The structure and basic knowledge of MP3 player

1. What is MP3 ? MP3 is a lossy digital audio compression format. The full name is Mpeg-1 audio Layer 3 , where MPEG is the abbreviation of Moving Picture Experts Group . The so-called " lossy compression audio format " means that digital audio is compressed in a way that causes loss of sound quality in order to reduce the file size and meet the needs of copying, storage, and transmission. The compression rate of MP3 can reach 1:12 , but it does not sound distorted to the human ear because it removes sounds that are beyond the hearing range of the human ear from the digital audio without changing the main sound. In addition, MP3 players can also upload and download computer files in any other format and have mobile storage functions.

2. The structure of MP3 player

An MP3 player is actually a small computer with specific functions. In its small body, it has memory (memory card), display ( LCD display), central processing unit [MCU (microcontroller) or decoding DSP (digital signal processor) ] , etc. Its basic structure is as shown in the figure below:

3. Workflow of MP3 player

When the micro operating system is loaded, the MCU begins to be controlled by the operating system and executes various functions specified by it. For MP3 players, the most important function is to play MP3 music. The playback process is the decoding process of MP3 music files. MCU uses its own CPU computing power to undertake the heavy task of MP3 music file decoding. When MP3 music is downloaded to the MP3 player, it is generally stored in the built-in flash memory or hard disk of the body. During the playback process, the MCU reads it from the storage medium, buffers it in the RAM , and plays it after decoding. Since the signal at this time is a digital signal, analog devices such as headphones cannot play it. At this time, a digital / analog converter ( DAC ) is needed to complete the work of converting digital signals into analog signals, and then beautiful music can be played through the headphones.

(II) MP3 performance parameters and technical knowledge 1. SRS ( WOW ) surround sound effect SRS (Sound Retrieval System) effect is simply a 3D stereo sound effect. WOW is composed of SRS, TRUBASS, and BRIGHTNESS, which can improve the sound quality to a panoramic 3D effect from low to high. SRS is the core technology of WOW, which can collect disturbed and scattered sound waves and then replay them in a normal way. It can be very close to the original level and live effect of the artist. TRUBASS can make practical adjustments to the low frequency, and can also raise the deep low frequency to the level of the octave and display it in all directions. BRIGHTNESS creates a higher and wider sound range through extended pleasant sound waves, thereby achieving a widening effect. 2. EQ Equalizer Equalization (equalization) modifies the overtone level of each frequency in the sound, optimizes it for a certain type of music, and enhances people's feelings. Common ones include: normal, rock, pop, dance, classical, soft, jazz, metal, heavy bass and custom. Custom means adjusting by yourself, without applying a fixed mode, and it is a real EQ according to personal preference. EQ sound effect can make up for the signal loss when MP3 is compressed, and also meet different personal listening preferences. 3. SNR (Signal to Noise Ratio) The signal-to-noise ratio refers to the ratio of the output signal voltage under the specified input voltage to the noise voltage remaining at the output when the input voltage is cut off. It can also be regarded as the ratio between the maximum undistorted sound signal intensity and the noise intensity emitted at the same time. It is usually expressed as S/N, generally in decibels (dB). The higher the signal-to-noise ratio, the better the audio product. The signal-to-noise ratio of common MP3 players is above 60dB, and the signal-to-noise ratio of OPPO MP3 is above 90dB, which has a higher advantage. 4. Frequency Respond Frequency response The frequency response range is the range between the lowest effective sound frequency and the highest effective sound frequency, in Hertz (Hz). It is directly related to the performance and price of the sound system. The smaller the value, the flatter the frequency response curve of the speaker, the smaller the distortion, and the higher the performance. The frequency response range of a general MP3 player is 20 to 20,000 Hz, which is exactly the sound frequency range that the human ear can hear.

