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Atmel provides detailed source code for an MP3 decoder that runs on its AVR32 UC3 MCU. This article explains in detail how to configure and use it.

This article will help the reader to use the AVR32? UC3 Audio Decoder over USB software. This software includes a software MP3 decoder, a file system, and USB host mass storage class support.

For more information about the AVR32 architecture, please refer to the appropriate documents available at http://www.atmel.com/avr32.

License

The MP3 audio decoder (MAD) is distributed under the terms of the GPL. The JPEG decoder (IJG) license can be found in the file /SERVICES/PICTURES/JPG/IJG/license.txt. The memory manager (dlmalloc) license can be found in the file /SERVICES/MEMORY/MEMORY_MANAGER/DLMALLOC/license.txt.

Requirements

The software referenced in this article requires several components:

• A computer running Microsoft? Windows? 2000/XP/Vista or Linux?

• AVR32 Studio and the GNU toolchain (GCC) or IAR Embedded Workbench? for the AVR32 compiler

• A JTAGICE mkII or AVROne! debugger

Theory of operation

Overview

Today, embedded MP3 decoders are everywhere for consumers listening to audio content on mobile devices.

MP3

MPEG-1 Audio Layer III, more commonly referred to as MP3, is a digital audio encoding format using a form of lossy data compression. Several bit rates are specified in the MPEG-1 Audio Layer III standard: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, and 320 Kbps. The available sampling frequencies are 32, 44.1, and 48 KHz, and a sample rate of 44.1 KHz is almost always used. 128 Kbps bitrate files are slowly being replaced with higher bitrates such as 192 Kbps, with some being encoded up to the MP3 maximum of 320 Kbps.

A tag, in a compressed audio file, is a section of the file which contains metadata such as the title, artist, album, track number, or other information about the file's contents.

The chosen MP3 decoder here is MAD (libmad), a high-quality MPEG audio decoder. It currently supports MPEG-1 and the MPEG-2 extension to lower sampling frequencies, as well as the so-called MPEG 2.5 format. All three audio layers (Layer I, Layer II, and Layer III) are fully implemented. MAD does not yet support MPEG-2 multichannel audio (although it should be backward compatible with such streams).

Block diagram

Figure 1 shows the UC3 interfacing to the USB stick, and outputting the audio stream from the key to the external DAC. The user can control the player using a keypad, or running a customizable Human Machine Interface (HMI).

Figure 1: Block diagram.

Software architecture
Figure 2 shows the basic software architecture of the application.

Figure 2: Software architecture.

The application does not require any operating system to run. The main() function is in charge of calling the software tasks (using a scheduler) that make audio decoding, HMI, and USB management possible. There are five tasks:

• The Communication Task contains the HMI of the application. This task holds the real intelligence of the user application and interfaces directly with the Audio Interface (AI). This is the task that the user should modify for their own application.

• Audio Interface Task: This task handles dynamic support for any newly plugged-in device types. For example, if a new device, using another class than the mass storage class, is plugged in, and if this class is supported by the application, then this task will tell the Audio Interface to link the Audio Interface API to a low level API that supports this class. All the mechanisms in place to dynamically link an API to this abstract API are located in the file: /SERVICES/AUDIO/AUDIO_PLAYER/audio_interface.[c|h]. In this application, only the USB mass storage class is implemented. Therefore, this function is not used.

• The last task is a macro used for feature tasks. In the MP3 audio player using USB, the modules involved will be the MP3 decoder, the USB stack, and the host mass storage task:

• The MP3 Codec Task: This task is in charge of Audio Decoding management.

• The USB Task: This task handles the USB stack and events.

• The Host Mass Storage Task: This task will check for newly connected devices and initialize them using the USB mass storage class.

The main loop of the application is a simple free-running task scheduler:

while (TRUE)
{

• ai_task();

• com_task();

• task();



• }

The Audio Interface can also support iPod audio decoding through the USB audio class. Please refer to the application note AVR32730: AVR32 UC3 iPod Support Over USB for more information.

