We are listening to some music every day. We can notice that always the professional audio recordings comes with a small data on it, figuring to be 44.1Khz. Many of us wonder what this is, and if some of us know that, it is sampling rate of music, some among us don’t know why it is exactly 44.1Khz and not some good round value. This post points to the reason for that.
Before going to the core reason, we need to know about two things. they are
- Nyquist Rate
- Audible range of human ear
- And Audio video interpretation in TV.
Nyquist Rate:
Every signal in nature is continuous signal . But we are in a digital Era. So we use to convert the continues signals into discrete and then to digital signal. Generally discretising the signal means, obviously sampling, that is collecting amplitude of the signal at some interval. This gives us the fact that here we lose some data of the signal. If we loose more data, our signal will become something different than original, and if we have more data, our signal memory, i mean the size will be increased a lot. So, we have to optimize the number of samples needed. We have a condition in this process called as Nyquist criteria, which states that
“if we sample the signal at twice the rate of maximum frequency in signal the signal, then the signal can be reproduced with quality near to the original signal.”
This can be practically understood as following. If we have a signal which has a maximum signal frequency as some 20Khz, we can reproduce the signal with at-most similarity by sampling the signal at 40Khz. I mean 40,000 samples per second.
Audible range of human ear: 20Hz – 20Khz
So, now , we can consider the audible frequency range of human ear. It is between 20 Hz to 20,000Hz. So as per our need, we need better quality with minimum size. As nyquist criteria, we need twice the maximum signal frequency. so we need minimum of 40Khz [ = 2 x 20Khz] sampling.
So here is the question. Then why we have most of the professional audio contents exactly at 44.1Khz and not 40 or 50Khz? I had this question for many days and got the answer recently in this site. It states its with concern to number of scanning lines and number of lines per field in Television. I have just quoted the exact content given in the site for better understanding.
In the early days of digital audio research, the necessary bandwidth of about 1 Mbps per audio channel was difficult to store. Disk drives had the bandwidth but not the capacity for long recording time, so attention turned to video recorders. These were adapted to store audio samples by creating a pseudo-video waveform which would convey binary as black and white levels. The sampling rate of such a system is constrained to relate simply to the field rate and field structure of the television standard used, so that an integer number of samples can be stored on each usable TV line in the field. Such a recording can be made on a monochrome recorder, and these recording are made in two standards, 525 lines at 60 Hz and 625 lines at 50 Hz. Thus it is possible to find a frequency which is a common multiple of the two and is also suitable for use as a sampling rate.
The allowable sampling rates in a pseudo-video system can be deduced by multiplying the field rate by the number of active lines in a field (blanking lines cannot be used) and again by the number of samples in a line. By careful choice of parameters it is possible to use either 525/60 or 625/50 video with a sampling rate of 44.1KHz.
In 60 Hz video, there are 35 blanked lines, leaving 490 lines per frame or 245 lines per field, so the sampling rate is given by :
60 X 245 X 3 = 44.1 KHz
In 50 Hz video, there are 37 lines of blanking, leaving 588 active lines per frame, or 294 per field, so the same sampling rate is given by
50 X 294 X3 = 44.1 Khz.
The sampling rate of 44.1 KHz came to be that of the Compact Disc. Even though CD has no video circuitry, the equipment used to make CD masters is video based and determines the sampling rate.
So we can get to the fact why we are going for exactly 44.1Khz.
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