This has already been posted to HN before, but its the best explanation of digital audio I have seen: http://xiph.org/video/vid2.shtml
http://xiph.org/video/ is probably a better link, the first episode has plenty of interesting and important stuff.
Explanation of 44.1 kHz CD sampling rate
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Re: Explanation of 44.1 kHz CD sampling rate
#12This confuses me because the vertical rate is not 60 Hz, it's 3579545 Hz / (525/2 * 455/2) = 59.94 Hz. In other words it's odd that they would have chosen to be compatible with black and white instead of color.. instead of 44.1 KHz, it would be 44.056 KHz. Edit: Well it turns out that 44.056 KHz was used for the "EIAJ digital-audio-on-videotape standard" http://recordingtheworld.infopop.cc/eve/forums/a/tpc/f/22260...
I believe all NTSC equipment is required to support Black&White System M signals, which are exactly 60Hz[1]. It probably made their equipment much simpler to forget about colour encoding entirely. (And it made the 44,100Hz fit too)
Re: Explanation of 44.1 kHz CD sampling rate
#13Also, the fact that 44,100 can be factored as 2^2 * 3^2 * 5^2 * 7^2 makes it very efficient to do Fast Fourrier Transforms on moving windows. Which we do a lot in Speech Recognition and DSP in general.
Actually 44.1kHz is unhelpful for that... a lot of infrastructure exists that expects things to work on 10 and 20ms intervals (and multiples there-of), e.g. the normal windows audio apis work in 10ms chunks.
10ms of 44.1kHz is 441 samples, which is odd and thus a not really fantastic size for easily implemented factorizations.
Of course, if you can pick your own analysis interval you can pick some nice power of two or other very smooth number. But thats true regardless of the sampling rate.
Re: Explanation of 44.1 kHz CD sampling rate
#14Re: Explanation of 44.1 kHz CD sampling rate
#15Also, the fact that 44,100 can be factored as 2^2 * 3^2 * 5^2 * 7^2 makes it very efficient to do Fast Fourrier Transforms on moving windows. Which we do a lot in Speech Recognition and DSP in general.
Actually 44.1kHz is unhelpful for that... a lot of infrastructure exists that expects things to work on 10 and 20ms intervals (and multiples there-of), e.g. the normal windows audio apis work in 10ms chunks. 10ms of 44.1kHz is 441 samples, which is odd and thus a not really fantastic size for easily implemented factorizations. Of course, if you can pick your own analysis interval you can pick some nice power of two o…
Almost all ASR engines use 25ms windows, not 10ms (they typically overlap and are processed every 10ms).
Re: Explanation of 44.1 kHz CD sampling rate
#16Also, the fact that 44,100 can be factored as 2^2 * 3^2 * 5^2 * 7^2 makes it very efficient to do Fast Fourrier Transforms on moving windows. Which we do a lot in Speech Recognition and DSP in general.
What you say actually makes no sense whatsoever. What is forcing you to consider 1 second windows? Also, 48kHz is 2^7 * 3 * 5^3 so it actually avoids the larger prime factor of 7. Not that anyone cares: just use an actual power of two as your window size to begin with!
What you say actually makes no sense whatsoever. Why are you talking about 1 second windows? If 44100 can be decomposed easily in small prime numbers, it means that you can split 44100 into windows that are themselves products of small prime numbers.
Re: Explanation of 44.1 kHz CD sampling rate
#17It seems strange to me that they would be re-purposing video tape for digital audio, given that digital tape technology was used in computers since UNIVAC in the 1950's [1]. [1] http://en.wikipedia.org/wiki/Tape_storage
The key innovation is 'helical scan': http://en.wikipedia.org/wiki/Helical_scan This came along for video applications. Up until then the idea of tapes for audio or data was to put as many linear tracks on them as possible. With video some new thinking was needed, hence the helical scan. As it turned out helical scan was the future for data and audio storage too.
Quad recording [1] preceded helical. In Quad, the mag stripes are across the tape almost perpendicular to the tape's movement. The head spun at an insane speed -- 14,400 RPM in open air. Helical was originally invented as a lower-cost recorder. Once helical gained the ability to do slow-mo and still-frame, Quad died.
Re: Explanation of 44.1 kHz CD sampling rate
#18This confuses me because the vertical rate is not 60 Hz, it's 3579545 Hz / (525/2 * 455/2) = 59.94 Hz. In other words it's odd that they would have chosen to be compatible with black and white instead of color.. instead of 44.1 KHz, it would be 44.056 KHz. Edit: Well it turns out that 44.056 KHz was used for the "EIAJ digital-audio-on-videotape standard" http://recordingtheworld.infopop.cc/eve/forums/a/tpc/f/22260...
The original American TV standard and TV recorders were 60Hz. When color was introduced, the frequency was shifted to 59.94 to avoid interference between the color signal and the sound signal.
(Color was encoded as a high-frequency sine-wave on top of the black-and-white signal which is mostly invisible on a black-and-white set (which allows for backwards compatibility). The phase of the fuzz indicates hue, and the amplitude indicates color intensity. This is why in the old system, if someone wore a shirt with vertical stripes on TV, viewers would see a rainbow of color over the shirt.)
Re: Explanation of 44.1 kHz CD sampling rate
#19Earlier quoted context omitted.
What you say actually makes no sense whatsoever. What is forcing you to consider 1 second windows? Also, 48kHz is 2^7 * 3 * 5^3 so it actually avoids the larger prime factor of 7. Not that anyone cares: just use an actual power of two as your window size to begin with!
What you say actually makes no sense whatsoever. Why are you talking about 1 second windows? If 44100 can be decomposed easily in small prime numbers, it means that you can split 44100 into windows that are themselves products of small prime numbers.
The length of a second is arbitrary and irrelevant to audio processing. The fact that there are 44100 samples in an arbitrary length of time is meaningless. You would only care about the prime factorization of 44100 if the second was some special amount of time, which it is not.
Re: Explanation of 44.1 kHz CD sampling rate
#20Earlier quoted context omitted.
What you say actually makes no sense whatsoever. What is forcing you to consider 1 second windows? Also, 48kHz is 2^7 * 3 * 5^3 so it actually avoids the larger prime factor of 7. Not that anyone cares: just use an actual power of two as your window size to begin with!
What you say actually makes no sense whatsoever. Why are you talking about 1 second windows? If 44100 can be decomposed easily in small prime numbers, it means that you can split 44100 into windows that are themselves products of small prime numbers.
Why are you splitting 44100 samples into windows? That's the number of samples in one second. Where did you get one second from? You could have started from any duration.