public
final
class
AudioFormat
extends Object
implements
Parcelable
| java.lang.Object | |
| ↳ | android.media.AudioFormat |
The AudioFormat class is used to access a number of audio format and
channel configuration constants. They are for instance used
in AudioTrack and AudioRecord, as valid values in individual parameters of
constructors like AudioTrack(int, int, int, int, int, int), where the fourth
parameter is one of the AudioFormat.ENCODING_* constants.
The AudioFormat constants are also used in MediaFormat to specify
audio related values commonly used in media, such as for KEY_CHANNEL_MASK.
The AudioFormat.Builder class can be used to create instances of
the AudioFormat format class.
Refer to
AudioFormat.Builder for documentation on the mechanics of the configuration and building
of such instances. Here we describe the main concepts that the AudioFormat class
allow you to convey in each instance, they are:
Closely associated with the AudioFormat is the notion of an
audio frame, which is used throughout the documentation
to represent the minimum size complete unit of audio data.
Expressed in Hz, the sample rate in an AudioFormat instance expresses the number
of audio samples for each channel per second in the content you are playing or recording. It is
not the sample rate
at which content is rendered or produced. For instance a sound at a media sample rate of 8000Hz
can be played on a device operating at a sample rate of 48000Hz; the sample rate conversion is
automatically handled by the platform, it will not play at 6x speed.
As of API M,
sample rates up to 192kHz are supported
for AudioRecord and AudioTrack, with sample rate conversion
performed as needed.
To improve efficiency and avoid lossy conversions, it is recommended to match the sample rate
for AudioRecord and AudioTrack to the endpoint device
sample rate, and limit the sample rate to no more than 48kHz unless there are special
device capabilities that warrant a higher rate.
Audio encoding is used to describe the bit representation of audio data, which can be either linear PCM or compressed audio, such as AC3 or DTS.
For linear PCM, the audio encoding describes the sample size, 8 bits, 16 bits, or 32 bits, and the sample representation, integer or float.
Developers
ENCODING_PCM_8BIT: The audio sample is a 8 bit unsigned integer in the
range [0, 255], with a 128 offset for zero. This is typically stored as a Java byte in a
byte array or ByteBuffer. Since the Java byte is signed,
be careful with math operations and conversions as the most significant bit is inverted.
ENCODING_PCM_16BIT: The audio sample is a 16 bit signed integer
typically stored as a Java short in a short array, but when the short
is stored in a ByteBuffer, it is native endian (as compared to the default Java big endian).
The short has full range from [-32768, 32767],
and is sometimes interpreted as fixed point Q.15 data.
ENCODING_PCM_FLOAT: Introduced in
API LOLLIPOP, this encoding specifies that
the audio sample is a 32 bit IEEE single precision float. The sample can be
manipulated as a Java float in a float array, though within a ByteBuffer
it is stored in native endian byte order.
The nominal range of ENCODING_PCM_FLOAT audio data is [-1.0, 1.0].
It is implementation dependent whether the positive maximum of 1.0 is included
in the interval. Values outside of the nominal range are clamped before
sending to the endpoint device. Beware that
the handling of NaN is undefined; subnormals may be treated as zero; and
infinities are generally clamped just like other values for AudioTrack
– try to avoid infinities because they can easily generate a NaN.
ENCODING_PCM_FLOAT for audio capture, processing,
and playback.
Floats are efficiently manipulated by modern CPUs,
have greater precision than 24 bit signed integers,
and have greater dynamic range than 32 bit signed integers.
AudioRecord as of API M and
AudioTrack as of API LOLLIPOP
support ENCODING_PCM_FLOAT.
For compressed audio, the encoding specifies the method of compression,
for example ENCODING_AC3 and ENCODING_DTS. The compressed
audio data is typically stored as bytes in
a byte array or ByteBuffer. When a compressed audio encoding is specified
for an AudioTrack, it creates a direct (non-mixed) track
for output to an endpoint (such as HDMI) capable of decoding the compressed audio.
For (most) other endpoints, which are not capable of decoding such compressed audio,
you will need to decode the data first, typically by creating a MediaCodec.
Alternatively, one may use MediaPlayer for playback of compressed
audio files or streams.
When compressed audio is sent out through a direct AudioTrack,
it need not be written in exact multiples of the audio access unit;
this differs from MediaCodec input buffers.
Channel masks are used in AudioTrack and AudioRecord to describe
the samples and their arrangement in the audio frame. They are also used in the endpoint (e.g.
a USB audio interface, a DAC connected to headphones) to specify allowable configurations of a
particular device.
