Sfront Reference Manual: Part I/3: Sfront Real-Time Operation
Description: A part of the Sfront reference manual, that describes how to use the program in real-time applications. Sfront is a program that translates MP4-SA files into C programs.
Keywords: MP4-SA, SASL, SAOL, MP4, render, midi, stream, timesync, real time, real-time, interaction, control, encode, encoding, decode, decoding, omissions, extensions

### From The `sfront` Reference Manual by John Lazzaro and John Wawrzynek.

# Part II/3: Sfront Real-Time Operation

## Sections Introduction. Real-time Playback. Sending audio directly to sound hardware. Real-time Interaction. Sfront engines with real time audio and control input.

## Introduction This chapter describes how to use `sfront` in stand-alone real-time applications. We begin with basic sound playback, and show how to route audio output to a speaker under different operating systems. We then describe real-time interaction, with a MIDI keyboard and with an audio input source. | ##

## Real-time Playback Sfront can create C programs that send audio output directly to the sound card of a computer. The `-aout` command-line option is used to specify the hardware driver for audio output. The right panel shows the currently supported hardware arguments to the `-aout` option, and describes an example program to test real-time playback. If your hardware is not supported, use the `-aout std` command-line option to create a C program that places audio samples on standard output. When executing the program on the command line, pipe the output to a sound player that can drive the audio hardware. #### Latency Real-time programs come in two flavors: interactive and streaming. Interactive programs create audio on the fly, in response to user input. These programs work best if the delay (or latency) from user input to audio output is as short as possible (2 ms is ideal, 10 ms is barely tolerable). However, if the latency is too short, audio output glitches may happen. Short latency is not as important in audio streaming work. It is usually better to use longer latency values, to minimize the chances of audio output glitches. For some audio hardware, 20-40ms of latency is optimal for streaming; other hardware prefers latencies in the 200-400ms range. Sfront chooses a default latency value, based on the nature of the SAOL program (interactive or streaming) and the type of audio hardware. The `-latency` command-line option overrides the default latency. Try setting the latency to a longer time value to minimize audio output artifacts. #### Temporal modes Sfront programs run under one of three temporal modes: render, playback, or timesync. These modes define a set of real-time behaviors, described on the right panel. In most cases, the default temporal mode for an audio driver works well. Users can override the default behavior by adding the `-render`, `-playback`, or `-timesync` flag to the sfront command line. Real-time audio drivers usually default to playback mode. Programs running in playback mode accurately compute audio output audio, as long as samples are being computed fast enough to keep the audio hardware supplied with data. However, if the audio hardware is in danger of running out of samples, the decoder deviates from MP4-SA compliance, in an effort to maintain the sample stream. Once the danger of sample underrun has past, the decoder returns to accurate computation. A SAOL program may also implement graceful degradation techniques directly, by monitoring the `cpuload` standard name and pre-emptively reducing CPU usage. In contrast to playback mode, render mode computes an accurate rendition of the MP4-SA performance at all times. The file rendering audio drivers use render mode as the default, and always set `cpuload` to zero. When render mode is used with a real-time audio driver, `cpuload` updates to reflect the current CPU usage. The timesync mode forces the program to use at least 1 ms of real time to compute 1 ms of audio, by temporarily halting program execution if audio is computing too fast. Timesync mode may be used to make an audio driver that is not real-time by nature (such as the `-aout null` dummy driver) to behave in a real-time fashion. #### Under Linux ... Under Linux, if the C program generated by sfront runs as root, audio glitches and dropouts are significantly reduced. The Linux features used in root mode are described in the `sched_setscheduler` man page. These features are used in other popular Linux audio programs such as xmms. We have tested sfront root mode carefully, and added special "watchdog" features to detect poorly written SAOL programs. However, any remaining bugs in sfront could have dangerous effects when the C files sfront creates are run as root. In particular, the machine could lock up, requiring a hard reboot, and files may be damaged as a result. Use this feature at your own risk. | ## Output Hardware Options `
    See reference section of
    sfront manual for details on
    using these options.

     option            Environment
    ------------------------------
    -aout linux        Linux.

