The origin of MP4
Compared with the well-known MP3, people do not know much about MP4. Although logically speaking, when we hear or see MP4, we will infer that this is the successor format of MP3, but the fact is much more complicated. In fact, MP4 and MP3 are too different. MP3 is an international standard for audio compression, while MP4 is a brand name. Although both belong to the category of network music formats, they also represent two completely different audio compression technologies and formats.
The emergence of MP4 makes the original MPEG standard series that is easy to confuse more difficult to distinguish. MP3 does not refer to the MPEG-3 standard, but an abbreviation for MPEG Layer 3, which is an ISO / IEC international standard and is a completely public audio compression technology. MP4 is neither an abbreviation for MPEG Layer 4 nor a MPEG-4 standard. It is GMO's copyright infringement of music publications against MP3. It uses AT & T authorized A2B music based on MPEG-2 AAC (Advanced Audio Coding), and named it MP4. Its purpose is probably to indicate that MP4 is following MP3. A kind of upgrading technology, which is just in line with people's thinking habits.
A2B technology is mainly composed of the following three parts. First, AT & T's patent for audio compression technology, with "perceptual encoding" as the key technology, can improve the AAC compression ratio to 20: 1 without losing sound quality; second, the security database, which can create a specific for your A2B music file Key, and put this key in its database, only A2B players can play music containing this key; third, the protocol certification, this certification includes the copy permission, the number of copies allowed, the total number of songs Information such as the time, the time the song can be played, and the operating and sales license.
Compared with MP3 and other previous music formats, MP4 has the following characteristics:
(1) Each MP4 music piece is an executable file with the extension exe. It has a built-in player and is easy to operate. It can be played by double-clicking directly in Windows, which is very convenient;
(2) Due to the use of advanced A2B audio compression technology, the size of MP4 files is only about three-quarters of MP3, better sound quality, greater compression ratio, from this perspective MP4 is more suitable for spreading on the Internet , And the sound quality is even better;
(3) Unique digital watermark, MP4 uses a digital watermark technology called "SOLANA", which can easily track and discover piracy, and any illegal decompression of MP4 may cause damage to the original MP4 file;
(4) Support for copyright protection, MP4 music has built-in copyright descriptions including texts and images related to the copyright holder of the work, which can not only explain the copyright, but also show respect for the composer and singer (player);
(5) Complete functions, MP4 can independently adjust the volume of the left and right channels, the built-in waveform / frequency dynamic audio display and music manager can support a variety of color images, website links and unlimited scrolling display text.
It can be seen that MP4 is a commodity. It uses the improved MPEG-2 AAC technology to compress the audio, and adds an intellectual property agreement directly authorized by the publishing company to form a new form of digital music standard.
MPEG-2 AAC
MPEG-2 AAC (Advanced Audio Coding) was proposed in 1997 by audio coding research led by Bosi, Brandenburg, Johnston, etc. It summarizes the strengths of MPEG-1, MPEG-2 and AC-3, etc. in the MPEG system A lot of new functions have been further improved on the above, which greatly enhances the flexibility of coding, while compressing the code rate to a greater extent while ensuring sound quality. At the same time, MPEG-2 AAC has become the core of high-quality audio coding in the MPEG-4 standard and is the next-generation audio compression standard.
In order to be able to adapt to different applications, three different complexity profiles (Profile) are defined in the AAC standard. They are:
Main Profile: This framework has the highest complexity and can be used in situations where storage capacity and computing power are sufficient. In this framework, all coding tools except gain control are used to improve compression efficiency.
Low Complexity Profile: This framework is used in applications that require compression under the conditions of limited storage space and computing power. In this framework, there are no two tools for prediction and gain control, and the order of TNS is relatively low.
SSR (Scalable Sample Rate) Profile: In this framework, gain control tools are used, but prediction and coupling tools are not allowed, with lower bandwidth and TNS order. No gain control tool is used for the lowest PQF subband. When the bandwidth is reduced, the complexity of the SSR framework can also be reduced, especially suitable for occasions where the network bandwidth changes.
