Sound Recording Solutions
Sound Card Recorder
Powerful voice activated microphone recorder for Windows. Click here to learn more.Aliasing
Aliasing is a major concern in the analog-to-digital conversion of video and audio signals: improper sampling of the analog signal will cause high-frequency components to be aliased with genuine low-frequency ones, and be incorrectly reconstructed as such during the subsequent digital-to-analog conversion. To prevent this problem, the signals must be appropriately filtered before sampling. It is also a major concern in digital imaging and computer graphics, where it may give rise to moire patterns (when the original image is finely textured) or jagged outlines (when the original has sharp contrasting edges, e.g. screen fonts). Anti-aliasing techniques are used to reduce such artifacts. The sun moves east to west in the sky, with 24 hours between sunrises. If one were to take a picture of the sky every 23 hours, the sun would appear to move west to east, with 24 ? 23 = 552 hours between sunrises. Note that both motions would result in the same pictures. The same phenomenon causes spoked wheels to apparently turn at the wrong speed or in the wrong direction when filmed, or illuminated with a flashing light source - such as fluorescent lamp, a CRT, or a strobe light. These are examples of temporal aliasing. If someone wearing a tweed jacket with a pronounced herringbone pattern was videoed, and the video played on a TV screen with a smaller number of lines than the image of the pattern or on a computer monitor with pixels larger than the elements of the pattern, then one would see large areas of darkness and lightness over the image of the jacket and not the herringbone pattern. This is an example of spatial aliasing, also known as a moire pattern; how it is produced is illustrated next. In the same way, when one measures a sinusoidal signal at regular intervals, one may obtain the same sequence of samples that one would get from a sinusoid with lower frequency. Specifically, if a sinusoid of frequency f (in cycles per second for a time-varying signal, or in cycles per centimeter for space-varying signal) is sampled s times per second or s intervals per centimeter, with s ? 2 f, the resulting samples will also be compatible with a sinusoid of frequency 2 f - s. In the area's jargon, each sinusoid gets aliased to (becomes an alias for) the other. Therefore, if we sample at frequency s a continuous signal that may contain both sinusoids, we will not be able to reconstruct the original signal from the samples, because it is mathematically impossible to tell how much of each component we should take. One way to avoid such aliasing is to make sure that the signal does not contain any sinusoidal component with a frequency greater than s/2. More generally, it suffices to ensure that the signal is appropriately band-limited, namely that the difference between the frequencies of any two of its sinusoidal components must be strictly less than s/2. This condition is called the Nyquist criterion, and is equivalent to saying that the sampling frequency (s) must be strictly greater than twice the signal's bandwidth, the difference between the maximum and minimum frequencies of its sinusoidal components. The term "aliasing" derives from the usage in radio engineering, where a radio signal could be picked up at two different positions on the radio dial in a superheterodyne radio: one where the local oscillator was above the radio frequency, and one where it was below. This is analogous to the frequency-space "wrapround" that is one way of understanding aliasing.
Phone Call Recorder
Must have software for voice modem. Record all phone calls automatically, watch Caller ID information, create you own powerful answering machine. Perfect sound quality. Click here to learn more.3d audio effect - 3gp - a-law - aac - acm - adc - aiff - aliasing - amplifier - amr-wb - amr-wb plus - amr - apev2 tag - asf - atrac - audio codec - audio compression - avi - bitrate - bitrate peeling - chord - codec - comfort noise - compact audio cassette - compact disc - compression - compression artifact - compression ratio - contact microphone - container format - dab - data compression - digital audio - digital camera - divx - dolby digital - dolby digital plus - dsp - dvd-audio - dvd - effects unit - equalization - ffmpeg - flanging - fourcc - frequency spectrum - hi-fi - high-end audio - hiln - id3 - joint stereo - laser microphone - line level - lossy - loudspeaker - matroska - mcf - microphone - midi - mixing console - mp2 - mp3 - mp3 surround - mp3 sx - mp3pro - mp4 - mpeg-1 - mpeg-21 - mpeg-3 - mpeg-4 - mpeg-7 - mpeg - mu-law - musepack - music - mxf - nut - osm - parabolic microphone - pcm - perception - phonograph - pink noise - pqf - psychoacoustics - qdesign - quadraphonic - radio receiver - ratdvd - realaudio - red noise - reverberation - rhythm - ribbon microphone - riff - rmvb - sbr - signal processing - sound - sound card - sound effects - sound recording - spdif - speech encoding - speech recognition - speex - stereo - subwoofer - surround sound - synthesizer - tape recorder - tdm - tweeter - video - video compression - vob - voice analysis - vorbis - wav - white noise - wma - woofer