5. Sampling Rate: The digital audio system reproduces the original sound by converting the sound wave into a series of binary data. The process of converting analog audio into digital audio is called sampling. The device used to implement this process is an analog/digital converter (A/D). It samples the sound wave at a rate of tens of thousands of times per second. Each sample records the state of the original analog sound wave at a certain moment, which is called a sample. By connecting a series of samples, a sound wave can be described. The number of samples taken per second is called the sampling rate, which is measured in Hz (Hertz). The higher the sampling rate, the higher the frequency of the sound wave that can be described, and the more guaranteed the sound quality is, which is most obvious when recording. Most MP3 players support the playback of 44.1KHz MP3 audio files. 6. Output Power: refers to the power output of the non-pure voltage output method, which is mainly voltage output, at the output port of the headphone of the walkman. There is usually a nominal value in the manual. The headphone must match the output power of the walkman. Simple analysis: The higher the impedance of the earphone, the greater the output voltage will be, the total power of the player will decrease, and the output power will decrease. When the volume is turned up high, the power reduction is more significant, and the so-called distortion phenomenon will occur. Under the condition of the standard impedance of 16Ω earphones, the maximum output power of a single channel of the current MP3 player is generally between 7 and 18mW. 7. USB interface USB stands for Universal Serial Bus. Currently, MP3 products generally use USB2.0 interface, which is divided into two types: USB2.0 Full Speed ​​and USB2.0 Hi Speed. The transmission rate of USB2.0 Full Speed ​​is 12Mbps (megabits per second), equivalent to 1.5MB/s. Most MP3s currently use this type of interface. The theoretical transmission rate of USB2.0 Hi Speed ​​can reach 480Mbps, equivalent to 60MB/s, which means that it only takes 2 seconds to fill a 128MB MP3 player! However, due to various reasons, the actual transmission rate is far from this value. Generally, the transmission rate does not exceed 10MB/s, but it can reach several megabits per second, which is fast enough. In addition, most early MP3 players use USB1.1 interfaces, and its general transmission rate is 1.5Mbps, which is equivalent to 1/8 of USB2.0 (full speed). 8. Line in (Line input/direct recording function) From a hardware perspective, Line in is an input terminal (jack) used to receive line-level signals. From a functional perspective, Line in is an MP3 direct recording function. It can directly obtain sound sources from external audio devices such as CD players, VCDs, and recorders through audio cables for recording, and then use the MP3 encoding function in the machine to compress it into MP3 format audio files. In this way, CDs and other sound sources can be recorded into MP3 music without going through a computer. 9. FirmWare (Firmware) Firmware (hardware with software functions) includes music decoding, interface control, display of various prompt information, and connection with computers through lines, etc., and its functions are very powerful. Firmware upgrades can solve existing errors and compatibility issues, improve the operation mode to make it more user-friendly, and provide more music format support. 10. Battery The battery is generally a lithium battery, No. 7 or No. 5 battery. No. 7 or No. 5 battery may be a nickel-metal hydride rechargeable battery. The biggest advantage of lithium batteries is that they have almost no memory effect and strong energy. 11. TTS TTS is an outstanding work that uses both linguistics and psychology. With the support of built-in chips, it intelligently converts text into natural speech streams through the design of neural networks. TTS technology converts text files in real time, and the conversion time can be calculated in seconds. Under the action of its unique intelligent voice controller, the voice rhythm of the text output is smooth, making the listener feel natural when listening to the information, without the indifference and awkwardness of machine voice output. TTS speech synthesis technology will soon cover the first and second level Chinese characters of the national standard, have an English interface, automatically recognize Chinese and English, and support mixed reading of Chinese and English. All sounds are pronounced in real-person Mandarin, achieving fast speech synthesis of 120-150 Chinese characters per second, and a reading speed of 3-4 Chinese characters per second, so that users can hear clear and pleasant sound quality and coherent and fluent intonation. Now a small number of MP3 players have TTS function. 12. TXT text reading TXT text reading means that MP3 can read text in TXT format, which allows consumers to read text while listening to music. 13. ID3 information ID3 tag is the additional information of songs in MP3 music files. It can add the performer, author and other category information of the song to MP3, which is convenient for the management of many songs. The lack of ID3 tag will not affect the playback of MP3, but if it is missing, it will be quite troublesome to manage music files. If you download MP3 music online, it will probably have preset ID3 information written in it. If you want to clear and reset it, use the "Alt + 3" hotkey in Winamp to modify the ID3 tag in the MP3 file. 14. Memory expansion slot Expanding memory is equivalent to increasing the memory capacity of an MP3 player. Depending on the MP3, there are many options, such as CF card, SM card, MMC card, and SD card. CF card was used in the early MP3, but now it is rarely used due to its size and power consumption; SM card is widely used in the field of MP3 players, but it is now relatively rare; MMC card is known as the smallest Flash Memory card in the world, but due to price reasons, there are very few MP3 players that use this card; SD card is developed based on MMC card, so it is also backward compatible - in other words, if your MP3 supports SD card, then it can generally support MMC card, but conversely, MP3 with MMC card slot does not support SD card. In addition to the above-mentioned cards, SONY uses a dedicated memory stick (Memory Stick), which is generally only used on SONY models and is rarely seen in other products.