Android APIs
public class

Visualizer

extends Object
java.lang.Object
   ↳ android.media.audiofx.Visualizer

Class Overview

The Visualizer class enables application to retrieve part of the currently playing audio for visualization purpose. It is not an audio recording interface and only returns partial and low quality audio content. However, to protect privacy of certain audio data (e.g voice mail) the use of the visualizer requires the permission android.permission.RECORD_AUDIO.

The audio session ID passed to the constructor indicates which audio content should be visualized:

  • If the session is 0, the audio output mix is visualized
  • If the session is not 0, the audio from a particular MediaPlayer or AudioTrack using this audio session is visualized

Two types of representation of audio content can be captured:

  • Waveform data: consecutive 8-bit (unsigned) mono samples by using the getWaveForm(byte[]) method
  • Frequency data: 8-bit magnitude FFT by using the getFft(byte[]) method

The length of the capture can be retrieved or specified by calling respectively getCaptureSize() and setCaptureSize(int) methods. The capture size must be a power of 2 in the range returned by getCaptureSizeRange().

In addition to the polling capture mode described above with getWaveForm(byte[]) and getFft(byte[]) methods, a callback mode is also available by installing a listener by use of the setDataCaptureListener(OnDataCaptureListener, int, boolean, boolean) method. The rate at which the listener capture method is called as well as the type of data returned is specified.

Before capturing data, the Visualizer must be enabled by calling the setEnabled(boolean) method. When data capture is not needed any more, the Visualizer should be disabled.

It is good practice to call the release() method when the Visualizer is not used anymore to free up native resources associated to the Visualizer instance.

Creating a Visualizer on the output mix (audio session 0) requires permission MODIFY_AUDIO_SETTINGS

The Visualizer class can also be used to perform measurements on the audio being played back. The measurements to perform are defined by setting a mask of the requested measurement modes with setMeasurementMode(int). Supported values are MEASUREMENT_MODE_NONE to cancel any measurement, and MEASUREMENT_MODE_PEAK_RMS for peak and RMS monitoring. Measurements can be retrieved through getMeasurementPeakRms(MeasurementPeakRms).

Summary

Nested Classes
class Visualizer.MeasurementPeakRms A class to store peak and RMS values. 
interface Visualizer.OnDataCaptureListener The OnDataCaptureListener interface defines methods called by the Visualizer to periodically update the audio visualization capture. 
Constants
int ALREADY_EXISTS Internal operation status.
int ERROR Unspecified error.
int ERROR_BAD_VALUE Operation failed due to bad parameter value.
int ERROR_DEAD_OBJECT Operation failed due to dead remote object.
int ERROR_INVALID_OPERATION Operation failed because it was requested in wrong state.
int ERROR_NO_INIT Operation failed due to bad object initialization.
int ERROR_NO_MEMORY Operation failed due to lack of memory.
int MEASUREMENT_MODE_NONE Defines a measurement mode in which no measurements are performed.
int MEASUREMENT_MODE_PEAK_RMS Defines a measurement mode which computes the peak and RMS value in mB, where 0mB is the maximum sample value, and -9600mB is the minimum value.
int SCALING_MODE_AS_PLAYED Defines a capture mode where the playback volume will affect (scale) the range of the captured data.
int SCALING_MODE_NORMALIZED Defines a capture mode where amplification is applied based on the content of the captured data.
int STATE_ENABLED State of a Visualizer object that is active.
int STATE_INITIALIZED State of a Visualizer object that is ready to be used.
int STATE_UNINITIALIZED State of a Visualizer object that was not successfully initialized upon creation
int SUCCESS Successful operation.
Public Constructors
Visualizer(int audioSession)
Class constructor.
Public Methods
int getCaptureSize()
Returns current capture size.
static int[] getCaptureSizeRange()
Returns the capture size range.
boolean getEnabled()
Get current activation state of the visualizer.
int getFft(byte[] fft)
Returns a frequency capture of currently playing audio content.
static int getMaxCaptureRate()
Returns the maximum capture rate for the callback capture method.
int getMeasurementMode()
Returns the current measurement modes performed by this audio effect
int getMeasurementPeakRms(Visualizer.MeasurementPeakRms measurement)
Retrieves the latest peak and RMS measurement.
int getSamplingRate()
Returns the sampling rate of the captured audio.
int getScalingMode()
Returns the current scaling mode on the captured visualization data.
int getWaveForm(byte[] waveform)
Returns a waveform capture of currently playing audio content.
void release()
Releases the native Visualizer resources.
int setCaptureSize(int size)
Sets the capture size, i.e.
int setDataCaptureListener(Visualizer.OnDataCaptureListener listener, int rate, boolean waveform, boolean fft)
Registers an OnDataCaptureListener interface and specifies the rate at which the capture should be updated as well as the type of capture requested.
int setEnabled(boolean enabled)
Enable or disable the visualization engine.
int setMeasurementMode(int mode)
Sets the combination of measurement modes to be performed by this audio effect.
int setScalingMode(int mode)
Set the type of scaling applied on the captured visualization data.
Protected Methods
void finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
[Expand]
Inherited Methods
From class java.lang.Object

Constants

public static final int ALREADY_EXISTS

Added in API level 9

Internal operation status. Not returned by any method.

