Symbol synchronization in convolutionally coded systems

Alternate symbol inversion is sometimes applied to the output of convolutional encoders to guarantee sufficient richness of symbol transition for the receiver symbol synchronizer. A bound is given for the length of the transition-free symbol stream in such systems, and those convolutional codes are characterized in which arbitrarily long transition free runs occur

On the inherent intractability of certain coding problems

The fact that the general decoding problem for linear codes and the general problem of finding the weights of a linear code are both NP-complete is shown. This strongly suggests, but does not rigorously imply, that no algorithm for either of these problems which runs in polynomial time exists

Two-dimensional burst identification codes and their use in burst correction

A new class of codes, called burst identification codes, is defined and studied. These codes can be used to determine the patterns of burst errors. Two-dimensional burst correcting codes can be easily constructed from burst identification codes. The resulting class of codes is simple to implement and has lower redundancy than other comparable codes. The results are pertinent to the study of radiation effects on VLSI RAM chips, which can cause two-dimensional bursts of errors

On the existence of optimum cyclic burst-correcting codes

It is shown that for each integer b >= 1 infinitely many optimum cyclic b-burst-correcting codes exist, i.e., codes whose length n, redundancy r, and burst-correcting capability b, satisfy n = 2^{r-b+1} - 1. Some optimum codes for b = 3, 4, and 5 are also studied in detail

Femtosecond Spectroscopy with Vacuum Ultraviolet Pulse Pairs

We combine different wavelengths from an intense high-order harmonics source with variable delay at the focus of a split-mirror interferometer to conduct pump-probe experiments on gas-phase molecules. We report measurements of the time resolution (<44 fs) and spatial profiles (4 {\mu}m x 12 {\mu}m) at the focus of the apparatus. We demonstrate the utility of this two-color, high-order-harmonic technique by time resolving molecular hydrogen elimination from C2H4 excited into its absorption band at 161 nm

Age Related Visual Pathologies among Nursing Home Residents: An Evaluation of Light Conditions and Recording in Client Files

Objective: Reflection on visual problems in nursing homes. Data Sources: Eye examinations, documented visual problems and illuminance levels. Study design: The optometric examinations and recorded visual problems were combined with illuminance data. Data collection: In seven nursing homes, 259 residents underwent an optometric examination. Their client records were analyzed for information regarding visual functioning. The illuminance data were ranked to set the quality of the lighting conditions. Principal findings: 50% of the referred residents had problems with cataracts, retinal problems (21%), suspected glaucoma (13%), and other pathologies (16%). The information was not current in 56% of the records. The quality of lighting conditions was low or moderate. Conclusion: The finding of poor lighting conditions in nursing homes in combination with a high prevalence of visual problems (with cataract found to be the most common age related pathology), stretches the need of enhanced awareness of eye care by professional caregivers

The effect of sodium fluorescein on anterior eye surface measurements

Purpose During image acquisition, certain topographers require the addition of sodium fluorescein (NaFl) dye to the tear film. This study investigates the effect of NaFl dye on corneal topography and tear surface quality. Method The E300 corneal topographer (Medmont International Pty Ltd., Victoria, Australia) was used to measure ocular surface topography and quality of 57 eyes of 57 healthy individuals without dry eye symptoms, age 35.1 ± 15.2 years (mean ± standard deviation) ranging between 19 and 65 years. The mean of three simulated keratometry values, a variety of corneal shape descriptors, and Tear Film Surface Quality (TFSQ) were measured under three different conditions; without NaFl (baseline), with the addition of a single dose NaFl, and using a double dose of NaFl. Results Compared to baseline, the Inferior-Superior (IS) index decreased significantly after a single dose (P = 0.034) or double dose of NaFl (P = 0.030). The corneal surface was significantly more regular without NaFl (P = 0.003) or one insertion of NaFl (P = 0.024) when compared to two doses of NaFl. There was no association with age, or dry eye signs or symptoms on the variance observed in any of the indices between baseline, intervention I, and intervention II (P > 0.05). Agreement between corneal surface indices reduced following the addition of NaFl. Conclusion In comparison to measurements taken without an ocular dye, one dose of NaFl resulted in increased reliability and consistency in corneal topography measurements using the E300 topographer, but 2 doses decreased reliability and consistency. Practitioners ought to be aware that tear film surface regularity and inferior-superior corneal power changed significantly following the addition of NaFl in those with healthy corneas. Its effect in diseased corneas is unknown

Comparison between subjective fluorescein breakup time and automated tear breakup time measurements using the E300 corneal topographer

Purpose Comparison of invasive fluorescein tear film breakup time (FBUT) and the automated measurement of tear film stability known as Tear Film Surface Quality (TFSQ) breakup time using placido disc videokeratography measured with and without fluorescein sodium (NaFl). Method In 57 eyes of 57 subjects (males n=23, females n=34), FBUT using a single dose of NaFl was measured three times and automated TSFQ breakup time was measured twice under two different conditions: non-invasively (without NaFl), and using a single dose NaFl, using the E300 corneal topographer (Medmont International Pty Ltd., Victoria, Australia). Mean age (± SD) was 35.1±15.2 years ranging from 19 to 65 years. There was no history of ocular diseases, contact lens wear, or previously diagnosed dry eye. Subjects were grouped by age (12 [n=21]). Results There were no significant differences between the three consecutive measures of FBUT (p=0.62), two measures of TFSQ breakup time without NaFl (p=0.67) or with NaFl (p=0.96). There were strong significant correlations between TFSQ without and with NaFl (r=0.709, p<0.0005) and moderate significant correlations between the FBUT and TFSQ breakup time without NaFl (r=0.583, p<0.0005) and with NaFl (r=0.432, p=0.001). Average FBUT was significantly shorter (8.1 ± 6.9 sec) compared to TFSQ breakup time without NaFl (12.6 ± 12.9 sec) and with NaFl (13.6 ± 12.6 sec; p=0.002, partially eta squared=0.21), irrespective of age group (p=0.36) or gender (p=0.60) or OSDI score (p=0.67). Conclusion Automated TFSQ break up time measured with the Medmont E300 topographer is repeatable when measured with and without the addition of NaFl dye; however, the TFSQ break up time is overall significantly increased compared to subjective FBUT. Both measures of tear breakup time are therefore not interchangeable