KEYWORDS: Image processing, Data storage, Data centers, Holography, Modulation, Data conversion, Holographic data storage systems, Signal processing, Spatial light modulators, Holograms
Optical holographic data storages (HDS) with high theoretical-capacity have been researched for more than two decades. Among them, coaxial HDS receives the most attention. Amplitude-based coding in coaxial HDS systems is fundamental and generally employs a 3:16 modulation scheme that sets merely 3 non-adjacent bright pixels within a 4*4 pixel-block, called as Three-Level Format (TLF). Unfortunately, TLF data frame recorded upon the disc is difficult to be retrieved accurately in the practice. In our previous work, we experimentally analyzed inaccurate TLF data frames recognition and its corresponding factors. The core issue is how to accurately and fast locate TLF data frame in encoded image. Thus, we have to use some image processing techniques such as Gaussian blur to assist in locating TLF data frame. However, using image processing techniques will inevitably introduce long processing-time. Therefore, we propose an efficient two-stage decoding scheme. In the first stage, we locate TLF data frame in encoded images and calculate scaling multiplier (about 1.9s). In the second stage, we calculate the coordinates of all data points and read them (about 0.08s). For the first image of a batch, we execute complete two-stage process, but the subsequence images are no longer to perform the first stage and only fine-tuning TLF data frame location. The experimental results show that the average latency of reading an image is reduced by 1.9s. The average data point error rate is 3.1%, and the average data block error rate is 7.8%, consistent with the results of executing complete process.
Optical holographic data storages (HDS) with the high theoretical-capacity density proportional to 1/λ3 have been researched for more than two decades. Among them, the coaxial HDS is current research hotspot. Amplitude-based coding in coaxial holographic storage systems is fundamental and generally employs a 3:16 encoding scheme that sets merely 3 non-adjacent bright pixels within a 4*4 pixels data block with the coding rate of 0.5. This data format is called three-level format (TLF). Unfortunately, the data frame recorded upon the disc is difficult to be recovered accurately in the practice due to the system inherent complexity such as light intensity and distribution, mechanical movement, dynamical focalizing, material distribution, servo subsystem, and component quality. This paper experimentally analyzes inaccurate TLF data frames recognition and its corresponding factors. We design an experimental approach to evaluate the quality of recorded data frames, and we also conclude several observations. First, the luminance among bright pixels within synchronization block consisting of 4×4 bright pixels are unevenly and distorted actually, making the synchronization block locating inaccurately. Second, the distance between two adjacent bright pixels heavily affects the recorded image quality. Third, the high aggregate brightness of a data frame severely reduces the frame’s signal-to-noise ratio. Forth, the recorded image suffers from the picture distortion, but the degree of distortion is slight within an acceptable range. Last, holographic material distribution upon disc also affects the brightness and pixel consistency. Based on these observations, we discuss several solutions to improve the reading accuracy for hologram data frames.
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