Constant Value: -2 (0xfffffffe)

public static final int ERROR

Added in API level 9

Unspecified error.

Constant Value: -1 (0xffffffff)

public static final int ERROR_BAD_VALUE

Added in API level 9

Operation failed due to bad parameter value.

Constant Value: -4 (0xfffffffc)

public static final int ERROR_DEAD_OBJECT

Added in API level 9

Operation failed due to dead remote object.

Constant Value: -7 (0xfffffff9)

public static final int ERROR_INVALID_OPERATION

Added in API level 9

Operation failed because it was requested in wrong state.

Constant Value: -5 (0xfffffffb)

public static final int ERROR_NO_INIT

Added in API level 9

Operation failed due to bad object initialization.

Constant Value: -3 (0xfffffffd)

public static final int ERROR_NO_MEMORY

Added in API level 9

Operation failed due to lack of memory.

Constant Value: -6 (0xfffffffa)

public static final int MEASUREMENT_MODE_NONE

Added in API level 19

Defines a measurement mode in which no measurements are performed.

Constant Value: 0 (0x00000000)

public static final int MEASUREMENT_MODE_PEAK_RMS

Added in API level 19

Defines a measurement mode which computes the peak and RMS value in mB, where 0mB is the maximum sample value, and -9600mB is the minimum value. Values for peak and RMS can be retrieved with getMeasurementPeakRms(MeasurementPeakRms).

Constant Value: 1 (0x00000001)

public static final int SCALING_MODE_AS_PLAYED

Added in API level 16

Defines a capture mode where the playback volume will affect (scale) the range of the captured data. A low playback volume will lead to low sample and fft values, and vice-versa.

Constant Value: 1 (0x00000001)

public static final int SCALING_MODE_NORMALIZED

Added in API level 16

Defines a capture mode where amplification is applied based on the content of the captured data. This is the default Visualizer mode, and is suitable for music visualization.

Constant Value: 0 (0x00000000)

public static final int STATE_ENABLED

Added in API level 9

State of a Visualizer object that is active.

Constant Value: 2 (0x00000002)

public static final int STATE_INITIALIZED

Added in API level 9

State of a Visualizer object that is ready to be used.

Constant Value: 1 (0x00000001)

public static final int STATE_UNINITIALIZED

Added in API level 9

State of a Visualizer object that was not successfully initialized upon creation

Constant Value: 0 (0x00000000)

public static final int SUCCESS

Added in API level 9

Successful operation.

Constant Value: 0 (0x00000000)

Public Constructors

public Visualizer (int audioSession)

Added in API level 9

Class constructor.

Parameters
audioSession int: system wide unique audio session identifier. If audioSession is not 0, the visualizer will be attached to the MediaPlayer or AudioTrack in the same audio session. Otherwise, the Visualizer will apply to the output mix.
Throws
java.lang.UnsupportedOperationException
RuntimeException
UnsupportedOperationException

Public Methods

public int getCaptureSize ()

Added in API level 9

Returns current capture size.

Returns
int the capture size in bytes.
Throws
IllegalStateException

public static int[] getCaptureSizeRange ()

Added in API level 9

Returns the capture size range.

Returns
int[] the mininum capture size is returned in first array element and the maximum in second array element.

public boolean getEnabled ()

Added in API level 9

Get current activation state of the visualizer.

Returns
boolean true if the visualizer is active, false otherwise

public int getFft (byte[] fft)

Added in API level 9

Returns a frequency capture of currently playing audio content.

This method must be called when the Visualizer is enabled.

The capture is an 8-bit magnitude FFT, the frequency range covered being 0 (DC) to half of the sampling rate returned by getSamplingRate(). The capture returns the real and imaginary parts of a number of frequency points equal to half of the capture size plus one.

Note: only the real part is returned for the first point (DC) and the last point (sampling frequency / 2).

The layout in the returned byte array is as follows:

  • n is the capture size returned by getCaptureSize()
  • Rfk, Ifk are respectively the real and imaginary parts of the kth frequency component
  • If Fs is the sampling frequency retuned by getSamplingRate() the kth frequency is: (k*Fs)/(n/2)
Index

0

1

2

3

4

5

...

n - 2

n - 1

Data

Rf0

Rf(n/2)

Rf1

If1

Rf2

If2

...

