Anti aliasing filter

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	Anti-aliasing filter - Wikipedia, the free encyclopedia
		An anti-aliasing filter is commonly used in conjuction with digital signal processing and is a filter to restrict the bandwidth to approximately satisfy the Shannon-Nyquist sampling theorem.
		Since anti-aliasing filters are analog, oversampling allows for the filter to be cheaper because the requirements are not as stringent.
		The purpose in oversampling is to relax the requirements on the anti-aliasing filter or to further reduce the aliasing.
	en.wikipedia.org /wiki/Anti-aliasing_filter (304 words)

	Filter Basics: Anti-Aliasing - Maxim/Dallas
		One advantage of an active filter is that for multi-order filters, the operation of the filter is less sensitive to the values of the external components, in particular, the 'Q' value of the filter.
		The key parameters that need observation are the amount of attenuation (or ripple) in the passband, the desired filter rolloff in the stopband, the steepness in the transition region and the phase relationship of the different frequencies as they pass through the filter (Figure 4a).
		When selecting a filter, the goal is to provide a cutoff frequency that removes unwanted signals from the ADC input or at least attenuates them to the point that they will not adversely affect the circuit.
	www.maxim-ic.com /appnotes.cfm/appnote_number/928 (2492 words)

	Anti-Aliasing Filter
		Operating at the higher sampling rate, the anti-aliasing filter is a low-pass filter with a passband from 0 to 4000 Hz (the folding frequency).
		One way of avoiding the problem of aliasing is to apply a low-pass filter to the signal, prior to the sampling stage, to remove any frequency components above the "folding" or Nyquist frequency (half the sampling frequency).
		The output from the filter is "downsampled" by a factor of six, simply by throwing away five samples in every six, giving a stream of sampled values effectively at the lower sampling rate of 8000 Hz.
	www.dsptutor.freeuk.com /analyser/aaf.html (369 words)

	Anti-aliasing filter circuit for oscilloscopes - Patent 4621217
		said digital transversal filter being responsive to a particular selected slower sweep rate for setting the parameters of said digital transversal filter to provide a diminshed pass band as said sweep rate and sampling rate are reduced in order to prevent aliasing in the presentation of an input signal waveform by said oscilloscope.
		Thus, the upper frequency of the pass band of filter 17 is approximately 100 megahertz, the upper frequency of filter 18 is approximately 50 megahertz, the upper frequency of filter 19 is approximately 25 megahertz and the upper frequency of filter 20 is approximately 12.5 megahertz.
		Filters 17-19 are selectively coupled by means of further switches 14e-g to first sample and hold circuit 24, the output of which is applied to analog to digital converter or digitizing circuit 26 via the upper contact of selection switch 14n.
	www.freepatentsonline.com /4621217.html (3139 words)

	Anti-aliasing in Fractal Domains
		These artifacts can be minimized by sampling the continuous image at a higher resolution than the final image desired, then applying an anti-aliasing filter that removes the high-frequency components that produce the artifacts before sampling down to the desired resolution.
		Aliasing occurs when a discrete grid (such as the pixel resolution of your display or printer) is superimposed on a continuous image which contains sharp edges or fine detail.
		The "jaggies" you see when a straight line or images with sharp discontinuities are displayed on a computer monitor are examples of artifacts caused by aliasing.
	www.fractaldomains.com /anti-alias/anti-alias.html (386 words)

	Anti-Aliasing Filter
		The low pass filter in the input circuitry of digital signal processing equipment such as FFT analyzers that eliminates all signal components higher in frequency than one-half the sampling frequency.
	www.dliengineering.com /vibman/gloss_antialiasingfilter1.htm (30 words)

	Linear Systems Image of the Future - Digital Imaging Digital Photography D.I.M.S
		When this same image was photographed with the anti-aliasing filter installed, the aliasing was removedbut the overall image was less sharp.
		It's important to understand that when the anti-aliasing filter is removed, an infrared (IR) cut-off filter must be installed in its place.
		These are an example of an image that was Photographed with and without the anti-aliasing filter installed.
	www.linear-systems.com /support/anti_aliasing.htm (890 words)

