Although there are clear differences among them, all tape transports
have a common goal to move magnetic tape smoothly and evenly
across a set of magnetic heads. From a tiny, handheld microcassette
recorder to a twenty-four track studio machine that uses two-inch
wide tape, they all work in a similar fashion. I'll lean more
toward the professional or semi-professional reel-to-reel tape
recorders in this first part of the discussion.
The open loop tape transport is the most common type.
Otari, Tascam, MCI/Sony, and many other companies use the open
loop transport design for their tape machines. Here, the supply
reel dispenses the tape. In professional quality machines, the
tape will first feed through a roller guide or perhaps
a dancer arm (with a roller guide attached to it) which
stabilizes the tape as it moves towards the head stack.
The tape then enters the head stack, where the erase,
record and playback heads are located. First there is a fixed
guide, which again serves to support the tape. The first
head will be the erase head, next is the record
head, and finally the playback or reproduce head.
As the tape passes out of the head stack, there is an additional
fixed guide to, once again, support the tape properly.
Now the tape is grabbed between the pinch roller and
the capstan. The capstan is actually the extended shaft
of a DC servo controlled motor. Its primary task is to grab the
tape and meter it through at an even pace.
The tape then flows past yet another roller guide or dancer
arm assembly to guide the tape toward the takeup reel.
The job of the takeup reel is simply to take up the slack and
to wind up the tape so it doesn't fall on the floor.
Notice the order of the magnetic heads. In the direction
of travel during recording or playback, the tape first reaches
the erase head, then the record head, and then the repro head.
This arrangement is simply common sense. Let's say that you have
recorded a person talking on tape and, for the sake of argument,
you want to redo the recording. The first thing you'll want the
tape recorder to do for you is to erase the previous information
and then record the new information. If the heads were arranged
differently, for example, if the erase head came second or third
in the stack, then it would be counter productive.
Okay, there you have the basic path of the tape across an open
loop transport. Let's rewind and look at a couple things we left
unmentioned. The supply reel has a second job to do. In addition
to dispensing the tape as the capstan/pinch roller assembly pulls
it off the reel, the supply reel motor is designed to actually
provide a bit of holdback tension. If it did not, the
tape would tend to flop around. That would be disastrous to the
recording being made or listened to, because having an even tension
of the tape across the head stack is vital. If you want to check
this point, try playing or recording a reel-to-reel tape and
upset the tension by touching the supply reel or the tape before
it gets to the head stack. The sound will jerk in and out because
your fooling with the tension will force the tape in and out
of alignment with the heads.
The tolerances for proper recording and playback are quite small,
so the tension of the tape is rather strong. The distance between
the record and playback head on, for example, a quarter-inch
tape machine may be less than one inch. It wouldn't seem that
this would leave room for a problem, but it does. The tape stretched
across that short distance tends to vibrate, almost like a violin
string. The vibration will show up as a high pitched flutter
which will cause an odd distortion-like sound to your recording.
To solve this problem, many tape deck manufacturers place a flutter
filter, a very small roller guide, between the record and
playback heads. The tape rolls against the flutter filter as
it passes across that span, and the filter smoothes out those
jitters.
To protect your hours of work invested in that piece of tape
sitting there on your tape deck, the tape load sensor
stands guard over it. Many machines use an optical sensor to
"know" if tape is on the machine or not. This works
by shining a tiny light across the tape path that will be picked
up by a sensor on the opposite side. If tape is in the path,
it will block the light from reaching the sensor, and this "tells"
it that the tape is present. If for any reason the tape falls
out of the path, or the tape reaches the end of the reel, the
sensor will disengage the transport. One word about preventative
maintenance can be said here make sure that you keep
this type of sensor clean. Small flakes of tape or even a buildup
of oxide can block the light path and make the machine think
there is tape properly installed, even if it is not. Periodic
cleaning with a cotton swab and perhaps some tape cleaning fluid
will help protect you from such a problem.
Another type of tape load sensor is a mechanical device called
a tension arm, and is similar to the dancer arm concept.
The tape tension keeps the dancer arm held up in a certain position.
