Broadband Noise Signal Research Articles

Localization and the law of the first wavefront in the median plane.

Test persons having their heads held in a fixed position were stimulated simultaneously from the front and the rear with identical broad-band (music and noise) signals. Between the front and rear signal, a time delay of maximum ±880 μsec could be set. It was found that the direction of the sound sensation coincided with the angle of incidence of the first wavefront for delay times greater than about ±550 μsec. For smaller delay times, however, the direction of the sound sensation depends on the spectrum of the ear signals and can be predicted by applying the concept of directional bands.

Signal Processing to Reduce Multipath Distortion in Small Rooms

The transmission between a sound source and a microphone in a small, hard-walled room is characterized by multiple peaks and sharp valleys. Sound received by such a microphone often is judged to be undesirably distorted or spectrally colored. Speakerphones and conference telephony systems are vulnerable to this acoustic distortion. A technique is described for combining the signals from two or more microphones to produce a single output having less multipath distortion than any of the individual microphone signals. The output from each microphone is filtered into a number of bandpass signals occupying contiguous frequency ranges. The microphone receiving greatest average power in a given frequency band contributes that signal band to the output. Each microphone, then, is used only for those frequency bands in which its received power is greater than any other microphone, so that the combined output contains less spectral coloration than the signal from any individual microphone. The processing method is studied by simulation on a digital computer. Broad-band noise and speech signals are used to test the method.

Modulated Langmuir Probe Characteristics

The presence of fluctuations in the discharge parameters introduces errors into Langmuir probe measurements. Previous work by Garscadden and Emeleus considered the effects of fluctuations in the individual parameters. In this paper a generalized small-amplitude theory of the effects of these fluctuations is given and also a large signal theory for single-frequency oscillations. The results of Garscadden and Emeleus can be deduced from these, but additional terms are predicted due to cross modulation occurring when the various fluctuations are correlated. The large signal theory has been checked experimentally in a mercury-vapor discharge at a pressure of about 1 μ. Further experimental measurements with injected broadband noise signals confirm certain features of the generalized theory. The errors caused by fluctuations are discussed and it is concluded that although electron temperatures may still be measured accurately in their presence, serious errors can be incurred in the measurement of electron saturation current and number density.

Highly linear amplification with transistors

Highly linear amplification can be obtained with HF diffused-base transistors in the common base configuration. It is the purpose of this paper to 1) present experimental results to demonstrate this, 2) demonstrate the causes for this high degree of linearity and derive expressions for optimum bias voltages and currents (these results can be extended with certain restrictions to the common emitter and common collector configurations), and 3) present a general method for analytically treating nonlinearity in all transistor configurations. A push-pull amplifier using 2N509- or 2N1195-type transistors provided 13 db of gain, with signal-to-distortion ratios of 70 to 80 db for broad-band white noise signals with +3 dbm of average output power or sine wave signals with +15-dbm peak output power. The stability of this gain and linearity with temperature and interchange of transistors is high. This high degree of linearity is shown to be due to 1) cancellation of current sensitive nonlinearity in the emitter and collector resistances, 2) collector voltage bias adjusted so that the derivative of the collector conductance with respect to collector voltage is zero, and 3) the use of a high alpha cutoff frequency transistor. An expression derived for optimum bias was in good agreement with experiment. The nonlinear distortion in transistors is shown to be different from that found in most devices, in that there are frequency-sensitive and frequency-insensitive nonlinearities. A general analysis carried out including these terms was in good qualitative agreement with experiment.