MPEG-2 AAC codec
When encoding, the time-domain signal is first passed through a gain control module (used only in scalable grades), the signal is divided into 4 frequency bands, the signal amplitude is gain-controlled, and then decomposed into 1024 frequency-domain spectral lines by the filter bank. At the same time, the psychoacoustic model is analyzed according to the input time domain signal, and the current masking domain value is calculated. In addition, the psychoacoustic model also outputs control information required for M / S stereo and intensity stereo processing. The TNS module performs linear prediction on part of the spectral lines output by the filter, replacing the original spectral lines with residual signals, and the intensity stereo module extracts the envelope of the high-frequency component of the signal. The M / S stereo module replaces the left and right channel signals with the sum signal M and the difference signal S. The quantization module performs bit allocation to try to make the quantization noise smaller than the masking threshold. The quantized signal and scale factor are Huffman coded, and finally AAC group coded. Reversing the above process is decoding. The specific process is shown in the block diagram shown in Figure 1.
MPEG-2 AAC combines high-resolution filter banks, prediction techniques, noise shaping techniques, joint stereo coding, non-uniform quantization, and Huffman coding to meet the following requirements:
(1) Support sampling frequency from 8kHz to 96kHz;
(2) Support input channel configuration as 1/0 (mono), 2/0 (dual channel stereo) and 3/2 + 1 (left / center / right, left surround / right surround, low frequency enhancement channel), etc. Different multi-channel configurations;
(3) Leave room for a larger number and a smaller number of channel configurations in the bitstream syntax;
(4) At a data rate of 384Kb / s and a 3/2 channel configuration, "indistinguishable" sound quality can be obtained;
(5) In order to improve the elimination of bit errors, support a mechanism to maintain the synchronization of the code stream in the presence of bit errors and some kind of bit error elimination mechanism.
to sum up
MP3 is one of the most popular music formats at present. It has occupied a lot of network resources and is loved by the majority of music lovers. Therefore, the promotion of GMO MP4 will encounter great difficulties. However, MP4 has its unique advantages, its compression technology is better than MP3, and its sound quality is better than MP3; from the perspective of copyright protection, it will be welcomed by the publishing industry. In particular, the AAC format, codec / decoder, and music, which are the same as the A2B compression technology, have appeared on the Internet and are being accepted and developed by the public. Moreover, MPEG-2 AAC has become the core of high-quality audio coding in the MPEG-4 standard and is the next-generation audio compression standard. Therefore, with the passage of time and the promotion of the MPEG-4 standard, MPEG-2AAC, which is one of the MPEG-4 audio, will surely be valued and widely used.
Compared with the well-known MP3, people do not know much about MP4. Although logically speaking, when we hear or see MP4, we will infer that this is the successor format of MP3, but the fact is much more complicated. In fact, MP4 and MP3 are too different. MP3 is an international standard for audio compression, while MP4 is a brand name. Although both belong to the category of network music formats, they also represent two completely different audio compression technologies and formats.
The emergence of MP4 makes the original MPEG standard series that is easy to confuse more difficult to distinguish. MP3 does not refer to the MPEG-3 standard, but an abbreviation for MPEG Layer 3, which is an ISO / IEC international standard and is a completely public audio compression technology. MP4 is neither an abbreviation for MPEG Layer 4 nor a MPEG-4 standard. It is GMO's copyright infringement of music publications against MP3. It uses AT & T authorized A2B music based on MPEG-2 AAC (Advanced Audio Coding), and named it MP4. Its purpose is probably to indicate that MP4 is following MP3. A kind of upgrading technology, which is just in line with people's thinking habits.
A2B technology is mainly composed of the following three parts. First, AT & T's patent for audio compression technology, with "perceptual encoding" as the key technology, can improve the AAC compression ratio to 20: 1 without losing sound quality; second, the security database, which can create a specific for your A2B music file Key, and put this key in its database, only A2B players can play music containing this key; third, the protocol certification, this certification includes the copy permission, the number of copies allowed, the total number of songs Information such as the time, the time the song can be played, and the operating and sales license.
Compared with MP3 and other previous music formats, MP4 has the following characteristics:
(1) Each MP4 music piece is an executable file with the extension exe. It has a built-in player and is easy to operate. It can be played by double-clicking directly in Windows, which is very convenient;
(2) Due to the use of advanced A2B audio compression technology, the size of MP4 files is only about three-quarters of MP3, better sound quality, greater compression ratio, from this perspective MP4 is more suitable for spreading on the Internet , And the sound quality is even better;
(3) Unique digital watermark, MP4 uses a digital watermark technology called "SOLANA", which can easily track and discover piracy, and any illegal decompression of MP4 may cause damage to the original MP4 file;
(4) Support for copyright protection, MP4 music has built-in copyright descriptions including texts and images related to the copyright holder of the work, which can not only explain the copyright, but also show respect for the composer and singer (player);
(5) Complete functions, MP4 can independently adjust the volume of the left and right channels, the built-in waveform / frequency dynamic audio display and music manager can support a variety of color images, website links and unlimited scrolling display text.
