Filter banks

Maintenance "indicators" are available to help facility staff determine when routine maintenance is required. For example, air filters are often neglected (sometimes due to reasons such as difficult access) and fail to receive maintenance at proper intervals. Installation of an inexpensive manometer, an instrument used to monitor the pressure loss across a filter bank, can give an immediate indication of filter condition without having to open the unit to visually observe the actual filter. Computerized systems are available that can prompt staff to carry out maintenance activities at the proper intervals. Some of these programs can be connected to building equipment so that a signal is transmitted to staff if a piece of equipment malfunctions. Individual areas can be monitored for temperature, air movement, humidity, and carbon dioxide, and new sensors are constantly entering the market. 

The filter banks incorporated require adequate access for cleaning and replacing. 

Approach velocity The velocity of airflow into a filter bank or heat exchanger. 

Filter life varies with make and type, and may be limited by the ability of the fan to operate against pressure drop. It is a function of the dirtiness of the air and the amount of material packed into the filter bank. Life may be quoted in terms of dust held in g/m face area. For fan selection knowledge of pressure drop is required. Typically, a panel filter might be quoted as operating from, say, 75 Pa initially at 2.5 m/s face velocity to 250 Pa when loaded and a HEPA might operate up to 700 Pa. The pressure drop... 

Very fine particles such as smokes can be caught by electrostatic precipitation. A high voltage is applied to plates or wires within the filter bank, to impart a static charge to dirt particles. These will then be attracted to earthed plates, and adhere to them. Impurities are generally cleaned off the plates by removing the stack and washing. 

The carbon analyzer has been used to analyze filters from 42 urban sites and 22 non-urban sites in the United States. These filters were obtained from the National Air Surveillance Network (NASN) filter bank for 1975. Carbon concentrations and mass fractions for Detroit, Michigan, are shown in Figures 5 and 6. Both the organic and elemental carbon concentrations are highly variable, and no seasonal trends are apparent. For this site elemental carbon constituted 38% of total aerosol carbon. Typical values for other sites ranged between 35 and 55%. 

Microwave. Millimetre and sub-millimetre wave radiometry have been very successfully used from a variety of platforms to measure stratospheric gases. By using microwave local oscillators (LOs) and filter banks, or appropriate spectrometers, the observed brightness temperatures can be measured at various frequencies or across an emission line from a particular molecule. 

A filter bank is used to decompose the input signal into subsampled spectral components (time/frequency domain). Together with the corresponding filter bank in the decoder it forms an analysis/synthesis system. 

Using either the time domain input signal or the output of the analysis filter bank, an estimate of the actual (time dependent) masked threshold is computed using rules known from psychoacoustics. This is called the perceptual model of the perceptual encoding system. 

In practice, the calculation of the PE requires an analysis filter bank and a perceptual model. The PE is defined as... 

The filter bank is the deciding factor for the basic structure of a perceptual coding system. Figure 2.6 shows the basic block diagram of an static n-channel analysis/synthesis filter bank with downsampling by k. If k = n, it is called a filter bank with critical sampling. A number of basic parameters can be used to describe filter banks used for audio coding ... 

Over the past years, two main types of filter banks have been used for high quality audio coding ... 

Low resolution filter banks (e.g. 32 subbands), normally combined with a quantization and coding module which works on blocks in time direction. These are frequently called subband coders. 

In an analysis/synthesis filter bank, all quantization errors on the spectral components show up on the time domain output signal as the modulated signal multiplied by the synthesis window. Consequently, the error is smeared in time over the length of the synthesis window / prototype filter. As described above, this may lead to audible errors if premasking is not ensured. This pre-echo effect (a somewhat misleading name, a better word would be pre-noise) can be avoided if the filter bank is not static, but switched between different frequency/time resolutions for different blocks of the overlap/add. An example of this technique called adaptive window switching is described below. 

The following section gives a short overview of filter banks which are currently used for audio coding purposes. 

QMF filter banks. Quadrature mirror filters (QMF, see [Esteban and Galand, 1977]) have often been proposed for audio coding. The most common configuration is the tree of filters with a two-way split. In one of the early examples [Theile et al., 1987] the 64d filter design from [Johnston, 1980] has been used. The decomposition tree is set up so that the filter bands resemble critical bands. The QMF halfband filters are non-perfect reconstruction, but with perfect alias cancellation by design. The reconstruction error of the analysis/synthesis pair can be held at small amplitudes by increasing the filter length. 

Figure 2.7 Window function of the MPEG-1 polyphase filter bank (Reprinted from [Sporer, 1998] 1998, courtesy of the author)...

Time domain aliasing cancellation based filter banks. The Modified Discrete Cosine Transform (MDCT) was first proposed in [Princen et al., 1987] as a sub-band/transform coding scheme using Time Domain Aliasing Cancellation (TDAC). It can be viewed as a dual to the QMF-approach doing frequency domain aliasing cancellation. The window is constructed in a way that satisfies the perfect reconstruction condition ... 

As a further advantage of MDCT-like filter banks it should be noted that the time domain aliasing property needs to be valid for each half of the window independently from the other. Thus hybrid window forms (with different types of window functions for the first or second half) can be used. This leads to the realization of adaptive window switching systems ([Edler, 1989], see below). 

The MDCT is known under the name Modulated Lapped Transform ([Malvar, 1990]) as well. Extensions using an overlap of more than a factor of two have been proposed [Vaupelt, 1991, Malvar, 1991] and used for coding of high quality audio [Vaupelt, 1991]. This type of filter banks can be described within the framework of cosine-modulated filter banks ([Koilpillai and Vaidyanathan, 1991][Ramstadt and T anem, 1991, Malvar, 1992]). 

In ISO/MPEG Layer 3, a different approach to hybrid coding has been used (see Figure 2.9. To ensure compatibility to Layers 1 and 2, the same polyphase filter bank is used as the first filter in the hybrid filter bank. Each of the 32 polyphase subbands is normally further subdivided into 18 frequency lines using an MDCT. By using the window switching technique described below the subdivision can be switched to 6... 

Adaptive filter banks. In the basic configuration, all filter banks described above feature a time/frequency decomposition which is constant over time. As mentioned above, there are possibilities to switch the characteristics of a filter bank, going from one time/frequency decomposition to another one. We explain the basic principle using the example of MPEG Audio Layer 3 ... 

To solve these tasks, perceptual models often work directly on the time domain input data thus allowing a time and/or frequency resolution of the model which is better than the time and/or frequency resolution of the main filter bank of the perceptual coding system. 

Filterbanks. There is still continued research on filter banks for high quality audio coding. Topics include wavelet based filter banks, low delay filter banks [Schuller, 1995] or variable filter banks allowing a higher degree of variability than classic window switching [Princen and Johnston, 1995],... 

---