This question throws us back into the days of the open-reel, or reel-to-reel, tape players.
Audio tape is threaded from its spindle, through the different tape heads and contacts of the machine, and into the empty takeup reel. As the tape is played, the takeup reel spins, gradually pulling the tape off the first reel. If you’re unsure what I’m talking about (I mean one doesn’t really run across them anymore) check out the Wikipedia page on the subject.
My dad, a musician, has a big reel-to-reel recorder, and I was always baffled by the appearance that one reel, the one pulling the tape, always seemed to spin faster than the second reel. Not by much, but certainly enough that you notice it.
You see this in 60s spy movies all the time – the audio tape containing our hero’s mission always seemed to have two different speeds on each spindle. You can even see this in a regular audio cassette or a VHS if you were to peer inside the VCR.
My question is, how is this possible? Is it an optical illusion of some sort? How can one reel spin faster than the other without stretching or breaking the tape, or causing continual pitch shifts? Why don’t they both seem to move at the same speed?
The spindle of the take-up reel rotates at a constant RPM, but as tape winds onto it, the diameter increases, therefore, the tape speed through the head increases. Each rotation of the take-up reel will wrap one circumference of tape onto the spool, but the spool is growing due to the thickness of the tape.
The diameter of the other reel is decreasing at the same time, as the tape winds off it, but (I didn’t do the math) that probably doesn’t exactly balance.
That the tape is pulled past the heads at a constant rate by the capstan (7.5 inches per second typical, other speeds twice or half that). The reel speeds, as Mangetout says, reflect the increasing or decreasing effective diameter of the reels as tape is added or subtracted.
This only happens at the start of a playback or recording, when the takeup reel’s diameter is smaller than the feed reel. The opposite happens at the end. This phenomena can be exacerbated by using non-equal-sized reels (not recommended). In any case, it’s not an illusion; your observation is correct.
Oh jeez! Of course! I completely didn’t think about the diameter of the spindle changing as it takes up and reels off tape. That makes all the sense in the world.
Thanks so much for clearing that up, Mangetout and Musicat! The mystery that’s been bubbling away since I was small enough to help unjam tape with my five-year-old fingers is finally solved. Thanks again!
I doubt there’s any kind of clutch in the take up reel mechanism. There wasn’t one in any of the assorted mechanisms I’ve studied and repaired over the years. It’s just a not very powerful motor turning as fast as it can and limited by the foot per minute feed speed of the tape reader and capstan.
The tape speed is controlled by the capstan, not the reel motors. The tape is squeezed between the rotating capstan and the pinch roller, the pinch roller spins because it is in contact with the capstan and gives the friction to pull the tape along.
As Alpha Twit says the reel motors are quite puny, you can easily stop them with a finger. The capstan motor is much beefier, try gripping the running capstan* you won’t be able to stop it.
The machine I had in the 1960’s was a Sony reel-to-reel tape recorder. It did have a slipping clutch (I guess that’s what you’d call it). The take-up mechanism consisted of:
– The motor.
– Rubber belts from the motor pulley to the take-up reel pulley.
– Take-up spindle had an inner ring (driven by the motor, via belt and pulleys).
– Outer ring surrounding the inner ring.
– Layer of felt separating inner and outer rings. This constituted the slipping clutch.
– All of the above is hidden below the deck.
– Above the deck, spindle for take-up reel is attached to the outer ring.
Yes, there is a simple clutch, often no more than felt on metal. Only a little pressure is put on the takeup reel to keep the tape taut.
The better recorders use a magnetic brake, with a separate motor for each reel and the capstan. The cheaper ones use a single motor with belts and a little friction to allow slippage. The cheapest have no brakes or friction at all on the feed reel during play.
On most tape decks, you didn’t need to unscrew the capstan and replace it, because they had a switch that set the tape speed. The most common choices were 7.5 and 3.75 inches / second, but some decks had speeds of 15 ips for high quality recording and/or 1.8 ips for low quality voice recording to get more on a single tape.
The capstan on those machines had multiple segments with different diameters, and the switch moved the proper segment into position.
When playing or recording the left reel (the take-off reel) is not powered at all. As said above the tape is pulled off this reel by the pinch & roller mechanism. The only time this reel is powered is when rewinding, the pinch & roller mech is opened & disengaged, the right-hand take-up reel goes into ‘neutral’ essentially, and the tape is pulled off it onto the left reel by a motor driving it (in the reverse direction as when tape is spooling off of it).
