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FLUTTER The general audio and broadcast field coined the term flutter to describe what you'll actually hear from the bad effects of this specification. FLUTTER DEFINITION Flutter is the result of non-uniform tape motion caused by variations in tape speed that produces frequency modulation of signals recorded onto magnetic tape. Flutter is usually expressed as a percent peak or a peak-to-peak value for instrumentation recorders and as a root-mean-square (RMS) value for audio recorders. It's caused by magnetic tape transports. Low-frequency flutter (below 1000 Hz) is caused by the rotating parts of a tape transport such as:
High-frequency flutter (above 1000 Hz) is caused by the fixed parts of a tape transport, such as fixed tape guides and magnetic heads. When the magnetic tape passes over a fixed tape guide or magnetic head, the transition from static to dynamic friction causes something called stiction. It's this stiction that causes the variations in tape speed which, in turn, cause the flutter. As you might guess, it's hard to prevent flutter. The only way to lessen flutter is through skilled engineering, machining, and design of magnetic tape recorders. FLUTTER MEASUREMENT There are many ways to measure flutter. Most are based on the fact that tape speed variations cause frequency modulation of a recorded tone. Figure 6-8 shows a typical setup for measuring the peak-to-peak value of flutter with a frequency-modulation (FM) demodulator and an oscilloscope. The technical manual for the magnetic tape recorder you're testing will tell you how to set up the signal generator to output the test signal. After setting up the test equipment, follow these procedures: Figure 6-8. - Test equipment setup for measuring flutter.
Record the test signal onto magnetic tape; then rewind the magnetic tape. This is necessary because you can't measure flutter as you're recording. Since the tape-speed variation past the record head is almost the same as past the reproduce head, the flutter level is too small to see. After you rewind the tape, play it back. During playback, the output signal from the tape recorder goes through the FM demodulator to remove the original test signal. The waveform you now see on the oscilloscope is the actual flutter signal that was modulated onto the test signal. Using the oscilloscope display, measure the peak-to-peak value of the flutter signal. TIME-BASE ERROR The time-base error (TBE) specification of magnetic tape recorders is closely related to the flutter specification. In fact, the TBE is a direct measure of the effects of flutter on the stability of recorded data. TIME-BASE ERROR DEFINITION The TBE is the time-relationship error between two or more events recorded and reproduced from the same magnetic tape. It's also defined as the displacement of a point on the magnetic tape from where it should have been, during a specific time interval. A typical TBE specification might read "+ / - 100 microseconds over a 10-millisecond time interval at a tape speed of 60 inches per second, referenced to a control tone." This means that the time-base error could cause a signal to jitter +/- 100 microseconds over a 10-millisecond period at a tape speed of 60 inches per second. TBE jitter introduces noise or unwanted frequency modulation (when using FM recording techniques) into the magnetic tape recording process. It can also cause a loss of accuracy in pulse-duration modulation (PDM), pulse-coded modulation (PCM), or other magnetic recordings where precise timing relationships exist between two or more signals. TIME-BASE ERROR MEASUREMENT The simplest way to measure the TBE is with an oscilloscope. Figure 6-9 shows a typical test equipment setup for measuring TBE. After you set up the test equipment, measure the TBE as follows: Figure 6-9. - Test equipment setup for measuring time-base error.
SKEW This magnetic tape recording specification only applies to multi-tracked magnetic tape recorders. SKEW DEFINITION Skew is the inter-track fixed and dynamic displacement, or change in azimuth, encountered by different tracks across the width of the magnetic tape as it passes the magnetic heads. In other words, it's the time difference between the tracks on a multi-tracked magnetic head. A typical skew specification might read "+/- 0.15 microseconds between adjacent tracks on the same head stack at 120 inches per second." This means that one of the tracks on a magnetic head could lead, or lag, the track next to it by as much as 0.15 microseconds at 120 ips. This specification applies to both fixed and dynamic skew. Fixed skew can be caused by
You can minimize most fixed skew by adjusting the magnetic recorder's electronics or by realigning the magnetic heads. Fixed skew errors usually do not show up when magnetic tapes are recorded and reproduced on the same tape recorder. Since fixed skew errors are additive, they'll usually show up when you record on one magnetic tape recorder and then reproduce on another. Dynamic skew errors are caused by either the magnetic tape transport or the magnetic tape itself. If the tape transport guides are worn or sticking, the magnetic tape won't properly pass over the magnetic heads. It'll drift and pass the magnetic head at an angle (like a car skidding on an icy road). If the magnetic tape itself is warped or isn't uniform across its width it, too, will cause dynamic skew. SKEW MEASUREMENT Skew is best measured with an oscilloscope. Figure 6-10 shows a typical test equipment setup for measuring skew. The technical manual for the magnetic tape recorder you're testing will tell you how to set up the signal generator. After test equipment setup, measure the skew as follows: Figure 6-10. - Test equipment setup for measuring skew.
Inject the test signal into a reference track and one other track of the multi-track magnetic tape recorder. (The reference track should be one of the two outside tracks of the magnetic head.) Connect the output from the reference track to the sync input of the oscilloscope to trigger the horizontal sweep. Connect the output from the other track to the vertical input of the oscilloscope. While recording and reproducing the test signal, measure the fixed and dynamic skews which are displayed on the oscilloscope. Figure 6-10 shows how this looks. Q.13 What causes flutter in a tape recorder's output? |
 
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