Film mid roll interrupt protection for a camera using magnetic azimuth recording on film

Data is recorded on the film by the manufacturer prior to exposure at a first magnetic head azimuth angle. The camera magnetically records information on the film frame-by-frame at a second head azimuth angle with each exposure or shortly thereafter. Before exposure of any frame, the camera determines whether the frame has already been exposed by sensing the recorded signal amplitude at one of the two magnetic head azimuth angles. In order to prevent a double exposure, the camera transports the film past any frame having data magnetically recorded therein corresponding to the second azimuth angle.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application contains subject matter related to subject matter 
described in U.S Pat. Application Ser. No. 417,347 filed on Oct. 5, 1989 
by Jeffrey R. Stoneham and Patricia D. Fairman, entitled "Camera Apparatus 
for Magnetically Recording on Film" and U.S. Pat. Application Ser. No. 
255,693 filed Oct. 7, 1988 by Robert Cloutier et al., entitled "Film 
Information Exchange System Using Dedicated Magnetic Tracks on Film", both 
applications being assigned to the present assignee. 
BACKGROUND OF THE INVENTION 
The problem of film cameras creating double exposures is a familiar one. 
After reinserting partially exposed film back into a camera. There is a 
need for a simple, reliable and inexpensive way of preventing a camera 
from creating double exposures, particularly when the film is reloaded 
after prior exposures have caused the film to be wound to near mid-roll. 
Magnetically recording data on film by the film manufacturer, the camera, 
the customer order station and the photofinisher is described in the 
above-referenced patent application by Cloutier et al. Typically, using 
the ability to magnetically record data on the film to provide mid-roll 
interrupt protection (i.e., to prevent the camera from double exposing the 
film) has required the camera to be able to interpret the recorded data. 
The disadvantage is that interpreting the data is an elaborate task, as 
described in the above-referenced Cloutier application. As such, a 
mid-roll interrupt protection system in a camera which reads and 
interprets the magnetically recorded data must be disadvantageously 
elaborate. Moreover, it is not possible to provide such a system in those 
cameras not having the ability to read and interpret magnetically recorded 
data. 
Accordingly, it is the object of the present invention to provide a 
mid-roll interrupt protection system in a camera which is simple and 
inexpensive and does not require that the camera to read, decode or 
interpret data magnetically recorded on the film in order to prevent 
double exposures. 
SUMMARY OF THE INVENTION 
The invention is a mid-roll interrupt protection system in a camera having 
a magnetic head (or heads) which can sense magnetic signals recorded on 
film loaded in the camera. In accordance with one embodiment of the 
invention, whenever a particular frame on the film is exposed by the 
camera, a controller on board the camera causes the magnetic head to 
record a predetermined signal at a first magnetic head azimuth angle. 
Whenever a particular frame on the film is to be exposed by the camera, 
the controller first causes that frame to be scanned by the head at the 
first azimuth angle to determine whether data has been previously recorded 
in that frame at the first azimuth angle. If so, the controller prevents 
exposure of that frame --by winding the film to the next frame, for 
example. 
In another embodiment of the invention particularly suitable for use in 
inexpensive cameras, a signal is magnetically recorded at a first azimuth 
angle in all frames in the film prior exposure in the camera. Each time 
the camera exposes a particular frame, it magnetically erases the 
prerecorded signal. A stationary magnetic head oriented at the first 
azimuth angle with respect to the film in the camera scans each film frame 
prior to the exposure of that frame. If simple electronics connected to 
the head sense a head playback signal above a certain threshold magnitude, 
then that frame is deemed not to have been exposed. Otherwise, in the 
absence of a playback signal of sufficient magnitude, the particular frame 
is deemed to have been previously exposed and magnetically erased by the 
camera, and a warning is issued to the camera user. (Alternatively, 
exposure of that frame may be automatically prevented.) 
In the preferred embodiment of the invention, the camera itself records 
data in each film frame with the exposure thereof, such data uniquely 
pertaining to information regarding the particular frame in which it is 
recorded. Such camera-recorded data is recorded at a second head azimuth 
angle. The camera only employs film having data pre-recorded in each frame 
thereof at a first head azimuth angle. The camera can move its magnetic 
head, in sequence, to the first and second azimuth angles to (1) sense 
whether the pre-recorded signal (of the first head azimuth angle) has been 
erased in a particular frame by virtue of that frame having been 
previously exposed and (2) to record (at the second azimuth angle) 
scene-related information in that frame with the exposure thereof. In this 
preferred embodiment, if the camera first senses the presence of data 
magnetically recorded in the current film frame at the first azimuth 
angle, it automatically searches for the next frame in which no such data 
is present.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a camera 100 transports a roll of film 102 by a 
transport motor 104 rotating two reels 106, 108 around which the film 102 
is wound. The film 102 includes a magnetic layer in which signals or data 
may be magnetically recorded. The camera includes a magnetic head 110 
which is rotatable between two head azimuth angles A and B. Such head 
rotation and apparatus for accomplishing it is disclosed in the 
above-referenced patent application by Jeffrey R. Stoneham et al. A 
magnetic track 114 containing a signal or data of a predetermined nature 
and which was pre-recorded at the head azimuth angle B extends along the 
length of the film 102. The track 114 may be prerecorded either by the 
film manufacturer or other entity prior to the film being loaded into the 
camera, or may be recorded by the camera head 110 by winding the entire 
length of film between the two reels 104 and 106 when the film is first 
loaded into the camera. Winding the entire length of the film through the 
camera 100 is ordinarily performed in many cameras in a so-called 
"pre-wind" operation. 
