mp3 audio format, the most popular
With the rapid development of file compression technology, MP3 has become the most popular music format today.
High-quality music quickly spreads to all parts of the world with the arrangement of 0 and 1, shaking people’s hearts. What is MP3? The full name of MP3 is MPEG Audio Layer 3. It is an efficient computer audio coding scheme. It converts audio files into smaller files with .MP3 extension with a higher compression ratio and basically maintains the sound quality of the file. original. MP3 is part of the ISO/MPEG standard. The ISO/MPEG standard describes audio compression using a high-performance perceptual coding scheme. This standard has been continuously updated to meet the pursuit of “high quality and small quantity”, and now has formed MPEG Layer 1, Layer 2. Layer 3 three audio encoding and decoding schemes. The compression rate of MPEG Layer 3 can reach from 1:10 to 1:12. A 1M MP3 file can be played for 1 minute, while a 1 minute CD-quality WAV file (44100Hz, 16bit, 2ch, 60sec) occupies 10M of space, so Calculated, the time The playback time of a 650M MP3 disc should be more than 10 hours, while the playback time of a CD with the same capacity is about 70 minutes. The advantages of MP3 are unmatched by CD. 2 Analysis of the principle of MP3 2.1 MPEG audio standard MPEG (Moving Picture Experts Group) is a moving picture expert group under ISO, and the MPEG standard formulated by it is widely used in various multimedia. MPEG standards include video and audio standards, among which MPEG-1, MPEG-2, MPEG-2 AAC, and MPEG-4 audio standards have been developed. The MPEG-1 and MPEG-2 standards use the same family of audio codecs: Layer 1, 2 and 3. A new feature of MPEG-2 is the use of low sample rate expansion kits to reduce data traffic , and another feature is the multi-channel expansion kit, which increases the number of main channels to five. Fraunhofer IIS and AT&T released the MPEG-2 AAC (MPEG-2 Advanced Audio Coding) standard in 1997 to significantly reduce data traffic. The MDCT (Modified Discrete Cosine Transform) algorithm adopted by MPEG-2 AAC, The sampling frequency can be between 8 KHz and 96 KHz, and the number of channels can be between 1 and 48. MPEG Audio Layer 1, 2 and 3 use the same filter bank, bitstream structure, and header information, and the sample rate is either 32 KHz, 44.1 KHz, or 48 KHz. Layer 1 is designed for DCC (digital compact cassette) digital compression tape, the data rate is 384 kbps, and layer 2 has made a compromise between complexity and performance, and the data rate has been reduced to 256 kbps- 192kbps. Layer 3 was designed for low data rate from the beginning, and the data rate is 128Kbps-112Kbps. Layer 3 adds MDCT transform, which makes its frequency resolution 18 times higher than that of Layer 2. Layer 3 also uses information averaging similar to MPEG video entropy coding to reduce redundant information. The vast majority of MP3 uses the MPEG-1 standard. 2.2 The purpose of audio compression The MP3 format began in the mid-1980s, and the Fraunhofer Institute in Erlangen, Germany, was committed to high-quality, low-data-rate audio coding. Let’s look at an example: You want to sample a song you like that is about 4 minutes long, store it on a disc, and sample it in CD-quality WAV format at a sample rate of 44.1 kHz, which means receiving 44100 per second. , stereo, each sample data is 16 bits (2 bytes), so the space occupied by this song is: 44100×2 channels x2 bytes x60 seconds x4 minutes=40.4MB If you download this song from the Internet, assume the transmission speed is of 56kbps, the download time is: 40.4x106x8/56x103x60=96 minutes. Even a 1M broadband network takes more than 5 minutes. It can be seen that audio compression is especially important to reduce the storage space of audio data. 2.3 MP3 encoding and decoding MP3 audio compression involves encoding and decoding in two parts. Encoding is turning the data in a WAV file into a highly compressed bitstream, and decoding is taking the bitstream and reconstructing it into a WAV file. MP3 uses a distortion algorithm called Perceptual Audio Coding. The frequency range of sound perceived by the human ear is from 20 Hz to 20 kHz. MP3 cuts out a lot of redundant and irrelevant signals.
