Wow and Flutter Not So
Funny
Here's another in an occasional series on the basic
principles and terms of audio and video. People new to the language of audio tend to find
the terms wow and flutter hilarious. They can, however, be among the most annoying things
to bedevil analog audio systems, although this is abating as more and more things become
digital.
What is it?
In audio recording systems, speed is very important. A
recording must be played back at precisely the speed it was recorded, or the musical pitch
will be off -- sometimes ludicrously so -- and the length will be wrong. But even if the
overall speed is correct, there can be speed variations of a smaller nature that can
degrade the sound.
Mechanical irregularities in turntables and tape decks can
cause a record or tape to go alternately faster and slower than the proper speed, even if
the net result is right. Slow variations -- five to ten a second, depending on who you ask
-- are called wow; faster ones are called flutter. They are kinds of inadvertent frequency
modulation, and can be annoyingly obvious even in very small amounts.
How is it specified?
Wow and flutter are usually lumped together as a single
number representing the average deviation from the proper speed, measured over a period of
time and expressed as a percentage.
For example, if a test recording of, say, 3000Hz is played,
and you measure it consistently swinging between 2994 and 3006Hz -- a 0.2 percent
deviation either way, the device has wow and flutter of 0.2%.
At one time, a figure like that, or even higher, would have
been acceptable, but it would be considered grossly inadequate today. Once they have
learned flutter's characteristic signature sound, even quite inexperienced listeners can
hear flutter of 1.5%; I'd consider about half that to be a maximum allowable level.
Fortunately, most of today's analog audio gear has little trouble with that.
How does it affect equipment?
Wow is usually perceived as a pitch variation, either a
once-around change caused by a warped or eccentric record, or higher, caused by some other
mechanical flaw. Serious wow, in truth, has never been that big a problem.
Flutter is not normally audible as a pitch change -- the
notes are still right -- but as an added character to the sound. Depending on the specific
nature of the cause, it can add a warbly, honky-tonk sort of sound, or sometimes a sort of
roughness.
To some extent, how obvious it is depends on the material.
We're most sensitive to things in the midrange, so if there's a lot of mids, you're more
likely to hear the flutter. An old, informal test was to play a test recording with a 3kHz
tone, because that frequency is one we are very sensitive to. If the sound was sharp and
clear, flutter was likely okay; if it sounded tinny or unstable, there was probably a
flutter problem. Most audiophiles have learned by experience that the most effective
flutter-revealers are sustained piano notes.
One particularly nasty aspect of wow and flutter is that
they can accumulate. You might have a turntable and a cassette deck with inaudible
flutter, but by the time you play a record, record it, and then play it back, there have
been three separate mechanical operations, each of which will have contributed some
flutter. The result may well be easily audible.
One of the advances of the development of digital audio
media is that such speed irregularities were essentially designed out of the system. The
compact disc player is a mechanical device, to be sure, but the relationship between its
rotation and the stability of the sound it produces is almost nonexistent.
The bits read from a CD are fed into a buffer -- a
temporary digital memory -- in order, but at whatever rate they come off the disc. After a
brief period of storage, they are released to the rest of the circuitry with absolute
regularity by a quartz crystal "clock" that operates at the same frequency that
was used in making the recording. The buffer controls the disc rotation speed to make sure
the data are arriving at approximately the right rate, but there's some leeway: as long as
the memory never quite empties, the precise behavior of the disc is irrelevant.
As a consequence, most digital audio devices specify wow
and flutter as being unmeasurable, or below some ridiculously teeny amount, which for
audiophiles who started out all analog is a kind of Valhalla. They always do specify it,
though.
...Ian G. Masters
ian@mastersonaudio.com
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