Dynamic magnetic information storage or retrieval – Recording on or reproducing from an element of diverse utility
Patent
1997-09-10
2000-06-06
Faber, Alan
Dynamic magnetic information storage or retrieval
Recording on or reproducing from an element of diverse utility
360 3, 360 46, 360 67, G11B 502
Patent
active
06072644&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a process for reading magnetically recorded signals from a photographic film provided with a transparent thin magnetic layer, and a device for carrying out this process.
A process of the kind mentioned for reading magnetically recorded signals is derived, for example, from the U.S. Pat. No. 4,964,139. The U.S. Pat. No. 4,977,419 discloses photographic films which have a very thin, transparent, surface-covering magnetic layer as an additional layer. On this magnetic layer the various signals can be recorded, including those recorded in the camera from the exposure process, which are helpful for the processing of the developed film. Normal devices for the detection of magnetic signals can be used for reading these magnetic signals.
It has been demonstrated, however, that the requirement for high transparency of the unexposed film after developing allows only for a magnetic layer with very little magnetic material. For this reason, the signals recorded with magnetic writing heads can deposit only extraordinarily weak magnetic fields and accordingly can induce only very small currents in the coils of movable magnetic heads opposite the magnetic layer. Devices normally used for reading magnetic impulses are designed for non-transparent magnetic layers and thus for those with much magnetic material which also deliver correspondingly strong signals. Signals readable from transparent magnetic layers may be weaker by a factor of at least 200 times than those from normal layers for recording sound or computer data.
SUMMARY OF THE INVENTION
The objective of the invention is thus to create a procedure suitable for reading extremely weak magnetic signals.
This objective is achieved by a process wherein said magnetic read head is operated through said pre-amplifying means in short-circuit, and wherein the pre-amplified signals are digitalized, filtered, differentiated and evaluated. Operating a magnetic read head by means of a pre-amplifier in short-circuit operation means that the internal resistance R.sub.i of the magnetic head is very great as opposed to the input resistance R.sub.e of the pre-amplifier connected immediately downstream of the magnetic head coil. When an operational amplifier connected to a resistance is employed according to an especially favorable embodiment of the invention for pre-amplification, the amplifier should have as high a no-load amplification A.sub.0 of more than 10.sup.6 as possible with very low intrinsic noise. An amplifier of this type can carry very high ohms, for example feedback coupled with a resistance in the order of 10 M.OMEGA. or more, so that with a very good low-ohm sound-scanning head coil on the order of 150 ohms an input resistance on the order of 4-5 ohms is attained. Further details of the invention and of a device for implementation of the claimed process can be found in the dependent claims. An exemplary embodiment of a circuit according to the invention is illustrated in the attached Figure.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In this FIG., 1 indicates the photographic film strip with the very thin magnetic layer which is led past a magnetic read head 2 with a coil 3 in a device for film processing, for example, a photographic printer. One of the problems is that both in the camera and in the process of printing, a constant relative speed between film 1 and magnetic head 2 cannot be ensured since the transport speed of the film is dependent on the operating conditions of the camera or of a printing device.
The internal resistance of the magnetic head coil 3 is represented symbolically by resistance 4 in the equivalent circuit diagram. Coil 3 is connected on one side to a ground and on the other to the negative input 5a of an operational amplifier 5. This operational amplifier has a very high no-load amplification A.sub.0 of more than 10.sup.6 --in particular of 5.times.10.sup.6 and low intrinsic noise. The negative input 5a of this operational amplifier is connected by a feedback resistance 6 with output
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Agfa-Gevaert Aktiengesellschaft
Faber Alan
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