The Broadway Audio System

Represents a piece of input or output hardware (microphone or speaker and the corresponding ADC or DAC)
Does not represent signal processing (DSP)
Unidirectional

Ports

Client access to buffers/buckets/devices
Format conversion (decompression / resample / etc.)
Synchronize source to device time
Over/underflow policy

Formats

Define characteristics of audio data:
- Encoding (linear, u-law, a-law, etc.)
- Sample rate (8000, 44100, 48000, etc.)
- Other attributes (sample size, number of channels, endian)

Example: Playing from file

Example: Record/playback

Buckets & waveforms

Other housekeeping

Security
Connection management

Time model

A timeline has 1 tick per sample
Separate timelines for the incoming data stream and device
The port object resolves the difference in timelines
- rate conversion
- adding or dropping samples

Time model - Transport delay

Normally, requests arrive ahead of the time at which they need to be played

Time model - Transport delay

Two ways of handling transport delay:
1. Stalling

Time model - Transport delay

2. Dropping samples from incoming data

Synchronization with other media

Low level in first release
Video or graphics are synchronized to audio
Inside X server, audio timeline presented as an XSync counter
More complex synchronization possible in the future

API

Most API encapsulated in 8 basic calls
Objects referenced by tag, not pointer
- Hides implementation, storage details
- Objects in either client or server

API: Connections

XaOpenAudio: open a connection

              connection = XaOpenAudio(networkID, 
                     argcInOut, argvInOut, 
                     errStringLength, errString);

XaCloseAudio: shut down a connection

              XaCloseAudio(connection);

API: Object manipulation

XaCreate: create an object

              tag = XaCreate(connection, 
                     objectClass, attribute, value, 
                     ... );

XaDestroy: destroy an object

              XaDestroy(connection, tag);

API: Object manipulation

XaSet: set attribute value(s) on an object

              XaSet(connection, tag, 
                     asyncCallbackFunc, 
                     asyncCallbackData, attribute, 
                     value, ...);

XaGet: get attribute value(s) from an object

              XaGet(connection, tag, 
                     asyncCallbackFunc, 
                     asyncCallbackData, attribute, 
                     &value, ...);

API: Audio data transfer

XaRead: read audio data from a port

              XaRead(connection, port, atTime, 
                     refTime, minBits, maxBits, 
                     &bitsRead, &leftPad, buff);

XaWrite: write audio data to a port

              XaWrite(connection, port, atTime, 
                     refTime, numBits, leftPad, buff);

API:
Synchronous vs. asynchronous

Synchronous: blocks until a reply is received

              XaGet(connection, object, NULL, 
                     NULL, ...);

Asynchronous: returns immediately, calls callback when reply is received

              XaGet(connection, object, 
                     callbackFunc, callbackData, ...);

API: Other calls

Locating objects/atoms
- XaFindAtom: Find a name
- XaFindObject: Find an object

API: Other calls

Manage the event queue
- XaPending: Check for pending events
- XaSync, XaCheckpoint: Return when pending requests are completed
- XaFlush: Flush the event queue
- XaMainLoop: Main loop for audio-only apps

Example: Play samples from buffer

              conn = XaOpenAudio("", NULL, NULL, 
                     256, errString);

              format = XaFindObject(conn, 
                     XaCformat, XaAname, XaNformatCD);

              port = XaCreate(conn, XaCport, 
                     XaNinputBuffer, XaTnone, XaNformat, 
                     format, NULL);

              XaWrite(conn, port, 0, XaNnowTime, 
                     numBits, leftPad, audioBuff);

              XaClose(conn);

Example: Play samples from file

              conn = XaOpenAudio("", NULL, NULL, 
                     256, errString);

              file = XaCreate(conn, XaCfile, 
                     XaApathName, 3/users/mw/
                     myFile.au2);

              player = XaCreate(conn, 
                     XaCfilePlayer, XaNfile, file, 
                     NULL);

              XaMainLoop();

Security

Broadway security
Security per connection (trusted/untrusted or other access group)
Access control attributes on each connection/device
Access settings: none, query, read-only, read-write

Audio Manager

May have a GUI
Not a window manager for audio
- Doesn't run continually
- Presence is not required
Sets preferences, persistent state
Configures access for devices/etc.

Events

Dispatched by event handler objects
Create / change / destroy object events
Plays well with Xt
- XaMainLoop or XtMainLoop
- Xa provides function and file descriptor for use with Xt event functions
- Xa equivalents to Xt basic event commands

Creating an event handler (API)

              handler = XaCreate(connection, 
                     XaCeventHandler, 
                     XaNhandlerProc, (XaArgVal) myFunc, 
                     XaNhandlerData, (XaArgVal) myData, 
                     XaNtargetObject, 
                     myFavoriteClassObject, 
                     XaNeventType, XaAcreate, 
                     NULL);

Extensions

May define new protocol requests
May add new classes
New devices
- Multicast/radio
- Telephony
New format encodings...

Implementing new encodings

Method 1: Xa client

Implementing new encodings

Method 2: Custom device

Futures

Flows
Synchronization/mixing of multiple input streams
Perform complex processing on streams
Additional synchronization facilities

Status

In active development
- Library & protocol specs
- Library, server software
Consortium standard in time for Broadway
Implementation release with Broadway

The Broadway Audio System

Ray Tice (ray@x.org)

Mark Welch (mw@x.org)

Postscript version of slides:

ftp://ftp.x.org/contrib/conferences/XTech96/audio_slides.ps