I received some damaged library audio tapes ,damaged by a 
magnetic field somwwhere.After filling in the "write 
protect " with cork and tape ,i demagnetised them with an
old Radio Shack demagnetizer.
 
  When I fast forward in a tape recorder with a counter,I
saw that many were 45 minute tapes.However,I needed 60 or
at least 55 minute tapes to record talk show programs on
investing,home repair,PC tips,car repair and special
Public radio interviews.
 
   I also had some short 20 minute tapes.So I wound one
to the half-way point,then open up the 20 and 45 minute 
tapes and used Scotch tape to add 10 min.from the short
tape to the longer tape.
    When I tested the modified tape in a tape recorder,
I got only 50 minutes.What gives? The rotation speed
of the motor is constant.There must be a non-linear 
effect.The diameter of the reels was involved.
 
   I measured the length of tape that unwound for one 
revolution:
 
Empty spool   2 5/8 "
Half spool    4 3/8 "
Full spool    6 6/8 "    O.D.=2.0 "
 
So I took a 20 min.tape-ran it for 5 min.,leaving 15 min.
left.Then added this 15 min. to a 45 min.tape.Now it was 
too large and was rubbing against the inside of the case.
Was over 2 " O.D.(outer diam.)
Now how do they make 60 min.tapes(120 min.total)?
 
Must be thickness involved.So I measured a 60 min.tape
from edge of center plastic spool to the outer edge-got
10/16 " or 0.625  divided by the number of turns on the
cassete counter ( 960),got 0.0006 " thick.
To verify,I measured the thickness of the tape with a
micrometer and got 0.0006 ",a good check
 
Then I did the same to a 50 min.tape-from plastic spool to 
edge = 11/16 "=6875 and for 810 turns,calculate 0.00085.
The micrometer gave me 0.0009 ",a good check.
 
The largest of these library tapes is 55 minutes and must 
be set by the thickness .The 60 minute tapes( sold as 120
minute) must get the longer time from thinner tapes.
 
So the most that I can add to a 45 minute tape is 10
minutes assuming the same thickness of film.I'll have 
to experiment to find the correct length.Still, 55 min. 
is useful.On 55 minute tapes,I can skip the 5 minute 
news leader.
 
If you are recording sound and the tape moves at a fast
rate,then the volume of sound must be spread over a larger
section of tape as compared to a short section.
Is this right?
 
 In repairing VCR video tapes for the library,I first check 
for obvious defects:cracked case,wrinkled tape,noise when
shaken,then I use a rewinder to check for unusual sounds.
I see the same non-linear effect on the counter depending
on the length of the tape.
   I sometimes get donated 30-40 minute commercial based 
tapes and I need one hour tapes to copy a special program
to pass on to the family.Maybe I'll use the same method 
above for extending video tapes.
 

--- Maximus 2.02