone record
A gramophone record, commonly known as a phonograph record , vinyl record , or colloquially, a record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove...
s and tape recorder
Tape recorder
An audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage...
s. In the latter, the collective expression wow and flutter is commonly used.
Gramophone records
When playing gramophone records, wow is a once-per-revolution pitchPitch (music)
Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...
variation which could result from warping of the record or from a spindle hole that was not precisely centered.
If the hole is not centered exactly, 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). Obviously, the more eccentric the positioning of the spindle hole, 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 an audio cassette 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 at the head. It adds a roughness to the sound that is not typical of wow & 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 recordingsVideotape
A videotape is a recording of images and sounds on to magnetic tape as opposed to film stock or random access digital media. Videotapes are also used for storing scientific or medical data, such as the data produced by an electrocardiogram...
, causing the picture to jitter slightly, or the top of the picture to wobble. This is known as flagwaving
Timebase correction
Time base correction is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media. Without time base correction, a signal from a videotape recorder or videocassette recorder cannot be mixed with other, more time stable devices...
.
Correction
Novel methods of digital signal processingDigital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
have been developed that correct wow and flutter by tracking various spuriae 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.
See also
- Rumble measurement
- PhonographPhonographThe phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
- Wow and flutter measurementWow and flutter measurementWow and flutter measurement is carried out on audio tape machines, cassette recorders and players, and other analog recording and reproduction devices with rotary components This measurement quantifies the amount of 'frequency wobble' present in subjectively valid terms...