[from now on, excuse me for my poor english]
eehiram, I agree with your skepticism...
I guess that a common, vulgar, typical cassette recorder or player, like a walkman, or the one built into an old hi-fi system, for tipical consumer music records, doesn't have the ability (control system) to do such precise work of controlling the linear speed of the tape passing in front of the head; I believe that the rotating speed of the motor is constant (as you can read at ..), so the tape linear speed in front of the head equals the tangent speed at the 'destiny' reel or capstan, so as time passes, the reel grows bigger in radius and so does the speed of the tape.
As music is recorded de same way, as it is, first slower and then faster, in the same ammounts, because its a constant rotating speed, we hear no changes in speed or pitch in the record as we play the tape.
And here it's i think the point of confussion we all reach at some point by reading several articles:
I think that when we or other people ussually recall a type of cassette or tape by its so called playing speed, like 1 7/8 inch/second (for me 4,76 cm/s) what we're doing is just a simple check, and that's dividing the length of the tape in inches/meters by the playing/program time, like 60 or 90 min, etc, but it doesn't describe the actual speed of the tape in front of the head at different playing positions over the playing time.
As planish says, and so I understand from some articles, big reel to reel tape desks do control linear speed at the front of the head, but that I don't think is the case for common cassette tapes. I might be wrong. If I am, someone please describe how the tape machines do control tape speed
One of the possibilities I can recall is: recently I knew that tape machines ussually AC bias the tape while recording with a supersonic frequency (40khz to 150khz, so neither we can hear it or players/speakers can reproduce it) for lineality and noise reasons, so I guess this means that it superimposes (sums) this signal. I think we can think of it as a carrier. Anyhow, the point is that that could be enough to read that signal and drive the motors properly, by means of an electronic control loop. That's just an idea.
But my hypothesis is: in the cassette tape (not reel-to-reel), the motor (or the capstan) rotates at a constant speed, varying the tape linear speed as the tape accumulates in the destiny reel, first playing slower and then speeding up as we reach the end of the music program.
I will try to prove or check this hypotesis later.
See you on the next post.
Leandro