VoIP is one of the key technologies for recent telecommunication services. The quality of its services should be discussed in subjective terms. Since subjective quality assessment is time-consuming and expensive, however, objective quality assessment which estimates subjective quality without carrying out subjective quality experiments is desirable. This paper discusses the performance of the objective quality measure that was standardized as ITU-T Recommendation P.862 and clarifies the quality factors that can be evaluated with satisfactory accuracy based on it. We found that P.862 can be applied to the evaluation of coding distortion, tandeming of codecs, transmission bit-errors, packet loss, and silence compression in a codec, at least for clean Japanese speech. In addition, we propose a method of estimating the subjective quality evaluation value from objective measurement results and show the validity of this method. We also evaluate the uniqueness of objective quality assessment based on P.862 from the viewpoints of the effect of measurement noise and the variation of test speech samples, and propose how to improve the reproducibility of objective quality assessment.

This paper assesses the media synchronization quality of recovery control schemes from asynchrony, which are referred to as reactive control schemes here, in terms of objective and subjective measures. We deal with four reactive control techniques: skipping, discarding, shortening and extension of output duration, and virtual-time contraction and expansion. We have carried out subjective and objective assessment of the media synchronization quality of nine schemes which consist of combinations of the four techniques. The paper makes a comparison of media synchronization quality among the schemes. It also clarifies the relations between the two kinds of quality measures.

In generating Frequency-Hopping (FH) sequences for Frequency Hopping Spread Spectrum Multiple Access (FH-SSMA) applications, binary maximal-length sequences (m-sequences) over GF(2m) have been preferred because of their characteristics of good Hamming correlation property, long period capability and high speed generation in association with simple hardware implementation based on Feedback Shift Registers (FSR). In practice, however, one difficulty of applying such sequences into the FH-SSMA communication systems with a wide bandwidth such as Military Satellite Communication (MilSatCom) is that the number of hopping frequency slots available may be far from a power of 2 in proportion to the spreading bandwidth. In that case, we can not make good use of the spreading bandwidth. In this paper, we propose a construction of some favorable FH sequences which deals effectively with the above difficulty using FSR and some nonlinear logic by introducing a re-mapping method. We show that the resulting sequences satisfy the (almost) uniform symbol distribution in one period and preserve a reasonably good Hamming correlation property so that they are appropriate for FH-SSMA applications.

Appropriate test signals defined by formula or generated by algorithm are used for measuring objective QoS (Quality of Services) for voice operated telecommunication devices such as telephone and speech codec (coder-decoder). However, that for measuring residual echo characteristics in hands-free telecommunications equipped with acoustic echo canceller is under study in ITU-T Recommendation G.167. This paper describes comparative assessment of test signals for measurement of residual echo characteristics. In hands-free telecommunications, acoustical echo canceller has been developed to remove a room echo signal through the loudspeaker to the microphone in the receiving end. Performance of the echo canceller system is evaluated by residual echo characteristics expressed in echo return loss enhancement (ERLE). The ERLE can be conventionally measured by putting white noise into the echo canceller system. However, white noise is not adequate as the test signal for measuring the performance of the echo canceller, since the performance may depend on the characteristics of input test signal, and the characteristics of the white noise differ from those of real voice. Therefore, this paper discusses appropriate characteristics of real voice required for objective quality evaluation of echo canceller system. The test signals used for this verification tests were real voice (RV), white noise (WN), frequency weighted noise (FWN), artificial voice (AV), and composite source signal (CSS) depending on the approximation of real voice characteristics. As the comparative assessment results, the ERLE characteristics measured by artificial voice conforming to ITU-T Recommendation P.50 having average characteristics of real voices in time and frequency domains are almost equivalent to those of real voice and best among those test signals. It is concluded that artificial voice P.50 is satisfied with measurement of residual echo characteristics.

The UCHT (Unified Complex Hadamard Transform) has been proposed as a new family of spreading sequences for DS-SSMA systems recently. In this Letter, the periodic autocorrelation (PAC) properties of the Unified Complex Hadamard Transform (UCHT) sequences are analyzed. Upper bounds for the out-of-phase PAC are derived for two groups of the UCHT sequences, namely the HSP-UCHT and the NHSP-UCHT sequences (the later is a more general representation of the well-known Walsh-Hadamard (WH) sequences). A comparison of the two bounds is performed. It turns out that the HSP-UCHT sequences have a lower upper bound for the out-of-phase PAC. This makes the HSP-UCHT sequences more effective than the WH sequences in combating multipath effect for DS-SSMA systems.

In this paper, we construct a software availability model considering the number of restoration actions. We correlate the failure and restoration characteristics of the software system with the cumulative number of corrected faults. Furthermore, we consider an imperfect debugging environment where the detected faults are not always corrected and removed from the system. The time-dependent behavior of the system alternating between up and down states is described by a Markov process. From this model, we can derive quantitative measures for software availability assessment considering the number of restoration actions. Finally, we show numerical examples of software availability analysis.

