libroot_build: *Actually* fix attribute usage on Haiku.

[haiku.git] / docs / user / midi2 / midi2intro.dox

/*!
\page midi2_intro Introduction to the MIDI2 Kit

The Midi Kit is the API that implements support for generating, processing, and 
playing music in MIDI format. <A HREF="http://www.midi.org/">MIDI</A>, which 
stands for 'Musical Instrument Digital Interface', is a well-established 
standard for representing and communicating musical data. This document serves
as an overview. If you would like to see all the components, please look at
\link midi2 the list with classes \endlink .

\section midi2twokits The two kits

The BeOS comes with two different, but compatible Midi Kits. This documentation 
focuses on the "new" Midi Kit, or midi2 as we like to call it, that was 
introduced with BeOS R5. The old kit, which we'll refer to as midi1, is more 
complete than the new kit, but less powerful.

Both kits let you create so-called MIDI endpoints, but the endpoints from midi1 
cannot be shared between different applications. The midi2 kit solves that 
problem, but unlike midi1 it does not include a General MIDI softsynth, nor 
does it have a facility for reading and playing Standard MIDI Files. Don't 
worry: both kits are compatible and you can mix-and-match them in your 
applications.

The main differences between the two kits:

- Instead of one BMidi object that both produces and consumes events, we have 
BMidiProducer and BMidiConsumer.
- Applications are capable of sharing MIDI producers and consumers with other 
applications via the centralized Midi Roster.
- Physical MIDI ports are now sharable without apps "stealing" events from each 
other.
- Applications can now send/receive raw MIDI byte streams (useful if an 
application has its own MIDI parser/engine).
- Channels are numbered 0..15, not 1..16
- Timing is now specified in microseconds instead of milliseconds.

\section midi2concepts Midi Kit concepts

A brief overview of the elements that comprise the Midi Kit:

- \b Endpoints. This is what the Midi Kit is all about: sending MIDI messages 
between endpoints. An endpoint is like a MIDI In or MIDI Out socket on your 
equipment; it either receives information or it sends information. Endpoints 
that send MIDI events are called \b producers; the endpoints that receive those 
events are called \b consumers. An endpoint that is created by your own 
application is called \b local; endpoints from other applications are \b 
remote. You can access remote endpoints using \b proxies.

- \b Filters. A filter is an object that has a consumer and a producer 
endpoint. It reads incoming events from its consumer, performs some operation, 
and tells its producer to send out the results. In its current form, the Midi 
Kit doesn't provide any special facilities for writing filters.

- \b Midi \b Roster. The roster is the list of all published producers and 
consumers. By publishing an endpoint, you allow other applications to talk to 
it. You are not required to publish your endpoints, in which case only your own 
application can use them.

- \b Midi \b Server. The Midi Server does the behind-the-scenes work. It 
manages the roster, it connects endpoints, it makes sure that endpoints can 
communicate, and so on. The Midi Server is started automatically when BeOS 
boots, and you never have to deal with it directly. Just remember that it runs 
the show.

- \b libmidi. The BMidi* classes live inside two shared libraries: libmidi.so 
and libmidi2.so. If you write an application that uses old Midi Kit, you must 
link it to libmidi.so. Applications that use the new Midi Kit must link to 
libmidi2.so. If you want to mix-and-match both kits, you should also link to 
both libraries.

Here is a pretty picture:

\image html midi2concepts.png

\section midi2mediakit Midi Kit != Media Kit

Be chose not to integrate the Midi Kit into the Media Kit as another media 
type, mainly because MIDI doesn't require any of the format negotiation that 
other media types need. Although the two kits look similar -- both have a 
"roster" for finding or registering "consumers" and "producers" -- there are 
some very important differences.
 
The first and most important point to note is that BMidiConsumer and 
BMidiProducer in the Midi Kit are NOT directly analogous to BBufferConsumer and 
BBufferProducer in the Media Kit! In the Media Kit, consumers and producers are 
the data consuming and producing properties of a media node. A filter in the 
Media Kit, therefore, inherits from both BBufferConsumer and BBufferProducer, 
and implements their virtual member functions to do its work.

In the Midi Kit, consumers and producers act as endpoints of MIDI data 
connections, much as media_source and media_destination do in the Media Kit. 
Thus, a MIDI filter does not derive from BMidiConsumer and BMidiProducer; 
instead, it contains BMidiConsumer and BMidiProducer objects for each of its 
distinct endpoints that connect to other MIDI objects. The Midi Kit does not 
allow the use of multiple virtual inheritance, so you can't create an object 
that's both a BMidiConsumer and a BMidiProducer.

This also contrasts with the old Midi Kit's conception of a BMidi object, which 
stood for an object that both received and sent MIDI data. In the new Midi Kit, 
the endpoints of MIDI connections are all that matters. What lies between the 
endpoints, i.e., how a MIDI filter is actually structured, is entirely at your 
discretion.

