Dolby Digital AC-3

Source: Internet
Author: User
Dolby Digital AC-3 is a new generation of home theater multi-channel digital audio system developed by Dolby. Dolby's targeted logic system is a simulation system. The four channels are decomposed from the encoded two channels, Therefore, it is inevitable that there will be poor separation, low signal-to-noise ratio, and a lack of three-dimensional effect on surround sound, In addition, the band of loop sound is narrow. AC (Audio Coding) refers to Digital Audio Encoding, which discards analog technology, The new digital technology is used.
Dolby AC-3 features

Dolby Digital AC-3. the surround sound system consists of five full-frequency channels and one subwoofer channel,So it is called 5.1 channels. The five channels include the "Left" and"Center channel, right channel, Left Loop and Right Loop". The frequency range of these channels is a full-frequency response of 3-20000Hz.The sixth sub-channel contains some additional sub-channel information,This makes some scenarios, such as explosion and impact sound, more effective.Because the frequency response of this channel is 3-120Hz, it is called ". 1.
Dolby Digital AC-3 is a multi-channel surround sound encoding system developed based on the feeling.It divides the frequency of each sound into many narrow-and Small-frequency bands based on the auditory characteristics of human ears,In the coding process, we analyze audio psychology principles to retain valid audio,Remove multiple signals and various noise frequencies to make the sound more pure,Extremely high degree of separation.
Dubi digital AC-3 system available front-left and right speakers,The center speaker produces a sound field with a deep sense and clear positioning,Use two rear or lateral surround speakers and ultra-low speakers to provide a wide and magnificent sound field,The information of the six channels is completely digitalized in the process of preparation and restoration,Information loss is rare. The full-band features rich details and real stereo sound.
Dolby Digital AC-3 has good compatibility, in addition to its own decoding,It can also serve Dolby's targeted logic decoding. Therefore,Dolby Digital AC-3. system reproduction. Due to Dolby Digital AC-3 system encoding is very flexible,Therefore, there are many formats. Currently, it has been used as a high-definition TV (HDTV) Audio System, the latest DVD machine also contains the Dolby Digital AC-3.Therefore, Dolby AC-3 surround sound system may be a promising technology.
Dolby AC-3 Principle

Since the first time Dolby stereo optical, 4-ch, was applied to a movie in 1976, It has become one of the most popular multi-channel stereo systems for home Dolby Surround Sound ( Dolby surround, 3-ch, 1982) and Dolby oriented logic surround sound ( Dolby Surround Pro logic, 4-ch, 1987 ), You can enjoy the sound of a cinema at home. In 1992, Dolby developed the latest generation of Dolby Digital surround sound system ( Dolby Digital) and applied to movies. The system is based on advanced sensory encoding ( Perceptual Coding) and digital compression coding technology, namely Dolby AC-3. Its excellent features enable it to be selected as a standard multi-channel sound for a DVD player in 1995 Frequency format (in ntsc TV country) The standard audio format of multiple other systems. Dolby Digital surround sound system has six completely independent channels, full band left, right, Center, left, right, plus 20 ~ Hz sub-woofer sound channel (in 01 channel), it is also called 5 1-channel. In Dolby ac- 3. The volume of super-high-definition audio channels is 10 dB higher than that of other full-band audio channels, It can achieve extraordinary low-frequency effect of attention.
Dolby ac- 3. In principle, it divides the audio of each sound channel into many optimal audio streams based on the characteristics of human ears. Narrow Band, using audio psychology "auditory masking effect ", Delete the unaudible or negligible parts and compress them with digital signals. At the same time, Using the division of narrow frequencies, some of the band noise can be almost completely filtered out during coding, Keep the spectrum of other noises close to the signal spectrum, These noises can be suppressed by signals. So Dolby ac- The 3 system is actually a selective and strong noise suppression system. It's hard to understand Dolby ac- 3 features: supports full-audio multi-channel at a low bit rate, With excellent playback sound quality and effect.
Dolby AC-3 is a sensor-based coding method, It divides the entire audio band into several narrow frequencies, Because human hearing has different sensitivity to different frequencies of sound, Different frequencies have different widths. Because useful signals are divided into narrow and frequent, Encoding noise filtering is always easier. Because for each frequency band, All signals other than this frequency can be filtered out without damaging useful signals. In the frequency band, the residual noise signal frequency is very close to the useful signal frequency, This is the region where the masking effect plays a major role. In this sense, like ac- 3. Such a sensor encoding system is a very effective noise reduction coefficient. These multi-channel digital audio signals that are divided into narrow frequencies need to be synthesized The whole band signal, but the data volume occupied by each band is not evenly distributed, The encoder has an "auditory masking module" to simulate the human auditory masking effect, It determines how data usage should be distributed at a certain time point based on the dynamic characteristics of the signal. It is most suitable for various frequencies. Spectrum intensive, Sound elements with a large volume should obtain a large amount of data, Sounds that cannot be heard due to the masking effect occupy less or no data volume.
AC-3 coding and decoding Technology

