Portable MP3 player design

Abstract: The portable MP3 player and its system structure and various functional units are introduced. On this basis, several existing design methods based on ARM, DSP and dedicated decoder chips are compared. The focus is on the choice of the chip and the issues that should be noted when using a dedicated decoder chip. Finally, some discussion on the development trend of portable MP3 players.

Keywords: MP3 decoder chip MP3 player SOC

For digital music product enthusiasts, portable MP3 players are no stranger. There are already many brands abroad, some of which have successfully entered the domestic market, such as Singapore Innovation Technology, South Korea's Samsung Electronics and the United States Diamond, and other companies in China and Lenovo and Malata. In addition, many companies in China are or are preparing to develop corresponding products.

The music CD has a 44.1kHz 16Bits stereo audio quality, and a CD can store 74min songs (about 15 or so). How to compress these songs losslessly or substantially without loss, so that more songs are stored in the same space is a research direction of audio signal processing. By using the MPEG1 Layer 3 encoding technique, a lossy music signal compressed at approximately 12:1 can be obtained. Although MP3 music is lossy, it loses the signal with weak power spectrum during compression, but it is not much different from CD original sound, and does not affect the appreciation of music by ordinary music lovers. MP3 greatly reduces the length of sound files, making music storage and transmission more convenient. At present, MP3 music resources on the Internet are very rich, various MP3 music CDs have been put on the market, and there are many application software packages that convert original CD music and tape recordings into MP3, which will promote the popularity of MP3 music. Although the current copyright disputes caused by MP3 continue, with the launch of the SDMI (Secure Digital Music Initiative) standard, copyright issues will eventually be resolved. Because portable MP3 players have the advantages of small size, light weight, long continuous playing time, etc., they are increasingly favored by music lovers, and will replace traditional cassette tape recorders and CD players.

1 System structure of portable MP3 player

The block diagram of the MP3 player is shown in Figure 1.

1.1 memory

There are three main types of memory types for portable MP3 players that are currently on the market:

·FLASH Nand Flash chip, Compact Flash card, SMC card, MMC card, SD card, Sony memory stick;

Magnetic memory: 2.5-inch hard drive, IBM Micro-Driver, Iomega CLICK disk;

Optical storage: Audio-CD, CD-ROM, DVD, Data-play.

The FLASH chip has the remarkable advantages of high reliability, small size, no mechanical contact and light weight, and is the mainstream memory of the current portable MP3 player. The capacity of single-chip FLASH chips is increasing. Samsung Semiconductor has introduced a 256MB single-chip Nand FLASH chip. Hitachi and Toshiba Semiconductor have introduced a 128MB single-chip Nand FLASH chip. Moreover, the price reduction and space of FLASH are very large. At present, the price of 32MB FLASH has dropped to about 10 dollars, only 20% of the price in early 1999, and there is still room for price reduction in the future. It is believed that Nand FLASH will remain the mainstream memory for portable MP3 players in the next few years. The reason why Nand FLASH is better than Nor FLASH in price depends largely on its standard of allowing 1% of bad sectors. Nor FLASH must be 100% qualified and does not allow any one to be damaged. This is a must note in the development of portable MP3. Firmware must have a method of marking and avoiding bad sectors.

1.2 Communication interface

Commonly used communication interfaces are as follows: the print port is generally set to ECP or EPP mode. Early MP3 players often use this method for data transmission, but because this method requires the user to set the computer's print port mode, the user It is inconvenient to use, so the new model MP3 player no longer adopts this interface mode; USB1.1 interface, because of its fast speed and plug and play, is the mainstream data transmission mode of MP3 player; IEEE1394 The interface, due to its transmission speed of up to 350Mbps, has been adopted in high-end MP3 players using large-capacity hard disks as memory. I believe that such products will be launched in the near future.

Of course, USB 2.0 will also become the future interface standard for hard disk-based MP3 players. In addition, some MP3 players do not need to use a built-in Modem to download music over the Internet without using a PC.

1.3 decoding chip

At present, the main MP3 decoder chip suppliers on the market are: ST company, Germany Micronas company, South Korea Samsung company. The performance comparison of the main chips is shown in Table 1.

In Table 1, the SOC (System On Chip) of the portable MP3 player refers to all functional modules except the data memory, including the USB interface and the audio CODEC and the headphone driving line. The SOC-only lacks the audio CODEC compared with the SOC. . The MP3CODEC chip can complete MP3 decoding and MP3 timely encoding. The CD/MP3 chip is integrated with a CD-ROM decoder function and is suitable for portable CD/MP3 products. Some IC design companies in China are also targeting the portable MP3 player market and are designing and developing their own MP3 decoder chips. But unfortunately, domestic IC design companies seem to pay more attention to the specific implementation of IC, and pay less attention to the initial ideas of IC. Therefore, they are always in a passive state in this fast-growing market. Even in the early stage of design, they tracked relatively advanced ICs abroad. After the design of its own IC is completed, foreign countries have already introduced more advanced and lower-cost SOC products. According to the author's work experience, it is recommended that when designing such a digital IC, once the verification work on the FPGA is completed, it should immediately communicate with the customer or potential customers to obtain the customer's opinion, and do not take it for granted.

1.4 DAC and headphone amplifier chip

If you choose an MP3 dedicated SOC such as the SAA7750, you don't have to consider the DAC and headphone amplifier chips because it integrates all of them. However, if the selected MP3 decoder chip does not have direct analog audio output, you must choose a suitable DAC chip and headphone amplifier chip. The current major suppliers of audio DACs include Philips, Cirrus Logic, TI and AKM. The choice of DAC and headphone amplifiers directly affects the sound quality and must be considered as one of the key points in design.