This tells the machine that there is tape on the machine. If
the tension should fail for a moment, the dancer arm will fall
below a certain point which will trip a relay that will disengage
the transport.
The tape load sensor provides a tremendous safety service.
For example, what if for some reason your takeup reel stopped
working while the tape was playing or (yikes!) in fast forward
motion? That would be a rather desperate time, especially if
you were not physically in the room to catch (literally) the
problem. If the tape falls out of the sensor, the machine will
disengage and the brakes will be applied to the reel motors which
should quickly but smoothly and evenly bring the tape to a halt.
A pair of tape lifters are designed to sit at rest
within the head stack, one between the erase and record heads,
the other between the record and repro heads. They serve to lift
the tape away from the heads whenever you shuttle the tape forward
or backward at high speed. This function helps in two ways. First,
it lessens the potential wear of the heads. Dragging that tape
across the tape path will wear down everything in its path over
time. Passing the tape at high wind speeds moves quite a bit
of tape, and that action would seriously shorten the life of
the heads in particular.
Secondly, lifting the tape from the heads at high wind speeds
will also protect your speakers and your ears. Tape carries a
changing magnetic field across the heads, and the frequency and
strength of the recorded field will determine the pitch and loudness
of what you hear over your loudspeakers or headphones. When tape
is passed over the heads at a speed faster than its normal playback
speed, more magnetic information is pulled across the playback
head in the same amount of time. This presents a stronger magnetic
field at the head, which will of course be amplified and sent
over your speakers. The tape lifters guard against this taking
place.
There may be times during high speed rewind or forward wind
when you do want to softly hear the tape with practice
you can know right where you are as you race through a song at
a high wind speed. Many machines allow you to manually release
the tension of the lifters just as much as desired, which will
allow you to hear what you need to hear. Others may force the
lifters to be only all the way out or all the way in, at which
point you are stuck with a less than optimum operation.
With such a long piece of tape, it's handy to know where
you are in the "pack" at any given time. The only sure
system for this is to use SMPTE time code, but most tape recorders
provide a tape counter of some sort. There are differing
methods of making a tape counter work. Often, either the roller
guide mounted before or the roller guide mounted after the head
stack is designed to also operate as the tape counter. If you
are able to unscrew the cap that holds the roller guide on its
spindle, you will see another type of optical sensor. The guide
has a pair of small posts hanging off the bottom of it, and the
sensor sits there and counts how many times the posts pass in
front of it. A small electronic circuit then calculates this
information into a representation for tape position.
In the older machines, this representation simply indicated
a relative position. That is to say, it simply counted up or
down in whole numbers. Later on, designers had it count in minutes
and tenths of minutes. Neither system was terribly useful, however,
because our time base is in hours and minutes and seconds. We
tend not to think in terms of "tenths of minutes".
Eventually, designers were able to find a way to display
the tape position in hours, minutes and seconds. Now, most current
professional machines and many semi-pro decks have this feature.
It still is only a "relative" position indication,
however. This because you can reset the tape counter to "zero",
or on some machines, even set it to read a certain position by
entering a number in with a keypad, regardless of where the tape
is in the pack as it sits on the machine. The counter will also
likely default to a "zero" position if the machine
is turned off for some reason. Still, it is a very workable arrangement
and most engineers would be quite lost without it.
A second popular type of tape transport is the closed
loop transport. The 3M Company makes a popular professional
model (the Isoloop), and Technics has a consumer version as well.
This transport works very much the same way as the open loop
transport, but with some rather obvious differences. Here, the
tape tension is applied by the capstan and a pair of pinch rollers.
The incoming and outgoing pinch rollers are slightly different
in size, such that the tape is pulled out of the head stack slightly
faster than it is allowed to enter. This constant pulling provides
the needed stability and tension across the head stack.
Look Ma, No Pinch Roller
In the early 1980's manufacturers developed a pressure roller-less
capstan system. They discovered that if the capstan diameter
was made quite large in comparison to earlier versions, and if
the capstan itself was coated with rubber, that a pinch roller
was no longer needed. The rubber coating provided the needed
traction to pass the tape through the transport. And by not squishing
the tape between the typical capstan and pinch roller, the tape
was subject to less wear. The Ampex ATR tape machine used this
type of transport, a version of the open loop style.