Audio Interface API

The audio player is a generic audio player interface and is designed to support multiple audio formats besides MP3.

Overview

The Audio Interface manages disk navigation, audio navigation, and audio control. Thus, the user does not have to directly interface with the file system and audio control APIs. This greatly simplifies the software architecture.

The Audio Interface can be used in two ways:

• Using asynchronous functions, the results or effects of which may not be produced in a single iteration, usually leads to the use of state machines in the user firmware (since one must wait for the completion of a command before launching a new one), and has the advantage of reducing the risk of audio underrun. Asynchronous functions always have the "ai_async" prefix.
  Note that only one asynchronous function can be used at a time.

• Using synchronous functions, which are executed immediately, this drastically simplifies the user firmware architecture (no use of state machines since the synchronous AI functions are immediately executed), but may produce audio underruns since the execution time of these functions may be too long. Synchronous functions just have the "ai_" prefix.

All functions of the Audio Interface have a synchronous and asynchronous interface. For example, the command which returns the number of drives is:

• ai_async_nav_drive_nb() in the asynchronous interface

• ai_nav_drive_nb() in the synchronous interface

Using asynchronous functions shall be the preferred solution in order to avoid audio underruns.

Audio Interface (AI) architecture

The AI commands to interface the audio player are divided into three categories (See Figure 3):

• Disk navigation (Disk Nav) to browse into the tree architectures of the USB device

• Audio navigation (Audio Nav) to manage a list of playable songs

• Audio control (Audio CTRL) to control the audio stream (such as play, pause, and fast forward)

Figure 3: Audio player navigators overview.

Disk Navigator: "Browse into the tree architectures of the mass storage device"

The Disk Navigator is similar to a file explorer. It is used to navigate in the tree architectures of the connected USB device. It will hide all files whose extension differs from *.mp3, *.pls, *.smp, or *.m3u. The file extension filtering is automatically updated according to the modules selected (such as MP3 and playlists).

The Disk Navigator is totally independent from the Audio Navigator.

The commands associated with this module are used to navigate into the disks/directories, and to synchronize the selected file/folder with the Audio Nav module. This synchronization is made with two commands: ai_nav_getplayfile and ai_audio_nav_playfile. See the "Disk Navigation" section later in this article for a complete list of commands.

Audio Navigator: "Manage a list of playable songs"

The Audio Navigator deals with a list of files. This list is defined once an ai_audio_nav_playfile command is executed. This command sets the list of files to be played according to the current selection in the Disk Navigator and to the Audio Navigator ai_audio_nav_expmode_set option.

When the ai_audio_nav_playfile command is executed:

• If the Disk Navigator is currently pointing to a playlist (*.m3u), then only the files of this playlist will be included in the Audio Navigator file list.

• If the Disk Navigator is currently pointing to a disk or a file, then the file list will depend on the audio explorer mode (set by the ai_audio_nav_expmode_set command) (See Table 1).

• If the Disk Navigator is currently pointing to a folder, the Audio Navigator will not enter into it. It will look for the first file that is in the current directory and build its file list according to the previous rules.

• If the Disk Navigator is not pointing to any files or folders (which is the case after a mount or a goto) when a directory or a playlist is selected, then it will select the first file of the file list.

Table 1: ai_audio_nav_expmode_set command behavior.

Note that the playing order can be changed at compilation time by enabling a define. This is fully explained in the "Example 3 — Change the playing order" section later in this article.

once the file list is set, two main commands are available to navigate into it:

• ai_audio_nav_next (select next file).

• ai_audio_nav_previous (select previous file).

Options can also be defined for the Audio Navigator (to change the repeat mode, or enable/disable the shuffle mode).

To synchronize the Disk Navigator with the Audio Navigator, i.e., to make the Disk Navigator select the file played by the Audio Navigator, the command ai_nav_getplayfile can be used. See the "Audio Navigation" section later in this article for a complete list of commands.

