I figured out what the issue was with your code. In method writeFile you do:

length = [track length]+3;
[contents appendBytes:&length length:4];


You can’t do that. You need to convert it into a char array just like you do in writeVarTime and appendNote.

Calling the following method will fix it:


- (NSMutableData *)appendUnsignedLong:(NSMutableData *)track longValue:(unsigned long)ul {
char ch[4];

return track;
}


So in your writeFile method you would say:


contents = [self appendUnignedLong:contents unsignedLong:length];


instead of:


[contents appendBytes:&length length:4];


Furthermore when you measure a track’s length, you have to include the 3 bytes in the end of track marker { 0xFF, 0×2F, 0×00}

brgds

This tutorial was very useful in explaining the Midi file specification, so thanks for that.

I’ve tried to follow it to create my own midi file, but I keep getting an error message that the midi file cannot be opened when I try to import the file into Logic Pro.

Below follows my code:

[code]- (void)midiTest {

NSString *fileAtPathName = [[NSString alloc] initWithFormat:@"%@MidiTest.mid", [mainDelegate documentsDirectoryName:YES]];

char header[18] = { 'M', 'T', 'h', 'd', 0x00, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00, 0x01, 0x00, 0x40, 0x4D, 0x54, 0x72, 0x6B };
char end[3] = {0xFF, 0x2F, 0x00};
char guitar[3] = { 0x00, 0xC0, 0x19};
int iLength;

NSMutableData *contents = [[NSMutableData alloc] init];
NSMutableData *track = [[NSMutableData alloc] init];

[contents appendBytes:header length:18];

char deltaChar[1] = {0x00};
char noteOn[3] = { 0x90, 60, 100 };
char noteOff[3] = { 0x80, 60, 100 };

[track appendBytes:deltaChar length:1];
[track appendBytes:guitar length:3];
[track appendBytes:deltaChar length:1];
[track appendBytes:noteOn length:3];
deltaChar[0] = 0x24;
[track appendBytes:deltaChar length:1];
[track appendBytes:noteOff length:3];

[track appendBytes:end length:3];

iLength = [track length];
[contents appendBytes:&iLength length:4];
[contents appendData:track];

[[NSFileManager defaultManager] createFileAtPath:fileAtPathName contents:contents attributes:nil];

}
[/code]

What gives?