Apparatus and method for cueing a video tape recorder

To position a cue location on a video tape to a scanner so that the information stored at the cue location can read by a transducer. The video tape is transported so that when the cue location arrives at the scanner, the tape is moving at a selectable desired velocity. A synchronous cueing function is also provided for, in which the cue location is advanced at the rate of the desired velocity during to cueing operation to allow the positioning to be synchronized with other devices when the cue operation is completed.

BACKGROUND OF THE INVENTION 
The present invention relates to the cueing, or positioning, of a location 
on a video tape moved by a tape transport. More particularly, the present 
invention relates to positioning a cue location on the video tape so that 
the cue location arrives at its destination with the tape moving at a 
selectable velocity. 
In video production, video tape recorders are used to store video 
information. Video information is composed of frames of video images. Each 
frame is in turn composed of two fields of video information. Video 
information is stored in discrete locations on the video tape. In the 
commonly used Type C video tape format, each field of video information is 
stored on the video tape in a helical track. In addition to these tracks 
of video information, there are other signals recorded on the video tape. 
These include control track, time code, and audio signals. Control track 
is typically a periodic signal on the tape used to accurately locate the 
beginning of each frame of video information. 
Video tape recorders perform a number of operations including the playing 
and recording of video information. To play or record video information, 
the tape is moved past a scanner, which contains transducers such as a 
play head, a record head, and an erase head. While the tape moves past the 
transducers, video information is either read with the play head, written 
with the record head, or erased with the erase head. The video information 
is sent to or received from a signal system of the video tape recorder, 
which interfaces between the video information stored on the video tape 
and the outside world. 
Video tape recorders are used to play and record video information that is 
stored at specific locations on the video tape. For example, it is common 
to record a large amount of video information during a video production. 
Only a small portion of the information is usually needed. The cue 
function is used to locate the beginning of the portion of needed video 
information stored on the tape. Similarly, it is often necessary when 
recording video information to begin recording at a specific location on 
the tape, for example, following a portion of video information previously 
recorded. The cue function of a video tape recorder is used to do this 
positioning of the video tape. 
The purpose of the cue function on a video tape recorder is to move a 
specific location on the video tape to the scanner, so that it can be 
played, recorded, or erased. This function is also known as the search 
function. For the purposes of the disclosure, this function will be called 
the cue function. This location is called the cue location, and the moving 
of the cue location to the scanner is called cueing. The cue location 
marks the beginning of a portion of video information to be played, 
recorded, or erased. This portion of video information might be as short 
as a field of video information or as long as many thousands of video 
frames. 
The cue function is implemented in present video tape recorders as follows. 
It is determined whether the cue location lies ahead of or behind the 
location on the tape presently at the scanner. If it is ahead, the tape 
transport accelerates the tape in the forward direction. If it is behind 
the present location, the tape transport accelerates the tape in the 
reverse direction. As the cue location approaches the scanner, the 
transports decelerates the tape. Ideally when the cue location reaches the 
scanner, the tape has been decelerated to a velocity of zero. Less 
accurate video tape recorders tend to miss the cue location on the first 
try and have to hop around the location until it is hit. 
Actually, arriving at the cue location at zero velocity can waste time 
because what is often desired is that the tape be moving at the velocity 
at which the playing or recording is to take place. A disadvantage of the 
present cue functions is that they can only cue to a zero velocity. 
Additionally, the transport cannot instantaneously jump the tape from zero 
to a desired velocity. Thus, the solution in present day recorders is to 
cue to a location a specific number of frames behind the actual cue 
location. From that location, the tape is accelerated from a zero velocity 
to the desired velocity in the distance before the actual cue location is 
reached. In effect, the transport takes a running start at the actual cue 
location. There are actually a number of other reasons to choose a 
location ahead of the actual cue location including allowing time for the 
various control systems to lock in place, and allowing the operator a 
visual reference before the cue location. 
There is another type of cueing operation that cannot be conveniently 
executed by present video tape recorders. This function is called 
synchronous cueing. Synchronous cueing differs from standard cueing in 
that the cue location changes during the cue operation. In synchronous 
cueing, the tape is cued to a moving target as the cue location is 
advanced at the rate of the desired velocity. The advancing cue location 
simulates a moving reference such as another video tape recorder. Tape 
movement of the tape recorder is synchronized with this simulated moving 
reference. 
Synchronous cueing can be used to synchronize the playing of video 
information by one video tape recorder with other video tape recorders or 
other devices. An example of this would be in a presentation using two or 
more video tape recorders which must be synchronized. To perform this 
synchronization with present video tape recorders, the present cue 
function would be used. After each recorder has been separately cued to 
the cue location for each recorder, all the recorders would be cued up to 
play speed. Using this method, time must be allowed for each recorder to 
cue, before the recorders will be synchronized. While this may be 
necessary in specific instances, this method is complex and requires that 
all the tape recorders wait for the last recorder to cue, which 
potentially may waste a great deal of time. 
Therefore, a need exists for a cueing technique in which a cue location can 
arrive at the scanner, at a desired velocity. There is also a need for a 
synchronous cueing technique in which the cue location changes during the 
cue operation at the rate of the desired velocity. 
SUMMARY OF THE INVENTION 
The present invention provides for a cue function which can position a cue 
location on a video tape to a scanner at a selectable desired velocity. 
