Saturday, December 18, 2010

【 Weak current College 】 CD standard specifications 】

CD specifications are Sony and Philips to develop. Sound signal with frequency-discuss 44.1KHz, each sampling point 16bit quantization, and then to LPCM (linear Pulse Code Modulation) way to encode the digital signal, digital signal by moulding approach saved in a specially crafted on the disc, make CD plate. CD tablets piece generally used plastic, one of the surface layer for molding of signal, the signal level has one pressure out of points, they hang point represents 0, 1, two messages. Signal layer and then coated with a layer of very thin aluminum membrane (also with gilt), for reads the signal from the enhanced laser reflection. CD patch has two dimensions, one of the most common diameter 650MB 12cm, data storage capacity, approximately 74 minutes of music; the other is called the MiniCD, diameter 8cm, data capacity approximately 185MB, can store 20 minutes of music.

Sampling, sampling rate, quantum, quantum precision, the term mean?

Sampling is also called sampling is continuous analog with one discrete points. Clearly, the need for adequate intensive sampling points in order to properly express the characteristics of the original analog signal. The number of samples per second is called sampling rate, sampling rate CD 44.1KHz, represents the second sampling 44100 times. The so-called quantization, popular to say, is to measure the magnitude of sampling discrete signals after, of course, the measurement results are represented using a binary number. Quantization precision is is the amount of time a measure, like a ruler on the Division of the number, it is clear that the rating, the more the more accurate measurement results. CD of quantization precision as 16bit (16-bit binary number), conversion to decimal, the number is equal to 65536 classification. That is, to a standard CD, you can tell 1/65536 level range changes. The problem comes if the signal amplitude variation ratio 1/65536 level also small? the answer is simple: not out of the results, like fine to 1 mm ruler to measure the diameter of the hair. Not the result would have been no data, restore to analog signals in the future it will form a background noise, the terminology called quantization noise. Quantization noise in digital sound source-to-noise ratio of the main constraints for CD specifications, assuming that the strongest signal as a unit, the size is 1 noise/65536 units, so that the SNR 65536 96dB.

CD specifications for what 16bit/44.1KHz? why not other number?

First sampling rate to 44.1KHz by, which is based on the famous "is the sampling theorem" Malmquist findings. "Is the sampling theorem" Rehnquist said: in the process of digitizing analog signals, sampling frequency greater than if that simulate signals highest frequency 2 x, we can 100% accurately and then restore the original analog signal. Audio of the highest frequency to 20KHz, sampling rates should be greater than at least, in order to stay a little 40KHz factor of safety, taking into account the fact that the habit of CD standard Xu finally chose 44.1KHz this value.

How 16bit? in quantization precision asking questions that the quantitative accuracy and final SNR has a direct link to the original standard, one major point of departure is to obtain as high signal to noise ratio. Philips engineers tend to 14bit, they think they already get 14bit 84dB Snr, compared to analog audio source 60dB maximum value around has been the improvement of quality. But advocates of specifications of Sony engineers think 14bit anyway enough, adhere to the proposed final 16bit in Sony's proposal was adopted. Why don't more quantization precision 24bit 20bit,?? because higher quantitative accuracy means that larger data volumes, CD's storage capacity is no longer enough.

16bit/44.1KHz, 24bit/192KHz what these figures mean?

Two sets of numbers are the specifications of the CD and VD-A, slash before the number indicates the most of accuracy, figures expressed after the slash sampling rate, as described in the quantitative accuracy and sampling rate.

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