Measurement Techniques
Measuring instruments use a frequency discriminator to translate the pitch variations of a recorded tone into a flutter waveform, which is then passed through the weighting filter, before being full-wave rectified to produce a slowly varying signal which drives a meter or recording device. The maximum meter indication should be read as the flutter value.
The following standards all specify the weighting filter shown above, together with a special slow-quasi-peak full-wave rectifier designed to register any brief speed excursions. As with many audio standards these are identical derivatives of a common specification.
- IEC 386
- DIN45507
- BS4847
- CCIR 409-3
- AES6-2008
Measurement is usually made on a 3.15 kHz (or sometimes 3 kHz) tone, a frequency chosen because it is high enough to give good resolution, but low enough not to be affected by drop-outs and high-frequency losses. Ideally, flutter should be measured using a pre-recorded tone free from flutter. Record-replay flutter will then be around twice as high, because worst case variations will add from time to time. When a recording is played back on the same machine it was made on, a very slow change from low to high flutter will often be observed, because any cyclic flutter caused by capstan rotation may go from adding to cancelling as the tape slips slightly out of synchronism. A good technique is to stop the tape from time to time and start it again, as this will often result in different readings as the correlation between record and playback flutter shifts. On high-end or most professional machines in a well maintained state, one may expect that it is impractical to not possible to find a tape made on a better machine. Therefore, a record-playback test using the stop-start technique, is, for practical purposes, the best that can be accomplished.
Read more about this topic: Wow And Flutter Measurement
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