    -aout coreaudio    Mac OS X

    -aout pa_win_wmme  Microsoft Windows
                       (WMME library).

    -aout pa_win_ds    Microsoft Windows
                       (Directsound).

    -aout irix         SGI Irix.

    -aout freebsd      FreeBSD.

    -aout hpux         HPUX.

    -aout std          Generic -- pipe
                       the output of
                       the C program to
                       a sound player app.

    -aout null         Generic -- ignore
                       the output of the
                       the C program.
` ## Example `
    sfront/examples/rtime/aatest
    contains a simple audio streaming
    test. To run this test under Linux,
    cd into aatest and type "make".

    To run aatest on other platforms,
    see the README file in the aatest
    directory for details.
` ## Temporal modes `
    The real-time behavior
    of an sfront program
    depends on the temporal
    mode it uses. Sfront
    sets the default mode to
    fit the chosen drivers.
    Use one of these options
    to override the default:

     option       description
    --------------------------
    -render       Execute the SAOL
                  program as
                  accurately as
                  possible. Update
                  the SAOL standard
                  name cpuload if a
                  real-time driver
                  is in use; set
                  cpuload to 0 if
                  no real-time
                  drivers are in use.

    -playback     Execute the SAOL
                  program to keep
                  up with real-time
                  constraints, even
                  if accuracy is
                  sacrificed. Update
                  the SAOL standard
                  name cpuload.

    -timesync     Use the -playback
                  accuracy semantics.
                  Also, force the
                  SAOL program to
                  take at least 1ms
                  real-time to compute
                  1ms audio. Use this
                  flag with the null
                  and std audio drivers
                  to force real-time
                  behavior.
`

## Real-Time Interaction Sfront can create C programs that can accept real-time audio input (which is placed on the input_bus) and control input (which provides SASL and MIDI commands to drive the program). The type of interactive input is specified by choosing an audio input and control driver. #### Audio Input The `-ain` command-line option is used to specify the hardware driver for audio input. The right panel shows the currently supported hardware arguments to the `-ain` option, and describes an example program to test real-time audio input. To reduce the time delay from input to output, use the `-latency` option to tune system performance. If your hardware is not supported, use the `-ain std` command-line option to create a C program that reads audio samples from standard input. When executing the C program on the command line, pipe the output of a separate sound recording program into the C program. #### Control Input The `-cin` command-line option is used to specify the driver for control input. The right panel shows the control input drivers present in sfront. Three drivers support real-time input from a MIDI In soundcard jack under Linux: the `linmidi` driver for OSS, the `alsamidi` driver for simple MIDI input under ALSA, and the `alsaseq` driver for the ALSA sequencer. The `coremidi` driver supports MIDI input under Mac OS X (i.e. from a USB, Firewire, PCI or Cardbus slot). Virtual MIDI sources are not supported (yet). The `win32` supports soundcard MIDI In under Microsoft Windows, and should work with both WMME and DirectSound Windows audio drivers (but has not been tested). Use the `examples/rtime/linmidi` program to test MIDI In performance under Linux. See the README in this example directory for details on using the example under Windows. The `ascii` driver provides simple real-time MIDI input from an ASCII keyboard. The `examples/rtime/ascii` example program shows how to use this driver under Linux, Mac OS X, and other Unix variants. The `gliss` driver is a platform-independent test driver for the control interface, and is used by the `examples/gliss` example. #### Linux/FreeBSD notes When used for interactive work, the Linux and FreeBSD audio drivers use the playback temporal mode by default, with a latency of 0.002 seconds. If audio artifacts are a problem, try increasing the latency time by using the `-latency` option. The Linux and FreeBSD audio driver prints out suggestions for the best sfront flags to use for a particular SAOL program and control driver setup, at the start of execution of the C program generated by sfront. Read these instructions and re-run sfront for best results. The linux audio driver detects if the C program generated by sfront is running as root, and uses special operating system features to reduce audio dropouts (see the man page for `sched_setscheduler` for details). Two root modes may be available for interactive use: Playback mode, which always keeps the mouse and ASCII keyboard "alive." The C program may be terminated at any time via control-C. Playback mode is the default root mode. Timesync mode, which disables mouse and alphanumeric keyboard input during use. Input is disabled once a NoteOn or NoteOff command is received via the MIDI In jack, and is restored 5 seconds after the last NoteOn or NoteOff command is received. Set this mode by using the `-timesync` sfront command-line option. The Linux audio driver will print out the root modes available for a given control driver and SAOL program. Timesync mode offers superior quality over playback mode, but is only available in certain configurations. To learn more about tuning your Linux system for real-time interactive performance, read the Linux Audio-Quality HOWTO, especially the sections on dropouts. We have tested sfront root mode carefully, and added special "watchdog" features to detect poorly written SAOL programs. However, any remaining bugs in sfront could have dangerous effects when the C files sfront creates are run as root. In particular, the machine could lock up, requiring a hard reboot, and files may be damaged as a result. In addition, using root mode with sfront networking has security implications. Use sfront root mode at your own risk. #### File Streaming If sfront reads an MP4 file using the `-bit` option, it compiles score information into the `sa.c` file from the entire file (configuration and streaming sections). For MP4 files with large streaming sections, the generated `sa.c` may be too large to compile, or may run inefficiently. The `fstr` control driver handles large MP4 files more efficiently. The driver opens the MP4 file during execution of the `sa.c` file, and inserts score information into the running decoder at the correct time. The `-bitc` option should be used with this driver, so that the streaming score information does not get compiled into the `sa.c` file: ``
    sfront -bitc file.mp4 -cin fstr
           -aout linux -playback
    gcc -O3 sa.c -lm -o sa
    ./sa
The `examples/claps` example shows the `fstr` control driver in action. Use the: ``
    make stream
command to demonstrate the driver. Note that this technique only helps for MP4 files with large streaming sections. If large amounts of score data are present in the configuration section, this method does not help the data-size problem. #### Writing New Control Drivers See Part II/1 for information on writing a control driver for sfront. | ## Audio Input Options `
    See reference section of
    sfront manual for details on
    using these options, including
    the stability of each driver.

