Guide for a thin tape-like recording medium, particularly a magnetic tape

A tape guide, especially a guide for thin magnetic tapes which are subjected to strong acceleration forces in the longitudinal direction, and a tape transport apparatus with a central capstan, comprising such a tape guide. The tape guide includes one or two rotatable, but fixedly arranged guide rollers having flanges arranged around part of the periphery thereof, which flanges each have an ingress and an egress chamfer for the tape. When one guide roller is used, the magnetic head is positioned between a tape reel and the guide roller and, when two guide rollers are used, it is positioned between the said two rollers, the magnetic head bearing against the unsupported span of tape in each case. The tape guide may be provided with means for producing cushions of air for stabilizing the roller as it rotates.

The present invention relates to a guide for a thin tape-like recording 
medium, particularly a magnetic tape which, when transported past a 
magnetic head, is subjected to strong acceleration forces in the 
longitudinal direction, wherein at least one fixedly arranged, rotatable 
guide roller is located between two fixed arcuate guide flanges. 
German Published Application DAS No. 1,625,592 discloses a rotatable guide 
roller for magnetic tapes which is located between stationary surfaces for 
guiding the longitudinal edges of the tape. However, the rotatable guide 
roller is also pivotable about its axis of rotation, so that the plane in 
which the tape is guided varies. 
German Pat. No. 1,249,340 discloses a tape guide wherein the surface of the 
guide roller which is rotatably mounted between two stationary disc-shaped 
members is covered with velvet to prevent longitudinal oscillations of the 
tape. The said disc-shaped members do not have any tape guiding function. 
Riding up of the tape onto the said disc-shaped members is therefore 
unavoidable. 
U.S. Pat. No. 3,317,104 describes a rotatable tape guide roller having a 
fixed and a spring-loaded flange for guiding the tape in the vertical 
direction. 
U.S. Pat. No. 4,098,446 relates to a stationary tape guide for thin 
magnetic tapes which are transported at high speed, a concave guide 
surface and a cushion of air, produced by introducing air through 
orifices, serving to guide the tape. Despite the use of this cushion of 
air produced by the introduction of extraneous air, it is not possible to 
prevent the tape from making contact with the guide surface in certain 
places, particularly in the case of very thin and flexible magnetic tapes. 
Further disadvantages of this type of tape guide are that there is 
considerable friction between the tape and guide surface at the points of 
contact, and that they are of complicated design. 
Other known rotatable guide rollers with integral flanges have the 
disadvantage that, when thin tapes are used, the tape is folded over by 
the rotating flanges or rides up on the flanges. 
Consequently, all prior art tape guides suffer from the drawback that they 
cause tape damage and provide no or only poor guidance of the tape in the 
vertical direction. 
An object of the present invention is to provide a low-friction tape guide 
which provides exceptionally good guidance of the tape in the vertical 
direction and is so designed that it can be manufactured simply and 
economically. A particular object of the invention is to provide a tape 
guide which is suitable for use in portable video recorders of small 
overall dimensions, and does not require any maintanance. 
We have found that these objects are achieved with a guide for a thin 
tape-like recording medium, particularly a magnetic tape which, when 
transported past a magnetic head, is subjected to strong acceleration 
forces in the longitudinal direction, wherein at least one fixedly 
arranged, freely rotatable guide roller is located between two fixed 
arc-shaped guide flanges, and the said guide flanges are arranged around 
part of the periphery of the guide roller and each have an ingress and an 
egress chamfer for the tape. 
Apart from offering precise tape guidance, this design has the advantage 
that air entrained by the tape can escape from between the tape and guide 
roller, and the tape lies uniformly on the guide surface of the guide 
roller. 
In a further embodiment of the invention, the fixed guide flanges are 
arranged symmetrically with respect to the central plane of rotation of 
the guide roller and at the same time around part of the periphery of the 
guide roller in an arc .beta. of at least 90.degree.-2.alpha., .alpha. 
being the arc length of one of the chamfers. 
