Wow and Flutter Audio Measurement
The sound recording and reproducing techniques had evolved from analog approaches, such as those used by phonograph cylinders, disc phonographs and taper recorders, into digital approaches using CDs or solid state memories as the recording mediums since 1980s. However, till today, the analog technique based devices have not vanished, as evidenced by the considerable new sales of turntables and vinyl records. Some audiophiles still love to listen to them.
What is Wow and Flutter? Flutter, wow, drift, and scrape flutter are all forms of distortion in analog recording and reproducing systems that use a moving medium. These are caused by undesired frequency modulation introduced into the signal by an irregular motion of the recording medium during the recording, duplicating, and reproducing processes. The measurement of wow and flutter quantifies the amount of frequency wobble (caused by speed fluctuations) present in subjectively valid terms. For example, when a 3150 Hz tone is replayed, the measured instantaneous frequency should be constantly 3150 Hz under ideal circumstances. However, with wow and flutter in the real world, the instantaneous frequency would vary constantly around some mean value (not necessary to be 3150 Hz). In other words, there are back-and-forth frequency deviations from that mean value over time. Drift refers to frequency modulation of the signal in the range below approximately 0.5 Hz resulting in distortion which may be perceived as a slow changing of the average pitch. Wow refers to frequency modulation of the signal in the range of approximately 0.5 Hz to 6 Hz resulting in distortion which may be perceived as a fluctuation of pitch of a tone or program. Flutter refers to frequency modulation of the signal in the range of approximately 6 Hz to 100 Hz resulting in distortion which may be perceived as a roughening of the sound quality of a tone or program. Scrape Flutter refers to frequency modulation of the signal in the range above approximately 100 Hz, which is caused by stick-slip motion (stiction) of the tape. It results in distortion which may be perceived as a noise added to the signal – that is, a noise not present in the absence of a signal. It is also called “friction noise” or “stiction noise”. Different standards exist for wow and flutter measurement, such as CCIR, DIN, NAB and JIS. AES6-2008 (r2012) is followed here. It is compatible with IEC 60386, IEEE Std-193, CCIR 409- 2, and DIN 45507. The standard test frequency is 3150 Hz. Other test frequencies such as 3000 Hz can also be used. Wow and flutter in analog domain has its counterpart in digital recording and reproduction, that is, the variation in sampling clock timing, often referred to as jitter. The measurement of clock jitter will be described in another article.