In this paper, we describe an experimental evaluation of visual fatigue in a binocular disparity type 3-D display system. To evaluate this fatigue, we use a subjective assessment method and focus on mismatching between convergence and accommodation, which is a major weakness of binocular disparity 3-D displays. For this subjective assessment, we use a newly-developed binocular disparity 3-D display system with a compensation function for accommodation. Because this equipment only allowed us to compare the terms of the mismatching itself, the evaluation is more accurate than similar previous works.

Cell loss is one of the most important metrics of quality of service in ATM mobile communication systems. This loss can be suppressed by introducing buffer memories in the network, but that sacrifices delay. This paper proposes a lossless handover scheme for ATM mobile communication networks that can suppress delay fluctuations, and presents a subjective evaluation of MPEG2 images with various buffer memory sizes.

This paper presents results of subjective assessment of the media synchronization quality in the virtual-time rendering (VTR) media synchronization algorithm. For the assessment, stored voice and video streams were transmitted as two separate transport streams from a source to a destination on various traffic conditions in an experimental system. At the destination, they were output after synchronization control. We subjectively assessed the quality of media synchronization in a systematic way. This paper examines the effects of the difference between methods of recovery from asynchrony on the media synchronization quality. The paper also clarifies the relationships between the subjective and objective performance measures. Furthermore, it examines the effect of the difference in scene between media streams and that of the modification of the target output time on the media synchronization quality.

M-ary/SS systems are compared with DS/SS systems applied the multicarrier systems and the RAKE receiver in the presence of AWGN, frequency selective Rayleigh fading and partial-band interference. In particular, the BER performance and SSMA capability are evaluated. Consequently, the M-ary/SSMA system using the multicarrier techniques is subject to the M-ary/SSMA system with the RAKE receiver in the presence of partial-band interference. The BER performance of the M-ary/SSMA system is better than that of the DS/SSMA system when the number of users is smaller than 20. And the spectral efficiency of the multicarrier M-ary/SSMA system is best in the four systems when JSR=20 [dB] and BER=10-3.

This comment rectifies the auto-correlation function of the raised-cosine pulse shaping filter response derived in the above paper, giving its exact expression. We give, not only the exact solution for the auto-correlation taken at multiples of the chip period, Φ(rTc), but also for its entire domain, Φ(τ). Nevertheless, due to an approximation made in [1], the conclusions reached therein remain valid.

In this paper, a simple frame synchronization system for M-ary Spread Spectrum (M-ary/SS) communication system is analyzed. In particular, synchronization performance, bit error rate performance, and Spread Spectrum Multiple Access (SSMA) performance are analyzed. The frame synchronization system uses the racing counters. The transmitted signal contains framing chips that are added to spreading sequences. In the receiver, the framing chips are detected from several frames. The authors have proposed the simple frame synchronization system that detects framing chips from consecutive 2 frames. In this system, as the number of framing chips increases, synchronization performance improves and bit error rate performance degrades. In this paper a frame synchronization system that improves bit error rate performance is treated and analyzed. As the rusult, when the number of reference frames is 3, the bit error rate is much improved than the conventional system.

In this paper we discuss the binary spreading sequences whose spectral distributions are DC free and spectral distribution's shapes can be easily controlled by a certain parameter denoted by δ. The newly developed sequences, referred to as modified antisymmetric M-sequences, are modified-versions of the conventional antisymmetric (AS)M-sequences. The proposed sequences are designed to increase the varieties of spectral distribution's shapes and improve the correlation properties when compared to those of the FM coded M-sequences which have already proposed by Tsuzuki et al. Some typical line coded M-sequences, i.e. the (differential) Manchester coded M-sequences and the FM coded M-sequences, and the conventional AS M-sequences are included in the set of proposed sequences. The improvement of the average BER (bit error rate) performance for asynchronous DS/SSMA (direct sequence/spread spectrum multiple access) systems using the proposed sequences in comparison to the system using the conventional AO/LSE (auto-optimal phase with least sidelobe energy) M-sequences is also shown.

The performance of a noncoherent parallel matched-filter (MF) acquisition scheme with a reference filter (RF) is evaluated for a direct-sequence/spread-spectrum multiple access (DS/SSMA) packet radio system in a mobile cellular environment. This acquisition scheme employs a RF to estimate the variance of interference at the output of detecting MF. Acquisition-based packet throughput of the parallel NM-RF scheme is derived for an AWGN and a Rayleigh fading channels. Packet throughput of a parallel MF-RF acquisition scheme is compared with those of a serial MF scheme, a serial MF-RF scheme, and a parallel MF. From the numerical results, it is shown that the packet throughput decreases with the number of users in the system, and increases with the preamble length. Imperfect power control causes packet throughput to decrease especially when the power control error is large. The considerations in this paper can be applied to the reverse link (mobile-to-base station) design of a DS/SSMA system for packet-type services.

