Pre-emphasis Network Research Articles

A novel eddy current compensation scheme for pulsed gradient systems

We describe a modified pre-emphasis network that uses a number of fixed time constants, only whose amplitudes are adjustable. In comparison with variable time constant pre-emphasis networks we have used, the fixed time constant network is more effective and easier to set on the fly. We have further developed a method to set the fixed time constant network mathematically using a linear least squares technique.

A four-stage FM broadcast audio peak limiter

The design of a peak limiter processor for FM broadcast audio is described. The limiter has four gain control stages. The basic peak limiting function is performed in the first and second stages by a slow-attach-slow-release stage followed by a fast-attack-fast-release stage. Preemphasis control is performed in the third and fourth stages, which are part of a filter that emulates the transmitter preemphasis network. A peak clipper at the output of the preemphasis controller provides protection from overmodulation by short duration peaks that are not controlled by the four limiter stages. >

Optimum Pre-Emphasis Network for Satellite Transmission of an FM TV Signal near or at Threshold

An optimun pre-emphasis network for the transmission of a FM television signal which is received at or near threshold is described. Objective and subjective measurements of video signal-to-noise ratio (SNR) versus received carrier-to-noise ratio (CNR) for an optimum and CCIR pre-emphasis network are given. The results show that there is an improvement of about 2 dB in SNR using the optimum pre-emphasis network over that achieved with the CCIR network. Subjective tests also show a preference for use of the optimum pre-emphasis network. These results are useful in the design of low cost earth terminals to receive higher quality television signals from a satellite transmitter.

Noise in an FM System Due to an Imperfect Linear Transducer

An approach to the calculation of intermodulation noise in FM systems due to imperfect transmission media is presented in this paper. The technique is essentially that originating with Carson and Fry. In this paper we extend a formulation due to Rice to include an arbitrary continuous pre-emphasis characteristic as well as an arbitrary gain and phase shape transmission medium which are representable by low-order polynomial series in radian frequency. Series approximations are carried out far enough to ensure accurate results for transmission characteristics normally encountered in broadband microwave radio systems. In many cases, only the second- and third-order noise is significant in broadband microwave radio systems. Hence, the analysis carried out in this paper considers only the second- and third-order distortion terms. A digital computer program concerning the intermodulation noise has been written. This analysis and the digital computer program are of aid in the design of microwave radio systems. With a slight modification, the calculation of noise due to AM -to-PM conversion caused by transmission deviation can also be accomplished. An optimum design of the pre-emphasis network may be achieved by using the computer programs through an iterative approach.

Analog communication over randomly-time-varying channels

The problem of analog communication over a randomly-time-varying channel is considered. An analog source generates a message which is assumed to be a sample function from a Gaussian random process. The message is passed through a linear realizable system before modulation. (This corresponds to the pre-emphasis network in FM.) The output of this system is the modulating signal for a no-memory modulator which, in general, will be nonlinear. The modulated signal is transmitted over a time-varying channel We restrict ourselves to Gaussian multiplicative channels. At the channel output, noise is added. The specific problem of interest is to find the optimum estimate of the message. The principle results are: \begin{enumeratge} \item An integral equation whose solution is the optimum estimate. \item A feedback demodulator whose output is the optimum estimate over a certain range of signal-to-noise ratios. \item A proof that the optimum demodulator corresponds to a joint channel and message estimator. This result is the continuous analog of the estimator-correlator result in digital systems. Some related problems and possible extensions are discussed briefly. \end{enumerate}

Optimum Angle Modulation

The problem of transmitting an analog message over a channel corrupted by additive Gaussian noise using angle modulation of a sinusoidal carrier is considered. The problem is formulated as a continuous estimation problem, where the desired output is a waveform that is the maximum a posteriori estimate of the transmitted message. The resulting structure is a nonrealizable, nonlinear system. We next find a demodulator structure that is realizable with delay, whose performance approximates the optimum system. Keeping the demodulator structure optimum, the transmitted signal is now optimized to obtain the best overall system. Two types of system constraints are considered. The demodulator is constrained to be above threshold, and the rms transmitted bandwidth is constrained. The modulator consists of a linear preemphasis network followed by a no-memory angle modulator. We find the linear network which causes the overall system to be optimum. Finally, we compare the optimum system to a phase modulation system using an optimum demodulator and properly chosen modulation index. For the class of message spectra, the difference is small.

