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Radio and Electronic Engineer

Issue 6 • Date June 1978

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Displaying Results 1 - 13 of 13
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  • The role and scope of digital signal processing in communications systems

    Page(s): 266 - 270
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (568 KB)  

    Trends in relative costs encourage sophisticated signal processing in order to make fuller use of the capacity of communications channels. This implies features of subtlety, adaptation, transformation and manipulation which are most expediently handled digitally. The variety, diversity and pervasive nature of these techniques in modern communications nets is illustrated, and their impact on overall system design is discussed. Three specific techniques are included for illustrative purposes: (1) The `design¿ of multi-valued discrete signals in the phase/amplitude plane. (2) The adaptation of Fourier analysis to equally-spaced narrow frequency `lines¿. (3) The compensation of tape recorders for both steady and transient differences in tape-speed between recording and replay. View full abstract»

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  • Partial correction of d.p.c.m. video signals using a walsh corrector

    Page(s): 271 - 276
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (866 KB)  

    A Walsh corrector is described for Walsh-transformed pictures. It enables all sequency errors > 4% of the sequency range to be corrected, provided that no more than one sequency is in error per block of length N. The penalty for this correction is a bandwidth increase of 100/(N¿1)%. When the Walsh corrector is applied to d.p.c.m. pictures, single sequency errors per block are usually removed while burst errors do not propagate, being confined to the block of length N. View full abstract»

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  • Interfacing intraframe d.p.c.m. with conditional replenishment coding for viewphone signals

    Page(s): 277 - 284
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1022 KB)  

    Conditional replenishment is a coding technique that is particularly efficient at exploiting the large frame-to-frame redundancies that exist in visual telephone or visual conferencing signals. However such techniques are necessarily complex and their cost of implementation may, in the case of lightly loaded routes, outweigh the saving in transmission capacity. It is therefore possible that on such routes, cheaper, less efficient, techniques such as differential p.c.m. (d.p.c.m.) will be used, thereby creating an interface between links employing different methods of source coding. The compatibility problems that exist at this interface are discussed and an experimental system is described that accepts a d.p.c.m. signal at 8 Mbit/s and uses a novel interfacing technique to take advantage of frame-to-frame correlation and the statistical distribution of the samples within the moving areas for transmission at 2 Mbit/s View full abstract»

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  • High-reliability data transmission to mobile vehicles

    Page(s): 285 - 292
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (872 KB)  

    The design for an adaptive-rate data transmission system providing very reliable two-way data transmission over a fading mobile channel is outlined and measurements are presented. A single u.h.f. or v.h.f. carrier is required. Operation with practical fading has been tested using a recording of variations in mobile-channel gain in conjunction with a hardware simulation of the system. The tests show that an error probability of 10¿10 can readily be achieved. For operation in 12.5 kHz bandwidth, four-phase keying at 10 kbauds is proposed. In this case the system will provide a mean data rate in each direction of 1.8 kbit/s with the mean power of the fading signal 7 dB above the receiver noise. With increasing received power the mean data rate in each direction tends to 4.8 kbit/s. These rates can be doubled by interleaving an identical scheme operating on a second carrier frequency. View full abstract»

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  • Low-pass digital filtering with the host windowing design technique

    Page(s): 293 - 300
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (833 KB)  

    This paper gives a detailed description of the host windowing technique of designing low-pass digital filters. The relationship between the quality of the host filter and the resulting low-pass filter is examined both theoretically and in measurement examples. New transfer function symmetry and error bounding results are obtained and experimentally confirmed for variable cut-off filters. It is demonstrated that variable cut-off low-pass filtering can be undertaken in a manner which, in contrast with the normal windowing approaches, gives the designer clearly specified worst case operating conditions across a family of filters for a selected host window. It is shown that the worst case performance of even-length filters built using a Hilbert transform host is better than the worst case obtained for odd-length differentiator hosted low-pass filters. However, for cut-off frequencies around the quarter sampling frequency, odd-length filters often perform significantly better than even ones. Kaiser and minimax hosts are studied and found to perform well; the designer can be sure that, for a given transition width, the peak ripple obtained with these hosts will not exceed about twice the peak ripple that could have been attained through direct (and much more costly) minimax low-pass design. Kaiser designs are slightly better host designs when the filter length is even, but minimax hosts are often better than Kaiser ones for odd-length filters. In all cases the one-off host filter design explicitly establishes the peak ripple and transition width that will arise for any conditions during the use of the host windowing technique. View full abstract»

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  • Near-maximum-likelihood detection processes for distorted digital signals

    Page(s): 301 - 309
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1037 KB)  

    The paper describes six detection processes that are developments of the Viterbi-algorithm detector and are suitable for use in a synchronous serial data-transmission system operating at a transmission rate of up to 20,000 bit/s over a slowly-time-varying channel. The processes are first described and the results of computer simulation tests are then presented, comparing the tolerances of the detection processes to additive white Gaussian noise with the tolerances of both the Viterbi-algorithm detector and a conventional non-linear transversal equalizer. Several different time-invariant channels are used in the tests. It is shown that, under the appropriate conditions, a near optimum tolerance to noise can be achieved by a detection process that involves very much less storage than the Viterbi-algorithm detector and requires only a very small fraction of the number of operations. View full abstract»

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  • A differential codec for monochrome-television signals

    Page(s): 310 - 312
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (313 KB)  

    The design of a digital encoder that will be used for connecting conference television signals to an experimental optical fibre is presented. The signal is transmitted as 4-bit words at 11 megasamples per second. The encoding makes use of psychophysical properties of human vision, but is not strongly dependent on picture content. Under normal viewing conditions, the only degradation in the received picture is a little blurring of sharp vertical edges. Transmission error rates of one per 106 bits are hardly noticeable View full abstract»

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  • IERE news and commentary

    Page(s): 315 - 316
    Save to Project icon | PDF file iconPDF (327 KB)  
    Freely Available from IEEE
  • Book review: The Wave and Ballistic Theories of Light - A Critical Review

    Page(s): 318
    Save to Project icon | PDF file iconPDF (190 KB)  
    Freely Available from IEEE
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  • Full text access may be available. Click article title to sign in or learn about subscription options.
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