The present invention provides for a cueing function which can efficiently 
position the tape to a cue location at a desired velocity without wasting 
the time to stop in order to cue. Additionally, the present invention 
provides for a synchronous cue function which can position a cue location 
on the video tape to the scanner at a selectable desired velocity, in 
which the cue location is advanced at the rate of the desired velocity. 
Using positional information provided from the transport, the location on 
the tape that is currently at the scanner is determined. Positional 
information is derived from reading the control track on the video tape, 
or alternately from tachometer information from the capstan, or both. The 
cue location that is desired is also determined. This location may be 
selected by a human operator, or may be calculated from other information. 
The difference between these two numbers is called the detected distance, 
and represents the distance and direction the tape must be moved in order 
to position the cue location at the scanner. 
From velocity information, also provided from the transport, the velocity 
at which the tape is currently moving is determined. Velocity information 
can be derived from tachometer information from the capstan. The desired 
velocity of the tape when the cue location reaches the scanner is 
selected. This velocity may be selected by a human operator, or may be 
calculated from other information. 
For the particular detected distance and selected desired velocity, an 
intermediate velocity is chosen. The intermediate velocity is the velocity 
at which the tape is ideally traveling so that when the cue location 
arrives at the scanner, the tape will be moving at the desired velocity. 
The intermediate velocity is selected by means of a velocity profile 
function which is optimized for the mechanical limitations of the tape 
transport and momentum of the tape and transport. This function limits the 
rate of change of the intermediate velocities, generated as the cue 
location approaches the scanner, to the maximum deceleration the transport 
is capable of. 
The intermediate velocity is compared to the velocity the tape is currently 
moving at, and a velocity error signal is generated. The velocity error 
signal indicates the difference between the velocity the tape is currently 
at and the velocity it should be moving at. The velocity error signal is 
used by the transport to adjust the velocity of the tape to minimize the 
velocity error signal. The process is continued until the cue location 
reaches the scanner. When the cue location reaches the scanner, the tape 
will be moving at the desired velocity thus saving the time necessary in 
the present cue functions which require a stop and running start. 
Synchronous cueing is accomplished by the process described above, with an 
additional step. Instead of the cue location being a constant, the cue 
location is advanced at the rate of the selected desired velocity.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a partial block diagram of a video tape recorder 2 is 
shown. A video tape transport 4 is used to transport video tape 14 past 
transducers located along the path of the video tape 14. Video tape 
transport 4 is of conventional design. 
When in a play mode, video tape 14 is fed off a video tape supply reel 12 
in the direction indicated. The video tape is then guided into contact 
with a longitudinal channel transducer 16. Longitudinal transducer 16 can 
be used to read any longitudinal signal on the tape, such as audio 
signals, longitudinal time code or control track information. Generally, a 
video tape recorder will have a number of longitudinal transducers. From 
longitudinal transducer 16, video tape 14 is guided around scanner 20. 
Scanner 20 contains at least one transducer 18 to read video information 
off video tape 14. Scanner 20 may also include other transducers, such as 
erase and write transducers. Scanner 20 is of conventional design and 
rotates in the direction indicated at a fixed, known velocity so as to be 
able to read helically recorded tracks on video tape 14. 
The precise velocity of rotation of scanner 20 and velocity of the video 
tape 14, at play velocity, vary for different recording formats. However, 
the values for each format are specified by well known standards. 
Generally, at normal play speed, the rotation of the scanner 20, and thus 
transducer 18, is about 100 times the linear velocity of the tape 14. 
After leaving scanner 20, video tape 14 is fed into contact with a capstan 
22. Capstan 22 is used to regulate the velocity of video tape 14 and is 
also of conventional design. After leaving capstan 22, video tape 14 is 
fed onto a take-up reel 24. The transport 4 includes a number of other 
devices which control the movement of the tape, such as the reel motors 26 
and tape tension arms 28. However, in a conventional video tape recorder, 
these other devices generally track the actions of the capstan 22, which 
is considered the primary tape motion control. As the exact control of 
these other devices is well known in the art, this disclosure will limit 
its discussion to control of tape movement to the capstan. 
Tape transport 4 is controlled by tape transport controller 10, which is in 
turn controlled by video tape recorder controller 6. Recorder controller 6 
is conventional and well known to those skilled in the art. 
Operation of tape transport 4 is in response to control input signals 7 as 
interpreted by recorder controller 6 and implemented by tape transport 
controller 10. Control input signals 7 may originate at a control panel 8 
operated by an operator, or may originate from a an outside source through 
the tape recorder's external interface. These signals specify an operating 
mode for the tape transport. Recorder controller 6 receives these signals 
and generates machine control signals that are sent to tape transport 
controller 10. Tape transport controller 10 interprets these signals and 
produces the signals necessary to control each element of tape transport 
4. 
Tape transport controller 10 is composed of two parts. The first part is 
the conventional transport controller which implements all non-cueing 
operating modes. A cueing function is one where a specific location on the 
tape must be positioned to the transducer 18 on the scanner 20. The part 
of the tape transport controller 10 that implements the non-cueing 
functions is conventional and well known to those skilled in the art. 
The second part of the tape transport controller 10 is the cue system, 
which embodies the present invention. The cue system is used when a 
specific location on the video tape 14 must be positioned to the scanner. 
