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Midi

The Midi implementation in ATL consists of reading and writing MidiMessages to and from a Stream.

The MidiMessage structure contains all information for all types of Midi messages - currently System Exclusive messages are not yet implemented. When reading Midi the information will be presented in MidiMessage instances and when writing the program must deliver the Midi information to sent as a MidiMessage instance.

The MidiMessage structure contains a somewhat complex union structure that declares appropriate member variables for all types of Midi messages - without taking more memory than strictly needed.

Streams

Before diving into Midi further it is important to note that ATL uses an abstraction for InputStreams and OutputStreams. The Midi implementation uses these abstractions. ATL comes with adapter classes to wrap an Arduino Serial stream as one of the ATL abstractions.

Writing Midi

The MidiWriter class implements the Midi protocol logic for writing a MidiMessage to a stream. Its BaseT is assumed to be an OutputStream that implements the Write(byte) method. Any class that has that method declaration can be used as a BaseT.

To write a MidiMessage to the underlying OutputStream simply call the Write(MidiMessage*) method. The writer class keeps track of running status to optimize the number of bytes actually sent to the OutputStream.

Reading Midi

The MidiReader class is a little more complex for it has to deliver the MidiMessages to the program - without using virtual methods. The MidiReader class implements the protocol for reading Midi messages from an InputStream.

The bool Read() method is meant to be called repeatedly in the program and pumps the bytes from the InputStream through the MidiReader class to the program. It is the only public method.

The BaseT of the MidiReader class is provided by the program and implements a couple of methods to receive the MidiMessages as they come in - currently System Exclusive messages are not implemented yet. The BaseT is also used to access the InputStream, so the program-supplied base class could also use the same 'BaseT-principle' to inherit from the required InputStream.

It is important to note that the MidiMessage* parameter that is provided in the 'callback' method to the program is only valid during that call. This means that the program must copy the information if it needs to store it. A MidiMessage::CopyTo is available for that purpose.

There are also 'callback' methods for real-time Midi messages (1 byte) and System Exclusive messages (not implemented).




Last edited Jan 6, 2015 at 11:01 AM by obiwanjacobi, version 3