Mechanical imperfections in every recorder cause speed variations (flutter) which have a frequency spectrum that is characteristic of both the model of recorder and of the individual recorder. The speed variation of the recorder cause fm sidebands around any signal recorded on that recorder. When the signal to be recorded contains a periodic signal, for instance power line hum, the sidebands on the recording can be measured, and converted into a flutter spectrum. By comparing the flutter spectrum found on the recording to that measured on the recorders, one may be able to identify the recorder which produced the recording. The theory is developed and a practical example is given.
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