When the camera is ready to operate, the head 110 is rotated to the azimuth 
angle A. As each film frame 102a, 102b, 102c, etc., is exposed by the 
camera 100, the head 110 erases the track 114 and records scene 
information (or a simple predetermined signal) in individual tracks 116a, 
116b, 116c, etc., in the respective frames 102a, 102b, 102c, etc. 
However, prior to the exposure of the film a controller 120 in the camera 
100 causes the head 110 to scan the film (e.g., by activating the 
transport motor 104) while the head is oriented at the first azimuth angle 
A, and monitors the amplitude of the playback signal from the head 110. If 
the amplitude is above a pre-determined threshold, this means that 
scene-related data is present on the film and therefore it has been 
previously exposed. Accordingly, the controller 120 causes the transport 
motor 104 to wind the film until the head no longer provides a playback 
signal exceeding the predetermined threshold magnitude. As soon as the 
absence of such a playback signal is detected by the controller 120, it 
stops the transport motor. The film frame now located adjacent the head 
110 has never been exposed by the camera. As a double check, the 
controller 120 can then rotate the head to the second azimuth angle B and 
make sure that the pre-recorded track 114 is still present in the film 
frame selected to be exposed (now located under the head 110). In either 
case, the controller 120 can now permit the camera 100 to operated as 
usual and sequentially expose the rest of the frames on the film 102. With 
each such exposure, the controller causes the head 110 to be oriented in 
the first azimuth angle A and to record scene-related information (or 
simply a predetermined signal) in the new tracks 116a, 116b, etc. in the 
subsequent frames. 
In an alternative embodiment of the invention, as shown in FIG. 2, the 
actuator 112 is eliminated in favor of two heads 110a and 110b oriented at 
the respective azimuth angles A and B. Instead of rotating a single head 
to the required azimuth angle, in this alternative embodiment the 
controller selects either one or the other of the two heads 110a, 110b for 
recording or playback as appropriate in accordance with the foregoing 
description. 
In yet another alternative embodiment of the invention, the head 110 is 
simply employed by the controller 120 as an erase head and as a playback 
head. As each frame 102a, 102b, etc. is exposed by the camera 100, the 
head 110 erases that portion of the pre-recorded track 114 lying within 
the current frame. Mid-roll interrupt protection is accomplished in this 
embodiment by the head 110 being oriented at the azimuth angle B of the 
pre-recorded track 114 and scanning the film 102 prior to camera 
operation. Such scanning may occur, for example, during film prewind in 
those cameras which normally prewind the film as a necessary prelude to 
taking pictures. (Alternatively, each individual film frame may be 
magnetically scanned prior to its being exposed.) During such scanning, 
the playback signal amplitude from the head 110 is monitored by the 
controller 120. If this amplitude falls below a predetermined threshold 
level, the controller issues an error signal, signifying that the current 
frame has been previously exposed. Such an error signal may command the 
transport motor to continue winding the film 102 until the magnetic head 
playback signal is no longer below the predetermined threshold, signifying 
that an unexposed section of the film has finally been located, and that 
normal camera operation may commence. 
As is well-known in the art, the azimuth angles A and B of the head 110 are 
simply the angle between the elongate magnetic gap in the head 110 and the 
perpendicular to the direction of the magnetic media (film) travel. As 
long as the azimuth angles A and B differ from each other by about 8.5 
degrees, there is a distinct and pronounced difference between the 
playback signal amplitude generated by the head reading data recorded at 
either one of the two angles. The maximum amplitude is obtained whenever 
the azimuth angle of the recording and playback heads are the same. Thus, 
the controller 120 can differentiate between signals recorded at different 
azimuth angles by simply employing a differential amplifier which compares 
the average playback signal amplitude from the head 110 with a pre-set 
voltage level. 
While the invention has been described in detail by specific reference to 
preferred embodiments thereof, it is understood variations and 
modifications thereof may be made without departing from the true spirit 
and scope of the invention.