Audio Streaming

Maxmod has a special capability to allow the user to stream customized data to the sound channels. This is called audio streaming.

Stream Information

Before you open the stream, you must understand the mm_stream structure.

typedef struct t_mmstream
{
    mm_word sampling_rate;
    mm_word buffer_length;
    mm_stream_func callback;
    mm_stream_formats format;
    mm_stream_timer timer;
    mm_bool manual;
} mm_stream;

This structure contains the neccesary information about how the stream will operate.

sampling_rate

This is the sampling rate of the stream in Hertz. Higher rates will provide better quality, but also more frequent requests to your filling routine. Pick according to which rate your filling routine will output.

buffer_length

This is the length of the wave buffer(s). It is specified in samples. You should make this a multiple of 16.

callback

This is a pointer to your filling routine. Your routine is responsible for handling fill requests from Maxmod.

format

This entry controls the output format for the stream. There are 4 settings which are listed in mm_stream_formats.

typedef enum
{
    MM_STREAM_8BIT_MONO      = 0x0, // 000b
    MM_STREAM_8BIT_STEREO    = 0x1, // 001b
 
    MM_STREAM_16BIT_MONO     = 0x2, // 010b
    MM_STREAM_16BIT_STEREO   = 0x3, // 011b
 
} mm_stream_formats;

timer

This entry controls which timer will be used for counting samples. Choose one of the 4 hardware timers available on ARM9.

typedef enum
{
    MM_TIMER0, // Hardware timer 0
    MM_TIMER1, // Hardware timer 1
    MM_TIMER2, // Hardware timer 2
    MM_TIMER3  // Hardware timer 3
 
} mm_stream_timer;

If your program disturbs the streaming timer while it is in use, then the stream will be corrupted.

manual

If this entry is true then the requests to your filling routine will not be automatic. You will then have to call mmStreamUpdate() manually to keep the stream filled. This is useful if you want to control when the stream will be filled.

Opening the Stream

Here is an example to open a monoural stream with 22KHz playback rate and 8-bit output.

void open_stream(void)
{
    mm_stream stream;
 
    stream.sampling_rate = 22000;        // 22khz
    stream.buffer_length = 800;          // Should be adequate
    stream.callback = fill_stream;       // Give fill routine
    stream.format = MM_STREAM_8BIT_MONO; // 8-bit mono
    stream.timer = MM_TIMER0;            // Use timer 0
    stream.manual = 0;                   // Auto filling
 
    mmOpenStream(&si);
}

And... another example that opens a stereo stream with 16KHz playback rate and 16-bit output.

void open_stream(void)
{
    mm_stream stream;
 
    stream.sampling_rate = 16384;         // 16KHZ
    stream.buffer_length = 800;           // Should be adequate
    stream.callback = my_stereo_filler;   // Give stereo filling routine
    stream.format = MM_STREAM_16BIT_STEREO; // Select format
    stream.timer = MM_TIMER0;             // Use timer 0
    stream.manual = 0;                    // Auto filling
 
    // open the stream
    mmOpenStream(&si);
}

Try to understand what's going on in these two examples by using the information explained above.

Caution

When opening the stream when music/sound is playing, some notes may be cut due to being interrupted by the starting of the stream. (if they happen to occupy the channels used for streaming)

Filling the Stream

After you open the stream (in auto mode), Maxmod will start sending you fill requests. These requests are handled by the function you specified in the callback entry.

Here is the format for the callback function.

typedef mm_word (* mm_stream_func)(mm_word length, mm_addr dest, mm_stream_formats format);

The dest parameter contains the address that you must output the new data to. length is the (recommended) amount of samples that should be output to the target. format is a copy of the value that you specified during setup. The return value of the callback is how many samples you have actually output. If you return a value less than 'length', then the remaining samples will be saved for the next stream update.

Here is a filling routine to output white noise (for the mono opening example above).

mm_word fill_stream(mm_word length, mm_addr dest, mm_stream_formats format)
{
    s8 *output = (s8*)dest;
    for ( ; length; length--)
    {
        *output++ = rand();
    }
    return length;
}

And... an example to fill the stereo output with white noise (16-bit).

mm_word my_stereo_filler(mm_word length, mm_addr dest, mm_stream_formats format)
{
    s16 *output = (s16*)dest;
    for ( ; length; length--)
    {
        *output++ = rand(); // Output left sample
        *output++ = rand(); // Output right sample (data is interleaved)
    }
    return length;
}

With these examples combined with the ones above, white noise should be produced through the speakers. Note that when using automatic filling you should always output the full 'length' amount unless you really know what you are doing.

Another note: The callback will only be called with a length that is a multiple of 4. The amount that you output must also be a multiple of 4 (zero is okay, to skip the update), the internal routines are optimized with this in mind and they do not handle situations where the callback returns an amount that is not a multiple of 4.

Manual Filling

Sometimes, when your filling code depends on certain data that is edited during your program logic, it is undesirable to have the filling routine called with random requests.

If this is the case, you need to switch the stream to manual mode. Specify a nonzero value in the manual entry of the stream information.

In this mode, you are responsible for issuing fill requests. The mmStreamUpdate() will refill the stream up to the point that is currently being played.

Here is an example program loop that will fill the stream after the program logic executes.

while(true)
{
    // ... Update game logic ...
 
    // Fill the stream with data
    mmUpdateStream();
 
    // Wait for new frame
    swiWaitForVBlank();
 
    // ... Update graphical data ...
}

Notice that this program only calls mmStreamUpdate() once per frame. You must ensure that the size of the wave buffer is able to hold enough data to be played until the next fill request.

ARM7/ARM9

The streaming functions can be used on both processors. If mmStreamOpen() is used on ARM9, then the ARM9 will be responsible for filling the stream, and the hardware timers on the ARM9 side will be used for timing. If mmStreamOpen() is called on the ARM7, then the ARM7 will be responsible to fill the wave buffer, and the ARM7 timers will be used.

Ending the Stream

When you want to stop the stream, call mmStreamClose().

Closing

For a better understanding, please have a look at the streaming example source code.