Analog Transmission Research Articles

An optical led transmitter for baseband video with JFET non-linearity compensation

An optical, intensity-modulated LED transmitter for analog transmission of baseband video signals is presented. A differential-gain reduction of more than 20 dB can be obtained by means of a novel compensation technique using a JFET as the compensation device. The circuit as described in this paper is capable of supplying a signal current of about 100 mA into an LED, resulting in an optical output power of −18 dBm coupled into a 50 μm graded-index fibre. A bandwidth of more than 20 MHz is available for high-resolution picture transmission. Differential gain amounts to about two per cent. Differential phase is less than one degree.

A Low-Cost Analog/Digital Proportional Control System for Home Computer and Robotics Applications

This paper describes an integrated circuit chip set* designed for low-cost transmission of analog and digital data. The system provides remote motion control capability and is designed for home computer or microprocessor control applications. The control system is based on a multichannel serial bus concept and uses pulse-position modulation (PPM) for information transfer. The input and output of the system interface directly with a home or personal computer.

Tomorrow's Telephony: Light and Clear

Tomorrow's Telephony: Light and Clear

Modulation characteristics of constricted double-heterojunction AlGaAs laser diodes

Detailed experimental results are presented for three types of constricted double-heterojunction (CDH) AlGaAs laser diodes. The dynamic properties (pulsed and RF modulation, noise response, and linearity characteristics) of these devices are given for the first time. Performance in analog and digital signal transmission systems is also described for the most useful types of such devices: the standard CDH [1], [2] and the high-power CDH large optical cavity (LOC) [3], [4]. With the latter, an order-of-magnitude increase in modulated power capability over standard devices has been achieved (40-mW peak power into 0.2 NA, 50-μm core graded-index fiber). Finally, we present a detailed experimental analysis of self-oscillation phenomena in a variant type of CDH structure (CDH-AMC) (asymmetric mode confinement) [5]. These observations, although uncommon in CDH and CDH-LOC devices, provide new experimental insight on the nature of self-pulsating laser diodes.

Symmetry restrictions for natural and induced circular dichroism and optical activity for all point groups

Group theoretical techniques are used to analyse the geometrical dependence of natural circular dichroism (CD) and circular dichroism induced by a linear interaction with either a magnetic (MCD) or electric field (ECD). For each of these effects the dependence of the spectra on the relative orientations of the crystal, radiation, and the external field, is given for each point group. At arbitrary orientations the spectrum is a linear superposition of a definite number of irreducible spectra. For example, MCD has axial symmetry in the groups Dn, Dnb, Cnv(n not=2), D2d, D3d while ECD is always forbidden in cubic symmetry, and also for non-cyclic groups if the field and radiation are parallel. The results apply equally to the analogous transmission effects, i.e. natural optical activity, electro-optical activity and the magnetic-field-induced Faraday effect.

Subscriber Stations in Service Integrated Optical Broad Band Communications Systems

Subscriber stations in service integrated communications systems can be implemented in many different ways. Within the HHI Broad-Band Communications System two extremely different solutions have been realized: on the one hand, subscriber stations in a conventional star type network with central exchange and analog transmission, and on the other hand, subscriber stations in a digital, loop-structured network employing distributed control. In both cases, the same services (telephone, data, videophone, TV, and stereo broadcasting) are carried to the subscriber via one single optical fiber. This paper describes these different realizations of service integrated subscriber stations in detail. The size of terminal equipment, power consumption, extendability to further subscribers, and services and reliability are the main parameters of an appropriate comparison. An analysis of both realizations, giving emphasis to the switching and multiplexing, and the feasibility of analog optical transmission will be made. Alternative realizations and an estimation concerning subscriber stations for future service integrated broad band communications systems will be presented.

Optical receiver front-end nonlinear distortion

The harmonic modulation products of a PIN-GaAs MESFET optical receiver front end are found to be about 20 dB smaller than those of a PIN-bipolar transistor front end, using the ‘two-frequency optical signal method’. It is pointed out that the design parameters in the PIN-GaAs MESFET front end, optimised from the high-sensitivity viewpoint, are compatible with the requirements for low distortion in analogue transmission systems.