AC-3 is in AC-1 and ac- 2. Based on the developed multi-channel coding technology, the original ac- 2. There are many features such as window function processing, exponential transformation encoding, and adaptive bit allocation, Added coupling and r using the stereo multi-channel encoding technology policy. Ematrix Algorithm . Generally, The left and right audio signals of a stereo sound are very similar in terms of hearing, There are many redundant information, Encode the signals of these two channels, Redundant signals can be removed without affecting the original sound quality. Here ac- Another effective method for three low bit rates.
AC-3 input PC sound data, output Compressed digital stream. The first step of encoding is to use tdac (Time Domain aliasing cancellation) The filter transforms PC sampling data in the time domain into a series of transform coefficients in the frequency domain. , Each transformation coefficient is expressed in binary exponent form, It is composed of an index and a ending number. The exponent part is coded to form the approximate spectrum of the entire signal, also known as the spectrum envelope. The bit allocation of each ending number is determined by the correlation of the line using the spectrum including and masking. Because the forward/ Backward adaptive bit allocation and public bit pools, Therefore, the limited bit rate can be, Different frequency components are reasonably allocated. During the quantification of the ending number, You can shake the ending number, The pseudo-random number generator used by Shake can obtain the same results on different platforms. Finally, the AC- 3. data frame format. Continuous frames are merged into digital stream output.
The selection of block lengths from Time Domain transformation to frequency domain transformation is the basis of exponential transformation encoding. In- 3 defines two types of length switching, one is the long block with 512 sample values, One is a short block with 256 sample values. In signal spectrum analysis, The longer the channel signal block area to be processed, the better, In this way, we can obtain better frequency resolution and higher coding efficiency. However, a long data block may contain some different noise that may be identified, For example, pre-Echo. That is to say, the masking effect of human ears in time and frequency is used for exponential transformation encoding. It is a matter of quality and cannot be considered at the same time. It must be handled in an integrated manner. For steady-state signals, The frequency changes slowly with time. To improve the coding efficiency, A filter group must have a good frequency resolution, that is, a long block; For fast-changing signals, good time resolution is required, A short block is required. In the encoder, After the input signal is removed from the DC component through a 3Hz high-pass filter, Then a 8 kHz high-pass filter is used to retrieve the high-frequency component, Compare its energy with the preset threshold to detect the transient state of the signal.
Improved discrete cosine transform (mdct) based on AC-3) Adaptive Transform encoding (ATC) algorithm. Although- 3. The mdct transformation is defined in the standard, but an N/ 4-point IFFT (Fast Fourier transformation), plus two simple pre- IFFT and post-IFFT as adjustments, To implement an imdct transformation of n points. An important consideration of the ATC algorithm is the critical band theory based on the auditory masking effect, That is, in the critical frequency band, the masking effect of one sound on another is the most obvious. Therefore, filter banks in the frequency band must respond quickly enough, This ensures that the noise attenuation outside the critical band is large enough, The noise in the time domain and frequency is limited to the following masking threshold values.
In the Bit Allocation Technology of AC-3 encoder, Adopts a widely used forward and backward adaptive bit allocation rule. The forward adaptive method is the bitwise allocation calculated by the encoder, And explicitly include the bit allocation information into the data bit stream, The feature is that the auditory model is used in the frontend encoding process, Therefore, modifying the model does not affect the decoding process on the receiving side. The disadvantage is that the encoding efficiency is reduced, Some valid bits are used to transmit the bit allocation information. The backward adaptive method does not obtain the bit allocation information specified by the encoder, Instead, it generates bit allocation information from the digital stream. The advantage is that it does not occupy valid bits, Therefore, it has higher transmission efficiency. Its disadvantage is to calculate the bit allocation from the received data, If the calculation is too complex, the cost of the decoder will increase. In addition, The decoder algorithm also changes with the change of the encoder's auditory model. Ac- 3. Hybrid forward/backward adaptive bit allocation, It balances the increase in bit rate and cost reduction.
The decoding principle of AC-3 decoder is basically the inverse process of coding, First, the decoder must be synchronized with the encoding data stream, Then, the control data is separated from the digital stream that has passed the data correction and verification, The system configuration parameters, the encoded spectrum envelope, and the quantified ending number, Generates bit allocation information based on the Sound Spectrum envelope, and reverse quantifies the ending part, Returns the index and tail of the transform coefficient, and then goes through the synthesis filter group, The data is transformed from the frequency domain to the time domain, and the reconstructed PCM sample value signal is output.
By understanding the AC-3, you can see the ac- 3. The technology makes full use of the sensory model of human ears for different types of signals, Effective algorithms are adopted, It achieves the expected goal of achieving a higher bit rate while ensuring a higher sound quality, It is a very efficient and economical digital audio compression system. Ac- 3 is a mandatory standard for digital television systems in the United States, Is the European Digital TV system Recommendation standard, at the same time, ac- 3. It is also the mandatory standard of the DVD system. At present, China is developing and promoting digital TV systems, and there is no reason to believe that ac- 3. technology will have a good prospect for application.
Apply in a DVD

