Daily News: MP3 SoC Design Challenges and Solutions

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Daily News: MP3 SoC Design Challenges and Solutions [Copy link]

In recent years, portable audio players (commonly known as MP3) have become the brightest spot in the consumer electronics market. At the same time, competition among chip suppliers has become increasingly fierce. The competition of consumer electronics products always focuses on the development of higher performance, lower cost, higher integration and lower power consumption, and portable audio player chips have more prominently demonstrated the demand for these aspects, making the contradiction between performance, cost and power consumption increasingly a challenge for chip design. The following uses Zhuhai Actions' ATJ2097 as an example to introduce how chip suppliers deal with these challenges.

· Happiness must also be safe Pentax waterproof DC drops below 2,000 · Competition with PSP: Ubit X1000 Review · MSI 512M MP3 is also cheap MS-5520 only 199 The cheapest CCD camera Fuji A400 is less than 1,000 yuan Recommended compact and powerful card cameras · The first choice for non-Japanese telephoto Kodak P850 dropped to 2850 600W +10X Aoji SP-500 reduced to 2500 yuan 2.2 - inch TFT Aino new product V600 will be launched soon · Only the kit is sold, the price is announced, Panasonic L1 is launched in Hong Kong Professional post-production software SILKYPIX is upgraded to be more compatible · Happiness must also be safe Pentax waterproof DC drops below 2,000 · Competition with PSP: Ubit X1000 Review · MSI 512M MP3 is also cheap MS-5520 only 199 The cheapest CCD camera Fuji A400 is less than 1,000 yuan Recommended compact and powerful card cameras · The first choice for non-Japanese telephoto Kodak P850 dropped to 2850 600W +10X Aoji SP-500 reduced to 2500 yuan 2.2 - inch TFT Aino new product V600 will be launched soon · Only the kit is sold, the price is announced, Panasonic L1 is launched in Hong Kong Professional post-production software SILKYPIX is upgraded to be more compatible · Happiness must also be safe Pentax waterproof DC drops below 2,000 · Competition with PSP: Ubit X1000 Review · MSI 512M MP3 is also cheap MS-5520 only 199 The cheapest CCD camera Fuji A400 is less than 1,000 yuan Recommended compact and powerful card cameras · The first choice for non-Japanese telephoto Kodak P850 dropped to 2850 600W +10X Aoji SP-500 reduced to 2500 yuan 2.2 - inch TFT Aino new product V600 will be launched soon · Only the kit is sold, the price is announced, Panasonic L1 is launched in Hong Kong Professional post-production software SILKYPIX is upgraded to be more compatible

　　In recent years, portable audio players (commonly known as MP3) have become the brightest spot in the consumer electronics market. At the same time, the competition among chip suppliers is becoming increasingly fierce. The competition of consumer electronic products is always focused on the development of higher performance, lower cost, higher integration and lower power consumption, and portable audio player chips have more prominently demonstrated the demand for these aspects, making the contradiction between performance, cost and power consumption increasingly a challenge for chip design. The following is an example of how chip suppliers deal with these challenges, taking Zhuhai Actions' ATJ2097 as an example. High system integration and few

　　peripheral components

　　For portable audio players, consumers have always pursued a smaller and more fashionable appearance, as well as as many functions as possible. This requires a high degree of integration of the system chip, and even some peripheral passive components (such as resistors and capacitors) should be as few as possible; and due to the relationship between cost and size, try to avoid using large components, such as large-capacity electrolytic capacitors. In addition, the size of the chip (not only the silicon wafer should be small, but also the packaged chip) must be as small as possible, and the fewer the pins of the chip, the better. For example, for both LQFP packages, a chip with 100 pins is 44% larger than a chip with 64 pins. The integration of numerous functional modules, the mixed integration of a large number of digital and analog circuits, the guarantee of the production yield and testability of large-scale integrated circuits, etc., all severely test the design and production capabilities of chip suppliers. At present, Zhuhai Actions can provide 64-pin portable audio playback system integrated chips (ATJ2085, ATJ2095, etc.).

　　The system architecture of ATJ2097, the system chip mainly includes six parts: main processor module, off-chip storage and control interface module, PC interface module (high-speed USB), audio input and output module, power module and storage unit module. Only a small number of components that cannot be integrated, such as FLASH memory and buttons, can form a portable audio player. Low power design

　　Portable

　　products have strict requirements on the power consumption of the whole machine. Not only does it require the player to have low power consumption under normal working conditions (which is manifested as long playback time on the end user side), but it also requires the player's standby current to be very small, because no one wants to unplug the battery when not listening to music.