Of course, because reel to reels were never really a consumer product but meant for professional use, many of them could do the above in reverse. IOW they could play a tape backwards as well as forwards. You could also move the tape across the heads manually (‘scrubbing’) so you could physically edit the tape with a razor blade.
BTW, the more modern cassette tape functions basically the same way in terms of only the take-up (i.e. right) side being gently powered while playing (or recording) and the tape being pulled across the head by a pinch & roller. The cassette was essentially simply an enclosed, small, portable, consumer-friendly reel to reel format.
In high-end decks, in the forward direction, yes, it is, to provide a slight back pressure to keep the tape taut over the heads. The same high grade units do not use pressure pads, but rely entirely on that back tension to keep the tape against the heads.
Conversely, in the reverse direction, a little back pressure is applied on the feed (formerly takeup) reel, too.
No reels go into neutral except when stopped or powered off, and even when stopped, most units hold a brake on them to prevent accidents.
And some decks have both an electrical speed control AND a capstan size change. I have an Akai 360D with 4 speeds; 3 are electrical (1.875, 3.75, 7.5 ips), but one of them requires a different capstan diameter (15 ips).
Nitpicker that I am, that’s 1.875 (1 7/8 ips). Traditionally, all speeds were 1/2 of the faster one. How 60, 30 or 15 ips got to be a standard at first, I really don’t know.
The same exact explanations apply to cassette tapes. The tape speed is enforced by the pinch roller, at a constant linear speed. The take up reel keeps just enough tension to take up the tape that’s been allowed to pass by the pinch roller. The two reels in a cassette turn at different rotational speeds depending on how far along you are in the recording.
reel to reel machines had many variations as noted. these were in use for a long period of time. many functions were accommodated from casual portable audio preservation, archiving of voice for hours of duration to high quality music (both playing and recording).
reel sizes of 3 1/2, 5, 7 inches were consumer market reels. tape speeds were 15/16, 1 7/8, 3 3/4, 7 1/2 inches per second were consumer market speeds.
the supply reel was dragged with a brake, mechanical or electrical. on all but some 3 1/2 reel machines a capstan governed the tape speed with the take-up reel providing just enough tension to pull in slack. on less expensive machines speed changes were done by changing the size of the capstan mechanically.
the linear tape speed was constant, on a capstan machine, with the reel speed changing with the tape diameter on it. some 3 1/2 machines without capstan the tape speed would change during the reel.
some higher priced machines might have a reverse direction switch so you didn’t have to flip the reels to play the other side. some of these would automatically reverse upon a reverse trigger tape splice passing across a sensing head.
On any reasonable quality reel to eel, both the feed and take-up reels are powered.
This is critical in ensuring a specific tape head contact pressure, which in turn is critical for the quality of the recording. Decent reel to reel machines can perform better than CD, but any slight loss in certain factors is highly noticeable, whereas they would not be so noticeable on lower standards of recording machines such as cassettes.
The back tension of the tape - from the head to the supply reel is controlled by the tension arm, which may either vary a magnetic brake, or change the speed of the supply reel motor. This is sound critical.
The torque applied to the tape from the take-up reel is also critical, too much can damage the tape by stretching it - this stretching is done ever so fractionally that you’d hardly notice it, and its why speed calibration tapes are supposed to be used only a limited number of runs - there are workarounds to this such as using a frequency counter but I’ll let that point alone.
The take-up reel torque is usually controlled by applying a signal derived from the take-up tension bar which feeds a controlling system that changes the power supplied to the take-up reel motor.Slightly cheaper systems use the take-up arm tensioner to operate a friction brake that controls the speed of that reel.
All this had to be considerably refined for use on video recorders, and the very final versions of reel to reel tape audio tape machines had fully closed loop systems, which was gained from the video recorder.
Only partly. If there wasn’t any back tension, the spring-loaded arms wouldn’t have any effect at all, would they? (There’s negligible friction on these contraptions.)
The arms serve another purpose – if the tape breaks, slacks, or runs out, the arm will usually trigger a shutdown. So it has to keep tension to prevent that under normal use, and that tension is supplied by a motor or a brake.
Casdave and johnpost covered it pretty well in posts after yours.
The spring-loaded arms also provide a function similar to the loop before and after the shutter on a film projector - they create a buffer between the reels and the heads so if the brake on the take-off reel is a bit sticky, or more commonly, the reel is warped, tape can flow past the heads smoothly while the tape is coming off the reel in spurts.
Oddly enough, the loops on a projector do just the opposite - they allow the film to pass through the shutter, stopping at each frame while the take-up reel is collecting the film smoothly.