JPS5916357B2 - Hologram memory readout device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reading device for a hologram memory in which digital information is recorded, and more particularly to a reading device that is equipped with a threshold calculation circuit and a determination circuit for reproduction bits and minimizes the error rate.

As with magnetic memories, it is best to evaluate hologram memories that record and reproduce digital information based on the bit error rate.

Conventionally, a method has been used in which the threshold level for determining reproduction bits of a hologram memory is set and fixed at a value between the minimum value of all "1" signal levels and the maximum value of all "o" levels.However, , this method does not improve the error rate even when the 1'' signal level is high or the o'' signal level is low.In order to minimize the bit error rate, the threshold level is changed for each reproduced bit. Ideally, this method would be impossible to use in a memory that records unknown information.In reality, therefore, in a hologram memory, each hologram must reproduce multiple bits. Therefore, if the threshold level can be made variable for each hologram, information can be read out with a low error rate.In other words, in hologram memory, the performance of each hologram differs depending on the conditions at the time of imaging and readout, but the The reproduced bits of each hologram are almost uniform. Therefore, by selecting a threshold level that minimizes the error rate for each hologram, it can be considered that the error rate of the entire hologram memory can be reduced to a minimum of 5. In particular, the error rate of the entire hologram memory can be reduced to at least 5. If a positional shift occurs between the hologram and the hologram, or if a defect is inserted into the hologram, the effect will spread to all reproduced bits, so the present invention is highly effective.The purpose of the present invention is to reduce the error rate of the reproduced bits of each hologram. An object of the present invention is to provide a hologram memory reading device in which a minimum threshold level is determined from reproduction bits of each hologram.

According to the present invention, in a hologram memory consisting of a plurality of micro holograms recording a plurality of bits, each micro hologram has a "J" signal "0" in addition to the information bit.
a circuit that records at least one bit of the '1' signal and detects the optical intensity of the '1' signal and the 'o' signal at the time of readout, and calculates an optimal threshold value that minimizes the error rate;
A reading device equipped with a circuit that determines whether the reproduced i0 bit signal is ``1'' or 0'' at the threshold level is obtained.

Generally, the signal level of one bit reproduced from a hologram is 51 for a ``1'' signal and 50 for a ``0'' signal.
Also, the noise of the hologram memory follows a Gaussian distribution, and if it is set as Ni or No depending on the respective signal state,
The probability P(V/N) that the Gaussian noise of the average power N2 exceeds the constant voltage V is given by:

Therefore, if the threshold level is set to , and the probability of appearance of the 61'' signal and the 60'' signal is assumed to be 0.5 each, the error rate P becomes e.

Therefore, the threshold that minimizes the error rate is . The threshold level T at this time is given by 0pt.

The Gaussian noise Nl, NO of a normal hologram memory consists of granular noise of the photosensitive material, shot noise of the photodetector, and thermal noise of the load resistor, but the thermal noise of the load resistor is the main one.

This means that even if the signal levels Sl and SO change, the noise N
1 and NO can be considered to be constant, so if the signal levels Sl and SO are known from equation (5), the threshold level T can be determined.

0pt Hereinafter, the present invention will be explained with reference to the drawings.

The figure shows an embodiment of the reading device of the present invention, in which numeral 1 denotes a hologram memory board which is made up of a plurality of minute holograms 2. Regarding the manufacturing method of this hologram memory board, for example, the Journal of the Institute of Electronics and Communication Engineers Vol.
49-256 (1972), ``Structure of 107-bit Hologram Memory''. Reference numeral 3 denotes a photodetector array for detecting reproduced bit point images, and at least one photodetector corresponds to each reproduced bit.

The photodetector 3a is for detecting a bit point image for generating a 1'' signal, and the photodetector 3b is for detecting a bit point image for reproducing a 10'' signal. Other photodetectors are for detecting bit point images of information reproduction bits. 4 is a circuit for calculating the threshold level T from the reproduced bit point image intensities Sl and SO from the photodetectors 3a and 3b.

Reference numeral 5 denotes a circuit that amplifies the information reproduction 0pt bit signal and determines whether the 611 signal is a 0° signal based on the threshold level from the threshold calculation circuit 4.

Now, when the readout light 6 is irradiated onto an arbitrary minute hologram on the hologram memory board 1, the reproduced bit point image is formed on the photodetector array 3.

If the outputs of the photodetectors 3a and 3b are input to the threshold calculation circuit 4, the threshold level T is calculated, and the output is 0 to the determination circuit 5-.
pt, the information bits are determined with the minimum error rate.

Since this operation is performed each time the readout light 6 illuminates a different microscopic hologram, the hologram memory is read out with the minimum error rate. As explained above, according to the present invention, information can be read out from a hologram memory with a minimum error rate by using only two reproduction bits of a minute hologram.

[Brief explanation of the drawing]

The figure shows an embodiment of the reading device of the present invention, in which 1 is a hologram memory board, 2 is a minute hologram, 3 is a photodetector array, 4 is a threshold calculation circuit, and 5 is a determination circuit.

Claims (1)

[Claims] 1. It includes a hologram memory board consisting of a plurality of micro holograms that record a plurality of bits, a photodetector array that detects a reproduced bit point image, a threshold calculation circuit, and a judgment circuit, and each micro hologram is provided with an external information signal. At least one bit of each “1” signal and “0” signal is recorded to determine the threshold value, and the detection signal levels of the “1” signal and “0” signal are used to determine the threshold level for each minute hologram. 1. A hologram memory readout device, characterized in that the hologram memory readout device calculates and determines other information reproduction bits.