Tape Handling
A father watches on with pride as his son displays his mastery
over a complex task. An engineer watches with pride at how wonderfully
his favorite tape recorder handles the tape. This is one of the
love affairs an engineer is allowed to have with his equipment.
How a tape machine handles the tape is a tremendously important
feature.
Some machines, particularly the older models, jerk the tape
back and forth. One finds himself thinking, "No way I'm
putting my master tape on that machine! No sir, that's 200 hours
of my strongest emotions wrapped up in that fragile piece of
tape. I'm taking no chances." Anytime I see one of those
older machines shuttling tape around, I wince when I hit the
stop button.
The tape tension we talked about earlier is also very important
when the tape is shuttled forward or backward at high speed.
A sign of a first class tape machine is to look at how evenly
the tape is packed on the reel after it has been wound up at
high speed. If it is very even, then the transport is top notch;
if it is very jagged, with ridges of tape showing up fairly often
and even about to unravel the pack, then the transport has problems.
In many studios it is customary to store quarter-inch tapes without
a reel; one simply ties off the end of the "pancake"
of tape and puts it in a box until the next usage. Obviously
this involves more handling of the tape, and if the pack is not
very even, you run the risk of damaging the tape. If the transport
will not pack the tape evenly at high speed, you will need to
run the tape its full length at one of the regular play speeds
to ensure a good pack.
Some machines have a shuttle control called an MVC (Manual
Velocity Control). Reel-to-reel machines today can zip tape back
and forth at a fairly good clip. It's asking a lot for even a
professional machine to pack the tape evenly at high wind speeds.
But packing a tape at play speeds takes forever. So the ability
to wind the tape at some in-between speed is rather handy. MCI/Sony,
Otari and other manufacturers offer this advantage in some of
their machines.
Braking is another important feature of any tape transport.
In fact, well-executed, it is one of those things that causes
an engineer to smile fondly at his machine when he presses the
stop button. Literally, one of the tests to see if the reel brakes
are working properly is to set up a worst case scenario. Imagine
placing a large reel of tape on the supply reel and threading
it up on the machine. Now, put the machine in a fast forward
wind, let it get up to full speed, and then simulate a power
failure by shutting off the tape deck power switch. If the brakes
are aligned properly, and the transport does its job, the tape
should coast quickly but smoothly to a rest. If they are not,
the inertia set up by all that mass on the supply reel should
spill lots of tape on the floor. Loading the tape mostly on the
takeup reel will setup the same test for the rewind condition.
Needless to say, don't try this test with your favorite master
tape.
Orientation
Although many reel-to-reel tape recorders are designed to
stand upright on a table, the more useful orientation is to lay
them on their back. It won't be long into a recording project
before you'll need to do some razor blade editing of the tape,
and you'll end up with a crick in your neck if you try to edit
on an upright machine. Do yourself a favor and lay the thing
on its back the editing will be much, much easier and I
think you'll find nearly everything about this arrangement to
be better than the other way.
Summary
Well, that's enough for now. Obviously the kinds of machines
I've talked about here are starting to be replaced with digital
counterparts, and tape transports aren't as exposed for discovery
as much as these older machines were. Still, there are a lot
of these machines out there in use everyday, so we hope
this information proves useful.
First published in the November/December
1989 issue of Soundcheck Magazine. Used with permission.
After making his living as a professional musician for twelve years, Curt Taipale returned to college and earned his Bachelor of Music degree from the University of Miami in 1980 (back in the days of analog tape machines). He has invested his career ever since as a recording and live sound engineer, a consultant, educator, and author. He served ten years on full time church production staff plus many more years as a guest sound engineer. He contributed three chapters to the Yamaha Guide to Sound Systems for Worship, has written numerous articles for several magazines, and is the Church Editor for Live Sound International. To learn more about Curt's background, see Who Are We?
