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I was wondering what set the rotational speed in a cassette recorder? I assume that the speed must be constant, but that means that the tape data would have different effective densities depending on where you were on the tape. Was it a stepper motor and a timer to create the rotations or was something more interesting used?

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    \$\begingroup\$ The battery voltage. \$\endgroup\$ – SDsolar May 30 '17 at 3:12
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    \$\begingroup\$ Just on stepper motors, they're pretty inefficient compared to a cheap brushed DC motor or more expensive-to-control brushless DC motor. They seem to superficially appeal to inexperienced engineers (did to a young me!), probably because they look digital and easy to position :-) But they don't get used for constant rotation systems, and certainly not for smooth spin system like this. Too expensive, jumpy, inefficient. Decent servo positioning systems don't use them, too imprecise to position and other motors are cheaper even with more sophisticated drive electronics,very generally speaking. \$\endgroup\$ – TonyM May 30 '17 at 5:20
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    \$\begingroup\$ @TonyM I suspected as much. The reason I asked had to do with the coil inductance of several, seemingly identical motors. \$\endgroup\$ – b degnan May 30 '17 at 10:59
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    \$\begingroup\$ There used to be a product in a cassette housing (perhaps still exists) to clean part of the tape path called a Capstan & Pinch Roller cleaner. This is the area that gets dirtiest. Some recorders used a crystal to control motor speed using a phase locked loop. This was necessary when recording audio for movies. \$\endgroup\$ – user56384 May 30 '17 at 13:47
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    \$\begingroup\$ See en.wikipedia.org/wiki/Tape_transport for pictures and details. \$\endgroup\$ – Thorbjørn Ravn Andersen May 30 '17 at 14:36
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The hubs do not rotate at constant speed in normal tape recorders - there is what is called a capstan that it kept in contact with the tape by a rubber pressure wheel. The capstan rotates at constant speed and so drives the tape at constant speed.

The take-up reel is usually driven through a friction wheel so that it keeps reasonably constant tension on the tape but the hub speed can vary as needed.

Some tape recorders such as ones for voice recording often do drive the tape by rotating the take-up reel at a constant speed - in that case the recording density will change depending upon how much tape is on the reel. The variations in quality for voice are usually acceptable.

The standard for cassettes is where the magnitude of the magnetic flux on the tape represents the instantaneous value of the signal being recorded. There are a couple of techniques used to improve the quality of the signal played back:

1) A high-frequency AC bias is added to the signal being recorded to avoid the non-linearity inherent in the flux recorded vs the current in the recording head, without this there would be a non-linear response around zero. A cheaper alternative where low quality can be tolerated is to use DC bias, that will give a lower signal to noise ratio on playback with more hiss.

2) Another technique is to use equalization where some frequencies are boosted in recording and attenuated on playback to improve the overall signal to noise ratio (i.e. reduce hiss). There are standards set by the RIAA so that tapes are interchangeable between different machines. Some also use noise reception techniques such as Dolby Noise Reduction.

The motors used for driving the tape were usually brushed DC motors with either a centrifugal governor (a small weight on a spring that open contacts at a defined speed to slow the motor) or by using the back-emf of the motor to sense and control the speed. (See another question I answered DC Motor speed control.)

Stepper motors were never used in conventional cassette recorders. They tend to be inefficient (i.e. consume more battery power), have unsteady speed (e.g. cogging) and are not so easy to drive as DC brushed motors.

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  • \$\begingroup\$ Since the OP asks what kind of motor is used, it might be nice to add that detail to your answer. In all the cassette decks I have looked at except very old ones, a simple brushed dc motor was used. Typically nothing more than motor emf was used for feedback. \$\endgroup\$ – replete May 29 '17 at 23:52
  • \$\begingroup\$ @KevinWhite Do you have any idea how the speed was set in manufacturing due to component mismatch? There was surely some automated process to trim the rotational velocity. Any idea what that was? \$\endgroup\$ – b degnan May 30 '17 at 11:01
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    \$\begingroup\$ @bdegnan A man with a test instrument, a screwdriver, and a bottle of potentiometer cement. (This is special stuff. This board had a question on it, but the answers were very wrong. Browsing the Web, the closest reco I've seen is Glyptal, but even Glyptal is runnier than I would like. I would mix Glyptal with West System collodial silica until it thickened up, but I stock that stuff for other reasons.) The point is to prevent accidental/incidental movement, while allowing adjustment, leaving a witness mark. \$\endgroup\$ – Harper - Reinstate Monica May 30 '17 at 14:45
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    \$\begingroup\$ @bdegnan You wouldn't. But you have an old one you need to set today? Easy as pie. The official speed of a Compact Cassette is 1-7/8" per second. At the factory I'm sure they had a tach or frequency counter listening to motor ripple. In the field you'd put marks on a piece of tape and use a stopwatch. More time consuming but very accurate. In fact, many cassette tapes had 5-10 second leaders with visible marks 1.875" (1 second) apart. \$\endgroup\$ – Harper - Reinstate Monica May 30 '17 at 18:16
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    \$\begingroup\$ Another way of calibrating the speed is to play a tape with a recording of a continuous tone at 1-2kHz that was recorded on a known good machine. Adjust the playback speed to get the correct frequency. Machines that used centrifugal governors weren't easily adjustable. \$\endgroup\$ – Kevin White Jun 1 '17 at 1:18
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In almost all tape recorders (cassette, open reel audio, various video cartridge types, 2" Broadcast videotape, etc...) the tape is driven by a capstan and pinch roller, not by the tension from the take-up reel.

In audio cassette recorders, the takeup reel is probably driven by a friction drive from the capstan motor. In professional open-reel machines, there are often a separate motors to drive the takeup reel and to maintain tension on the supply reel. These motors and their drive systems are designed to maintain a suitable tension on the tape during record/play operation but can produce higher torque for fast forward or rewind operations.

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I seem to recall someone (Colecovision?) had a cassette recorder without a capstan, where they dragged tape across heads using fairly precise control of the drive reels.

The drive mechanism was surprisingly naked, with much of the typical guts of a cassette transport just conspicuous in its absence. It was obvious they had reduced production cost considerably.

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The repair shop I worked in used a special calibration tape to set the speed precisely. It had a number of prerecorded tones on it used for various adjustments and measurements. The speed was set using a section on the tape recorded with a 1 kHz tone. The tone was monitored with a frequency counter which allowed precise speed adjustment using the technique described by Harper above. Other sections of the tape were used to measure frequency response and adjust the head azimuth (tilt) ensuring good high frequency response. Wow (slow speed variations) and flutter (fast speed variations) could also be measured if the right equipment was available.

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The speed for a conventional audio cassette is set at 1 7/8 inches per second. As others have said, this is by controlling the rotation rate of the capstan. The take-up spool is on a friction drive, and runs at whatever speed it needs to in order to take up the tape.

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