The development of speech codec technology is somewhat static, but audio codec technology has been evolving forward (see Figure 1). For example, the development of more surround sound channels is a trend. One of the biggest technology trends is the addition of technology for simulating multi-channel audio in stereo systems to recreate the "live feel" of a special live concert venue. This creates the challenge of completing all of these processes, because you can no longer use a DSP engine powered by a high-powered power supply in an AV receiver or DVD player, but must use a battery-powered DSP engine in a cell phone or PDA. to fulfill. All of the above factors bring an interesting story to two different but fused applications (personal audio and home theater).
Personal audio
Randy Cole, chief technology officer of Texas Instruments' portable audio and infotainment business unit, points out that the most popular audio codec in personal audio is MP3 (see Figure 2). MP3 was once limited to PCs and portable multimedia players, but it is still abound in the mobile phone industry. The reason is that mobile phone manufacturers are constantly chasing new features to induce end users to perform product replacement every six months. .
MP3 is a standard published by ISO. It is the third in a series of MPEG-1 codec standards developed by MPEG. MPEG-1 has three layers, each of which includes the previous layer. Thus, layer 3 implements layers 1 and 2. For this reason, MPEG-1 and Layer 3 are well-known MP3s that provide a bandwidth and data rate for portable applications, albeit low enough.
In the past 10 years, MP3 has been the dominant audio codec for downloading music. But Apple's iPod broke this convention in a new MPEG development called Advanced Audio Encoder (AAC). MPEG began working on the AAC standard in the mid-1990s as part of the next generation of MPEG-2 research and implemented the best new design concepts developed by AT&T, Dolby, Fraunhof and Sony. At first, it was intended to be backward compatible with MP3, but this goal could not be achieved.
Therefore, because the entire industry is deeply involved in MP3, no company is willing to allocate resources to produce a large number of audio programs for the new AAC program, so it has declined. That is to say, until Apple chose AAC's MPEG-4 version for the iPod, the technology has been in a state of decline. (The next MPEG working group established after MPEG-2 is MPEG-4, which goes beyond MPEG-3. The MPEG-4AAC used by Apple is an enhanced version of MPEG-2AAC with a slightly lower data rate and improved quality. )
Other proprietary encoders exist outside of the MP3 and AAC categories. They have some penetration in the PC and personal multimedia devices, but they are less important in the mobile phone industry because mobile phone manufacturers prefer standard encoders and their fixed royalty. A familiar proprietary encoder is WindowsMediaAudio (WMA). It is mainly used for PCs and competes with MP3 and AAC in it. However, from a data rate perspective, it is flexible (there is an appropriate quality difference from low to high). Currently, there is WMA's multi-channel version of WMA-Pro, and Microsoft released a lower-loss WMA in June 2005.
The other important proprietary encoder is Dolby Digital, also known as AC3. This type of encoder is used for DVD and (US) digital TV. Until recently, its operating rate was still too high for the Internet and mobile phones. However, what needs to be remedied is a new version that lowers the data rate.
According to Mohsin ImTIaz, marketing manager of TI's high-performance audio business, the main codecs in the home theater are Dolby and DTS. Dolby's Dolby DigitalPlus is targeting the high-definition DVD and broadcast markets. However, there is a certain overlap between portable standards such as MP3, AAC, and WMA. For the next generation of DVDs, Microsoft is pushing WMA.
Parsing a codec
Let's break down a codec. To clarify the whole thing, let's take a look at a paper presented at the Audio Engineering Society Annual Meeting in October 2004, which describes the Dolby DigitalPlus technology.
The paper says the new Dolby DigitalPlus codec is based on an earlier version of Dolby Digital, also known as AC-3. The Dolby Digital Plus or Enhanced AC-3 (E-AC-3) retains the metadata carrier, filter library and frame structure. Current data rates range from 32 Kb/s to 6.144 Mb/s. At a sampling rate of 32 kHz and a six-module conversion frame, the resolution of the data rate control can reach 1/3 of a second. (The resolution of the data rate is proportional to the sampling rate and inversely proportional to the size of the frame.)
E-AC-3 retains the six 256 coefficient conversion frame structure of AC-3, but it allows the existence of shorter frames containing one, two and three 256 coefficient conversion modules. As a result, audio transmission can be performed at rates above 6?0 Kb/s, which is suitable for some DVDs that limit the amount of data per frame.
The E-AC-3 can support the current 5.1, 6.1 or 7.1 channels and continue to the cinema's 13.1 channel. The main audio program bitstream plus up to eight additional substreams are multiplexed into a single E-AC-3 bitstream. The coding errors caused by matrix subtraction are eliminated by channel replacement. Compared to AC-3, E-AC-3 can transmit seven independent bit streams.
The increase in coding efficiency can also be achieved through a new filter bank, better quantization, enhanced channel coupling, spectral spreading and a technique called "transient pre-noise processing".
When an audio with stable characteristics appears, the filter bank adds a secondary DCT after the existing AC-3 filter bank. This converts the six 256-factor conversion modules into a single 1536-coefficient hybrid conversion module with improved frequency resolution. This increased frequency resolution combined with six-dimensional vector components (VQ) and gain adaptive components (GAQ) improves the coding efficiency of "difficult to encode" signals, such as clarinet and harpsichord.
VQ is used for frequency bands that require lower accuracy. GAQ is more efficient when higher accuracy components are needed. In addition, the coding efficiency can be improved by the coupling of channel and phase preservation. Where AC-3 uses a high frequency single synthesis channel as the high frequency portion of each channel, the addition of phase information and encoder controlled spectral amplitude processing allows this high frequency single synthesis channel to process lower frequencies, thereby reducing Effective coding bandwidth and increased coding efficiency.
Spectral expansion replaces the higher frequency conversion coefficients with lower frequency segments upconverted in the frequency domain. The spectral features of the converted spectral segment are matched to the original form by spectral modulation of the conversion coefficients.
To improve audio quality at low data rates, the E-AC-3 uses transient pre-noise processing. This post-decoding process minimizes the pre-noise error by using a time-scale synthesis technique that reduces the duration of the pre-noise, thereby reducing the audibility of transient disturbances. The metadata computed by the encoder and transmitted in the E-AC-3 bitstream provides the parameters required for the post-decoding process, the time-scale synthesis process, and the time-scale synthesis process uses the auditory scenario analysis technique.
Laser Radar contains LSPD Safety Laser Scanner and LS laser radar. LSPD safety laser scanner is type 3 with CE certificate. It can be used for agv safety and industrial area protection. LS laser radar is for agv guide. Many famous agv manufacturers has installed LS laser radar to guide their agvs. Our cooperating brand contains Quicktron, Mushiny, Aresbots, etc. Feedback from customers are quite posotive.
Laser Radar
Laser Radar,Auto Guided Vehicle Guide Radar,Sick Laser Radar,Safety Scanner,Safety Laser Scanner,Ls Series Laser Radar
Jining KeLi Photoelectronic Industrial Co.,Ltd , https://www.sdkelien.com