![]() ![]() Thus we need 200°⁄ 100 = 2° between the lines on the disc, or 180 lines in a 360° circle.Ĥ. So a light on 50Hz AC power is a light-source pulsating at 100Hz. Remember that a normal electric light pulsates twice in one cycle of AC power once on the positive half-cycle, and once on the negative. For the pattern to appear stationary, we need an advance of 1 line per pulsation of the light source. Take the example of 33⅓ RPM record illuminated by a light fed by 50Hz AC power. It's not difficult to calculate the number of lines required on the stroboscope disc. The note on this page discusses the stability of the mains AC supply frequency.ģ. The advantage of 33⅓ RPM is that it divides exactly into 30, so the speed is precise in regions which have either a 50Hz or a 60Hz AC supply. In regions that use 50 Hz current, the standard 77.92 RPM is derived from a two-pole, 3000 RPM motor and a 77:2 ratio. ![]() In 60 Hz regions a two-pole, AC synchronous motor running at 3600 RPM geared down by a 46:1 ratio will produce 78.26 RPM. Motor speeds were standardised in the pre-LP era. Available from the International Electrotechnical Commission webstore.Ģ. This may be employed to check turntable speed as described above.ġ. Cut round the perimeter circle and make a 7mm hole in the centre. 1.Ĭlick on the stroboscope disc image to download a. The number of lines required to fulfil this criteria is also specified in IEC 60098:1987 and the numbers are given in Table. In order that the pattern appears stationary, the lines have to advance exactly by one space between the repetitive pulses of the alternating current light 4. ![]() When illuminated with alternating current lighting, the pattern of bars on the stroboscope disc appears stationary when the turntable speed matches the standard speed³. 1.Ī stroboscope disc is a paper, cardboard or plastic disc with a spindle hole in the centre which may be placed on the turntable like a record to measure the turntable's rotational speed. Because the majority of both disc lathes and record-players used synchronous motors², the exact speeds vary according to the frequency of the alternating current supply of the country in question as annotated in Table. The standard speeds for records are specified in the international standard IEC 60098:1987¹. Standard Speeds for Phonograph (Gramophone) Records Standard Speeds for Phonograph (Gramophone) Records ![]()
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