In an advantageous embodiment of the invention, the guide flanges are 
arranged around part of the periphery of the guide roller in an arc .beta. 
of 180.degree.-2.alpha.. 
In a practical embodiment, the chamfers, i.e. the ingress and egress zones 
for the tape, each have an arc length .alpha. corresponding to an angle of 
from 5.degree. to 30.degree., preferably from 10.degree. to 15.degree.. 
In a further advantageous embodiment of the invention, a single guide 
roller is provided, so that the magnetic head cooperates with the 
unsupported span of tape extending between a tape reel and the guide 
roller. 
To obtain symmetry with respect to the magnetic head, two guide rollers 
arranged in spatial relationship are provided, the head cooperating with 
the unsupported span of tape extending between the two guide rollers. 
In a further embodiment, the guide flanges, when viewed in plan, are each 
provided with a ring of holes, so that a pumping action is obtained when 
the guide roller rotates, with the result that cushions of air are 
produced for the stabilization of the guide roller as it rotates. 
In yet another advantageous embodiment of the invention, the opposite outer 
arcuate edges of the guide flanges are provided with bevelled guide 
surfaces for centering the tape in the central plane of rotation of the 
guide roller. 
It is also advantageous to provide the guide roller with a convex guide 
surface, so that a centering action is exerted on the tape. 
The above-described tape guide can be used with particular advantage in a 
tape transport apparatus having a central capstan. Accordingly, the 
present invention also relates to a transport apparatus of the following 
design: 
A tape transport apparatus comprising a central capstan and take-up and 
supply reels which can be urged toward the capstan by biasing means and 
are rotatably mounted on supports that can be moved toward and away from 
the capstan, the capstan being provided with a resilient peripheral 
portion, so that compressive forces between the capstan and the take-up 
and supply reels and hence driving forces for rotating the reels can be 
produced, and the tape leaving the supply reel being guided over at least 
one fixedly arranged, rotatable guide roller and a magnetic head to the 
take-up reel where it is wound up, wherein the guide roller is located 
between two fixed guide flanges, and the said flanges are arranged around 
part of the periphery of the guide roller. 
In a further practical embodiment of the tape transport apparatus of the 
invention, the guide flanges of the tape guide each have an ingress and an 
egress chamfer for the tape, and are arranged around part of the periphery 
of the guide roller in an arc .beta. of at least 90.degree.-2.alpha., 
preferably in an arc .beta. of 180.degree.-2.alpha., .alpha. being the arc 
length of one of the chamfers.

FIG. 1 shows a tape transport apparatus with a central capstan 4, swing 
arms 1 and 2, tape guide 3 according to the present invention, hubs 6 and 
8, tape reels 5 and 7, and deck 9 on which the parts are mounted. The tape 
10 is provided with, for example, a large number of parallel magnetic 
tracks; it is fed from reel 7, which is urged against capstan 4 by a 
biasing means 31, over tape guide 3, mounted on a shaft, to hub 6 which is 
also so urged against capstan 4 and where the tape is wound up, and vice 
versa, the tape being transported in both directions at high speed. 
Magnetic head 26 cooperates with the unsupported span of tape extending 
between tape guide 3 and tape reel 5 or capstan 4. 
FIG. 2 is a side view, partly in section, of tape guide 3, also showing 
capstan 4 and drive motor 17. Tape guide 3 consists essentially of guide 
roller 14 which is rotatably mounted on shaft 16, and stationary guide 
flanges 11 and 15 which are arranged symmetrically with respect to the 
central plane of rotation of guide roller 14 (hereinafter referred to as 
"fixed flanges"). As can be seen from this Figure, there is a gap of 
preferably 0.05 to 0.3 mm between the guide roller and each fixed flange 
11, 15. These gaps may be produced by using, for example, disc-shaped 
distance pieces (not shown in the drawing). The ensure trouble-free 
running, the guide roller 14 is provided with a suitable bearing, 
advantageously a sintered metal bearing 13. The usual material of 
construction of the guide roller 14 is metal. If it is desired to 
additionally damp tape oscillations, a heavy metal, such as brass, or a 
metal shell filled with lead should advantageously be used. The guide 
surface of the guide roller 14 advantageously has an average 
peak-to-valley height R.sub.z of about 0.5 to 5 .mu.m, preferably 2.5 
.mu.m (according to DIN 4768). It is also expedient to provide the guide 
roller with a slightly convex guide surface. 
