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Pocket-Sized MP3 player

Nov 7, 1999
Update: Jan. 7, 2001


MP3 Player

The MP3 player is a new kind of portable audio player. It will be shipped simultaneously from many audio makers in the early part of 2000, and will start a new generation of portable audio. I have omitted a general description of the MP3 player and format because it is public knowledge.

Well, I built an MP3 player. Let's build something in fashion like that. I aimed to build it small as such as it can be called pocket-size.

I chose an MP3 decoder (key part of the MP3 player) taht was found at the parts shop Wakamatsu Tsusho in Akihabara electoric-town, Tokyo. The player also uses an "Atmel AT90S8515" as a system controller, and SmartMedia as data strage.

SPECS

Main LSIs MCU: AT90S8515 (ATMEL)
DSP: MAS3507D-D8/-F10 (MICRONAS INTERMETALL)
DAC: DAC3550A (MICRONAS INTERMETALL)
Recording media SmartMedia (NAND type flash memory card)
Capacity: 8MB, 16MB, 32MB, 64MB, 128MB
Audio data format MPEG1/2, Layer-2/-3 (depending on the specs of MAS3507D)
Up to 256kbps of MPEG data at 4MHz operation.
Up to 384kbps of MPEG data at 6MHz operation.
The VBR encoded data can also be played.
Control buttons PLAY/PAUSE, STOP, NEXT, PREV,
VOL+, VOL-
Track management All interaction, such as transfer audio data,
delete track, format medium, etc..., is controlled
with a PC via an ISP cable.
Dimensions 90mm(W) x 70mm(D) x 23mm(H)
Weight: 97g (excludes battery)
Power requirments 2 x AAA type alkaline battery
External DC power supply: 2.0-3.5V
Power consumption Playing: 310mW (210mW if used -F10 version).
Pause/Online: 75mW
Standby: 0
Cost 8,000-9,000 JPY (excludes memory card)

The table to the left shows key features into the built MP3 player. For "play" mode, the player is controlled with six buttons and supports only minimal functions.

For the removable media, it uses SmartMedia, which is the most popular memory card for digital cameras. The cards will not be compatible in other equipments because the SmartMedia is formatted in local format when use on this MP3 player.

HARDWARE


Parts Side | Solder Side | QFP on the board
Block diagram
Main board circuit diagram (Jun. 29, '00)
Optional poewr supply and charger
Parts list

Circuit Diagram

The circuit for the MP3 player is composed of three LSI chips plus capacitors, resistors, switches, etc. The DSP decodes input MPEG data and output as IIS digital audio data. While the DAC converts IIS data from DSP to an analog audio signal. The micro-controller controls all of components, such as the other LSI chips, SmartMedia and data flow.

The circuit diagram of the MP3 player is very simple. However, the same can be said of the manufactured MP3 players. Actually, most manufactured MP3 players are also cheaply designed, and add a LCD at the best. They are expensive in that they have built in or attached "memory".

The MAS3507D includes a step-up DC-DC converter. Therefore, it can produce constant voltage without external DC-DC converter from variable voltage such as batteries. It seems the MAS3507D is designed for portable use such as MP3 players. In this player, step up to 3V from 2-3V input voltage for the system power.

This player doesn't have a power switch, instead, the PLAY and STOP buttons serve as the power switch. The PLAY button is connected to power holding circuit. When the PLAY button is pressed, Q1 will be turned on and system powered up. Q1 will be kept by the micro-controller after the PLAY button is released.

P4 is a pin header of six pins and is an ISP connector for the AVR. This connector will be used to program the firmware, and for communication to the host to transfer the sound data or control of the player.

P2 is an interface connector for the memory cards. It is designed to connect directly to SmartMedia, however, I think it can connect to any strage device that works as an MP3 data source such as CompactFlash, MultiMediaCard or PC (LPT port). Of course, the firmware and circuit diagram must be modified when connecting any other device.

Building a main circuit board

The upper right image shows the main board. All LSI chips are QFP to keep the design small and pocket-sized. Also, pin converter boards could not be used due to size limitations, the QFP LSI chips where mounted onto the protoboard directly. To mount the QFP chips, please refer to wiring technic page.

The capacitors and resisters are also chip sized, and they are mounted on both surface of the circuit board. Mounting only single surface is not good, when use a double surfaced protoboard, it should be used both surfaces to mount components.

Building a memory card socket

soket

This was most difficult part because I could not find a socket for the SmartMedia at the market.

Therefore, I built it myself. However, it could be built with the materials found in my junk box and I could build it smaller than any other sockets :-). Now, the memory card socket seems apper on the market.

Audio characteristics

I measured the audio characteristics of the MP3 player with an audio analyzer. Here are the results:

The SNR (signal to noise ratio) is 92dB. Very good considering this is a portable MP3 player!

The THD (total harmonic distortion) at 1 kHz is 0.06%. Not too good :-( It may be due to MP3 compression.

The kind of the MP3 encoder affects THD. Most free encoders cannot generate MP3's with good quality. The 0.06% is a THD when measured with an MP3 file genereted by Fraunhofer's encoder mp3enc.exe, 0.09% or above with any other encoders.

One cell operation

When use the F10 version of MAS3507D, it can also operate with only one cell of AA type battery. If you wish to build as one cell operation, change parts value as follows:

FIRMWARE

This player works in two different operating modes that called Stand Alone Mode and Online Mode.

Stand alone mode

Normaly, the MP3 player operates in this mode, which is a generic portable audio player. All tracks will be played in an infinite loop. This player includes minimun funcitons so that track jump or volume can be adjusted with only six buttons.

Online mode

Power is turned on with the PLAY button. When an ISP cabled is attached to the ISP connector (MOSI is pulled low) the player will enter Online-Mode. The monitor program can then be started on the host PC.

In Online Mode, operations such as format a card, write sound data, delete a track and other opereations are done online commands.


Example of the monitor screen

Connects with an ISP cable, serial data format is N81, 38.4kbps. The terminal program must be running before player powered is on. If not, ER signal will not go high, and the power switch will not be held because reset will not be released.

When adding a track, MPEG data is transferred with fast synchronous mode. It is recommended that a DOS terminal program be used (provided later on this page). Transfer speed in synchronous transfer is between 25k bytes/sec throuh 35k bytes/sec.

Descriptions for the monitor commands are as follows:

To program the firmware

Firmware (hex file) is programmed with AVRSS.EXE via an ISP cable. To program the firmware, power on by pushing the PLAY button. Of course, the PLAY button must be held down until end of programming because controller cannot hold the power state while in ISP mode.

SPECIAL ATTENTIONS

This MP3 player doesn't support specifications of the SmartMedia standard. This player is using the memory card in local format. If a memory card is formatted with this player, it will be unusable on other equipments.

The design of the memory card socket does not allow for hot-plugging. Be sure that the power is turned off when inserting and removing a card. If a card is inserted or removed while player power is on, the card or player could be damaged.

TECHNICAL DATA

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