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Thursday, May 7, 2009

Graphic MP3 Player

The MP3 players consist of only semiconductor parts and no complex mechanics. This is a great feature for electronics handicrafts because it can be built easy with the same performance as commercial products except for the appearance. After a long blank in MP3 project, I built a new one again as the second MP3 project.

The first MP3 project was Pocket sized MP3 Player built with an MP3 chipset that obtained by chance. I have completed some projects of MP3 application for my business but not for hobby because I have no practice of listening music at the outdoors. Why did I built it in pocket size? Because that was only an impulse and nothing else. BTW, the MP3 player is being used in my car as a car MP3 player :-)

Now, there are many easy-to-use MP3 decoders that integrates DSP, DAC and amplifier on a chip. As the result, the MP3 player becomes to a popular project for electronics handiworks and everybody is enjoying to build it as their original project. One day I got to want to build an MP3 player by a reason (described below) and decided to start a new project. This is a regular project on the MP3 player after 8 years. I designed it as a desktop player because portable player is not useful for me.


Below image shows the block diagram and the circuit diagram of built MP3 player. It has a feature that it has a large color LCD and a touch screen. Followings describe on each block.


A V850ES/JG2 (NEC Electronics) is used for system control. This is a 32-bit RISC microcontroller with 256KB flash and 24KB RAM. It was not that well known for electronics handiworks but somebody will be interesting in it because the V850 board was bundled as supplement of magazine in this year. For ordinary MP3 players, most 8-bit microcontroller is sufficient to build it. However this project requires a microcontroller with external memory interface because the controller must handle large amount of image data. Of course any popular microcontrollers, such as Renesas SH2 and H8, will able to be used as well. The reason why I chose the V850 is from its very low power consumption and the serial interface is easy-to-use better than Renesas's one. The V850ES/JG2 can run at 20 MHz but it is used at 14.7 MHz (4xPLL from 3.68 MHz xtal) to deliver a clock signal to MP3 decoder and LCDC.

Storage Media

SD Memory Card is the de facto standard of flash memory card. It can be attached to the microcontroller via a few signal lines. FAT format is used to store data files in it so that the project using the memory card must implement the FAT file system. Fortunately, there are various FAT libraries on the web as freeware so that everybody can use the memory card in their project with ease.

LCD Module

A 4″ color STN-LCD module in resolution of 320x240 is used for the display. Recently the price of color TFT-LCD are going falling and color STN-LCD will soon be shut out of the market. This project was started to to use this LCD module before it decays in the junk box.

A touch screen is attached on the LCD module, so that command buttons can be omitted. A CCFL is used for the back light and a CCFL inverter is required to drive the back light.

LCD Controller

The LCD module in this degree of resolution with built-in display buffer is not available and it must be refreshed by external circuit like CRT display system. Therefor it requires an additional LCD controller on the board to drive the graphic LCD module. The display buffer (RAM) is integrated in the LCDC or attached externally. In this project, an S1D13705 (EPSON) is used for LCD control. The S1D13705 has 80KB integrated display buffer and it can display in resolution of 320 by 240 with 8-bit color depth (256/4096 indexed color). It can be attached to the host controller via a 16-bit SRAM like interface.

The LCD module requires a 3.3V logic supply and an LCD bias supply (21-25V/3mA). Generally, the contrast of the STN-LCD in high drive duty ratio is affected by ambient temperature, so that a contrast dial is required to adjust the contrast. The contrast dial varies the LCD bias voltage. In this project, the contrast is adjusted automatically in software with a thermister put on the LCD module and a D-A converter.

MP3 Decoder

Recently VS10xx family (VLSI Solutions) is used for most home-built MP3 projects because it is easy to obtain and use. The VS10xx is designed for portable audio equipments and can drive a headphone directly. However it has only analog outputs and no digital (I2S) output, so that the VS10xx is not good when require an I2S output to attach an external DAC besides the analog performance is not good for Hi-Fi audio.

I chose STA013 (ST Microelectronics) for MP3 decoding. The STA013 have been released at the dawn of the MP3 format and widely used as a well known MP3 decoder chip. It has only digital (I2S) output, so that a proper audio DAC is required. This is an advantage on electronic handiworks because it can use various DAC chips and output audio data in SPDIF format with DAI encoder.

The STA013 has an integrated PLL oscillator to generate an audio timing clock (384fs) depends on DSP clock. If there is a jitter on the sampling clock, SNR of the analog output will be worse especially on Sigma-Delta DAC. Therefore the PLL power should be tightly filtered and pay attention to parts layout around the loop filter.

Analog Block
A PCM1748KE (BurrBrown) is used. The analog performance on the data sheet is not so bad but it is a little difficult to achieve expected performance because it is a Sigma-Delta DAC.

A post-filter is required at DAC output but only a slow roll-off one will do due to integrated 8x over sampling digital filter. In this circuit, the DAC output is filtered with a LPF+buffer and then output it as a line output and there is no speaker out. Therefore the MP3 player is used with any audio power amplifier.

The line output is tied to ADC input of the microcontroller. This is to get amplitude of the line output and display it as a level meter on the LCD. Generally an envelope detector is used for the audio level meter. In this project, to eliminate the envelope detector, the microcontroller samples waveform in sampling rate of 1kHz and detects peak-to-peak value from 20 samples. There will be dip points on the frequency response but there is no problem because this is for only a visual effect.

Building the MP3 Player

The circuit board must be embedded into a space in height of 10mm, so that the allowable height of the components on the circuit board is less than 6mm. Most of components used in this project ware surface mounted device. They are mounted on the proto-board directly and wired with UEW. This method requires soldering skill and practices but there is an advantage that it can achieve the density of double layered PCB or more. The FPC connector (0.5mm) easily creates solder bridge due to its terminal forms, so that solder the wire to the terminal via a stripe PCB instead of solder the wire directly.

The built circuit board is embedded with the LCD module into the case. The case is a clear acrylic case SK-16 (110x78x32mm) sold from Akizuki. Its depth was too long for this project, so that I cut down it to 25mm, paint black from inside and put an aluminum sheet for electromagnetic shield.

Free FirmWare

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