FIG. 2a shows in cross-section part of the tape guide of the invention with 
bevels 30 on the fixed flanges 11 and 15 which thus have a centering 
action on the tape 10. 
As shown in FIGS. 3 and 4, the upper and lower flanges 11 and 15 each have 
two chamfers 18 in the areas at which the tape enters and leaves the tape 
guide, the chamfers being arranged at points located at an angle .alpha. 
behind the line of ingress and egress of the tape 10, the line of ingress 
and egress of the tape being central axis 31, so that in the case of the 
single roller variant shown in FIG. 1 arc length .beta. corresponds to an 
angle of 180.degree.-2.alpha.. Angle .alpha. is from 5 to 30.degree., 
preferably from 10.degree. to 15.degree.. In the case of the embodiment of 
FIG. 7 having two guide rollers 27 and 28 arranged symmetrically with 
respect to magnetic head 26, arc length .beta. corresponds in each case to 
an angle of 90.degree.-2.alpha., angle .alpha. being of the same order of 
magnitude as indicated above. 
Although the fixed flanges 11 and 15 shown in FIGS. 1 to 4 and 7 are 
circular in shape, the effective guiding zone thereof is arc-shaped (arc 
.beta.). 
The above-described design ensures that any air caught between the magnetic 
tape 10 and guide rollers 14 and 27 and 28 can escape, that the tape lies 
uniformly on the surface of the guide roller, and that the tape is in a 
stabilized position on the guide roller, from which position it is brought 
gradually into the position in which it is guided in the vertical 
direction by flanges 11 and 15; the position in which the tape is 
stabilized on the guide roller usually does not coincide with the position 
in which it is guided in the vertical direction because of tape width 
variations and manufacturing tolerances of the tape transport apparatus. 
Height h corresponds to the largest tape width plus up to a maximum of 0.3 
mm, preferably plus 0.1 mm. In the case of a guide roller having a convex 
guide surface, the preferred value is from 0 to 0.1 mm. 
FIG. 3 shows a variant of the guide flange; in this variant the guide 
flange is provided with a ring of holes 19. As a result of this 
arrangement of holes 19 in fixed flanges 11 and 15, the flanges and the 
guide roller 14, when it rotates, act like a self-priming pump. Cushions 
of air are thus formed between the guide roller and the lateral surfaces 
of the fixed flanges 11 and 15, which air cushions stabilize the guide 
rollers 14 and 27 and 28 as they rotate, and have the additional advantage 
that they prevent the magnetic tape from entering the gaps between the 
guide rollers 14 and 27 and 28 and the fixed flanges 11 and 15. For 
economic reasons fixed flanges 11 and 15 are advantageously identical. The 
surfaces of the fixed flanges 11 and 15 which face one another may also be 
provided with a highly abrasion-resistant coating, e.g. a coating of 
tungsten carbide or ceramic material, by for example vacuum deposition, 
spraying, electroplating or laminating. It is of course also possible to 
make the fixed flanges 11 and 15 entirely of ceramic material, tungsten 
carbide or precious stone. 
The following two fundamentally similar tape transport apparatus having a 
central capstan were compared with one another: 
1. A tape transport having a stationary crescent-shaped two-part tape guide 
employing a film of extraneous air, the concave side of the tape guide 
being arranged adjacent to part of the periphery of the capstan having a 
diameter of 3 cm, and the magnetic head being disposed between the two 
parts of the guide. The width of the guide surface was 6.28 mm, and the 
mean distance between the flanges for guidance of the tape in the vertical 
direction was 6.30 mm. 