	Sensors Magazine Online - July 1999 - Demystifying Piezoresistive Pressure Sensors
		As with the anti-aliasing filter, the cutoff frequency of the decimation filter is selected to some value larger than the highest frequency of interest and lower than half the final sampling rate.
		It may also include an analog anti-aliasing filter that limits the bandwidth of its output to a bandwidth of interest for the signal.
		Filters with a sharp cutoff at an exact frequency require a large number of operational amplifiers and highly accurate resistors and capacitors.
	www.sensorsmag.com /articles/0799/55/main.shtml (2256 words)

	Canon 10D's anti-aliasing filter question, and AA filters in general
		One real-world constraint with anti-aliasing filters is that it is difficult to design filters that approach the ideal brick wall approach.
		Kodak used to supply a removable filter for their DCS cameras; this was apparently more aggressive than filters built in to other cameras, but could be removed for ultimate sharpness, at the risk of aliasing.
		Fortunately, however, the aliasing is most objectionable in the color components (at least for non-Foveon cameras), and with some insight into human perception and the 3-color sampling process, as well as perceptual limitations, can often be masked or filtered out by software.
	www.thehelparchive.com /new-2559869-77.html (1890 words)

	Sensors Magazine Online - February 2000-Get the Mostout of Your Data Recorder with the Right Anti-Aliasing Filter
		Filter specifications--especially frequency response, phase, and delay plots--are important for correct measurements.
		To analyze a filter's technical specifications and to determine the effect of a filter on a signal, manufacturers generally provide three plots: frequency response, phase vs. frequency, and group delay.
		Note that the Bessel filter is the only one that does not display ringing at the rising and falling edge, and the linear phase--although having a much smaller ringing amplitude--displays ringing at all four corners.
	www.sensorsmag.com /articles/0200/50/main.shtml (2048 words)

	Medusa Preamp Upgrade
		This filter is functionally the same as an anti-aliasing filter, but in this case it eliminates high-frequency "images" of the signal spectrum caused by the staircase jumps in reconstruction.
		Comparing the filtered and non-filtered spectral plots, it is evident that filtering greatly reduces the total amount of signal content immediately beyond the 0 to 10kHz frequency range of interest; this in turn greatly reduces the sampling frequency required to avoid aliasing.
		This filter might be a separate analog filter or built in to the device (as with sigma-delta A/D and D/A converters).
	www.tdt.com /Upgrade_Path/Preamp_Upgrades.htm (853 words)

	Anti-aliasing
		See the articles on signal processing and aliasing for more information about the theoretical justifications for anti-aliasing; the remainder of this article is dedicated to anti-aliasing methods in computer graphics.
		Anti-aliasing in digital signal processing is the technique of minimizing aliasing when representing a high-resolution signal at a lower resolution.
		However, in many instances, the filtering should not be uniform in each direction (it should be anisotropic, as opposed to isotropic), and a compromise resolution is used.
	www.brainyencyclopedia.com /encyclopedia/a/an/anti_aliasing.html (1777 words)

	Illustrations
		Anti-Aliasing filter: Typically a low-pass filter that is applied before sampling to ensure that no components with frequencies greater than half the sample frequency remain.
		To remove the extra components we apply an ideal analog low-pass filter as shown in figure 5 As we see the ideal filter is rectangular in the frequency domain.
		The consequence of aliasing is that we cannot recover the original signal, so aliasing has to be avoided.
	cnx.rice.edu /content/m11443/latest (1029 words)

	EE448 DSP - LAB 2
		The anti-aliasing filter is an analog filter which is designed to limit the frequency range of the input signal prior to sampling.
		The bandwidth or cutoff frequecy of the input filter is set at 3600 Hz for the switched-capacitor filter frequency (SCF) of 288 kHz.
		The input bandpass filter is bypassable and can be programmed for a desired cutoff frequency or bandwidth based on the sampling frequency.
	cil.ece.uic.edu /~dliu/DSP/Sampling.html (1533 words)

	Anti-aliasing filter
		Hence, the purpose of the anti-aliasing filter is to filter out any high frequency noise which may be introduced by external sources, and which would get aliased into the 0-25kHz band.
		The anti-aliasing filter is included to remove any frequencies above 20kHz from the signal, thus preventing them from being aliased upon sampling (aliasing gives rise to frequencies above the 25kHz Nyquist frequency being transposed down in to the 0-25kHz band).
		Spectra of the input pulse and the stepped tube/coupler reflections, along with the resultant input impulse response, measured without the filter in place (figures 3.15, 3.16 and 3.17) are virtually identical to the spectra and the input impulse response measured with the filter in place (figures 3.11, 3.12 and 3.14).
	acoustics.open.ac.uk /david/node31.html (286 words)

