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I've two related questions.

[1] Analogue record players rotate the disk at 16⅔, 33⅓, 45 or 78 RPM - apart from the first two values, there's no obvious relationship between all those figures. Why were those particular values chosen?

[2] Analogue tape speeds are 30, 15, 7½, 3¾, 1⅞ IPS (inches per second) and so on. Again, why those particular figures, and why the progressive halving? Why not a stepwise speed change instead, e.g. maybe 30, 25, 20, 15, 10, 5 IPS?

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    Try googling "history of audio recordings" . And as Tom's answer shows, the tape-speed relative selections should be obvious Feb 15 at 17:15
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    @CarlWitthoft I disagree about tape speeds. There are many factors that influence tape format and to me it’s not at all obvious how those would interact to make 2:1 proportions in speeds a given. Between various reel sizes, widths, and thicknesses, there are many parameters to account for, including quality, running time, portability, and reliability. I have a guess that perhaps a popular and reliable stepper motor circuit was easily doubled or halved in speed, but haven’t found anything in searches to confirm that or offer other theories. Feb 16 at 2:30
  • They wouldn't have used stepper motors at all. It was one motor spinning at the same speed all the time. (Steppers don't run smoothly. Also consider how hard it would be to run one with a mechanical or vacuum tube drive.) To change speeds, they used belts and pulleys. That means to change the speed, you just need to change the ratios of the diameters of two pulleys, much like in the transmission of a car.
    – Duston
    Mar 18 at 18:48

3 Answers 3

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Emile Berliner created the first vinyl player with an electric motor, in 1888, which just happened to run at 3,600 rpm. That made the disc run close to 78rpm, allowing about 5 mins of play per record. It could have been anything - that's what it was. That was 1925, when it became an industry standard.

On a few years and new techniques developed, meaning smaller discs, due mainly to smaller grooves. But it took until after the War, 1948, years after it was launched, that Columbia took a market hold, as a concerto or such-like could be played on one side - no need to flip over. Hence 33rpm was the standard.

Faster revs meant better sound reproduction, and 45 was found to be good, with a reduction in size of the disc. Hence pop records of ~3-4 mins.

There's also the marketing wars similar to Beta/VHS which played their part.

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  • "marketing wars" -- remember the very short-lived dual-stylus stereo records? Feb 15 at 17:17
  • @CarlWitthoft - no, I must have blinked. What year? By whom?
    – Tim
    Feb 15 at 17:31
  • several examples (few of which made it out of the lab) benbeck.co.uk/firsts/1_Technology/sound2t.htm Feb 15 at 18:08
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    It's also worth noting that Columbia first released 33RPM records, while RCA Victor invented the 45RPM ones. So that had no desire to be compatible with each other.
    – Simon B
    Feb 15 at 19:15
  • @Tim Apparently you're fairly old ;)
    – Tom
    Feb 15 at 23:59
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According to a Wikipedia article that has no citation listed for this fact:

Tape recording first became common enough for the issue of compatibility between tape deck manufacturers to become an issue in the 1950s. At this time the most common speeds for professional recording were 30 ips and 15 ips, and some machines already supported both speeds. As the tape speed was determined by the speed of a synchronous motor driving a capstan, one way of achieving this was to switch the poles of the motor to a different configuration, halving or doubling the speed.

This system was extended to domestic tape decks, and so slower speeds as they were adopted tended to be exactly half the previous slowest speed

Despite there being no source for this, it is an extremely likely explanation, so I’m adding it as an answer while I look for support.


I’m starting to believe it’s just simple practicality. A synchronous motor with two poles in the circuit that is connected to North American 60 Hz power will rotate at 3600 RPM (60 revolutions per second). Adding two poles (which is electronically easy to do with a switch) cuts the motor speed in half. Building a more complicated circuit to divide speeds any other way is not worth the effort.

The only thing left unexplained is why 60 RPS was mechanically translated to 30 IPS. A likely explanation is that it is a good round number with a simple mathematical relationship to the motor speed.

Source

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  • How would that translate to UK 50Hz mains?
    – Tim
    Feb 16 at 10:06
  • @Tim I’m not sure. I suspect after the initial systems were designed and built, someone had the same thought and came up with a different way to sync that was not based directly on the mains power, but they retained the base speed for backwards compatibility and the easy division by two works everywhere. Feb 16 at 12:05
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    @Tim Tape speed is a function of spindle rpm AND spindle circumference. If using a synchronous motor, the UK model could be supplied with a somewhat fatter spindle. Though I'm not at all sure this is how it's done in practice. Is mains frequency stable enough to be used as an 'audio clock'?
    – Laurence
    Feb 16 at 15:20
  • @Laurence - not sure, but the first idea is probably true. The last turntable I had (sold for stupid money) was designed to run fast, but had a stick in some oil to get it to the right speed, though I'm sure that's not what the marketing called it...
    – Tim
    Feb 16 at 16:15
  • "Varispeed" is a standard feature on pro tape decks, Studer, Otari, Revox etc. I think we're barking up some wrong trees here. Though my Akai 4000 does change basic speed with a slip-on capstan sleeve!
    – Laurence
    Feb 16 at 17:34
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At least for the second question, it makes way more sense to halve the speed because then you halve the resolution. This is the idea behind logarithmic scales: doing a normal (linear) step, say 5, would produce very little difference if your initial value was 100 (5%) but would produce a lot if your initial value was 10 (50%).

Doing it by halving (or any multiplier) gives you a something with respect to your initial value which makes way more sense

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    Why does it matter if one preset tape speed is twice the resolution of the next one down or (say) 1.97 of it? Possibly it might be mechanically expedient to build a multi-speed tape transport that way. Though my Akai 4000 has a constant speed capstan and changes from 3¾ to 7½ ips by adding a sleeve over it. Any other higher speed would have been equally easy to arrange.
    – Laurence
    Feb 16 at 0:26
  • @Laurence Usually because small increments in technology do not make it through. The point of lowering the speed is usually to lengthen the record, or make some economy on the record material. It has to be compensated by an increased rendering quality of the medium. So changes should be significantcompared to the previous to be interesting market-wise, manufacturing wise and so on.
    – Tom
    Feb 16 at 6:59
  • Todd's also sounds very plausible on that even thought, mechanically it also fairly easy to do
    – Tom
    Feb 16 at 7:06

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