2. A tape transport having a tape guide according to the present invention 
as shown in FIGS. 1 to 4, the capstan and guide roller each having a 
diameter of 3 cm. The height of the guide roller was 6.10 mm, and the 
distance between the flanges was 6.30 mm. 
The two tape transport apparatus did not have any kind of special device 
for preventing or reducing tape flutter or any kind of device for 
correcting time base errors. 
The same test circuit of the type shown in FIG. 5 was used for all 
measurements. In the drawing, block 20 denotes the electronic 
playback/record components of the tape transport apparatus, block 21 a 
band-pass filter having a bandwidth of 1 to 30 kHz, block 22 an 
oscillograph, and block 23 a spectrum analyzer. 
Band-pass filter 21 serves to suppress unwanted HF signal components and is 
connected to the demodulator of the electronic playback/record components. 
Spectrum analyzer 23 is set to a bandwidth of 0.3 kHz. 
In position 2' of switch 29, a composite video signal in the form of a 
black-to-white transition was recorded on the tape 10, and in the test was 
reproduced by means of playback head 25 and the electronic playback 
components. 
In switch position 2' all time base errors are visible on monitor 24 as 
jitter of the black-to-white transition. 
In switch position 1' a fixed frequency f.sub.o of 3 MHz is recorded. The 
screen of oscillograph 22 shows the demodulated signal and hence the time 
base errors in the form of a shift in the zero crossovers of the 
demodulated signal. The flutter frequency spectra of the two tape 
transport apparatus shown on spectrum analyzer 23 are given in FIG. 6 in 
the form of curves a and b, dotted curve a belonging to tape transport 1 
and unbroken curve b to tape transport 2. The spectrum analyzer shows the 
disturbances in the frequency and amplitude of the signal caused by, for 
example, oscillations of the tape in the longitudinal direction (scrape 
flutter). The third dashed curve c shows the demodulation noise. The first 
pulse at 0Hz is the zero beat of the analyzer; it can be seen from curves 
a to c that the main flutter frequency is at about 7 kHz. 
The percentage scrape flutter factor S.sub.F can be calculated with the aid 
of the following equation: 
##EQU1## 
where 
f.sub.0 denotes the frequency of 3 MHz (the frequency of the peaks of the 
horizontal line pulses), 
f.sub.1 `denotes the flutter frequency concerned, 
USF denotes the voltage at the determined flutter frequency f.sub.1, 
U.sub.maxdemo denotes the maximum output D.C. voltage of the demodulator 
(in this instance 2 volts), and 
f denotes the entire frequency deviation (in this instance 1.3 MHz). 
The results obtained with the two tape transport apparatus are given in the 
following Table: 
TABLE 
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Tape transport 
U.sub.SF(mV) 
S.sub.F (%) 
.DELTA. t (.mu.sec) 
.DELTA.t (%) 
______________________________________ 
1 18 0.39 0.8 1.25 
2 4 0.09 0.2 0.3 
______________________________________ 
The .DELTA.t (.mu.usec) column lists the visible high frequency time base 
errors which were read off on the oscillograph, and the .DELTA.t (%) 
column the percentage errors based on a line duration of 64 .mu.sec 
(=100%). The limit for visual detection of time base errors on the screen 
of oscillograph 22 is 0.2%, corresponding to 1.28.mu.usec, so that the 
above value of 0.3% can be read off without difficulty. 
To sum up, the percentage scrape flutter factor was able to be reduced from 
0.39% to 0.09%, i.e. a value which is about 4.5 times less. A distinct 
reduction in the percentage scrape flutter factor, i.e. to one third, was 
achieved with a tape transport of the first-mentioned type which was 
provided with an expensive electronic time base error correction device 
based on a charged coupled device. 
As can be seen from FIG. 6, there is a reduction in the amplitude of the 
unwanted signal of from 10 to about 20 db, corresponding to factors of 
from 3.3 to 10.