Also, rather than use token structs like media_node to make connections via the 
MediaRoster, the new kit makes the connections directly via the BMidiProducer 
object.

\section midi2remotelocal Remote and local objects

The Midi Kit makes a distinction between remote and local MIDI objects. You can 
only create local MIDI endpoints, which derive from either BMidiLocalConsumer 
or BMidiLocalProducer. Remote endpoints are endpoints that live in other 
applications, and you access them through BMidiRoster.

BMidiRoster only gives you access to BMidiEndpoints, BMidiConsumers, and 
BMidiProducers. When you want to talk to remote MIDI objects, you do so through 
the proxy objects that BMidiRoster provides. Unlike BMidiLocalConsumer and 
BMidiLocalProducer, these classes do not provide a lot of functions. That is 
intentional. In order to hide the details of communication with MIDI endpoints 
in other applications, the Midi Kit must hide the details of how a particular 
endpoint is implemented.

So, what can you do with remote objects? Only what BMidiConsumer, 
BMidiProducer, and BMidiEndpoint will let you do. You can connect objects, get 
the properties of these objects -- and that's about it.

\section midi2lifespan Creating and destroying objects

The constructors and destructors of most midi2 classes are private, which mean 
you cannot directly create them using the C++ <CODE>new</CODE> operator, on the 
stack, or as globals. Nor can you <CODE>delete</CODE> them. Instead, these 
objects are obtained through BMidiRoster. The only two exceptions to this rule 
are BMidiLocalConsumer and BMidiLocalProducer. These two objects may be 
directly created and subclassed by developers.

\section midi2refcount Reference counting

Each MIDI endpoint has a reference count associated with it, so that the Midi 
Roster can do proper bookkeeping. When you construct a BMidiLocalProducer or 
BMidiLocalConsumer endpoint, it starts with a reference count of 1. In 
addition, BMidiRoster increments the reference count of any object it hands to 
you as a result of \link BMidiRoster::NextEndpoint() NextEndpoint() \endlink or 
\link BMidiRoster::FindEndpoint() FindEndpoint() \endlink. Once the count hits 
0, the endpoint will be deleted.

This means that, to delete an endpoint, you don't call the <CODE>delete</CODE> 
operator directly; instead, you call \link BMidiEndpoint::Release() Release() 
\endlink. To balance this call, there's also an \link BMidiEndpoint::Acquire() 
Acquire() \endlink, in case you have two disparate parts of your application 
working with the endpoint, and you don't want to have to keep track of who 
needs to Release() the endpoint.

When you're done with any endpoint object, you must Release() it. This is true 
for both local and remote objects. Repeat after me: Release() when you're done.

\section midi2events MIDI events

To make some actual music, you need to \link BMidiProducer::Connect() Connect() 
\endlink your consumers to your producers. Then you tell the producer to 
"spray" MIDI events to all the connected consumers. The consumers are notified 
of these incoming events through a set of hook functions.

The Midi Kit already provides a set of commonly used spray functions, such as 
\link BMidiLocalProducer::SprayNoteOn() SprayNoteOn() \endlink, \link 
BMidiLocalProducer::SprayControlChange() SprayControlChange() \endlink, and so 
on. These correspond one-to-one with the message types from the MIDI spec. You 
don't need to be a MIDI expert to use the kit, but of course some knowledge of 
the protocol helps. If you are really hardcore, you can also use the \link 
BMidiLocalProducer::SprayData() SprayData() \endlink to send raw MIDI events to 
the consumers.

At the consumer side, a dedicated thread invokes a hook function for every 
incoming MIDI event. For every spray function, there is a corresponding hook 
function, e.g. \link BMidiLocalConsumer::NoteOn() NoteOn() \endlink and \link 
BMidiLocalConsumer::ControlChange() ControlChange() \endlink. The hardcore MIDI 
fanatics among you will be pleased to know that you can also tap into the \link 
BMidiLocalConsumer::Data() Data() \endlink hook and get your hands dirty with 
the raw MIDI data.

\section midi2time Time

The spray and hook functions accept a bigtime_t parameter named "time". This 
indicates when the MIDI event should be performed. The time is given in 
microseconds since the computer booted. To get the current tick measurement, 
you call the system_time() function from the Kernel Kit.

If you override a hook function in one of your consumer objects, it should look 
at the time argument, wait until the designated time, and then perform its 
action. The preferred method is to use the Kernel Kit's 
<CODE>snooze_until()</CODE> function, which sends the consumer thread to sleep 
until the requested time has come. (Or, if the time has already passed, returns 
immediately.)

Like this:

\code
void MyConsumer::NoteOn(
    uchar channel, uchar note, uchar velocity, bigtime_t time) 
{ 
    snooze_until(time, B_SYSTEM_TIMEBASE); 
    ...do your thing...
}
\endcode
 
If you want your producers to run in real time, i.e. they produce MIDI data 
that needs to be performed immediately, you should pass time 0 to the spray 
functions (which also happens to be the default value). Since time 0 has 
already passed, <CODE>snooze_until()</CODE> returns immediately, and the 
consumer will process the events as soon as they are received.