　　ATJ2097 can control the standby current of the whole machine under lithium battery power supply mode to less than 160uA by precisely controlling each module in the chip, strictly stopping the clocks of different functional modules and shutting off some bias currents, etc. That is to say, if a common 720 mAh lithium battery is used, the player can still work normally after being placed for half a year.

　　To reduce the power consumption of the player under normal working conditions, efforts need to be made in two directions. First, try to improve the conversion efficiency of the power supply; the player chip usually integrates a high conversion efficiency step-up and step-down switching power supply (DC-DC); second, the power consumption of the decoding circuit (or processor) and the audio circuit should be low. Taking MP3 playback as an example, the following focuses on the efforts to reduce the power consumption of the MP3 decoding circuit.

　　The functions with large computational volume in the MP3 decoding algorithm are mainly: HUFFMAN (about 20%), DEQUANTIZE (about 18%), IMDCT (about 18%), SYNTHESIS (about 20-25%), which account for about 70-90%, almost determining all the workload of the decoder. ATJ2097 has made great optimizations to these functions. Here we will only briefly describe the optimization methods of some functions.

　　As we all know, HUFFMAN is a classic and effective variable-length lossless coding method proposed in the early 1950s. It constructs the shortest average length code based on the probability of character occurrence, but its decoding complexity is higher than encoding. MP3 uses 34 HUFFMAN tables (some of which are the same), 32 of which are used for simultaneous encoding of two data, and the other 2 are used for simultaneous encoding of 4 data.

　　When decoding, if the traditional HUFFMAN tree is used for bitwise expansion, that is, 1 bit is input each time, if the value is 1, it is represented as the right node, and if it is 0, it is represented as the left node. By searching the HUFFMAN tree until a leaf node is found (the value in the table is 0 for the leaf node, otherwise it is the offset value of the next node in the table relative to the current node), when the codeword length is long, the decoding cycle required is also long. ATJ2097 uses parallel HUFFMAN decoding to solve this problem well. The basic idea is to reconstruct a new HUFFMAN table by saving HUFFMAN codewords. When decoding, the decoded data stream with a fixed length of 24 bits is input each time, and the decoding is performed by searching the new code HUFFMAN table. This significantly speeds up the decoding speed and saves the storage space of the HUFFMAN tree.

　　For example, the MDCT function, which is only used in MP3, uses 18 samples in long blocks and 6 samples in short blocks in MP3 to have a good resolution in both time and frequency domains. Therefore, the required amount of calculation and storage space will be quite large. In ATJ2097, the algorithm is fully optimized, and the focus is mainly on optimizing the algorithm by making full use of the address generator and other characteristics of the DSP processor. Finally, the amount of calculation and storage space of the MDCT function are reduced by nearly half, which reduces the cost and significantly reduces the power consumption of the player.

　　On the other hand, ATJ2097 is also carefully designed in the hardware circuit. As we all know, the power consumption of digital circuits such as processors, memories, and logic interfaces is approximately equal to F×C×V2. Therefore, in order to reduce power consumption, in addition to reducing the amount of calculations to reduce the operating frequency, a lot of efforts can be made to reduce the number of logic gates and compress the circuit's working voltage. Reducing the number of logic gates is one of the efforts of almost all digital circuit designers, and the methods of various chip suppliers have their own focuses, which are nothing more than improving the efficiency of the system architecture, reducing unnecessary functions, and writing hardware description statements in more specific details. The same is true for ATJ2097.

　　In terms of compressing the circuit's working voltage, when the working voltage is reduced, the power consumption of the logic device is reduced, and the reaction speed will also decrease significantly. Therefore, the corresponding technology used by ATJ2097 is to improve the timing margin of the logic circuit, reduce the absolute delay in the unit clock cycle, and reasonably split the time-consuming logic operations into different clock cycles. The more obvious manifestation is that the core operating voltage of the system chip designed and produced using the 0.25um process should usually be 2.5V, while the ATJ2097 can easily work at 1.8V or even 1.6V, which naturally greatly reduces the power consumption of the core.

　　Provide Hi-Fi sound quality

　　Hi-Fi is the abbreviation of High-Fidelity, which means "high fidelity" for the audio system. As the name suggests, it is to reproduce the playback sound that is highly similar to the original sound. Of course, the "Hi-Fi" generally mentioned now is already a relative concept, usually referring to a high-quality audio recording and playback system. For general audiophiles, due to the lack of instruments and measurement methods, the sound quality of the player is more based on subjective listening.