This study was conducted to assess the relationship between fatigue and pupillary responses. Pupillary responses, ECG and blood pressure were measured for 24 hours every 30 min in 8 subjects. A questionnaire was used to rate subjective feeling of fatigue. Twenty-four hours were divided equally into four 6-hour blocks. Subjective feeling of fatigue increased markedly in the fourth block, and the difference in subjective fatigue between fourth and first blocks was significant. Of nine pupillary responses, the pupil diameter was found to decrease with time. With respect to the function of the autonomic nervous system such as heart rate, systolic blood pressure and diastolic blood pressure, only heart rate was found to be sensitive to the increased subjective feeling of fatigue. A significant difference was found in the mean pupil diameter and mean heart rate between the last and first blocks. This result indicates that pupil diameter is related to fatigue and can be used to assess fatigue. Possible implications for fatigue assessment are discussed.

The performance of an M-ary spread-spectrum multiple-access (M-ary/SSMA) scheme in the presence of carrier frequency offset is discussed in this paper. The influence of carrier frequency offset on the non-coherent reception of M-ary/SSMA signals is examined and it is shown that the carrier frequency offset degrades the performance remarkably, yet. this influence has a distinctive property. Making use of this property, we propose a new M-ary/SSMA scheme that can mitigate the influence of the carrier frequency offset. The scheme is based on the assignment of two distinctive Hadamard codes to in-phase and quadrature components of the transmitted signal. The effect of simultaneous transmission is evaluated in terms of bit-error-rate performance with the carrier frequency offset. As the result, it is observed that the satisfactory bit-error-rate performance can be achieved in the presence of carrier frequendy offset.

A new matched-filter (MF) acquisition scheme with adaptive threshold is proposed for a frequency-hopped/spread-spectrum multiple-access (FH/SSMA) system. Detection and false alarm probabilities are derived for combined interference environments. The combined interference consists of partialband noise jamming or tone jamming, multiple access interference (MAI), multipath interference, and thermal noise. We use Gaussian approximation for modeling the MAI and multipath interference. Equal power assumption of the users is employed which is typically used in the SSMA system analysis. In the proposed scheme, MF output is compared to an adaptive threshold determined by the number of jammed frequency slots. It is shown that the proposed adaptive-threshold acquisition scheme achieves higher detection probability and lower false alarm probability than a conventional fixed-threshold scheme for each jammed fractional bandwidth, JSR, the number of multipaths, and the number of users. It is also shown that adaptive threshold achieves faster acquisition and higher packet throughput than fixed threshold in application to FH/SSMA packet radio system.

We develop a software availability model incorporating software failure-occurrence and fault-correction times, under the assumption that the hazard rate for software failure-occurrence decreases geometrically with the progress in fault-removal process. Considering that the software system alternates two states, i.e. the operational state that a system is operating and the maintenance state that a system is inoperable due to the fault-correction activity, we model the time-dependent behavior of the system with a Markov process. Expressions for several quantities of software system perfomance are derived from this model. Finally, numerical examples are presented for illustration of software availability measurement.

We propose an objective measure from assessing low-rate coded speech. The model for this objective measure, in which several known features of the perceptual processing of speech sounds by the human ear are emulated, is based on the Hertz-to-Bark transformation, critical-band filtering with preemphasis to boost higher frequencies, nonlinear conversion for subjective loudness, and temporal (forward) masking. The effectiveness of the measure, called the Bark spectral distortion rating (BSDR), was validated by second-order polynomial regression analysis between the computed BSDR values and subjective MOS ratings obtained for a large number of utterances coded by several versions of CELP coders and one VSELP coder under three degradation conditions: input speech levels, transmission error rates, and background noise levels. The BSDR values correspond better to MOS ratings than several commonly used measures. Thus, BSDR can be used to accurately predict subjective scores.

Considering the trend towards adopting high efficiency picture coding schemes into digital broadcasting services, we investigate objective picture quality scales for evaluating digitally encoded still and moving pictures. First, the study on the objective picture quality scale for high definition still pictures coded by the JPEG scheme is summarized. This scale is derived from consideration of the following distortion factors; 1) weighted noise by the spatial frequency characteristics and masking effects of human vision, 2) block distortion, and 3) mosquito noise. Next, an objective picture quality scale for motion pictures of standard television coded by the hybrid DCT scheme is studied. In addition to the above distortion factors, the temporal frequency characteristics of vision are also considered. Furthermore, considering that all of these distortions vary over time in motion pictures, methods for determining a single objective picture quality value for this time varying distortion are examined. As a result, generally applicable objective picture quality scale is obtained that correlates extremely well with subjective picture quality scale for both still and motion pictures, irrespective of the contents of the pictures. Having an objective scale facilitates automated picture quality evaluation and control.