mp3 playerAlma Rico and Amy WibowoMay 13, 2005TA Charlie KehoeClass 6.111Abstract Have you wondered exactly how your mp3 player accomplishes the tasks of downloadingmusic onto your mp3 player and the playing of the music itself? We did, and that’s why for our6.11 final project, we wanted to make our own. It’s a cool device, it’s useful, and additionally, itmakes for a good project because it modularizes itself quite nicely into well-defined subtask units.We wanted to implement the basic functionalities of an MP3 player: stop, play, pause, select song, and display playlist. An MP3 decoder was used to translate songs from MP3 format into digitalsound information and a D/A converter to change this information into an analog signal to be fedinto speakers. The results were not as we anticipated, but in this report, we discuss our design, test attempts, and the difficulties we had.11 Overview of SystemMp3, which stands for MPEG audio layer 3, is a compression scheme for audio files. It is is acommon format to store songs in, since it takes up little memory. And after decoding the file backinto audio signals, the song can be played through speakers with relatively high quality. For ourfinal project we wanted to create a device that stored mp3 files and decoded them back into audiofiles. The user interface would include ability to view song list and select song for play, along withplay, pause, and stop buttons.Mp3 decoding is a complicated process, so we chose to facilitate the decoding process and insteadfocus on the user interface by using an STA013 MP3 decoding chip. The audio file is stored as thecontents of a rom. The output of the mp3 chip is fed to speakers, so the user can hear the song.The song list is displayed on a 640 x 480 vga monitor, or a 1 line, 16 character led display. Thesong information can be found in the last 128 bytes the mp3 file. The title information specificallyis retrieved from the 4th through 33rd of these bytes. The ascii codes are stored into an SRAM,which is used by a video controller module to paint the picture on the display screen. Up and downbuttons change the song that is selected (indicated by reverse video in the case of the 640 x 480 vgamonitor, or by being in the viewing frame of the 1-line led display), and the song which is playingis indicated by a playing arrow next to its title.2 Description of SystemThe system was divided into an audio component and a video component.2.1 Audio Component (by Alma Rico)MP3 Audio Compression MP3 formatting reduces the amount of bytes that make up a songthrough compression. A perceptual noise shaping scheme is used to convert audio data into MP3formatted music to prevent hurting the quality of the music. By taking into account what thehuman ear can hear, the compression algorithm can reduce the size of a CD-quality song by afactor of 10. For example, MP3 formatting eliminates sounds that the human ear cant detect. Thistype of music compression allows you to download music faster and store more songs in your harddisk without taking up too much space (Brain, Marshall. How MP3 Files Work.).The MP3 audio format is composed of various parts called frames, each containing a headerand audio information. The header information contains the MPEG audio version ID, the layerdescription, the bit rate index, and sampling rate frequency index, and other data relating to theapplication. Meanwhile, the frame contains the actual audio data. At the end of the audio data isan ID tag that contains information describing the audio file (Supurovic, Predrag. MPEG AudioCompression Basics. )STA013 MP3 Decoder Chip By extracting the bit rate information from audio data, the 3Vchip can decode an MP3 data stream into digital audio data. Also, through careful configurationthe decoder can send the control signals to a digital to analog converter.IC Communication Protocol Since the STA013 has a wide range of functionalities, thechip must be configured for a specific application using Philips Semiconductor IC communication2Figure 1: General block diagram of system.protocol. In addition, ST requires that the decoder chip be initialized prior to use. They provide afile with 2007 internal register addresses and data. The need for this initial configuration is neverexplained. The communication bus protocol consists of two lines: SDA (data) and SCL (clock).IC is designed for multiple master-slave interactions, but in this case only the FPGA acts as themaster and the chip as the slave.The main operations used to communicate with the MP3 decoder are: start condition, stopcondition, send a byte, and receive a byte. These four steps compose the read and write functions.Both the SDA and SCL lines should be idle prior to communication, where both are set to high.A master initializes communication with a start condition and ends with a stop condition. A startcondition consists of setting the SDA line low while SCL is high, while a stop condition is a changefrom low to high of the SDA line while SCL is high. Other than the start and stop conditions,SDA should never change during the high period of the SCL. Since the SCL line behaves as aclock during the master-slave communication, it dictates the time at which bits are sent. Whetherthe byte is sent from master to slave or slave to master, the SDA line sends one bit every lowclock period for eight clock periods. During a ninth cycle, the device receiving the data sends anacknowledge signal on the SDA line by setting the line low. The figure below shows the behavior3of the four basic operations.Figure 2: Basic IC operations schematic. (Philips Semiconductors, “The IC Bus Specification.”)Tri-state buffer and MuxThe operations described above are combined to create read and write functions. The FPGAcan read from the MP3 decoder by following these steps:(1) Start Condition(2) Send 7-bit Device Address plus R/Wbar bit set to 0(3) Send Sub-Address Byte(4) Stop Condition(5) Start Condition(6) Send 7-bit Device Address plus R/Wbar bit set to 1(7) Receive a Byte(8) Stop ConditionThe STA013 device address assigned by Philip’ss Semiconductors is 0x86. The least significantbit during a send a byte operation is used to indicate a read or a write. A logical 1 indicates a readwhile a 0 indicates a write. Likewise, the write function consists of these steps:(1) Start Condition(2) Send 7-bit Device Address plus R/Wbar bit set to 0(3) Send value of Address to write byte4(4) Send value of Data
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