A broadband echo-ranging system has been employed to collect echoes from fish schools off the southern California coast. The system uses an implosive impulse source called Hydroshock(R)and a broad-band, constant beamwidth towed array. The equipment is capable of measuring target strength from 500 Hz to 50 kHz, with minimal frequency distortion due to transducer beamwidth effects. Averaged frequency spectra for echoes from several schools are presented, showing different cutoff and resonance characteristics.

A dual classification of variable delays 1) by their explicit dependence on time or through a state variable and 2) through their origins as ``pipe'' or ``nonpipe'' problems is presented. Techniques of simulation through standard analog components and, more particularly, transport devices are analyzed and further classification made of the latter to distinguish those suitable for state-variable delay systems from those that are not. The construction and testing of a simple variable delay are described. An application to a system in which the delay is a sinusoidal function of time is discussed in detail. A theory is presented that predicts what frequencies will be present in the output of such a device when excited by a sinusoid of commensurate frequency; experimental results corroborating this are presented. A scheme for converting a problem with variable delay to one with fixed delay is further analyzed and its range of application delimited. Conclusions include a discussion of the best type of simulator to be used for variable delay problems.

Shannon's definition for the information content of a Gaussian, time-continuous process in Gaussian noise is extended to the case where the observation interval is finite, and where the processes may be nonstationary, in a straightforward way. The extension is based on a generalization of the Karhunen-Loeve Expansion, which allows both the signal and noise processes to be expanded in the same set of functions, with uncorrelated coefficients. The resultant definition is consistent with that of Gel'fand and Yaglom, and avoids the difficulties posed by Good and Doog to Shannon's original definition. This definition is shown to be useful by applying it to the calculation of the information content of some cases of stationary signals in stationary noise, with different spectra, and to one case where both are nonstationary. Limiting relations are derived, to show that this reduces to previously established results in some cases, and to enable one to obtain rule-of-thumb estimates in others. In addition, both the matched filter and the Wiener filter are related to the information; the matched filter in a very direct way, in that it converts a time-continuous process to a set of random variables while conserving the information.

The maximum average mutual information in the observation of the output,y(t), of a channel over the time interval[T_3,T_4]about the signal (input),s(t), in the interval[T_1, T_2]is taken as the definition of channel capacity for the time-continuous case. In the case where the channel introduces additive Independent Gaussian noise of known correlation function, the capacity is evaluated subject to the constraint that the signal process have a given correlation function. For this evaluation a new joint expansion of the processesy(t)ands(t)is introduced which has the property that all coefficients in the expansion are uncorrelated. Thus, the expansion is a generalization of the Karkunen-Lo'{e}ve expansion to which it reduces when the noise is white and the time intervals coincide. The channel capacity is shown to be directly related to results in the theory of optimum filtering over a finite time interval. Closed form results for the capacity of several channels are given as well as some limiting expressions and bounds. For the case of white noise of spectral densityN_o, the capacity is always bounded by\bar{E}/N_owhere\bar{E}is the average signal energy.