Rf(n-1)/2

If(n-1)/2

Parameters
fft byte: array of bytes where the FFT should be returned
Returns
int SUCCESS in case of success, ERROR_NO_MEMORY, ERROR_INVALID_OPERATION or ERROR_DEAD_OBJECT in case of failure.
Throws
IllegalStateException

public static int getMaxCaptureRate ()

Added in API level 9

Returns the maximum capture rate for the callback capture method. This is the maximum value for the rate parameter of the setDataCaptureListener(OnDataCaptureListener, int, boolean, boolean) method.

Returns
int the maximum capture rate expressed in milliHertz

public int getMeasurementMode ()

Added in API level 19

Returns the current measurement modes performed by this audio effect

Returns
int the mask of the measurements, MEASUREMENT_MODE_NONE (when no measurements are performed) or MEASUREMENT_MODE_PEAK_RMS.
Throws
IllegalStateException

public int getMeasurementPeakRms (Visualizer.MeasurementPeakRms measurement)

Added in API level 19

Retrieves the latest peak and RMS measurement. Sets the peak and RMS fields of the supplied Visualizer.MeasurementPeakRms to the latest measured values.

Parameters
measurement Visualizer.MeasurementPeakRms: a non-null Visualizer.MeasurementPeakRms instance to store the measurement values.
Returns
int SUCCESS in case of success, ERROR_BAD_VALUE, ERROR_NO_MEMORY, ERROR_INVALID_OPERATION or ERROR_DEAD_OBJECT in case of failure.

public int getSamplingRate ()

Added in API level 9

Returns the sampling rate of the captured audio.

Returns
int the sampling rate in milliHertz.
Throws
IllegalStateException

public int getScalingMode ()

Added in API level 16

Returns the current scaling mode on the captured visualization data.

Returns
int the scaling mode, see SCALING_MODE_NORMALIZED and SCALING_MODE_AS_PLAYED.
Throws
IllegalStateException

public int getWaveForm (byte[] waveform)

Added in API level 9

Returns a waveform capture of currently playing audio content. The capture consists in a number of consecutive 8-bit (unsigned) mono PCM samples equal to the capture size returned by getCaptureSize().

This method must be called when the Visualizer is enabled.

Parameters
waveform byte: array of bytes where the waveform should be returned
Returns
int SUCCESS in case of success, ERROR_NO_MEMORY, ERROR_INVALID_OPERATION or ERROR_DEAD_OBJECT in case of failure.
Throws
IllegalStateException

public void release ()

Added in API level 9

Releases the native Visualizer resources. It is a good practice to release the visualization engine when not in use.

public int setCaptureSize (int size)

Added in API level 9

Sets the capture size, i.e. the number of bytes returned by getWaveForm(byte[]) and getFft(byte[]) methods. The capture size must be a power of 2 in the range returned by getCaptureSizeRange(). This method must not be called when the Visualizer is enabled.

Parameters
size int: requested capture size
Returns
int SUCCESS in case of success, ERROR_BAD_VALUE in case of failure.
Throws
IllegalStateException

public int setDataCaptureListener (Visualizer.OnDataCaptureListener listener, int rate, boolean waveform, boolean fft)

Added in API level 9

Registers an OnDataCaptureListener interface and specifies the rate at which the capture should be updated as well as the type of capture requested.

Call this method with a null listener to stop receiving the capture updates.

Parameters
listener Visualizer.OnDataCaptureListener: OnDataCaptureListener registered
rate int: rate in milliHertz at which the capture should be updated
waveform boolean: true if a waveform capture is requested: the onWaveFormDataCapture() method will be called on the OnDataCaptureListener interface.
fft boolean: true if a frequency capture is requested: the onFftDataCapture() method will be called on the OnDataCaptureListener interface.
Returns
int SUCCESS in case of success, ERROR_NO_INIT or ERROR_BAD_VALUE in case of failure.

public int setEnabled (boolean enabled)

Added in API level 9

Enable or disable the visualization engine.

Parameters
enabled boolean: requested enable state
Returns
int SUCCESS in case of success, ERROR_INVALID_OPERATION or ERROR_DEAD_OBJECT in case of failure.
Throws
IllegalStateException

public int setMeasurementMode (int mode)

Added in API level 19

Sets the combination of measurement modes to be performed by this audio effect.

Parameters
mode int: a mask of the measurements to perform. The valid values are MEASUREMENT_MODE_NONE (to cancel any measurement) or MEASUREMENT_MODE_PEAK_RMS.
Returns
int SUCCESS in case of success, ERROR_BAD_VALUE in case of failure.
Throws
IllegalStateException

public int setScalingMode (int mode)

Added in API level 16

Set the type of scaling applied on the captured visualization data.

Parameters
mode int: see SCALING_MODE_NORMALIZED and SCALING_MODE_AS_PLAYED
Returns
int SUCCESS in case of success, ERROR_BAD_VALUE in case of failure.
Throws
IllegalStateException

Protected Methods

protected void finalize ()

Added in API level 9

Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.