Sample, tuning

Sample Rate Conversion Techniques. The simplest form of sample rate conversion is called either drop sample tuning or zero order hold interpolator. This technique is the basis for the table lookup phase increment oscillator, well known in computer music [Moore, 1990a],... 

Drop Sample Tuning table lookup sampling playback oscillator 323... 

Figure 6.4 Sample tuning curves for single units in the auditory nerve of the cat...

We can look at an equivalent hardware block diagram. Here we have a wavetable being addressed by what is essentially a counter whose rate is changed to vary the pitch. The term drop sample tuning refers to the fact that samples are either dropped (skipped) or repeated to change the frequency of the oscillator. The phase increment is added to the current value of the phase register every sample, and the integer part of the phase is used as an address to lookup a sample in waveform memory to output to aDAC. 

Figure 8.5 Drop Sample Tuning table lookup sampling playback oscillator. The phase Increment Register adds an increment to the current phase, which has a fractional part and an integer part. The integer part is used to address a wavetable memory, and the fractional part is used to maintain tuning accuracy.

Drop sample tuning can introduce significant artifacts from changing the pitch of a waveform. This method originated in the design of early computer music oscillators... 

Tuning the probe assures that the resonant frequency of the probe coil is the same as the RF frequency you will be using and matching the probe matches the probe coil as a load to the impedance (internal electrical resistance) of the amplifiers. This gives maximum efficiency of transfer of RF power from the amplifiers to your sample nuclei and maximum sensitivity in detecting the FID. Each sample modifies the resonant frequency and matching of the probe, so these have to be reoptimized with each new sample. Tuning the probe is not necessary for routine XH spectra, but for advanced experiments it is important if you wish to use standard values for pulse widths without the need to calibrate for each sample. 

As was discussed earlier in the chapter, probes typically include two coils H and X nucleus (e.g., or N). The inner (or observation) coil is more sensitive and requires more careful adjustment. Normal NMR spectra usually have such good signal-to-noise ratios that probe tuning is not critical for relatively concentrated samples. Tuning, however, is very important for both one- and two-dimensional X-nucleus-detected experiments and for many two-dimensional H-detected techniques. A surprising number of these experiments have failed simply because the X coil was not tuned to the correct nucleus In addition, if X-nucleus detection is to be conducted with proton broadband decoupling, as is usually the case (Section 1-5), then it is important that the H decoupling coil also be tuned optimally. 

The absolute measurement of areas is not usually usefiil, because tlie sensitivity of the spectrometer depends on factors such as temperature, pulse length, amplifier settings and the exact tuning of the coil used to detect resonance. Peak intensities are also less usefiil, because linewidths vary, and because the resonance from a given chemical type of atom will often be split into a pattern called a multiplet. However, the relative overall areas of the peaks or multiplets still obey the simple rule given above, if appropriate conditions are met. Most samples have several chemically distinct types of (for example) hydrogen atoms within the molecules under study, so that a simple inspection of the number of peaks/multiplets and of their relative areas can help to identify the molecules, even in cases where no usefid infonnation is available from shifts or couplings. 

The heart of an NMR spectrometer is the probe, which is essentially a tuned resonant circuit with the sample contained within the main inductance (the NMR coil) of that circuit. Usually a parallel tuned circuit is used with a resonant frequency of coq = The resonant frequency is obviously the most important probe... 

Before sample preparation, the laboratory must demonstrate that the mass spectrometer is operating satisfactorily. First, the instrument must be tuned by calibration using one of two compounds. 

The MPC control problem illustrated in Eqs. (8-66) to (8-71) contains a variety of design parameters model horizon N, prediction horizon p, control horizon m, weighting factors Wj, move suppression factor 6, the constraint limits Bj, Q, and Dj, and the sampling period At. Some of these parameters can be used to tune the MPC strategy, notably the move suppression faclor 6, but details remain largely proprietary. One commercial controller, Honeywell s RMPCT (Robust Multivariable Predictive Control Technology), provides default tuning parameters based on the dynamic process model and the model uncertainty. 

Miniaturisation of various devices and systems has become a popular trend in many areas of modern nanotechnology such as microelectronics, optics, etc. In particular, this is very important in creating chemical or electrochemical sensors where the amount of sample required for the analysis is a critical parameter and must be minimized. In this work we will focus on a micrometric channel flow system. We will call such miniaturised flow cells microfluidic systems , i.e. cells with one or more dimensions being of the order of a few microns. Such microfluidic channels have kinetic and analytical properties which can be finely tuned as a function of the hydrodynamic flow. However, presently, there is no simple and direct method to monitor the corresponding flows in. situ. 

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