As of API M, there are two types of channel masks:
channel position masks and channel index masks.
BASE.
For input and output, they imply a positional nature - the location of a speaker or a microphone
for recording or playback.
| channel count | channel position mask |
| 1 | CHANNEL_OUT_MONO |
| 2 | CHANNEL_OUT_STEREO |
| 3 | CHANNEL_OUT_STEREO | CHANNEL_OUT_FRONT_CENTER |
| 4 | CHANNEL_OUT_QUAD |
| 5 | CHANNEL_OUT_QUAD | CHANNEL_OUT_FRONT_CENTER |
| 6 | CHANNEL_OUT_5POINT1 |
| 7 | CHANNEL_OUT_5POINT1 | CHANNEL_OUT_BACK_CENTER |
| 8 | CHANNEL_OUT_7POINT1_SURROUND |
CHANNEL_OUT_STEREO is composed of CHANNEL_OUT_FRONT_LEFT and
CHANNEL_OUT_FRONT_RIGHT.
M. They allow
the selection of a particular channel from the source or sink endpoint by number, i.e. the first
channel, the second channel, and so forth. This avoids problems with artificially assigning
positions to channels of an endpoint, or figuring what the ith position bit is within
an endpoint's channel position mask etc.
CHANNEL_OUT_QUAD device, but really one is only interested in channel 0
through channel 3. The USB device would then have the following individual bit channel masks:
CHANNEL_OUT_FRONT_LEFT,
CHANNEL_OUT_FRONT_RIGHT, CHANNEL_OUT_BACK_LEFT
and CHANNEL_OUT_BACK_RIGHT. But which is channel 0 and which is
channel 3?
1 << channelNumber.
A set bit indicates that channel is present in the audio frame, otherwise it is cleared.
The order of the bits also correspond to that channel number's sample order in the audio frame.
0xF. Suppose we wanted to select only the first and the third channels; this would
correspond to a channel index mask 0x5 (the first and third bits set). If an
AudioTrack uses this channel index mask, the audio frame would consist of two
samples, the first sample of each frame routed to channel 0, and the second sample of each frame
routed to channel 2.
The canonical channel index masks by channel count are given by the formula
(1 << channelCount) - 1.
CHANNEL_OUT_FRONT_LEFT,
CHANNEL_OUT_FRONT_CENTER, etc. for HDMI home theater purposes.
AudioTrack
to output movie content, where 5.1 multichannel output is to be written.
AudioRecord may only want the
third and fourth audio channels of the endpoint (i.e. the second channel pair), and not care the
about position it corresponds to, in which case the channel index mask is 0xC.
Multichannel AudioRecord sessions should use channel index masks.
For linear PCM, an audio frame consists of a set of samples captured at the same time,
whose count and
channel association are given by the channel mask,
and whose sample contents are specified by the encoding.
For example, a stereo 16 bit PCM frame consists of
two 16 bit linear PCM samples, with a frame size of 4 bytes.
For compressed audio, an audio frame may alternately
refer to an access unit of compressed data bytes that is logically grouped together for
decoding and bitstream access (e.g. MediaCodec),
or a single byte of compressed data (e.g. AudioTrack.getBufferSizeInFrames()),
or the linear PCM frame result from decoding the compressed data
(e.g.AudioTrack.getPlaybackHeadPosition()),
depending on the context where audio frame is used.
Nested classes | |
|---|---|
class |
AudioFormat.Builder
Builder class for |
Inherited constants |
|---|
 From
interface
android.os.Parcelable
|
Fields | |
|---|---|
public
static
final
Creator<AudioFormat> |
CREATOR
|
Public methods | |
|---|---|
int
|
describeContents()
Describe the kinds of special objects contained in this Parcelable instance's marshaled representation. |
boolean
|
equals(Object o)
Indicates whether some other object is "equal to" this one. |
int
|
getChannelCount()
Return the channel count. |
int
|
getChannelIndexMask()
Return the channel index mask. |
int
|
getChannelMask()
Return the channel mask. |
int
|
getEncoding()
Return the encoding. |
int
|
getSampleRate()
Return the sample rate. |
int
|
hashCode()
Returns a hash code value for the object. |
String
|
toString()
Returns a string representation of the object. |
void
|
writeToParcel(Parcel dest, int flags)
Flatten this object in to a Parcel. |
Inherited methods | |
|---|---|
From
class
java.lang.Object
| |
From
interface
android.os.Parcelable
| |
int CHANNEL_CONFIGURATION_DEFAULT
This constant was deprecated
in API level 5.