	[music-dsp] Anti-Aliasing Filter With FFT/iFFT
		The antialiasing filter is a function of the > resampling ratio, which in turn depends on sample rate and the > playback frequency of your sample data.
		I don't understand why the filter is a function of the resampling ratio.
		Can't you resample at an oversampled rate with whatever interpolation you want to use (linear works fine for a high oversampling ratio), brick wall filter this oversampled, resampled data, and then drop the extra samples to get back to the host samplerate.
	shoko.calarts.edu /pipermail/music-dsp/2004-September/028223.html (162 words)

	Nyquist-Shannon sampling theorem
		The anti-aliasing filter is to restrict the bandwidth of the signal to satisfy the sampling condition.
		The theorem is satisfied when downsampling by filtering the signal appropriately with an anti-aliasing filter.
		Note that if the FM radio band is sampled at >40 MHz then a band-pass filter is required for the anti-aliasing filter.
	www.worldhistory.com /wiki/N/Nyquist-Shannon-sampling-theorem.htm (1418 words)

	447lab11.doc
		For a 1st-order filter with a break frequency of 32 Hz, calculate the peak-to-peak voltage output when the input is a 100 Hz signal with an amplitude of 1 volt peak-to-peak.
		Adjust the sample period in the vicinity of 100 Hz until the period of oscillation of the motor shaft due to aliasing is between approximately 1 Hz and 3 Hz.
		Problems with aliasing can result from the presence of noise in both commands and feedback; here, a constant command upon which a noise signal is superimposed will be sampled using an A/D converter.
	mechatronics.me.wisc.edu /me447/447lab11.doc (600 words)

	Aliasing
		In order to avoid aliasing artefacts, a suitable low-pass filter as the one shown above should be used to only keep the blue part of the spectrum of the resampled signal (the red parts are meerly repetitions of the blue part).
		The aliasing is visible on the graphs as a "warping" of the frequenties against a "mirror" at the Nyquist frequency (1kHz).
		This is due to the aliasing of the frequencies that were higher than the Nyquist frequency (half the sampling frequency).
	www.sic.rma.ac.be /~xne/el401/aliasing (488 words)

	Digital anti-aliasing filter - Patent 4970637
		This maximum filter serves as an LPF (anti-aliasing filter) having a cutoff frequency of fb/2.
		The filter width of maximum filter 3 is determined in accordance with fa/fb (fb is the frequency of a clock pulse supplied to sample circuit 4, and fa.gtoreq.fb).
		Thus, it is understood that the maximum filter exhibits the characteristics of an LPF having frequency fN as a cutoff frequency.
	www.freepatentsonline.com /4970637.html (3497 words)

	Anti-aliasing filter to soften edgeline roughness and jagged edges
		Filter edge-line roughness and pixelation (this is known as "anti-aliasing").
		The anti-aliasing filter plugin is one of a kind for filtering edge-line roughness.
		The amount of pixels - and which - selected for filtering is determined by the plug ins second group of filter-controls "Jaggie definition".
	www.powerretouche.com /Antialias_plugin_tutorial.htm (1713 words)

	Sweetwater inSync
		An anti-aliasing filter is needed whenever analog signals are sampled, or when a digital signal is sample rate converted from a high sample rate to a lower sampling rate.
		Ideally an anti-aliasing filter will be a brick-wall filter, allowing everything under the Nyquist frequency to pass while filtering everything over the Nyquist frequency.
		The filter used in an analog to digital converter that is designed to filter all components of the signal that are above the Nyquist frequency in order to prevent aliasing from occurring at the digital sampling stage.
	www.sweetwater.com /insync/word.php?find=AntiAliasingFilter (219 words)

	totalpower.txt
		Science IPT may prefer a smaller number of parameters, but that might result in a less satisfactory implementation that limits the potential science." There is a requirement for the phase response of the anti-aliasing filter to be fairly linear over the range of frequencies of interest.
		If this Bessel filter, with F(3dB)=3.74 kHz, is sampled at 18.3 kHz, then noise products aliasing back within the passband of interest (0-663 Hz) are at 1% or less.
		(With this filter, even at the -6 dB point, the departure from linear phase/frequency response is less than 3 degrees, although it deteriorates rapidly beyond that point.) The scientific requirement is only that the departure from linearity is less than 13 degrees at the highest frequency of interest (663 Hz), which is easily met.
	www.cv.nrao.edu /~awootten/mmaimcal/totalpower.txt (855 words)

