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Bitrate Part 2

The amount of information transmitted through the channel per unit of time is called the bit rate, and the unit is bits per second (bit/s), called the bit rate.

Bitrate is often used in communications as a synonym for connection speed, transmission speed, channel capacity, peak throughput, and digital bandwidth capacity. The higher the bit rate, the higher the data transfer. Bit rate in video refers to the sampling rate at which an analog signal is converted to a digital signal [4] . Video file quality is often measured in terms of bitrate. [4] .
Distinction of conceptedit transmission
Baud rate is also known as waveform rate or modulation rate. The code for a data unit is represented by a finite combination of numbers, each of which is a symbol (or code point). In electrical communication, an electrical waveform is often used to represent one or more symbols. Waveforms with different characteristics may represent different symbol values ​​or symbol combination values, and the duration of the waveform corresponds to the duration of the symbol or symbol combination it represents. Obviously, the shorter the duration of an electrical waveform, the more waveforms are transmitted in a unit of time, or the more data is transmitted, that is, the higher the data rate. Therefore, we can define the baud rate as follows: In the process of data transmission, the number of waveforms transmitted per unit time on the line is the baud rate, and its unit is “baud” [5] .
“Bit rate” and “baud rate” are speed units defined in two different concepts, and it is often easy to confuse them when you are not careful. When binary waveform is used, baud rate and bit rate have the same value, but their meanings are different [5] .
Difference: Both bit rate and baud rate are units that measure the transmission rate of a modem. In data transmission, data information is represented by binary numbers “0” and “1”, and each binary number is called 1 bit. The number of bits transmitted through the channel per unit of time is called the bit rate, expressed in bits per second, usually abbreviated as bit/s. The number of symbols transmitted through the channel per unit of time is called the baud rate, also called the modulation rate. Bit rate and baud rate are consistent only when modulated with two values. For example, in quadrature modulation, every two bits of the data signal form a symbol, and there are 4 values: 00, 01, 10 and 11, which represent the phase changes of the 4 types of carrier signals respectively, for Therefore, send such a symbol. It is equivalent to transmitting two bits of data, and the baud rate is equivalent to half the bit rate. The usual transmission rates of 300, 600, 1200 and 9600, etc., refer to the baud rate, which indicates that the number of binary numbers transmitted per unit of time is 300, 600, 1200 and 9600 [6] .

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Sample rate and bit rate of MP3 Part 2

The number of digits in the sound is equivalent to the number of colors on the screen, indicating the amount of data per sample.

Of course, the larger the amount of data, the more accurate the playback sound, so as not to confuse the sound. of the teapot with the train whistle. In the same way, it is more clear and precise for the image, so as not to confuse blood and ketchup. [However, limited by the function of human organs, 16-bit sound and 24-bit image are basically the limits of ordinary humans, and the higher digits can only be distinguished by instruments. For example, the phone has 7-bit sound sampled at 3 kHz and the CD has 16-bit sound sampled at 44.1 kHz, so the CD is clearer than the phone. ]

When you understand the above two concepts, bitrate is easy to understand. Take the phone as an example, 3000 samples per second, each sample is 7 bits, then the phone’s bit rate is 21000. And the CD is 44100 samples per second, two channels, each sample is 13 bit PCM encoded, so the CD bit rate is 44100*2*13=1146600, which means the CD data volume per second is about 144KB. the capacity of a CD is 74 minutes equal to 4440 seconds, which is 639360KB=640MB.

Sound is actually a type of energy wave, so it also has the characteristics of frequency and amplitude, with frequency corresponding to the time axis and amplitude corresponding to the level axis. The wave is infinitely smooth, and the string can be considered to be made up of innumerable points. Since the storage space is relatively limited, in the process of digital encoding, the points of the string must be sampled. The sampling process consists of extracting the frequency value of a certain point. Obviously, the more points that are extracted in one second, the richer the frequency information that can be obtained. To restore the waveform, there must be two sampling points in one vibration. The highest frequency that can be felt is 20kHz, so to meet the auditory requirements of the human ear, at least 40k samples per second, expressed at 40kHz, and this 40kHz is the sample rate. Our common CD has a sample rate of 44.1 kHz. It is not enough to have only frequency information, we must also obtain and quantify the energy value of this frequency to represent the strength of the signal. The number of quantization levels is an integer power of 2, and the sample size of our common CD bit is 16 bits, that is, 2 to the power of 16. Sample size is harder to understand than bit rate. sampling, because it makes it seem abstract. For a simple example: suppose a wave is sampled 8 times, and the energy values ​​corresponding to the sampling points are A1-A8, but we only use 2-bit sampling size, as a result we can only keep the 4 point values ​​in A1-A8 and discard the other 4. If we use the 3bit sample size, all 8 point information is recorded. The higher the sample rate and sample size values, the closer the recorded waveform is to the original signal.

MP3 sample rate and bit rate

When we listen to mp3 and watch movies, we will notice two parameters.

The most common ones are 44.1 KHz sample rate and 192 Kbps bit rate. So what is the sample rate and what is the bit rate? What is the relationship between them? Explain:

The process of converting an analog audio signal to a digital audio signal is called sampling. In a nutshell, how many data points does it take to record a 1 second long sound via waveform sampling. For example: the sound sample rate of 44.1 KHz is equivalent to spending 44,000 data points to describe the sound waveform for 1 second. In principle, the higher the sample rate, the better the sound quality; sampling frequency is generally divided into three levels: 22.05KHz, 44.1KHz and 48KHz; 22.05KHz can only achieve FM radio sound quality, and 44.1KHz is the theoretical limit of CD sound quality, 48KHz has reached DVD quality.

Sampling rate refers to the sampling frequency when converting sound (analog signal) to mp3 (digital signal), i.e. how many data points are sampled per unit of time. (The data for a sample point is 8 (or even more) bits long.)

Bit rate refers to the number of bits (bits) transmitted per second. The unit is bps (bit per second). The higher the bitrate, the more data transmitted and the better the sound quality.

It can be said that the sample rate and bit rate are like the horizontal and vertical coordinates on the coordinate axis. The sampling frequency on the abscissa represents the data points sampled per second. The bit rate on the ordinate represents the precision when quantizing analog quantities with digital quantities.

The sample rate is similar to the number of frames of moving images. For example, the sampling rate of movies is 24 Hz, the sampling rate of PAL format is 25 Hz, and the sampling rate of NTSC format is 30 Hz. When we play back the still images sampled at the same rate as the sampling frequency, we see a continuous image. In the same way, when a CD recorded at a sampling rate of 44.1 kHz is played back at the same rate, a continuous sound can be heard. Obviously, the higher the sample rate, the more coherent the sound will be heard and the picture will be seen. [Of course, the sampling rate that human auditory and visual organs can distinguish is limited, which is basically higher than sound sampled at 44.1kHZ, and most people haven’t noticed the difference. ]