Once the cue system has accomplished the desired positioning, control is 
released back to the conventional transport controller. 
By controlling the velocity and direction of capstan 22, supply reel 12, 
and take-up reel 24, a range of tape velocities can be obtained. Tape 
transport 4 can be operated in the play mode, stop mode, and shuttle mode 
(including variable play speed mode). In play mode, the direction of the 
tape is forward and the velocity of video tape 14 is fixed and equal to 
the velocity normally used to record video information. In stop mode, the 
video tape velocity is zero, but the scanner 20 is still rotating and 
video information can be read off tape. Shuttle mode is used to control 
movement of the tape in either direction and at any velocity. As a 
practical matter, shuttle velocity is limited to plus or minus 50 times 
play velocity in even the fastest video tape transports currently being 
used. 
Video information read off video tape 14 is fed to a signal system 30, 
where it is processed. Video information that is to be recorded is input 
to the signal system 30. Signal systems for video tape recorders are well 
known. 
Referring now to FIG. 2, a partial block diagram of the video tape recorder 
of FIG. 1 is shown. The elements included in this figure are parts of the 
video tape transport 4, the video tape recorder controller 6, cue system 
32, and parts of transport controller 34. 
Referring to tape transport 4, in FIG. 2, video tape 14 is shown in contact 
with capstan 22 and a control track head 16. Control track head 16 is used 
to read magnetically recorded control track signals 44. These signals are 
used to mark the beginning of each frame of video information recorded on 
video tape 14, and can be used to identify the location of each frame of 
video information on video tape 14. A control track decoder 46 decodes the 
control track signal 48 from control track head 16 into a control track 
position signal 50. Control track position signal 50 indicates the current 
frame of video information presently located at the scanner 20 (FIG. 1). 
Because presently used control track signals only occur every frame, 
particular fields within a frame cannot be identified from the control 
track signal alone. 
Also shown in tape transport 4, is the capstan 22. Capstan 22 is driven by 
motor 40. The direction and speed of drive motor 40 is controlled by motor 
control signal 52. Motor control signal 52 is generated by the transport 
controller 34. 
The motion of the capstan 22 is detected by tachometer 42. Ideally, there 
should be no slip between the capstan 22 and the tape 14, in order for 
tachometer 42 to accurately detect movement of tape 14. There are a number 
of capstan designs which are well suited for this task, such as vacuum 
capstans. A tachometer signal 54 is generated by tachometer 42 and is fed 
to transport controller 34. 
Transport controller 34, parts of which are shown in FIG. 2, has two 
primary functions. The first is to determine various information about the 
position and movement of tape 14 on tape transport 4. The second is to 
control the velocity at which the tape is transported. The position of any 
particular location on tape can be completely controlled by controlling 
the velocity of the tape. There are two servo loops used in the cue 
function. There is an outer loop which includes the cue system 32 and the 
transport controller 34. There is also an inner loop that includes the 
tape transport 4 and the transport controller 34. These two loops provide 
a velocity and positional servo system. 
The elements shown in FIG. 2 of the transport controller are only the 
elements necessary for the discussion of the present invention in the cue 
system 32. As was discussed above, there are a number of functions of the 
transport control 34, and there are elements for these functions that are 
not shown. These elements are well known to those skilled in the art. 
Tachometer signals 54 are fed to tachometer decoder 60. Tachometer decoder 
60 decodes these signals into a signed tape movement signal 62, and a 
direction signal 63. The tape movement signal 62 indicates the velocity of 
tape movement. This signal is fed to current position detector 66 along 
with direction signal 63. The tape movement signal 62 and direction signal 
63 are also fed to a velocity comparator 68. A counter 69 also receives 
these two signals. Tachometer decoder 60 is of conventional design and is 
commercially available as an integrated circuit package. 
The function of the current position detector is to generate a current 
position signal 65, which indicates the location on the video tape that is 
currently at the scanner 20 (FIG. 1). This detector is shown receiving the 
control track position signal 50 and tape movement signal 62. The 
information provided by control track position signal 50 is sufficient 
alone to generate a current position signal 65 that is accurate to a frame 
of video information. Using this signal, the cue system 32 will only be 
able to position the tape to within one frame of accuracy. This degree of 
resolution may be sufficient for many recorders. However, it is desirable 
to position tape with the resolution of a field of video information. Thus 
the preferred embodiment of the present invention uses a current position 
signal with at least field position resolution. 
The tape movement signal 62 from the tachometer decoder can be used to 
augment the resolution of the control track position signal 50. In order 
to do this, the tachometer 42 must be of higher resolution than the 
control track 44. Tape movement signal 62 is in the form of a pulse train 
whose frequency indicates the velocity. Knowing the number of pulses that 
are generated during the length of each frame, it is possible to determine 
the position of the tape 14 between the beginning of each frame, as marked 
by the control track position signal 50, by counting the number of pulses 
since the beginning of the frame. The current position signal 65, at 
whatever resolution desired, is fed to the cue system 32. 
A complete disclosure of this method of determining tape location by using 
a combination of control track and capstan tachometer signals can be found 
in U.S. Pat. No. 4,692,819 entitled Method and Apparatus for Controlling 
the Position of a Transported Web, issued on Sept. 8, 1987 (this patent 
issued from an application which is a continuation-in-part of U.S. 
application Ser. No. 646,619, filed on August 31, 1984, now abandoned), by 
the present inventor and assigned to the same assignee of the present 
application. That application is hereby incorporated by reference into the 
present application. 