Use CHANNEL_OUT_DEFAULT or CHANNEL_IN_DEFAULT instead.
Constant Value: 1 (0x00000001)
int CHANNEL_CONFIGURATION_INVALID
This constant was deprecated
in API level 5.
Use CHANNEL_INVALID instead.
Constant Value: 0 (0x00000000)
int CHANNEL_CONFIGURATION_MONO
This constant was deprecated
in API level 5.
Use CHANNEL_OUT_MONO or CHANNEL_IN_MONO instead.
Constant Value: 2 (0x00000002)
int CHANNEL_CONFIGURATION_STEREO
This constant was deprecated
in API level 5.
Use CHANNEL_OUT_STEREO or CHANNEL_IN_STEREO instead.
Constant Value: 3 (0x00000003)
int CHANNEL_INVALID
Invalid audio channel mask
Constant Value: 0 (0x00000000)
int CHANNEL_IN_BACK_PROCESSED
Constant Value: 512 (0x00000200)
int CHANNEL_IN_FRONT_PROCESSED
Constant Value: 256 (0x00000100)
int CHANNEL_IN_LEFT_PROCESSED
Constant Value: 64 (0x00000040)
int CHANNEL_IN_RIGHT_PROCESSED
Constant Value: 128 (0x00000080)
int CHANNEL_IN_VOICE_DNLINK
Constant Value: 32768 (0x00008000)
int CHANNEL_IN_VOICE_UPLINK
Constant Value: 16384 (0x00004000)
int CHANNEL_OUT_7POINT1
This constant was deprecated
in API level 23.
Not the typical 7.1 surround configuration. Use CHANNEL_OUT_7POINT1_SURROUND instead.
Constant Value: 1020 (0x000003fc)
int CHANNEL_OUT_7POINT1_SURROUND
Constant Value: 6396 (0x000018fc)
int CHANNEL_OUT_BACK_CENTER
Constant Value: 1024 (0x00000400)
int CHANNEL_OUT_BACK_LEFT
Constant Value: 64 (0x00000040)
int CHANNEL_OUT_BACK_RIGHT
Constant Value: 128 (0x00000080)
int CHANNEL_OUT_DEFAULT
Default audio channel mask
Constant Value: 1 (0x00000001)
int CHANNEL_OUT_FRONT_CENTER
Constant Value: 16 (0x00000010)
int CHANNEL_OUT_FRONT_LEFT
Constant Value: 4 (0x00000004)
int CHANNEL_OUT_FRONT_LEFT_OF_CENTER
Constant Value: 256 (0x00000100)
int CHANNEL_OUT_FRONT_RIGHT
Constant Value: 8 (0x00000008)
int CHANNEL_OUT_FRONT_RIGHT_OF_CENTER
Constant Value: 512 (0x00000200)
int CHANNEL_OUT_LOW_FREQUENCY
Constant Value: 32 (0x00000020)
int CHANNEL_OUT_SIDE_LEFT
Constant Value: 2048 (0x00000800)
int CHANNEL_OUT_SIDE_RIGHT
Constant Value: 4096 (0x00001000)
int CHANNEL_OUT_SURROUND
Constant Value: 1052 (0x0000041c)
int ENCODING_AC3
Audio data format: AC-3 compressed
Constant Value: 5 (0x00000005)
int ENCODING_DEFAULT
Default audio data format
Constant Value: 1 (0x00000001)
int ENCODING_DTS
Audio data format: DTS compressed
Constant Value: 7 (0x00000007)
int ENCODING_DTS_HD
Audio data format: DTS HD compressed
Constant Value: 8 (0x00000008)
int ENCODING_E_AC3
Audio data format: E-AC-3 compressed
Constant Value: 6 (0x00000006)
int ENCODING_IEC61937
Audio data format: compressed audio wrapped in PCM for HDMI
or S/PDIF passthrough.
IEC61937 uses a stereo stream of 16-bit samples as the wrapper.
So the channel mask for the track must be CHANNEL_OUT_STEREO.
Data should be written to the stream in a short[] array.
If the data is written in a byte[] array then there may be endian problems
on some platforms when converting to short internally.
Constant Value: 13 (0x0000000d)
int ENCODING_INVALID
Invalid audio data format
Constant Value: 0 (0x00000000)
int ENCODING_PCM_16BIT
Audio data format: PCM 16 bit per sample. Guaranteed to be supported by devices.
Constant Value: 2 (0x00000002)
int ENCODING_PCM_8BIT
Audio data format: PCM 8 bit per sample. Not guaranteed to be supported by devices.