It can be seen that MP4 is a commodity. It uses the improved MPEG-2 AAC technology to compress the audio, and adds an intellectual property agreement directly authorized by the publishing company to form a new form of digital music standard.
MPEG-2 AAC
MPEG-2 AAC (Advanced Audio Coding) was proposed in 1997 by audio coding research led by Bosi, Brandenburg, Johnston, etc. It summarizes the strengths of MPEG-1, MPEG-2 and AC-3, etc. in the MPEG system A lot of new functions have been further improved on the above, which greatly enhances the flexibility of coding, while compressing the code rate to a greater extent while ensuring sound quality. At the same time, MPEG-2 AAC has become the core of high-quality audio coding in the MPEG-4 standard and is the next-generation audio compression standard.
In order to be able to adapt to different applications, three different complexity profiles (Profile) are defined in the AAC standard. They are:
Main Profile: This framework has the highest complexity and can be used in situations where storage capacity and computing power are sufficient. In this framework, all coding tools except gain control are used to improve compression efficiency.
Low Complexity Profile: This framework is used in applications that require compression under the conditions of limited storage space and computing power. In this framework, there are no two tools for prediction and gain control, and the order of TNS is relatively low.
SSR (Scalable Sample Rate) Profile: In this framework, gain control tools are used, but prediction and coupling tools are not allowed, with lower bandwidth and TNS order. No gain control tool is used for the lowest PQF subband. When the bandwidth is reduced, the complexity of the SSR framework can also be reduced, especially suitable for occasions where the network bandwidth changes.
MPEG-2 AAC codec
When encoding, the time-domain signal is first passed through a gain control module (used only in scalable grades), the signal is divided into 4 frequency bands, the signal amplitude is gain-controlled, and then decomposed into 1024 frequency-domain spectral lines by the filter bank. At the same time, the psychoacoustic model is analyzed according to the input time domain signal, and the current masking domain value is calculated. In addition, the psychoacoustic model also outputs control information required for M / S stereo and intensity stereo processing. The TNS module performs linear prediction on part of the spectral lines output by the filter, replacing the original spectral lines with residual signals, and the intensity stereo module extracts the envelope of the high-frequency component of the signal. The M / S stereo module replaces the left and right channel signals with the sum signal M and the difference signal S. The quantization module performs bit allocation to try to make the quantization noise smaller than the masking threshold. The quantized signal and scale factor are Huffman coded, and finally AAC group coded. Reversing the above process is decoding. The specific process is shown in the block diagram shown in Figure 1.
MPEG-2 AAC combines high-resolution filter banks, prediction techniques, noise shaping techniques, joint stereo coding, non-uniform quantization, and Huffman coding to meet the following requirements:
(1) Support sampling frequency from 8kHz to 96kHz;
(2) Support input channel configuration as 1/0 (mono), 2/0 (dual channel stereo) and 3/2 + 1 (left / center / right, left surround / right surround, low frequency enhancement channel), etc. Different multi-channel configurations;
(3) Leave room for a larger number and a smaller number of channel configurations in the bitstream syntax;
(4) At a data rate of 384Kb / s and a 3/2 channel configuration, "indistinguishable" sound quality can be obtained;
(5) In order to improve the elimination of bit errors, support a mechanism to maintain the synchronization of the code stream in the presence of bit errors and some kind of bit error elimination mechanism.
to sum up
MP3 is one of the most popular music formats at present. It has occupied a lot of network resources and is loved by the majority of music lovers. Therefore, the promotion of GMO MP4 will encounter great difficulties. However, MP4 has its unique advantages, its compression technology is better than MP3, and its sound quality is better than MP3; from the perspective of copyright protection, it will be welcomed by the publishing industry. In particular, the AAC format, codec / decoder, and music, which are the same as the A2B compression technology, have appeared on the Internet and are being accepted and developed by the public. Moreover, MPEG-2 AAC has become the core of high-quality audio coding in the MPEG-4 standard and is the next-generation audio compression standard. Therefore, with the passage of time and the promotion of the MPEG-4 standard, MPEG-2AAC, which is one of the MPEG-4 audio, will surely be valued and widely used.
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