Counter 69 receives the tape movement signal 62 and the direction signal 63 
from the tachometer decoder 60. By counting the pulses in the tape 
movement signal 62, a velocity signal 71 is generated, where the direction 
signal 63 determines the sign of the velocity signal 71. The velocity 
signal 71 indicates the current velocity of the tape and is fed to the cue 
system 32. 
The second function of the transport controller, in its role of the present 
invention is to control the motor 40, thereby controlling the velocity of 
the tape 14 by means of the capstan 22. Cue system 32 provides transport 
controller 34 with an intermediate velocity signal 72. This velocity 
signal indicates the velocity that the cue system 32 desires the tape 14 
to presently be moving at. Intermediate velocity 72 is converted to the 
same form as velocity signal 62. Frequency generator 70 performs this 
task. Frequency generator is of conventional design and can easily be 
constructed by those skilled in the art. 
The output of the frequency generator 70 is a motion signal 74 and a 
direction signal 75, and is fed to velocity comparator 68 which compares 
tape motion signal 62 and direction signal 63, indicating the current 
velocity of the tape 14, with motion signal 74 and direction signal 75, 
indicating presently desired velocity. The result of this comparison is 
the difference between the current velocity of tape 14, and the velocity 
presently desired by the cue system. This difference is represented by a 
velocity error signal 76. Velocity comparator 68 is of conventional 
design. 
The velocity error signal 76 is fed to a motor drive amplifier 80. Motor 
drive amplifier 80 generates the motor drive signal 52 and thereby adjusts 
the velocity of the tape 14 to minimize the velocity error signal 76. 
Motor drive amplifiers are well known to those skilled in the art. 
Cue system 32 receives the current position signal 65 and current velocity 
signal 71 from transport controller 34, and a cue control signal 82 from 
recorder controller 6. Cue control signal 82 specifies the desired 
position at which the tape is to be cued, the desired velocity and 
direction the tape should be moving when cued, and framing requirements, 
if any. These parameters can be derived from operator input at the control 
panel 8, or can be derived from a control signal provided by a source 
external to the video recorder through an external input interface. 
A cue controller 100 receives the cue control signal 82 and generates a 
desired position signal 102, a frame select signal 104, and a desired 
velocity signal 106. The desired position signal 102 indicates the desired 
cue location, that is, the location on the tape that is to be positioned 
at the scanner 20 (FIG. 1). The desired velocity signal 106 indicates the 
desired velocity at which the tape 14 is to be moving when the desired cue 
location reaches the scanner 20 (FIG. 1). The frame select signal 104 
indicates whether a particular type of frame lock is desired. 
Framing selector 108 receives the desired position signal 102 and frame 
select signal 104. It generates a framed desired position signal 110. To 
perform the standard cueing operations that have been discussed, framing 
selector 108 may be omitted from the cue system 32. 
There are several desirable play modes for a video tape recorder. One mode 
is the non-framed play mode. This mode simply requires the tape 14 to be 
moving at the standard play speed without regard to synchronization to any 
particular location on the tape 14 with any reference signal. This 
function can be executed by the tape transport control 10 (FIG. 1), 
without the assistance of cue system 32. 
A second play mode is the framed play mode. The requirement of this play 
mode is that positioning of the beginning of a frame at the scanner 20 
(FIG. 1) is synchronized to a reference signal. This is a variation on the 
standard cue function in that instead of one particular cue location, the 
beginning of any frame is a potential cue location. 
The third play mode is the color framed play mode. Depending on the 
television standard, the phase of the color information in the video 
signal changes from field to field, and from frame to frame. It takes a 
certain number of frames before the color phase repeats. For example, in 
the NTSC television standard, the color phase repeats every two frames, 
and in the standard, it repeats every four frames. In certain 
situations it can be very desirable to synchronize the play mode at the 
beginning of a color frame sequence with a reference signal. 
To provide these framed play modes, the framing selector 108 takes the 
modulus of the desired position 102, with the frame select 104 being the 
modulo. Restated, the desired position 102, which must be selected to be 
the beginning of a frame or color frame sequence, is divided by the frame 
select value 104, which must be chosen to be the length of the frame or 
color frame sequence, and the remainder is produced as the framed desired 
position signal 110. The framing selector 108 can easily be implemented as 
a digital divider with remainder output. When framing is not desired, the 
frame select signal 104 is set to a large number, such as the length of 
the tape, and the remainder produced will be the desired position signal 
110. 
The framed desired position signal 110 is fed to a position comparator 112. 
Position comparator 112 also receives the current position signal 65. 
Position comparator calculates the difference between these two positions 
by subtracting the framed desired position signal 110 from current 
position signal 65, and generates a detected distance signal 114, which 
indicates the distance between the desired position and the current 
position. The sign of the detected distance signal 114 indicates which 
direction the desired position is from the current position. Position 
comparator 112 can be implemented as a digital subtractor. 
The detected distance signal 114 is fed to an adjustable velocity selector 
116. The adjustable velocity selector 116 also receives the desired 
velocity signal 106. From these two signals, adjustable velocity selector 
116 initiates the generation of the intermediate velocity signal 72 of 
previous mention. 