2 Design method

At present, portable MP3 is basically divided into three types based on a dedicated decoding chip, a DSP or an ARM chip.

2.1 Based on dedicated MP3 decoder chip

There are many kinds of dedicated MP3 decoder chips on the market, which are indicated in Table 1 as ASICs. In general, the ASIC chip completes the task of MP3 decoding. The designer only needs to select the commonly used 51 series or other 8-bit low-end CPUs as the main controller for management work. The basic functions of the main controller are communication, memory management, LCD display and keyboard management, as well as providing MP3 data streams for the ASIC chip. Of course, choosing a CPU with both USB interface and LCD driver can reduce PCB size and reduce total system cost. The principle of selecting a decoder chip is to prioritize price and power consumption under the premise of guarantee function.

The advantage of this design method is that the development is relatively simple and the market is fast. The disadvantage is that the system cannot be upgraded, it is difficult to adapt to the new audio compression format, and the system cost is relatively high.

The AT89C51SND1A from American ATMEL and the S3FB42F from South Korea's Samsung integrate integrated functions such as MP3 decoder, USB interface, 8-bit CPU and Firmware memory, and only need external audio DAC and data memory, but its basic structure remains. It belongs to the low-end CPU plus decoding chip mode, but only integrates various components to reduce system cost.

2.2 DSP-based design method

At present, TI and ADI have introduced a DSP-based MP3 solution, which uses the same DSP to perform all functions, and only needs an external audio DAC. SingmaTel's MP3 chip based on the Motorola 56K DSP integrates an audio DAC and headphone amplifier circuitry.

The advantage of this design method is that the system has strong upgrade capability, can be compatible with multiple audio compression formats and even future audio compression formats, and the system cost is low; the disadvantage is that the development is difficult, the designer needs to transplant the MP3 decoding algorithm to the corresponding DSP. Go in the chip.

2.3 ARM-based design method

The ARM 32-bit CPU is currently the most popular 32-bit embedded CPU. The ARM-based MP3 decoding program developed by ARM has been licensed to many companies such as Philips of the Netherlands, Micronas of Germany and Cirrus Logic of the United States. Philips' SAA7750 is the most integrated ARM720T-based MP3 player SOC. It integrates an ARM720T core, a 24-bit DSP core (providing a variety of audio processing), 284KB FLASH and 64KB SRAM, USB1.1 interface and audio CODEC, and also integrates a CD-ROM decoder, so it is also currently the most suitable for Portable CD/MP3 chip. Cirrus Logic's EP7312 family of ARM chips requires an external USB interface chip, Firmware memory and an external audio DAC.

The advantages and disadvantages of the ARM-based solution are the same as those based on the DSP.

Since ARM is a company that does not produce final IC products and only sells IP (Intelligence Property), it can sell various ARM core IPs to any IC designer and electronics manufacturer. Perhaps the purchase of ARM's IP plus its own special features is the best way for domestic IC design companies to design their own dedicated digital audio player ICs.

3 Development direction

3.1 Small size and light weight

Compared with traditional music players, the main advantage of MP3 players is their small size and light weight. There are now MP3 players with only BP and watch sizes on the market. The smaller the volume, the lighter the weight is one direction of MP3 player development.

3.2 Nand Flash as the mainstream data storage

As a portable MP3 player, price, memory capacity and continuous playback time are all important considerations for users. But the storage capacity is not as big as possible. I believe that not many users will like thousands of songs at the same time. Based on FLASH memory, the capacity is 256MB, and the MP3 player can store 4 hours of songs. Due to its price, size, weight and power consumption advantages, it will be more popular than the ultra-large capacity MP3 player based on hard disk. This is because even if the hard disk capacity is large, you still need to get a new song on the PC. Moreover, the more songs are stored at the same time, the more inconvenient it is to listen to songs.

3.3 USB mass storage class

Most portable MP3 players currently need to install the corresponding PC software to download songs, which requires developers to spend a lot of time writing PC programs. If the user wants to use the same MP3 player in both the company and the home, the corresponding software must be installed in both places, which is inconvenient for the user. Microsoft's Windows 2000 and Windows XP operating systems can directly support the USB mass storage class. Now many foreign companies have begun designing MP3 players that support the USB mass storage class. I believe that products will be available soon. This kind of MP3 player, the user does not need to install any software and drivers, directly connect the player to the USB port of the PC, the operating system will find a new mobile memory, the user only needs to simply copy the file to the player. . This allows users to download music anywhere, on any computer, without having to install any new software.

3.4 Multi-functional design

Some additional features can be added to increase the selling point without increasing system cost or adding little system cost. Additional features that can be considered are: FM radio, long-time recording, and some PDA functions.

3.5 integrated speaker

Integrating two speakers on the existing portable MP3 to replace the traditional cassette recorder, similar products will soon be available.

3.6 USB Audio Class

The portable MP3 player that supports the USB Audio Class can be a portable external PC sound card. Since Microsoft's Windows 2000 and later operating systems support the USB Audio Class, users do not need to install any programs. There are portable MP3 players that can support voice recording. Just add audio input interface and upgrade Firmware to support USB Audio Class. The USB interface supports isochronous transfer mode (Isochronous) is a prerequisite for supporting USB Audio Class. The USB ports of both SAA7750 and PUC3030A chips support Isochronous transfer mode.