Waveform generation unit

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like Wurst , Sneeze , Iburp , and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed. 

A discussion of the instrumental aspects of voltammetry and leading references to the original literature can be found in some of the monographs already cited in the introduction [1-4]. The essential units are the potentiostat, a triangular waveform generator, and a recording device. The latter is most conveniently a digital oscilloscope or a transient recorder. Commercial equipment with the units combined into one instrument controlled by a PC is available from a number of manufacturers. Also, homebuilt instrumentation... 

By definition, H gives the transfer function and frequency response for a unit impulse. In reality of course, the vocal tract input for vowels is the quasi-periodic glottal waveform. For demonstration piuposes, we will examine the effect of the /ih/ filter on a square wave, which we will use as a (very) approximate glottal source. We can generate the output waveforms y[n] by using the difference equation, and find the fi equency response of this vowel from //(e/ ). The input and output in the time domain and frequency domain are shown in figure 10.26. If the transfer function does indeed accurately describe ihe frequency behaviour of the filter, we should expect the spectra oiy[n, calculated by DFT to match H eJ )X(eJ ). We can see fiom figure 10.26 that indeed it does. 

One of the fundamental problems in unit selection is that the specification items lack the acoustic description(s) that would make matching them with units a fairly easy process. We can approach this problem in two ways. Firstly, we can just ignore the fact that the units have acoustic features and just match on the linguistic features alone. Alternatively, we can try and perform a partial synthesis, where we attempt to generate some or all of the acoustic features and then match these with the acoustic features derived by signal processing fi-om the waveforms. 

Once we have found the units by our search, the actual S5mthesis process is often trivial. In pure unit selection, we have stored our units as waveforms, and so a simple overlap and add technique at the waveform boundaries is sufiicient. If we have stored the units in a more parametric representation (e.g. LP coefiicients and residuals), these can be S5mthesized in the same way as with second generation diphone synthesis. Some systems perform spectral smoothing between units, but again this can be performed with the same techniques as with second generation systems. 

A particularly Important example of this Interactive capability occurs in the analysis of time-of-flight diffraction inspection data files. Following display of the set of ultrasonic rf waveforms stored in each file on the colour graphics unit, a screen cursor is used to select points on identified waveforms for display as time-delay ellipses on the appropriate nozzle profile. The intersection point of the arcs generated from the same point on several waveforms corresponds to the location of the defect extremity producing the observed signal. 

---