The adjustable velocity selector 116 implements a function, which for any 
particular detected distance signal 114 and desired velocity signal 106, 
generates an intermediate velocity, which is the velocity at which the 
tape 14 should presently be traveling in order for the cue position to 
reach the scanner at the desired velocity represented by signal 106. This 
function can take many forms, as will be discussed below, but must 
generate the desired velocity signal 106 in the form of the intermediate 
velocity 163 when the detected distance signal 114 is zero. This should 
not be considered a limitation on this function, but rather is the purpose 
of the function. That is, when the detected distance is zero, indicating 
the cue location on the tape 14 has arrived at the scanner 20 (FIG. 1), 
the velocity of the tape 14, as dictated by the intermediate velocity 
signal 72, should be at the desired velocity of signal 106. 
Referring to FIG. 3, a block diagram of the adjustable velocity selector 
116 is shown. The detected distance signal 114 is fed to a velocity 
selector 160. Velocity selector 160 generates a selected velocity signal 
161 in response to the detected distance, which is the velocity at which 
the tape 14 should presently be transported in order for the cue location 
to reach the scanner at a velocity of zero. This function must generate a 
selected velocity and signal 161 of zero when the detected distance signal 
114 is zero. Functionally, for any detected distance 114, velocity 
selector 160 selects a selected velocity signal 161. Velocity selector 160 
can be implemented as a look-up table of previously calculated values, or 
can be implemented as a mathematical function that is calculated for each 
value of the detected distance. Details of the velocity selector 160 will 
be discussed below. 
The selected velocity signal 161 from velocity selector 160 is fed to adder 
162. Also fed to adder 162 is the desired velocity signal 106. These two 
signals are added by adder 162 to generate a non-limited intermediate 
velocity signal 163. Adder 162 can be implemented as a digital adder. 
Referring back to FIG. 2, the non-limited intermediate velocity signal 163 
is fed to a maximum velocity limiter 164. The function of maximum velocity 
limiter 164 is to limit the intermediate velocity to the maximum velocity 
the transport 4 is capable of. This may not be necessary, and would not be 
needed where the transport's capabilities are greater than the expected 
intermediate velocities. 
The output of maximum velocity limiter 164 is fed to overshoot limiter 165. 
The function of overshoot limiter 165 is to limit the non-limited 
intermediate velocity signal 161 so as to prevent an overshoot of the 
desired position because of inability of the transport 4 to 
instantaneously change velocity. If this ability is also not be required, 
this element could be eliminated. Overshoot limiter 165 receives the 
output of the maximum velocity limiter 164 and the current velocity signal 
71 from the counter 69. The function of overshoot limiter 165 will be 
described below. Such limiters are well known to those skilled in the art. 
The output of overshoot limiter 165 is the intermediate velocity signal 
72. 
Referring now to FIG. 4, a graph of the function implemented by velocity 
selector 160 (FIG. 3) is shown. There are a limitless number of functions 
that meet the sole requirement that a detected distance of zero must yield 
a selected velocity of zero. For example, a linear function might be used. 
Such functions are common called velocity profiles by those skilled in the 
art. The design of a velocity profile is tailored to mechanical 
limitations of the tape, the potential amount of tape packed on the tape 
reels, and the transport. There are also ballistic considerations relating 
to the same elements. Creating a velocity profile based on these 
considerations is well known to those skilled in the art. 
The preferred velocity profile for the present invention is composed of two 
velocity profiles. The first velocity profile is the square root function 
of the form velocity equals a constant multiplied by the square root of 
the detected distance. The constant can be determined from mechanical and 
ballistic parameters. The differential of this function forms an 
indication of acceleration, which should reflect the maximum acceleration 
the transport is capable of for any particular velocity. 
A graph of this function is shown in FIG. 4. The vertical axis is velocity 
and the horizontal axis is distance. Line A represents the square root 
function. The origin point of the axis is zero velocity and zero distance. 
This function would be implemented by velocity selector 160 such that for 
a detected distance, a selected velocity could be read off the graph. For 
a negative detected distance, indicating the cue location is ahead of the 
current position, a positive selected velocity would be chosen, causing 
the tape to be transported forward. For a positive detected distance, 
indicating the cue location is behind the current position, a negative 
selected velocity would be chosen, causing the tape to be transported in 
reverse. 
The use of the square root curve for a velocity profile is well known to 
those skilled in the art, and is commonly used for this purpose. There is 
a problem, however, in using the square root curve. As the curve 
approaches its zero crossing, its slope, that is the acceleration, 
approaches infinity. This requires that the transport 4 be capable of 
infinite acceleration or deceleration, which is unfortunately not 
possible. The maximum acceleration for the transport can be determined. 
Line B on the graph has a slope which is the maximum acceleration of 
transport 4. The selected velocity should not be greater than the values 
allowed by this line. Thus, where curve A would produce a greater selected 
velocity than line B would, the selected velocity should be produced by 
line B. This creates a composite function, shown as composite curve C 
which is shown in FIG. 5. 
Composite curve C should be selected to be less than the maximum curve the 
transport can handle. This difference provides headroom for the transport 
so that when a velocity is overshot, the servo system remains in control. 
If this was not done, the servo system would saturate when a velocity is 
over the curve C, and control would be lost until the velocity returns 
under the curve. 
Whereas FIG. 5 shows the range of outputs from velocity selector 160, FIG. 