Trends in telecommunications

There are two main evolutionary trends in communication today. First, the mechanical switching systems and analogue transmission techniques of the public telephone network are being swept aside by digital switching and transmission, with both techniques making heavy use of silicon-chip technology. Secondly, a host of new services is becoming available, mostly for business: some are led by new opportunities opened up by digital transmission and switching, and others are driven by the entry of silicon technology into commercial practice

Orthogonal Transform Coding for TV Signals

Binary coded television signals require high transmission rates if no loss of quality is permissible in comparison with analog transmission. An important class of coding scheme that use the spatial correlation of picture elements (Pels.) in reducing data redundancy is the transform coding techniques. A review of the field of transform image coding is presented for data compression with the comsideration of dimensionality and optimality criteria.

Generation of harmonic distortion at fibre connectors

Harmonic distortion generated by the Fabry-Perot resonator at fibre connectors is described. The maximum second-harmonic distortion is calculated as a function of end separation for GaAlAs lasers. A −42 dB second-harmonic distortion is generated in 100 μm end separation, which is harmful in analogue transmission systems.

Optimizing Optical Transmitters and Receivers for Transmitting Multichannel Video Signals Using Laser Diodes

Analog transmission using laser diodes is quite attractive for transmitting multiplexed video signals, owing to the wide modulation bandwidth of laser diodes. Optimizations for transmitting color-TV signals (7 channel frequency division multiplexed) in the VHF band using buried heterostructure laser diodes are investigated. The major transmission objectives are a carrier to noise ratio of larger than 48 dB and intermodulation products of less than -55 dB. Design curves for minimizing distortion at the transmitter are demonstrated. A modulation index of 70 percent is shown to be capable of attaining the objective. Both the avalanche photodiode (APD) and the pinphotodiode (PD) are considered for use in the receiving amplifier. Minimum receiving power of -11 dBm obtained with the PD receiver is in good agreement with theoretical values. Deterioration of signal to noise ratio due to fiber guided transmissions is also reviewed in comparison with aerial transmissions.

Efficient implementation of a frequency‐domain technique for frequency scaling of speech signals

Frequency scaling of speech signals is a useful technique for reducing bandwidth requirements in analog and digital speech transmission systems. A recently developed frequency domain scaling (FDS) technique is based on scaling the phase of the signal's short‐time Fourier transform (STFT), evaluated at a discrete set of equally spaced frequencies. For the particular scale factor of 2, a simple sign tracking algorithm is used which avoids explicit phase computation and unwrapping. A computationally efficient implementation of this technique is presented. It is based on performing the STFT analysis, modification and synthesis by means of the FFT algorithm, using specially chosen analysis and synthesis window functions. In addition, efficient decimation and interpolation of the complex bandpass signals, along with the required change in output sampling rate are embedded in the implementation scheme. Simulation results show that this FDS technique is particularly robust to environmental conditions such as background noise, room reverberation, and multiple speakers. Processed examples will be played along with results obtained with the earlier reported pitch‐adaptive, time‐domain harmonic scaling (TDHS) technique. A hybrid scheme, which combines the two techniques will also be discussed and demonstrated.

Time diversity by pitch-synchronized interpolation

In mobile communications such as automobile telephone systems, an instantaneous interruption is caused by rapid fading, and speech quality is markedly deteriorated. A pitch-synchronized interpolation method is proposed to improve this quality. This method utilizes the fact that most speech signal regions have a pitch period. In this method, when an instantaneous interruption is detected at the receiver, a speech signal, which includes instantaneous interruption noise, is deleted and interpolated by repetition of the speech signal received one pitch interval wave before the instantaneous interruption. This method can be applied to receivers used in analog and digital transmission systems. A receiver using this method has been constructed, and it was shown that a 10-dB carrier-to-noise ratio (CNR) gain can be obtained by this receiver.

Wide-band optical fibre local distribution systems

The concept of a wide-band optical local network is presented and the technical aspects are discussed in detail. The types of service are outlined and grouped according to the requirements for transmission capacity. The components available and their characteristics are presented. The modulation characteristics of the semiconductor laser are scrutinized and show that the laser is unfit for high-quality analogue intensity modulation. The aspects of digital and analogue transmission of television are examined and compared. Analogue transmission can provide transmission of only a few TV channels at the present level of quality whereas digital transmission can provide improved quality with a comfortable system margin. However, none of the schemes are suited for simultaneous transmission of many TV channels, which shows that the direct optical equivalent of a conventional CATV network cannot yet be constructed. The concepts of central switching and customer switching are outlined together with the options for multiplexing and network structure. Finally a review is made of known activities. The review covers eleven studied, planned and implemented experimental networks. Nearly all of the networks employ central switching. The older experiments use exclusively analogue transmission whereas there is a growing tendency to use digital transmission in recently planned networks.

