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MP3 PLAYERS FOR YOUR CAR

by Rick Prescott

As the MP3 format continues to surge in popularity, a growing number of digital-music devotees are taking their music on the road. In the past few months, an informal network of people building relatively inexpensive systems has sprung up. Most of them use Linux, and all of them are ready to rock.

The homemade MP3 car-player movement has grown because most of the people building systems post specs on their web sites. They feature detailed, technical discussions of everything from motherboards to custom metal cases, including pictures, component charts, and prices.

Consumer models of mobile MP3 players are in the works. The owners of Great Britain-based Empeg plan to start shipping mass-produced MP3 car players in March. "About 4000 people (60% from the United States) have already signed up to buy the $999 players," Empeg's Steve Sanders wrote in an e-mail. The player is roughly the size of a car radio and holds about 35 h of music. (more)

What is MP3?

MP3, or MPEG3, stands for Moving Pictures Expert Group Layer III. The Moving Pictures Expert Group is the organization responsible for developing the format, and "Layer III" refers to the third revision of the compression format. The current version, MP3, has a compression ratio of 1:12 and a minimum bandwidth of 128 Kbps, meaning every 8 s of audio requires 128 Kb of storage space. Compressing an audio file to MP3 can trim the file size to less than a tenth of the original file with minimal loss in quality. In English, the MP3 technology makes large audio files much smaller, making them easier to download, copy, and store. The MP3 encoder cheats a bit, by removing the parts of the recording not audible to the human ear. Although MP3 isn't quite as clear and rich as CD sound, it's pretty close. MP3 files can be created from music on CDs (or any analog or digital recording from .WAV files to DAT) or downloaded from thousands of sites on the Internet.

So, what's involved in building an MP3 player?

1) What is the source of the MP3 files?
2) What microprocessor are you going to use?
3) What features do I want?
4) What overall parts do I need?
5) How do I assemble everything together?

If you can answer all of these questions easily, then you shouldn't have any real difficulty in designing and building your own MP3 player. If you're stuck on any of these questions, you can simple see how other people did their project to get good ideas.

1) What is the source of the MP3 files?

In other words, where are the MP3 files going to be retrieved? In most cases, people want the MP3 files to come off of a compact disc or a hard drive because of their mass storage capabilities. This method is extremely complicated and requires reading through hundreds of pages of standards and technical documents. Fortunately, there's already software code and schematics available to make this process easier.

And for portable reasons, you might decide on using a flash memory card (like 90% of all portable commercial players). Unfortunately, getting a hold of the materials needed to use and program a memory card can be difficult and far too expensive.

Another simple (yet, not very practical) option is to download the MP3 from the computer parallel port. This method should probably be your first choice because you don't need a lot of hardware, and it's good for "testing" your player. After your player is capable of decoding MP3 files from the parallel port into music (thus performing a successful test) you can then add a CD-ROM or hard drive interface.

2) What microprocessor are you going to use?

This is probably going to be the most critical decision about your player that you're going to make. It will require reading manuals and doing research to try to decide which processor is right for you. When choosing a processor you need to keep these things in mind...

- Can I get the processor and how much does it cost?
- If needed, can I get a hold of a C compiler for it?
- Does it have enough I/O pins?
- If needed, can I interface an IDE device to it?
- Am I able to easily mount it to a project board? PLCC or PQFP?

Remember, the processor is going to be responsible for taking the MP3 data from the source and sending it to the MP3 decoder chip. If needed, it will also be responsible for controlling a small keyboard, LCD display, and I2C communication to and from the MP3 decoder chip.

However, if you just want to make a simple MP3 player that will decode MP3 files coming out of the computer parallel port, you may not need a processor at all.

3) What features do I want?

If you want to be really fancy, you can implement additional features to the MP3 player, features that will make it more practical and user friendly. Here's a list of ideal features that your player should have:

- Some kind of display or LCD panel to show which MP3 file is currently being played, what's the current status of the song (time remaining, etc.), whether the song is playing in MONO or STEREO. The same kind of information that WinAmp would display (maybe not as fancy though).
- A simple keyboard interface with all the buttons that are similar to a regular CD-player (play, stop, next, pause, volume, etc.)
- A variety of inputs and outputs (outputs to a stereo system, headphone jack, parallel port connector to download songs (if a hard drive is being implemented), remote control, etc.)

4) What overall parts do I need? Here's a sample of what your block diagram should look like assuming you're going to build the ideal MP3 player:

Here are some example parts that you need to build your player (I'll list more than one option under each heading):

SOURCE:
- regular computer IDE CD-ROM drive
- regular computer IDE hard drive
- DB-25 female connector for parallel port

MAIN PROCESSOR:
- Microchip PIC processor (make sure it has enough I/O pins)
- Motorola 68HC11
- Motorola 68000 series
- Atmel AT90S8515
- Texas Instruments

TMS320C31 MP3 DECODER CHIP:
- Micronas Intermetall MAS3507D (most popular)
- STMicroelectronics STA013
- Cirrus Logic EP7209

STEREO DAC:
- Micronas Intermetall DAC3550A (most popular)
- Crystal Semiconductor CS4334
- Burr-Brown PCM1723

LCD DISPLAY:
- really any plain LCD panel will do, make sure it has enough room to display all the information you want, and MAKE SURE you have documentation with it.

5) How do I assemble everything together?

This part of the design is really up to which parts you decide to choose. This is where everything starts to get complicated, and you really should have knowledge of microprocessor interfacing and troubleshooting. Sure, anybody can get to the point of knowing which parts to use, but actually putting them all together is what it all comes down to.

Knowledge of soldering or wire wrapping, digital glue logic, noise filters and decoupling, ability to manipulate signals, measurements and readings, machine-level programming, technical document comprehension, and so on. If you are unable to perform these electronic tasks, then you are not ready to build your project. Just one simple mistake can go unnoticed for months, and you're left wondering why your player isn't performing. Even if you have somebody else's complete schematics and software code, you will not be able to build the device unless you understand how it works.

A hardware MP3 player should only be made by a skilled electronics hobbyist.

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