6 shows the range of outputs from adjustable velocity selector 116 for a 
particular desired velocity. As can be seen, the effect of adder 162 is to 
shift the composite curve C upward on the velocity axis by the amount of 
the desired velocity. It should be noted that the center point of 
composition curve C, when the detected distance is zero, is now at the 
desired velocity, rather than zero velocity. Restated, for a given 
detected distance of zero, the intermediate velocity generated from an 
implementation of composite curve C is the desired velocity. This is 
exactly the requirement stated for adjustable velocity selector 116, 
discussed above. 
Also shown in FIG. 6 is the effect of maximum velocity limiter 164, which 
is to limit the selected velocity to the maximum velocity the transport 4 
is capable of. Line D1 indicates the maximum forward velocity, and Line D2 
indicates the maximum reverse velocity. 
FIG. 7 is a graph of a simple cueing operation. Only the left hand side of 
the previous graphs is shown. Composite curve C is shown intersecting with 
the distance and velocity origin at point F. This point represents the 
desired position and the desired velocity. The current position of the cue 
location and current velocity of the tape are shown on the graph as point 
I, which indicates a current velocity of zero. For the detected distance, 
which is the distance on the graph between point I and point F on the 
distance axis, an intermediate velocity is selected, whose value is 
indicated by point J. Because of the forces of inertia and momentum, the 
transport cannot instantaneously implement the intermediate velocity and 
the current velocity distance curve, shown as curve E, does not track the 
composite curve immediately. 
As the detected distance is reduced, inertia is overcome by the transport 
and the current velocity increases to meet the intermediate velocity 
curve. When the current velocity equals the intermediate velocity, at 
point K, momentum carries the current velocity above the intermediate 
velocity. As the detected distance is further reduced, momentum is 
overcome by the transport and the current velocity decreases to meet the 
intermediate velocity curve. Depending on the abilities of the transport 4 
(FIGS. 1 and 2), the current velocity curve E may only approximate the 
composite curve C as the detected distance reaches zero and the current 
velocity reaches the desired velocity at point F, however curve C is 
selected so that curve E can follow curve C with only some small 
difference. 
As was discussed above, the function of overshoot limiter 165 is to limit 
the selected velocity so as to prevent overshoot. An example of this 
problem is shown in the graph of FIG. 8. Composite curve C is shown 
intersecting with the distance and velocity origin at point F. This point 
represents the desired position of the cue location and the desired 
velocity. The current position and velocity is shown on the graph as point 
I, which indicates a current velocity of zero. For the detected distance, 
which is distance on the graph between point I and point F on the distance 
axis, an intermediate velocity is selected, whose value is indicated by 
point J. Once again, because of the forces of inertia and momentum, the 
transport cannot instantaneously implement the intermediate velocity, the 
current velocity/distance curve, shown as curve E, does not track the 
composite curve C. 
As the detected distance is reduced, inertia is overcome by the transport 4 
and the current velocity increases to meet the intermediate velocity 
curve. When the current velocity equals the intermediate velocity at point 
K, the momentum of the transport causes it to overshoot the composite 
velocity curve C. Because the detected distance is shorter than in the 
previous example, there is insufficient distance for the transport to 
overcome the momentum before reaching a detected distance of zero. Thus, 
the transport overshoots the desired velocity at the detected distance of 
zero and must reverse direction and come back as shown through points H, 
G, M and N to reach point F. This is not necessarily a problem, as the cue 
location is eventually reached. The only real problem is that this method 
is inefficient and wastes time. 
This problem can be overcome as shown in the graph of FIG. 9. Overshoot 
limiter 165 performs a calculation to determine, for any given current 
velocity, the distance necessary to overcome momentum and return to that 
current velocity if the intermediate velocity is held to that particular 
current velocity. This calculation is based on the assumption that the 
transport has a fixed known acceleration, which is generally the case when 
the servo system is locked to the capstan. This would not be the case 
where the tape transport is limited by the reel servo system. This 
distance, the response limit distance, is indicated with line P for the 
particular current velocity shown. The response limit distance is 
subtracted from the detected distance. This is called the response limit 
difference. 
A velocity is selected for the response limit difference, in the same 
manner as a velocity is selected for the detected distance. If the 
non-limited intermediate velocity is equal to or greater than this 
response limit velocity, then momentum will carry the velocity of the 
transport beyond the curve C and an overshoot will occur. Thus, when the 
non-limited intermediate velocity is equal to or greater than the response 
limit velocity, overshoot limiter 165 limits the non-limited intermediate 
velocity to the current velocity to prevent overshoot. The output of 
overshoot limiter 165 is the intermediate velocity signal 72. 
Referring now to FIG. 10, the cue system 32 is shown with the addition of a 
desired position counter 130. This cue system 32 functions identically 
with the cue system 32 discussed in FIG. 2 with the exception that the 
desired position now is advanced at the rate of the desired velocity by a 
new element, the desired position counter 130. This provides a synchronous 
cueing ability that will be discussed below. 
Instead of the desired position signal 102 being fed directly to the 
framing selector 108, it is first fed to desired position counter 130. 