Nonlinear distortions in the current modulation of non-self-pulsing and weakly self-pulsing GaAs/GaAlAs injection lasers

Nonlinear distortion characteristics of GaAs/GaAlAs injection lasers under analog current modulation were experimentally investigated. In non-self-pulsing lasers, relaxation oscillation (R.O.) plays a heavy role in generating harmonic distortions, in good agreement with theory. Additional distortions generated by microwave mixing in weakly self-pulsing lasers make such lasers unacceptable for wideband analog transmissions unless suitable negative feedback is applied. With such measure taken, both fundamental and the harmonic responses of a self-pulsing laser is indistinguishable from that of a laser with no R.O. resonance.

High-Definition Television System — Signal Standard and Transmission

Since 1968, NHK (Japan Broadcasting Corp.) has been carrying out research into a high-definition wide-screen television system, including development of equipment. The standards and signal transmission for broadcasting of high-definition TV are discussed. Several TV systems and the desired signal-to-noise ratios that will fit in with various viewing conditions from the viewpoint of the characteristics of the human visual system are briefly described. Also described are the transmission primaries of the system and the signal forms being used for a 1125-scanning-line system, which is the tentative standard chosen by NHK. The parameters necessary for analog and digital transmission for satellite broadcasting using this system are considered. In November 1978 and March 1979, transmission tests of the high-definition TV system were carried out via the broadcasting satellite “Yuri” in Japan.

Measuring and evaluating performance

The subject of system performance measurement and evaluation has undergone as many generations of changes as the systems themselves. The problem of what to measure and evaluate is complicated by the fact that computing and communications, having become technically similar (digital), will undergo further fusion. Because the technologies are merging, a comparision of their respective origins is instructive. Communications and computing do not share a common history. Communications performance evalution began as a turn-of-the-century issue. Important performance attributes of voice communications systems were accessability and reliability. The general public and communications system analysts always viewed the voice communications systems as a bundled service, with little emphasis on the characteristics of its individual components. Performance was "engineered" into communications systems for given workload capacity levels (traffic). A reliable service offering evolved over two decades (1920's and 1930's) and was expanded to include data as well as voice communications. The voice network used primarily analog transmission techniques, because voice traffic grew far more rapidly than data. Pulse code modulation (PCM) techniques, employing digital transmission, reversed the trend of analog circuitry. In the future, communications transmission, switching, and integrated services networks (voice, data, facsimile, picture) will be implemented exclusively with digital techniques.

Superimposed pulse modulation for fibre optic analogue video transmission using semiconductor laser diodes

The letter proposes a new modulation technique to reduce speckle noise in multimode optical fibre analogue transmission systems using semiconductor laser diodes. The feasibility of analogue video transmission systems using the proposed modulation technique has been confirmed.

Fibre optic analogue transmission experiment for high-definition television signals using semiconductor laser diodes

The transmission characteristics of 30 MHz analogue colour t.v. signals are investigated using a semiconductor laser diode at the 1.3 μm wavelength. Transmission over 17 km with 40.3 dB unweighted s.n.r. is confirmed, and transmission over 30 km is feasible using a 1.3 μm laser diode and low-loss single-mode fibres.

The LT-1 Connector--A Conventional Transmultiplexer

The LT-1 connector has been designed to provide an economical means of terminating analog transmission facilities on the No. 4ESS, a digital toll switching machine. It performs a bilateral conversion between two 12-channel analog group signals in the 60-108-kHz band and a 24-channel digital digroup at 1.544 Mbits/s. In addition, signaling information in the form of a 2600-Hz tone must be converted to the digital format. The connector has been implemented with conventional per-circuit technology but achieves significant savings compared to back-to-back channel banks. This is accomplished by eliminating redundant circuitry and using a number of custom designed silicon integrated circuits. This paper gives a description of the design and some typical performance data.