Taking the C-CUBE DVD decompression chip developed by American ZIVA-D6 Company as the core, a DVD player (Dolby Digital Class C Product) solution is formed. The Block Diagram 1 shows the audio decoding and output. This solution supports Decoding of audio from multiple origins, with an optical fiber digital output, One 2-channel analog output and one 5.1-channel analog output. It not only meets the output Configuration Requirements of Dolby AC-3, It also meets the requirements of different power amplifiers.
1. audio source decoding and Output Mode
Ziva can detect audio sources based on the sample frequency and size changes detected in the audio source code. Don't, mainly MPEG-1 system code stream, MPEG-2 program code stream, The sample code of the lpcm and the fan area code of the VCD. Then, report the MCU to control the digital-to-analog converter.
Sound Source CD-DA, maid, Mpeg1 and so on are recorded in the CD in the format of 1 or 2 channels, and Ziva- D6 does not contain the Dolby Pro Logic decoder, so it is output in 1 or 2 channels.
Audio source Dolby Digital is decoded to generate multi-channel digital to the D/A converter, You can configure the number of audio speakers based on the source content of the program, Set to generate the corresponding multi-channel analog audio output. For example, 3/2, LCR/sl sr; 2/2, LR/sl sr; 3/1, LCR/s; 2/1, LR/s; 3/0, LCR; 2/0, LR; 1/0 C; 2/0, ltrt and other Eight Digital Mixed output modes. The above various modes are produced by the ZiVA-D6 decoder through matrix mixing, Sub channels are output independently because they are not mixed.
On the surface, the output mode of the ZiVA-D6 decoder is equivalent to the Dolby output mode, But at present its technical specifications cannot fully meet the specifications of the AC-3. Therefore, you need/ After a special circuit design to meet the requirements of Dolby AC-3 specifications, see the four.
2 DVD audio output
There are two types of output for DVD Audio: digital output and analog audio output.
There are two formats for digital output: Optical Fiber Digital Output ( Iec958 or iec1937) and digital RF output. This solution selects the Optical Fiber Digital iec958 output, using S/pdif ( Sony/Phil IPS digital interface) optical splitters (including electro-optic converters) generate optical signals, Transmit to other A/V products, such as AC-3 power amplifier.
Audio analog output: The 2-CH output conforms to Dolby ac- 3. Required for mixed output. When the sound source is Dolby Digital, it must be in the 2/0, ltrt (Dolby-oriented logic mode) Output. The initial intention of linear output is to connect to an external stereo or surround sound replication system, Without directly connecting the power amplifier. So there should be a fixed sound output that meets the standard level, This standard level is controlled by application software. Therefore, DVD and other video players cancel the linear output of volume control. Multiple outputs are optional.

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