　　Consumers' requirements for the sound quality of portable audio players are mainly: the timbre of musical instruments and vocals is real, clear positioning, rich sense of space and three-dimensionality, appropriate reverberation, high signal-to-noise ratio, sufficiently large dynamic range, balanced tone ratio, sufficient power reserve, etc. Only when these requirements are met can it be called a Hi-Fi playback system.

　　As a digital audio playback device, the audio player must first decode and restore the digital audio files on the storage medium, usually generating PCM data, then converting it into analog audio signals through a digital-to-analog converter (DAC), and finally outputting it through a power amplifier to drive headphones or speakers to produce sound. It can be seen that there are two links that affect the sound quality. The first is to decode the source file with high quality to minimize the distortion and damage to the sound quality; the second is that the audio indicators of the digital-to-analog converter (DAC) and the power amplifier must meet or even exceed the Hi-Fi standards.

　　From the above ATJ2097 system architecture diagram, we can see that the player system chip contains a 24-bit fixed-point DSP. By carefully handling the overflow and the choice of mantissa in the operation process, the digital precision of 24-bit word length is sufficient to ensure that the decoding process does not cause any damage to the sound quality.

　　The decoded PCM data enters the 18-bit Sigma-Delta architecture digital-to-analog converter integrated in the system chip, generating high-quality analog audio signals, and directly drives the headphones to sound after passing through the integrated volume adjustment unit and the power amplifier up to 14 milliwatts. Since the player chip contains a large number of digital circuits and storage units, they will generate serious noise interference during operation; and in order to obtain higher power conversion efficiency, portable audio players almost all use step-up or step-down switching power supplies (i.e. DC-DC), so the power supply noise of the analog circuit inside the chip is as high as hundreds of millivolts; coupled with the restrictions on the number of pins and the number of peripheral components mentioned above, the power supply and ground of each analog circuit in the chip cannot be separated very cleanly, and the peripheral power supply filter circuit is also very limited due to the size; therefore, the electromagnetic environment and power supply pollution faced by the audio analog circuit part of the chip are very bad.

　　ATJ2097 uses three technologies to deal with this problem. First, make full use of the N-well and P-well of the integrated circuit. On the one hand, the low-resistance layer on the surface of the substrate is cut off by the depth of the well to reduce the internal resistance between different grounds. On the other hand, many independent N-wells are reverse biased separately to form independent and relatively pure power islands, so as to obtain better interface isolation between digital circuits and analog circuits and chip substrate isolation. Second, the power supply and ground loops of each part of the analog circuit are cleverly planned to make the impedance of the power supply and ground loops low and the plane area of the entire loop small to suppress electromagnetic induction pollution. Third, the power supply noise suppression capability (PSRR) of the analog circuit is improved by adopting a fully differential circuit structure, adjusting the size and static working current of the circuit components, and selecting a suitable operating frequency. The focus is on the operating frequency of the switching power supply, which is a frequency band of hundreds of kilohertz to several megahertz. Through the efforts in the above three aspects, ATJ2097 finally achieved a signal-to-noise ratio of more than 90dB (excluding A-Weight), and when the output power is as high as 14mW to drive the 16-ohm impedance headphones, the THD can still be as low as -85dB, and the channel isolation is as high as 80dB, which fully exceeds the hardware indicators required by Hi-Fi audio equipment.

　　In addition, providing the function of equalizer to render the music appropriately is also an important function of the audio player. The general equalizer can only adjust the three frequency bands of high frequency, medium frequency and low frequency respectively. ATJ2097 can adjust the 7 frequency bands in the range of 20-20 kHz for various styles such as ROCK, POP, CLASSIC, SOFT, JAZZ, DBB, etc. according to the response characteristics of each sound band. At the same time, it can also provide the function of SRS WOW sound effect. WOW is a patented technology that can significantly enhance the quality of stereo audio playback. Through this set of technology, the sound field can be widened to show a more spatial stereo surround sound and the shocking effect of the subwoofer.

　　Of course, to produce high-quality audio players, downstream machine manufacturers must also reasonably arrange the structure inside the machine, strictly select the components of the chip peripheral circuit according to the requirements of the chip supplier, and carefully arrange the layout and wiring of the PCB. In this regard, ATJ2097 also comes with a complete set of standard application solutions and design guidelines for manufacturers to refer to.

　　Conclusion

　　At present, the market for portable audio players is still rising, and with the continuous improvement of consumer requirements and the continuous improvement of chip supplier design technology, future portable players will continue to develop in terms of sound quality, sound effects, cost, playback time, and expansion of application areas. Audio players that truly have CD sound quality, as well as multimedia players with functions such as video playback, games, and mobile TV will become more and more common.