摘要:

A new way of localising two simultaneous voices, called thef-pmethod, is introduced. Test results of investigations carried out in the free-field (i.e. on a flat roof with no obstructions) are presented. Thef-pmethod applies a neural,N-tuple, adaptive windowing technique to new binary image representation of the phase information derived from two microphones. It is closely related to a psychoacoustic theory of the human interaural time difference localisation mechanism. The microphones need not be fixed for the purposes of speech enhancement, and so the method has, in principle, the same spatial freedoms as the human system. A crucial factor in the success of the device is the presence of gaps in the spectrum of each source at different times and frequencies. The voices can, on the free-field case, be almost completely separated by using the localisation information. In the two-voice free-field case each voice is localised approximately seven times per second. The method involves no complex calculations and it is likely that it could be implemented in real-time, to allow tracking of moving sources, with a DSP board and a fast microprocessor. There is room for improvement in the values of thef-pparameters.

DOI:10.1049/ip-i-2.1993.0034

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The security of frequency domain analogue speech scramblers is investigated. It is shown that a vector codebook similar to that deployed in vector quantisation of speech can be used to attack the speech scramblers even under the stringent condition that no section of the original speech is available to the attacker and that the encryption key of the system is varied frequently in a random and unknown manner. Subjective as well as objective results demonstrating the success of the attack is given. A technique for improving the security of the frequency domain encryption scheme under the proposed attack is discussed.

DOI:10.1049/ip-i-2.1993.0035

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The paper examines several pitch estimation and adaptation algorithms in forward and backward adaptive CELP systems. The authors compare open-loop and closed-loop pitch estimation structures, the use of integer and non-integer delay pitch prediction, and the implementation of block adaptive and sample recursive pitch tracking algorithms. Forward adaptive systems are found to perform better with a closed-loop structure in conjunction of non-integer delay pitch prediction. However, temporal and noise feedback problems suggest that open-loop structures with hybrid pitch tracking are better suited for backward adaptive systems

DOI:10.1049/ip-i-2.1993.0036

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A Reed-Solomon decoder for errors-and-erasures correction, based on a new algebraic decoding algorithm, is presented. This high-speed decoder requires onlynclock cycles for decoding each receivedn-symbol block. A serial structure that requires very few multipliers and provides a general expression to calculate the coefficients of the erasure-locator polynomial is also presented. A (15, 11) RS decoder and its shortened version (7, 3) RS decoder are used as design examples to illustrate the operating procedure of the new decoding algorithm.

DOI:10.1049/ip-i-2.1993.0037

出版商:IEE    

年代:1993

数据来源: IET

摘要:

Markov models and values of channel capacity are presented for multilevel sequences with the number of signal levels ranging from four to seven, subject to three classes of channel-input restrictions. Several new line codes suitable for use in digital-communication systems are presented, together with their error-propagation properties and power spectral densities. These codes satisfy restrictions regarding runlength and zero-frequency components.

DOI:10.1049/ip-i-2.1993.0038

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The coded performance of a slow-frequency hopped differential PSK (SFH/DPSK) system in the presence of both tone jamming and additive white Gaussian noise (AWGN) is studied. The error correlation due to DPSK demodulation and the effect of tone jamming is considered when evaluating the block and decoded error probabilities. The decoded error probabilities in some special cases are computed. The effect of interleaving on system performance is addressed under various conditions. A nearly optimum code rate for a Reed-Solomon code is determined for systems employing interleaving techniques. The effect of partial-band tone jamming is demonstrated.

DOI:10.1049/ip-i-2.1993.0039

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The performance of a concatenated coding scheme is studied over nonlinear channels when additive white Gaussian noise is considered in both the up and downlink. Bandwidth-efficient trellis codes using a 32-QAM signal set are used as inner codes and 3/4 rate convolutional codes as outer codes to achieve an overall bandwidth efficiency of 3 bit/s/Hz. Simulation results show that this coding system compared with an uncoded 8-PSK scheme, 3/4 convolutionally encoded 16-QAM systems and some TCM techniques, provides significant coding gains over linear and nonlinear channels.

DOI:10.1049/ip-i-2.1993.0040

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The authors investigate the performance of packet loss and delay in a nonblocking backpressure-type packet switch with input and output queueing that is able to transfer up toLpackets per slot to a given switch output. The switching fabric allows limited queueing at the output ports. Overflow at the output queues is prevented by using a backpressure mechanism and additional buffers at the input ports. The authors develop an analytical model of the switch architecture and analyse the impact of the backpressure effect on the switch performance for arbitrary output and input buffer sizes and different speed-up factors. The model is based on the output contention process and Geom/PH/1/K input queueing process. The output contention involved by packets at the head of input queues can be viewed as an independent phase type process. The result indicates that, when there are reasonable sizes of buffers at both input and output port and the backpressure mechanism is used, the switch with output buffers having a relatively small capacity and a speed-up factor of L = 2 approaches closely the ideal delay and packet loss performance of a pure output queueing switch at a reasonable input load condition.

DOI:10.1049/ip-i-2.1993.0041

出版商:IEE    

年代:1993

数据来源: IET

摘要:

Broadband ISDN using asynchronous transfer mode as the transport mode carries traffic of several different classes, including one or more real-time traffic classes, each with its own set of traffic characteristics and performance requirements. To support multiple classes of traffic in ATM networks, multiple priority levels are provided, and different priority levels are given to different classes of traffic. A dynamic priority scheme is proposed as an efficient scheduling method at a switching node in an ATM network serving multiple classes of delay-sensitive traffic. The advantage of this protocol lies in that, by properly adjusting the control parameters, it can meet the average delay requirement for different classes and thus guarantee the quality of service. In addition, each class of service can be guaranteed a minimum of bandwidth, which can prevent the low priority class from starvation.

DOI:10.1049/ip-i-2.1993.0042

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The author addresses a multiple header detection issue for the centralised spread-spectrum packet radio system with common directsequence/ binary phase-shift-keying (DS/BPSK) modulation. Chernoff bounds on the false alarm and missed detection probabilities of the header are derived and computed for a specific class of spreading codes and header sequences. Using these performance bounds, the author compares the header sequences and finds a good one which can be received with small probability of false alarm and high probability of correct detection. He also determines a proper randomisation time to minimise the two bounds for given system parameters, and then calculates the allowable number of transmitting radios subject to a constraint on the bounds for this randomisation time.

DOI:10.1049/ip-i-2.1993.0043

出版商:IEE    

年代:1993

数据来源: IET