Subtractive noise

SPC techniques are hardly affected by additive noise and multiplicative noise is absent. However, subtractive noise due to the collection efficiency and transmission of optics and the quantum efficiency of the detector do play a role. In addition, at high count rates, the efficiency goes down due to pileup effects. 

E represents the combined collection and detection efficiency of the system and F the intrinsic photon-economy of the technique. The factor 77 accounts for the subtractive noise, tA is the dead-time of the detector and Q the count rate of the system. 

Block diagram of a spectral-subtraction noise-reduction system. 265... 

The traditional method of noise subtraction has been revived and used very effectively for ion spectroscopy.°9 The laser beam is split into two parts, one for spectroscopy and the other for reference and detected by two matched infrared detectors. The signals from the two infrared detectors are elctronically combined with opposite phase to subtract noise. This method is particularly effective for a color center laser and a difference frequency laser system where the... 

It is obvious from (47), as well as from the equation (12), that the relations (46) describe a transfer channel 1C with negative values of noise added, and thus with the subtractive noise H Y X) < 0, which is caused by the draining off of the heat AQo t (—AQqi < 0) from the medium . With regards to the information loss H(X Y) from the transferred information H X) in any input message x AQ v within the reversible part O) of the irreversible O ), the heat AQox is that noise which causes another (average) information loss H(Y X) [the sign - in (47)] in O". 

This necessitates plant capable of control of temperature by being able to add or subtract heat from the air and control of humidity by being able to add or subtract moisture. The system also comprises fan(s), filtration, and a distribution system and may include noise control. Other terms such as cooling or comfort cooling may be met and these can be taken to mean an ability to lower the temperature of the air by refrigeration but without full control of humidity. Moisture may be removed as an incidental characteristic of the cooling coil. The term air conditioning is sometimes used where control of humidity is not included. It is essential to employ clear specifications of performance. 

Reactant and product analyses were obtained from the intensities of infrared absorption bands by successive subtraction of absorptions by known species. Low noise reference spectra for UDMH and several reaction products were generated for this purpose in order to minimize the increase in the noise level of the residual spectrum with each stage of subtraction. 

For infrared spectroscopy, 20-50 mg of the cobalt-exchanged zeolite was pressed into a self-supporting wafer and placed into an infrared cell similar to that described by Joly et al. [21], Spectra were recorded on a Digilab FTS-50 Fourier-transform infrared spectrometer at a resolution of 4 cm-i. Typically, 64 or 256 scans were coadded to obtain a good signal-to-noise ratio. A reference spectrum of Co-ZSM-5 in He taken at the same temperature was subtracted from each spectrum. 

We also have noted before that adding or subtracting noisy data causes the variance to increase as the number of data points added together [2], The noise of the first derivative, therefore, will be larger than that of the underlying absorbance band by a factor of the square root of two. 

Transmission infrared spectra of species adsorbed on the catalyst were taken with a Digilab FTS-10M Fourier-transform infrared spectrometer, using a resolution of 4 cm-l. To improve the signal-to-noise ratio, between 10 and 100 interferograms were co-added. Spectra of the catalyst taken following reduction in H2 were subtracted from spectra taken in the presence of NO to eliminate the spectrum of the support. Because of the very short optical path through the gas in the reactor and the low NO partial pressures used in these studies, the spectrum of gas-phase NO was extremely weak and did not interfere with the observation of the spectrum of adsorbed species. 

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