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Introduction to Audio Measurement

Wow & Flutter Measurement

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Wow and flutter measurement is carried out on audio tape machines or cassette recorders and players in order to quantify the amount of 'frequency wobble' (caused by tape speed fluctuations) present in subjectively valid terms. It is also used on turntables (for vinyl recordings), which tend to suffer mainly slow Wow. In digital systems, which are locked to crystal oscillators, wow and flutter are usually irrelevant, and need not be measured.

While the terms Wow and Flutter used to be used separately (for wobbles at a rate below and above 4Hz respectively), they tend to be combined now that universal standards exist for measurement which take both into account simultaneously. Listeners find flutter most objectionable when the actual frequency of wobble is 4Hz, and less audible above and below this rate. This fact forms the basis for the weighting curve shown here.

Audible Effects
Wow and flutter are particularly audible on music with oboe or piano solo playing. While wow is perceived clearly as pitch variation, flutter can alter the sound of the music differently, making it sound ‘cracked’. There is an interesting reason for this. A recorded 1kHz tone with a small amount of flutter (around 0.1%) can sound fine in a ‘dead’ listening, but in a reverberant room constant fluctuations will often be clearly heard. These are the result of the current tone ‘beating’ with its echo, which since it originated slightly earlier, has a slightly different pitch. What is heard is quite pronounced amplitude variation, which the ear is very sensitive to. This probably explains why piano notes sound ‘cracked’. Because they start loud and then gradually tail off, piano notes leave an echo that can be as loud as the dying note that it beats with, resulting in a level that varies from complete cancellation to double-amplitude at a rate of a few Hz: instead of a smoothly dying note we hear a heavily modulated one. Oboe notes may be particularly affected because of their harmonic structure. Professional tape machines can achieve a weighted flutter figure of 0.03%, which is practically inaudible, but the best cassette decks struggle to manage around 0.08%, which is still audible under some conditions. Average cassette decks and car players often have around 0.2% or more flutter.

The term ‘flutter echo’ is used in relation to a particular form of reverberation that flutters in amplitude. It has no direct connection with flutter as described here, though the mechanism of modulation through cancellation may have something in common with that described above.

Absolute Speed
Absolute speed error causes a change in pitch, and it is useful to know that a semitone in music represents a 6% frequency change. This is because Western music uses the ‘equi-tempered scale’ based on a constant geometric ratio between twelve notes; and the twelfth root of 2 is 1.05946. Anyone with a good musical ear can detect a pitch change of around 1%, though an error of up to 3% is likely to go unnoticed, except by those few with ‘absolute pitch’. It is interesting to note that most ‘Movie’ films shown on UK Television are speeded up by 4.166% because they were shot at 24 frames per second, but are scanned at 25 frames per second to match the PAL standard of 50 frames/sec 25 fields/sec. This causes a noticeable increase in pitch on voices, which often brings surprised comment from the actors themselves when they hear their performance on video. It can also frustrate attempts to play along with film music, which is closer to a semitone sharp than its intended pitch. Recently, digital pitch correction has been applied to some films, which corrects the pitch without altering lip-sync, by adding in extra cycles of sound. This has to be regarded as a form of distortion, as there is no way to change the pitch of a sound without also slowing it down that does not change the waveform itself.

Scrape Flutter
High frequency flutter, above 100Hz can sometimes result from tape vibrating as it passes over a head, as a result of rapidly interacting stretch in the tape and stiction at the head . This is termed 'scrape flutter'. 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 10kHz tone.


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