To schedule MIDI events for a performance time that lies somewhere in the 
future, the producer must take into account the consumer's latency. Producers 
should attempt to get notes to the consumer by or before 
<I>(scheduled_performance_time - latency)</I>. The time argument is still the 
scheduled performance time, so if your consumer has latency, it should snooze 
like this before it starts to perform the events:

\code
snooze_until(time - Latency(), B_SYSTEM_TIMEBASE);
\endcode

Note that a typical producer sends out its events as soon as it can; unlike a 
consumer, it does not have to snooze.

\section midi2ports Other timing issues

Each consumer object uses a Kernel Kit port to receive MIDI events from 
connected producers. The queue for this port is only 1 message deep. This means 
that if the consumer thread is asleep in a <CODE>snooze_until()</CODE>, it will 
not read its port. Consequently, any producer that tries to write a new event 
to this port will block until the consumer thread is ready to receive a new 
message. This is intentional, because it prevents producers from generating and 
queueing up thousands of events.

This mechanism, while simple, puts on the producer the responsibility for 
sorting the events in time. Suppose your producer sends three Note On events, 
the first on t + 0, the second on t + 4, and the third on t + 2. This last 
event won't be received until after t + 4, so it will be two ticks too late. If 
this sort of thing can happen with your producer, you should somehow sort the 
events before you spray them. Of course, if you have two or more producers 
connected to the same consumer, it is nearly impossible to sort this all out 
(pardon the pun). So it is not wise to send the same kinds of events from more 
than one producer to one consumer at the same time.

The article Introduction to MIDI, Part 2 in <A 
HREF="http://open-beos.sourceforge.net/nsl.php?mode=display&id=36">OpenBeOS 
Newsletter 36</A> describes this problem in more detail, and provides a 
solution. Go read it now!

\section midi2filters Writing a filter

A typical filter contains a consumer and a producer endpoint. It receives 
events from the consumer, processes them, and sends them out again using the 
producer. The consumer endpoint is a subclass of BMidiLocalConsumer, whereas 
the producer is simply a BMidiLocalProducer, not a subclass. This is a common 
configuration, because consumers work by overriding the event hooks to do work 
when MIDI data arrives. Producers work by sending an event when you call their 
member functions. You should hardly ever need to derive from BMidiLocalProducer 
(unless you need to know when the producer gets connected or disconnected, 
perhaps), but you'll always have to override one or more of 
BMidiLocalConsumer's member functions to do something useful with incoming 
data.

Filters should ignore the time argument from the spray and hook functions, and 
simply pass it on unchanged. Objects that only filter data should process the 
event as quickly as possible and be done with it. Do not 
<CODE>snooze_until()</CODE> in the consumer endpoint of a filter!       

\section midi2apidiffs API differences

As far as the end user is concerned, the Haiku Midi Kit is mostly the same 
as the BeOS R5 kits, although there are a few small differences in the API 
(mostly bug fixes):

- BMidiEndpoint::IsPersistent() always returns false.
- The B_MIDI_CHANGE_LATENCY notification is now properly sent. The Be kit 
  incorrectly set be:op to B_MIDI_CHANGED_NAME, even though the rest of the 
  message was properly structured.
- If creating a local endpoint fails, you can still Release() the object 
  without crashing into the debugger.

\section midi2seealso See also

More about the Midi Kit:

- \ref Midi2Defs.h
- Be Newsletter Volume 3, Issue 47 - Motor Mix sample code
- Be Newsletter Volume 4, Issue 3 - Overview of the new kit
- <A HREF="http://haiku-os.org/documents/dev/introduction_to_midi_part_1">Newsletter
  33</A>, Introduction to MIDI, Part 1
- <A HREF="http://haiku-os.org/documents/dev/introduction_to_midi_part_2">Newsletter
  36</A>, Introduction to MIDI, Part 2
- Sample code and other goodies at the 
  <A HREF="http://haiku-os.org/about/teams/midi_kit">Haiku Midi Kit team page</A>

Information about MIDI in general:

- <A HREF="http://www.midi.org">MIDI Manufacturers Association</A>
- <A HREF="http://www.borg.com/~jglatt/tutr/miditutr.htm">MIDI Tutorials</A>
- <A HREF="http://www.borg.com/~jglatt/tech/midispec.htm">MIDI Specification</A>
- <A HREF="http://www.borg.com/~jglatt/tech/midifile.htm">Standard MIDI File Format</A>
- <A HREF="http://www.io.com/~jimm/midi_ref.html">Jim Menard's MIDI Reference</A>

*/

/*!
\addtogroup midi2

Please have a look at the \link midi2_intro introduction \endlink for a more
comprehensive overview on how everything ties together.
*/