     option           Environment
    ------------------------------
    -ain linux        Linux.

    -ain coreaudio    Mac OS X

    -ain pa_win_wmme  Microsoft Windows
                      (WMME library).

    -ain pa_win_ds    Microsoft Windows
                      (Directsound).

    -ain irix         SGI Irix.

    -ain freebsd      FreeBSD.

    -ain hpux         HPUX.

    -ain std          Generic -- pipe
                      the output of
                      the C program to
                      a sound player app.

    -ain null         Generic -- ignore
                      the output of the
                      the C program.
` ## Example `
    sfront/examples/rtime/linain
    contains a simple reverb system
    that uses real-time audio input
    and output.

    To run this test under Linux,
    cd into linain and type "make".

    To run aatest on other platforms,
    see the README file in the aatest
    directory for details.
` ## Control Input Options `
    See reference section of
    sfront manual for details on
    using these options, including
    the stability of each driver.

     option       function
    --------------------------
    -cin linmidi  uses the MIDI port
                  of soundcards under
                  Linux (OSS). Generates
                  MIDI commands.

    -cin alsamidi uses the MIDI port
                  of soundcards under
                  Linux (ALSA). Generates
                  MIDI commands.

    -cin alsaseq  supports the ALSA
                  sequencer under
                  Linux, for more
                  sophisticated MIDI
                  control than alsamidi.

    -cin coremidi supports external MIDI
                  input in Mac OS X.

    -cin freebsdmidi

                  uses the MIDI port
                  of soundcards under
                  FreeBSD (pcm). Generates
                  MIDI commands.

    -cin ascii    maps the ASCII
                  keyboard as a
                  MIDI device.

    -cin gliss    plays piano
                  glissandos,
                  up uses MIDI
                  control, down
                  uses SASL.

    -cin fstr     plays MP4 streaming
                  packets from a file.

    -cin win32    uses the MIDI port
                  of soundcards under
                  Windows. Generates
                  MIDI commands.
`

Next section: Part I/4: Networking |

Copyright 1999 John Lazzaro and John Wawrzynek.