	ElectronicsWeekly.com - Anti-aliasing filter combines functions
		The FMS6407 is designed for anti-aliasing, removing high frequency noise prior to digitising the signal, or for reconstruction filtering, removing artefacts introduced during D/A conversion in the encoder in LCD TV, set-top box, DVD and HD/Blu-ray applications.
		These filters can be bypassed allowing the output amplifiers to accommodate higher bandwidth (80MHz) PC applications.
		Fairchild Semiconductor’s video filter/driver offer the combined functionality of high definition, progressive scan and standard definition filters in one chip.
	www.electronicsweekly.co.uk /Articles/2004/09/03/32945/Anti-aliasingfiltercombinesfunctions.htm (230 words)

	Electronic Music Mailing List Archives: pd-list - Re: [PD] Anti-aliasing filter
		I programmed a dual similar filter for adding the low > partials of a sound to the high partials of another sound (the limit is as > vertical as possible).
		> > not just frequencies in the audible range) > > You can make a 'brick' filter using an FFT transform, but the sound will be > more or less altered (depending on the window size and the overlap you use > for the FFT).
		Once aliased frequencies are introduced, the game is over, because the frequencies have already been aliased.
	elists.resynthesize.com /pd-list/2004/05/668671 (348 words)

	KODAK: Cleaning Imager Coverglass and Filters
		Determine if the anti-aliasing filter needs to be cleaned.
		When reinstalling the filter on the camera, alternate the tightening sequence so that each screw is tightened a little at a time until both screws are snug.
		When the sensor, filters, and other delicate mechanisms are exposed to tools, they are at risk of being damaged.
	www.kodak.com /global/en/service/professional/tib/tib4232.jhtml (1195 words)

	[music-dsp] Anti-Aliasing Filter With FFT/iFFT
		And I don't resample after oversampling, I combine the oversampling and interpolation into one step, then filter, and downsample by dropping say 1/4 of the samples for 4x oversampling for example.
		Of course I can optimize the filter to not generate the 3/4 of samples left when doing 4x oversampling.
		I think I'll resample directly as you suggested as being the most common method, as I did a test with FFT and found it slow, although the plot of the oversampled/filtered/downsampled square wave looked really nice and bandlimited, but the result should be identical with straight windowed sinc FIR filtering (and hopefully faster).
	shoko.calarts.edu /pipermail/music-dsp/2004-September/028247.html (311 words)

	Elimination of Aliasing: Anti-Aliasing Filter
		Anti-aliasing filter: lowpass filter of bandwidth B Hz (B: effective bandwidth)
		Aliasing can be eliminated by bandlimiting a signal before sampling
		Non-bandlimited signal: Alias results regardless of sampling rate
	www.ics.uci.edu /~rgupta/iec/hardware-sp/sld015.htm (29 words)

	000list
		// ------------------------------------------------------------------------------ File: noise/m109.data.Z M109 tank noise sampling rate: 19.98 KHz A/D: 16 bit pre-filter: anti-aliasing filter pre-emphasis: none filter: none duration: 235 sec length (uncompressed): approx 24 Mb M109 noise acquired by recording samples from 1/2" B&K condensor microphone onto digital audio tape (DAT).
		------------------------------------------------------------------------------ File: noise/babble.data.Z Voice Babble sampling rate: 19.98 KHz A/D: 16 bit pre-filter: anti-aliasing filter pre-emphasis: none filter: none duration: 235 sec length (uncompressed): approx 24 Mb Voice Babble acquired by recording samples from 1/2" B&K condensor microphone onto digital audio tape (DAT).
		The noise was recorded at the co-pilot's seat in a two-seat F-16, traveling at a speed of 500 knots, and an altitude of 300-600 feet.
	spib.rice.edu /spib/data/signals/noise/000list (817 words)

	anti-aliasing
		This is a filter which is used to prepare an analogue signal for conversion to digital.
		It is designed to filter out frequencies which are greater than 1/2 the sampling rate, i.e.
		those frequencies which according to the Nyquist theorem will cause aliasing.
	www.sfu.ca /sca/Manuals/ZAAPf/a/anti_aliasing.html (43 words)

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