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先引用一段话
It's not a secret that the Internet and file-exchanging networks are literally overloaded with music. The plenty of various formats which compress music files just favours this situation. Files in formats with lossy encoding and the highest degree of compression, like MP3, have got the greatest prevalence. While playing such files using cheap acoustic systems one can hardly distinguish their dissimilarity from the original ones, but all the distortions become evident when listening such disks on a Hi-Fi systems or using stereo headphones, especially if one has an opportunity to compare a record with its original. That is why when purchasing traditional audio CDs with music a question naturally arises whether this or that record is an authentic one or just a fake made by somebody who has just downloaded lossy encoded records from the Internet and recorded them to the blank CDs under the guise of a solid firm.
Here is an example collection of audio CDs, bought during 2000-2004 years in Russian stores. As everyone can see, approximately 25% of them are so called fake audio CD, produced from lossy audio source, usially from low quality MPEGs. Tau Analyzer can help you to detect such CDs.
To detect these CDs we should found the MPEG-algorithm artefacts in audio data it contains.. The most useful and simple artefact is frequency cut-off - signal is missed or hardly deformed at frequencies about 18 kHz. The frequency cut-off (produced by ear-model of the sound, used by the MPEG algorithm) usially is pretty sharp at one or more high frequencies (16-20kHz). Music, produced from such MP3 has less distinct sound (for example, drums sounds, and other sharp sounds).
Other artefacts of MPEG-coding add specific type of noise, corresponding to the MPEG-coding errors (numerical noise), Fourier transformations artefacts, and decreased correlation between channels (so called sound center fluctuations).
Some studio companies, for example when making live records from concertos removes live noise very inaccurately, by simple cutting-off all the frequencies above 16-18 kHz using digital filtration. So the music produced have some same sound artefacts, as a popular MPEG algorithms.
Usually, MPEG-coding errors could be decreased using so-called dithering technique (it is a kind of smart smoothing the music). One could make sound more distinct by adding additional noise and smoothing the signal spectrum, the technique is known as noise-shaping. All these techniques does not allows you to listen original music, all you listen is music of less quality with some processing and artefacts old (MPEG) and new (different sound quality improving techniques, described above). All these problems could be detected without special tools if you have original and such a copy of the same record.
Please note: Tau Analyzer does not provide an absolutely warranty in results of detection. Sometimes the program can show a wrong result for the track (please see Testing Results). Thus, to make sure you should check all of the CD contents. In this case, the program can produce more trustable CD conclusion, except some special cases. |
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发表于 2006-12-18 14:32
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