There are three other signals which are fed to desired position counter 
130. These are a desired time signal 134, which specifies the point in 
time at which the synchronization is to take place, a synchronize select 
signal 135, which enables the synchronization, and the desired velocity 
106, which is used by the desired position counter 130 to provide the rate 
the desired position is to be advanced. These signals are provided by the 
cue controller 100 from the cue control signal 82 it receives. The output 
of desired position counter 130 is fed to the framing selector 108 as 
current desired position signal 132. From this point, the current desired 
position signal is treated just as the desired position signal in the cue 
system 32 of FIG. 2. 
Referring to FIG. 11, an implementation of the desired position counter 130 
is shown. The desired velocity is fed to a frequency generator 170, which 
produces a period clock signal 172 which has a frequency at the rate of 
the desired velocity. The frequency generator 170 is enabled by the 
synchronize select signal 135. If synchronization is selected, the 
frequency generator 170 is enabled. Frequency generators are well known to 
those skilled in the art. 
Clock signal 172 is fed to a counter 174 which uses this signal as its 
clock signal. Counter 174 is preset with the desired position signal 102 
when the desired time signal 134 is active. The output of the counter 174 
is the current desired position signal 132. 
In operation, the desired velocity signal is provided to frequency 
generator 170. The desired position signal 102 is provided to the preset 
of counter 174. At the point in time from which the desired position is to 
be advanced, the desired time signal 134 is activated and counter 174 
presets with the desired position signal 102. At the same time, the 
synchronize select 135 is activated and frequency generator 170 is 
enabled. The counter now increments the original desired position 102 at 
the rate of the desired velocity. This allows the synchronous cueing 
operation. 
In the standard cueing operation, the tape is cued to a fixed desired 
position. In synchronous cueing, the tape is positioned to a moving 
target. The advancing desired position simulates a moving reference such 
as another video tape recorder. It is this simulated moving reference that 
tape movement is synchronized with. 
For example, it is desired that a cue location, frame 3000, on the tape be 
at the scanner, moving at play speed, at the present time. Instead, frame 
1000 is at the scanner and the tape is moving in high speed shuttle. 
Obviously it is not possible to instantaneously obtain the desired goal, 
and it will take a certain amount of time to arrive at frame 3000. Thus 
when we arrive at frame 3000 it will no longer be at the original present 
time, but rather the original present time plus the time it took to 
position the tape. If the tape had been at the desired frame 3000 at play 
speed at the original present time, the tape would have moved a certain 
distance during the positioning time, and thus the tape should be at frame 
3000 plus the number of frames that would have been moved during this 
period, which might be frame 3400. 
The solution to this problem is to advance the desired position at the rate 
of the desired velocity, so that when the desired position arrives at the 
scanner at the desired velocity, the time it took to get there is 
compensated for. This is the synchronous cueing function. 
There are a number of uses for this function, including synchronizing 
several video tape recorders, however there is a particularly useful 
application for this ability. This application is in frame locking that 
has been discussed above. Using the cue system 32, of FIG. 10, with the 
synchronous cueing ability, the desired position moves allowing accurate 
frame locking. 
Referring now to FIG. 12, the graph shown is a representation of the effect 
of the framing selector 108 used in combination with synchronous cueing. 
Line R is representative of the distance specified by the frame select 
signal 104. This distance is selected according to which type of frame 
lock is desired. For example, for a simple frame lock, the distance chosen 
is two fields, or one frame. For an NTSC color lock, a distance of four 
fields is chosen, and for a color lock, a distance of eight fields is 
chosen. While only one curve, curve C1, is used to select the intermediate 
velocity, the net effect of framing selector 108 is to create a number of 
velocity distance curves, shown as C1 through C4 over the distance the 
tape travels. 
FIG. 13 illustrates a path that the velocity of the tape might take as the 
transport obtains a frame lock from high speed shuttle. Curve S indicates 
the change in velocity and distance of the tape. Coming from high speed 
shuttle on the upper left portion of the Curve S, intermediate velocities 
are selected first from curve C1. Curve C1 has little direct effect on 
curve S because the current velocity is much greater than intermediate 
velocity. When curve C1 is overshot, intermediate velocities are selected 
from curve C2. Because curve S is closer to curve C2, an effect on its 
path can be seen as curve S attempts to follow curve C2. When curve S 
overshoots curve C2, intermediate velocities are selected from curve C3. 
Curve C3 is followed by curve S and the tape is frame locked when it 
reaches the desired velocity. 
While the present invention has been described in terms of discrete 
components, in which it is easily implemented, the preferred 
implementation is in computer software executed by a microprocessor 150, 
as illustrated in FIG. 14. The discrete elements of the cue controller are 
replaced by a microprocessor, although any computing apparatus could be 
used. The preferred microprocessor is a Motorola 68000 microprocessor, and 
the software is preferably written in the C programming language. 
Microprocessor 150 receives the cue control information 82, the current 
position signal 65, and the velocity signal 71. It outputs the 
intermediate velocity signal 72. The program executed by microprocessor 
150 is illustrated by the logic diagrams in FIGS. 15, 16 and 17. 
Referring now to the logic diagram in FIG. 15, the program begins at start 
block 300. The first step is represented by block 302, in which the cue 
information is received and decoded into a desired position variable, a 
desired velocity variable, a frame select variable, a desired time flag, 
and a synchronize select flag. 
After the decoding block 302, the status of the desired time flag is 
checked in block 303. If it is active, then block 304 is branched to. If 
it is not active, block 306 is branched to. The desired time flag 
indicates the point in time at which synchronization is to start. In block 
304, which is branched to if the desired time flag is active, the desired 
position variable is loaded into a desired position counter procedure. 