Constant Value: 3 (0x00000003)
int ENCODING_PCM_FLOAT
Audio data format: single-precision floating-point per sample
Constant Value: 4 (0x00000004)
int SAMPLE_RATE_UNSPECIFIED
Sample rate will be a route-dependent value. For AudioTrack, it is usually the sink sample rate, and for AudioRecord it is usually the source sample rate.
Constant Value: 0 (0x00000000)
int describeContents ()
Describe the kinds of special objects contained in this Parcelable
instance's marshaled representation. For example, if the object will
include a file descriptor in the output of writeToParcel(Parcel, int),
the return value of this method must include the
CONTENTS_FILE_DESCRIPTOR bit.
| Returns | |
|---|---|
int |
a bitmask indicating the set of special object types marshaled by this Parcelable object instance. |
boolean equals (Object o)
Indicates whether some other object is "equal to" this one.
The equals method implements an equivalence relation
on non-null object references:
x, x.equals(x) should return
true.
x and y, x.equals(y)
should return true if and only if
y.equals(x) returns true.
x, y, and z, if
x.equals(y) returns true and
y.equals(z) returns true, then
x.equals(z) should return true.
x and y, multiple invocations of
x.equals(y) consistently return true
or consistently return false, provided no
information used in equals comparisons on the
objects is modified.
x,
x.equals(null) should return false.
The equals method for class Object implements
the most discriminating possible equivalence relation on objects;
that is, for any non-null reference values x and
y, this method returns true if and only
if x and y refer to the same object
(x == y has the value true).
Note that it is generally necessary to override the hashCode
method whenever this method is overridden, so as to maintain the
general contract for the hashCode method, which states
that equal objects must have equal hash codes.
| Parameters | |
|---|---|
o |
Object:
the reference object with which to compare. |
| Returns | |
|---|---|
boolean |
true if this object is the same as the obj
argument; false otherwise. |
int getChannelCount ()
Return the channel count.
| Returns | |
|---|---|
int |
the channel count derived from the channel position mask or the channel index mask. Zero is returned if both the channel position mask and the channel index mask are not set. |
int getChannelIndexMask ()
Return the channel index mask.
See the section on channel masks for more information about
the difference between index-based masks, and position-based masks (as returned
by getChannelMask()).
| Returns | |
|---|---|
int |
one of the values that can be set in setChannelIndexMask(int) or
CHANNEL_INVALID if not set or an invalid mask was used.
|
int getChannelMask ()
Return the channel mask.
See the section on channel masks for more information about
the difference between index-based masks(as returned by getChannelIndexMask()) and
the position-based mask returned by this function.
| Returns | |
|---|---|
int |
one of the values that can be set in setChannelMask(int) or
CHANNEL_INVALID if not set.
|
int getEncoding ()
Return the encoding. See the section on encodings for more information about the different types of supported audio encoding.
| Returns | |
|---|---|
int |
one of the values that can be set in setEncoding(int) or
ENCODING_INVALID if not set.
|
int getSampleRate ()
Return the sample rate.
| Returns | |
|---|---|
int |
one of the values that can be set in setSampleRate(int) or
SAMPLE_RATE_UNSPECIFIED if not set.
|
int hashCode ()
Returns a hash code value for the object. This method is
supported for the benefit of hash tables such as those provided by
HashMap.
The general contract of hashCode is:
hashCode method
must consistently return the same integer, provided no information
used in equals comparisons on the object is modified.
This integer need not remain consistent from one execution of an
application to another execution of the same application.
equals(Object)
method, then calling the hashCode method on each of
the two objects must produce the same integer result.
equals(java.lang.Object)
method, then calling the hashCode method on each of the
two objects must produce distinct integer results. However, the
programmer should be aware that producing distinct integer results
for unequal objects may improve the performance of hash tables.
As much as is reasonably practical, the hashCode method defined by
class Object does return distinct integers for distinct
objects. (This is typically implemented by converting the internal
address of the object into an integer, but this implementation
technique is not required by the
JavaTM programming language.)
| Returns | |
|---|---|
int |
a hash code value for this object. |
String toString ()
Returns a string representation of the object. In general, the
toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
| Returns | |
|---|---|
String |
a string representation of the object. |
void writeToParcel (Parcel dest, int flags)
Flatten this object in to a Parcel.
| Parameters | |
|---|---|
dest |
Parcel:
The Parcel in which the object should be written. |
flags |
int:
Additional flags about how the object should be written.
May be 0 or PARCELABLE_WRITE_RETURN_VALUE.
|