Audio Control: "control the audio stream
(play/pause/fast forward/...)"

This module controls the audio stream of the selected file (selected by the Audio Navigator). Commands like play, pause, stop, fast forward, and fast rewind are available. See the "Audio Control" section later in this article for a complete list of commands (See Table 2).

Limitations
Speed

Speed navigation (such as file browsing) in directories may be affected if:

• A high-bitrate file is played at the same time.

• Directories have many files.

• The playlist includes many files.

Disk navigation

The exploration is based on a selector displacement. The file list is the list of the files in the current directory according to the extension filter (i.e., .mp3, .m3u, .pls, and .smp).

The file list:

• Is updated when you exit or enter a directory or a disk

• Starts with the directories then the files

• Is sorted in the creation order

Table 2: Disk Navigator commands.

Audio navigation

This navigator sets the list of files to be played. It can be seen as a playlist. Before accessing this navigator, an ai_audio_nav_playfile command must be issued.

Table 3: Audio Navigator commands.

Modes used by the interface

Ai_repeat_mode defines the repeat mode:

Ai_shuffle_mode defines the shuffle/random mode:

Ai_explorer_mode defines the explorer mode (see the "Example 3 — Change the playing order" section later in this article for more information).

Audio Control

The audio controller is used to control the audio stream of the audio file selected by the Audio Navigator.

Table 4: Audio Control commands.

Using asynchronous or synchronous API

Using synchronous functions is straightforward. once finished, synchronous functions return, with or without a result. Using asynchronous functions is more complicated; they may not produce the requested task in a single shot. Thus, these functions need some other functions to operate properly:

• void ai_async_cmd_task (void): This function is the entry point of all asynchronous commands. It must be called to execute the current state of the internal state machine of the current asynchronous function.

• Bool is_ai_async_cmd_finished (void): This function returns TRUE if the last command is finished.

• U8 ai_async_cmd_out_status (void): Returns the status of the last executed command (CMD_DONE, CMD_EXECUTING or CMD_ERROR).

• U32 ai_async_cmd_out_u32 (void): If the last executed command should return a 32-bit result or less, this function will return this value.

• U64 ai_async_cmd_out_u64 (void): If the last executed command should return a 64-bit result, this function will return this value.

• 16 ai_async_cmd_out_SizeArrayU8 (void): If the last executed command should return an extra result (e.g., a song name), this function returns the size in bytes of the extra result (no size limit).

• U8 ai_async_cmd_out_PtrArrayU8 (void): Returns a pointer to the extra result (assuming that the last executed command returns an extra result). This pointer can be freed by the application with the ai_async_cmd_out_free_ArrayU8() function.

• void ai_async_cmd_out_free_ArrayU8 (void): This function may be called to free the allocated buffer that holds the extra result. Note that the Audio Interface will automatically do this before executing a new command that needs extra results. This ensures that the application will not have memory leakage. Allowing the application calling this function to free the extra results sooner and improve allocated memory usage. Figure 4 shows the flow of the asynchronous function use.

Figure 4: Asynchronous function flow.

Examples

The code for the next a three examples can be found in the following directories:

Let's take this disk as disk number 0 for the system.

Example 1 - Play "file1.mp3"

Table 5: Example: Play "file1.mp3."

Example 2 — Play while browsing

Table 6: Example: Play while browsing.

Example 3 — Change the playing order

The playing order can be changed at compilation time by enabling the NAV_AUTO_FILE_IN_FIRST define (see the "Project Configuration" section later in this article).

Table 7: Playfile sequence.

If the NAV_AUTO_FILE_IN_FIRST define is not set, the sequence will play audio files in the order shown in Table 8.

Table 8: Playfile sequence with NAV_AUTO_FILE_IN_FIRST undefined.

If this define is set, the sequence will play audio files starting with files on the root, as shown in Table 9.

Table 9: Playfile sequence with NAV_AUTO_FILE_IN_FIRST defined.

Source code architecture

Package

AUDIO-PLAYER-