Optimum Pre-Emphasis and De-Emphasis Networks for Transmission of Television by PCM

The advantages attendant to the use of pre-emphasis and de-emphasis networks in a PCM television system are examined. The optimum pre-emphasis network is obtained by maximizing the ratio of peak-peak signal to rms weighted noise with the coder input constrained to prevent overload. The results show that for a system using a 12-Mc sampling rate the weighted quantizing noise in the video band can be reduced about 3.7 dB, which corresponds to adding half a digit. This particular figure is based on the assumptions that quantizing noise is subjectively equivalent to thermal noise and that we have a reasonable estimate of the video signal spectrum. Networks approximating the optimum pre-emphasis and de-emphasis networks have been constructed and the performance with these networks compared to that without pre-emphasis (and de-emphasis). A visual comparison of the received pictures shows the improvement, particularly in breaking up contours.

The Structure of Efficient Demodulators for Multidimensional Phase Modulated Signals

The structure of optimum receivers for multilink diversity reception of analog, nonlinear modulated signals is outlined. The receivers are optimum in the sense that they are maximum a posteriori probability estimates (an unconditional maximum-likelihood estimate) of the signal over a finite time interval. For the single link case, Youla has developed a pair of integral equations which specify the optimum estimator. For the multiple-signal, multiple-link case, the corresponding two matrix integral equations have been developed by Thomas and Wong. In both cases, the authors treat the AM case in detail and indicate that the equations for the PM case suggest mechanization schemes. This paper is, in essence, a study of the multidimensional implementation schemes and some of their ramifications. Several useful modulation schemes are discussed. For the multilink PM case with additive white Gaussian noise, we show that the optimum demodulator reduces to a generalized maximal-ratio combiner cascaded with an optimum one-dimensional demodulator. The optimum one-dimensional demodulator contains a nonrealizable filter in the feedback loop. By replacing the loop with an iterative structure the nonrealizability is eliminated. Next, a common multiple-carrier modulation scheme <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PM_{n}/PM</tex> is discussed. The optimum demodulators are shown. Once again, these loops contain nonrealizable filters. Here it is also possible to replace the feedback structure with an iterative structure to eliminate the nonrealizability. Alternately, one can use realizable filters and have a suboptimal demodulation scheme. In currently used demodulation schemes, the threshold of the main carrier is a primary system limitation. A heuristic explanation of the apparent threshold improvement of these suboptimum demodulators is given. An extension of the analytical development from no-memory modulation schemes such as PM to integral modulation schemes such as FM requires some justification. However, in most modulation schemes, the modulation waveforms are passed through pre-emphasis networks before being modulated onto the carrier. For this case, we show that our estimators are essentially optimum for the multilink FM, and FM,/FM case.

The Equalization of Base-Band Noise in Multichannel FM Radio Systems

Pre-emphasis and de-emphasis networks are frequently incorporated in the radio equipment of multichannel systems in order to improve the distribution of noise among the channels. The pre-emphasis network has a rising frequency amplitude characteristic and this produces a redistribution of both signal power and nonlinear noise power over the working spectrum. The improvement in signal-to-noise ratio in any channel which the technique of equalization affords is dependent not only on this effect, but also on the contribution of the multiplex equipment to the total channel noise. This is discussed for typical networks having a maximum slope of attenuation of 6 db per octave and the general case when second-order distortion is predominant. Under practical conditions, the maximum possible improvement is almost entirely obtained when the mean power of the multichannel signal is unchanged by the presence of the networks; a convenient way of establishing this condition with sufficient accuracy is to maintain constant signal energy at a base-band frequency which is 55 per cent of the highest used. Practical considerations usually dictate that the networks do not have a maximum insertion loss exceeding 26 db. With this value and with the multiplex contributing 25 per cent of the total channel noise, an improvement of 3.6 db in the signal-to-noise ratio of the top channel can be expected; provided this loss is not exceeded, the relative noise power, in general, will still be greatest in the top channel.