From block 304, block 306 is executed. 
In block 306 the status of the synchronize select is checked. This flag 
indicates whether synchronization is desired. If it is desired, then the 
flag will be active. If it is active, then block 308 is executed. In block 
308, the desired position counter procedure is enabled to increment the 
desired position variable at a rate specified by the desired velocity 
variable. The effect of this procedure is to advance the desired position 
at the rate of the desired velocity. This counter procedure continues to 
increment the desired position variable until it is disabled by 
inactivating the synchronize select flag. The next block executed is block 
312. 
Back in block 306, if the synchronize select flag is inactive, then block 
310 is executed. In block 310, the desired position counter procedure is 
disabled. The next block executed is block 312. 
The desired position variable is read and stored in a current desired 
position variable in block 314. This is done because desired position 
variable might be changing during the execution of this program, which 
would give inaccurate results. The effect copying the desired position 
into the desired position variable is to freeze its value. 
Next, in block 314, the current desired position variable is divided by the 
frame select variable. The remainder is loaded into a framed desired 
position variable. This operation is used to implement the frame select 
procedure discussed above. The length of the frame selection desired is 
chosen as the frame select value. If not frame select is desired, the 
frame select value is chosen to be a number larger than the current 
desired position, such as the length of the tape. When the frame select 
number is larger than the current desired position, the current desired 
position is the remainder. 
The next block executed is block 316, in which the current position is 
received and stored into a current position variable. In the next block, 
block 318 of FIG. 16, the difference between the current position 
variable, and the framed desired position is calculated. This difference 
is stored in a detected distance variable. This is the distance between 
the current position and desired position, and is the distance the tape 
must be moved to position it. 
The next block is block 320. In this block, a velocity value is selected 
for the detected distance. Preferably, this selection is done according to 
the velocity/distance profile discussed with regard to FIGS. 4 through 6. 
Whatever function is implemented, it can be executed in a number of ways. 
One method is a pre-calculated table of values, that is used as a look-up 
table. In the preferred embodiment, the calculation is made according to 
the composite function discussed. If the detected distance is in the 
linear range of the function, the linear function is used. If the detected 
distance is outside the linear range, the square root function is used. 
The selected velocity is stored in a selected velocity variable. 
Next, in block 322, the selected velocity variable is added to the desired 
velocity variable and the result is stored in a non-limited distance 
variable. The purpose of this operation is to add the offset of the 
desired velocity to the selected velocity, so that when the detected 
distance reaches zero, the current velocity is the desired velocity. 
The next block executed is on FIG. 16, and it is labeled 324. The absolute 
value of the non-limited velocity variable is compared to maximum velocity 
constant. If it is greater, then block 326 is executed, which limits the 
value of the non-limited velocity variable to the value of the maximum 
velocity while preserving the variable's original sign, which is used to 
determine direction of travel. The tape transport has a maximum velocity. 
This tends to be the same value in both directions. The next block is 238. 
If the non-limited velocity was not greater than the maximum value 
constant, then block 238 is also the next block. 
In block 328, a distance is calculated in which the tape transport, 
accelerating the tape at maximum acceleration, can return to the current 
velocity, if it was given the current velocity as the intermediate 
velocity. This distance is stored in a response limit distance variable. 
Because the acceleration of the transport is a fixed number, the only 
variable in the calculation is the current velocity. The maximum 
acceleration constant is divided by the current velocity. This calculation 
is performed as part of the steps necessary to prevent overshoot. 
Next, in block 330, the response limit distance variable is subtracted from 
the detected distance variable. In the next block, block 332, a velocity 
is selected for this difference, just as it was selected for the detected 
distance. This velocity is stored in a response limit velocity variable. 
If the response limit velocity is greater than the non-limited velocity, 
then an overshoot will occur if the non-limited velocity is output as the 
intermediate velocity. This comparison is performed in block 334 of FIG. 
17, and if the non-limited velocity is greater, then block 336 is branched 
to, and if it is less or equal, then block 338 is branched to. 
In block 336, the current velocity is outputted as the intermediate 
velocity value and in block 338, the non-limited velocity is outputted as 
the intermediate velocity. After either of these blocks, block 340 is 
executed. 
Block 340 checks whether the positioning is completed by checking to see if 
the detected distance is zero, and whether the current velocity is equal 
to the desired velocity. If both are true, block 342 is branched to, which 
is the end box and control of the transport is passed to other software. 
If these conditions are not met, then block 302 is branched to, and the 
procedure is repeated until the conditions are met. 
In summary, the present invention provides a technique for cueing a 
particular cue location, such as the beginning of a certain frame, to the 
scanner on the tape transport, at a selectable desire velocity. 
Additionally, the cue location on the video tape that is being positioned 
can be advanced at the rate of the desired velocity to allow a synchronous 
cueing ability at an arbitrary velocity. 
While the present invention has been described in terms of a video tape 
recorder, it will be appreciated by those of ordinary skill in the art 
that the present invention can be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. The 
presently disclosed embodiments are therefore considered in all respects 
to be illustrative and not restrictive. The scope of the appended claims 
rather than the foregoing description, and all changes that come within 
the meaning and range of equivalents thereof are intended to be embraced 
therein.