In theory an 44.1 KHz file, whether it came from a CD or a digital download, can contain audio from 0 Hz (DC) 22.05 KHz (to one half the sample rate).
For the low limit: At one point, the usual policy when mastering CDs was to feed everything through a digital high-pass filter to remove any DC component, because it could cause mistracking with certain players. I don't think they worry about it anymore. I've certainly seen (looking at CD rips with Cool Edit 2000) several with substantial DC offsets, so the capabilities of CDs certainly go all the way down to 0 Hz. These days, a substantial DC offset is generally viewed as bad, and most good AtoD converters have fairly low DC offsets. Back at the beginning of digital audio, the AtoD's DC offset may have actually made an improvement, because it eliminated low level crossover distortion, offsetting it to a bit higher audio level, but today's converters have much lower crossover distortotion, so DC offset is currently frowned upon. Certainly it's a problem for "hard" digital edits; no matter where one puts the edit, there will be a slight click due to the change in DC offset. Just about all modern editing programs do a fast crossfade to eliminate this problem. The high frequency "limit" is the result of the anti-alias filters in the recorder and playback device. Since a practical filter doesn't have an infinitely steep cut-off, the "maximum" high frequency will be somewhat less than 22.05 KHz, depending on the steepness of the filter's "skirt". CD players and other devices which play 44.1 KHz audio files usually use "brickwall" filters with a very steep slope so that the passband can extend up over 20 KHa. These days, such filters are almost always implemented by a mathematical function that resamples the audio to a much higher sampling rate where an analog filter with a much shallower "skirt" can be used. The point to realize is that the CD is just a data carrier, so it's largely unimportant whether the audio data has "seen a CD" or not; it should still be the same data, bit-for-bit, and will produce exactly the same audio. I say "largely unimportant"; uncorrected read errors from the CD may cause audible differences. That is why a sophisticated program such as Exact Audio Copy is needed so that the data is read correctly from the disc. But programs such as AccurateRip can attest to the fact that, in most cases, Exact Audio Copy is able to extract bit-accurate data from an audio CD. Bottom line, a file properly extracted from a CD will be identical, bit-for-bit, with the original file that was used to make that CD, so merely recording audio on a CD makes no difference in the audio. -- Timothy Stockman ------------------------------------------------------------------------ Timothy Stockman's Profile: http://forums.slimdevices.com/member.php?userid=8867 View this thread: http://forums.slimdevices.com/showthread.php?t=41668 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/lists/listinfo/audiophiles
