ILMAACEncoder Interface

This is the interface for the LEAD AAC Encoder.

Interface Properties:

Type

Property name

Description

long

OutputBitrate

The output bitrate, in kbps, of the compressed audio data. Bitrate denotes the average number of bits that one second of audio data will take up in the compressed bitstream. The output quality is related to the selected bitrate; the higher the bitrate, the better the quality.

Supported bitrates are: 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, 448, 512, 576, and 640 kbps.

eOutputChannels

OutputChannels

The number of output channels. Possible values are (members of the eOutputChannels enumeration):

Value
Meaning

LAAC_OC_UNCHANGED

[0x00] Same as input.

LAAC_OC_MONO

[0x01] One channel; mono.

LAAC_OC_STEREO

 

[0x02] Two channels; stereo.

LAAC_OC_6CHANNELS

[0x03] Six channels; 5.1.

 

VARIANT_BOOL

SimulateSurround

Enables or disables surround feature simulation. Possible values are:

Value
Meaning

VARIANT_TRUE

Enables surround feature simulation.

VARIANT_FALSE

Disables surround feature simulation

This property has effect only if the OutputChannels property was set to LAAC_OC_6CHANNELS.

eMPEGVersion

MPEGVersion

The AAC audio MPEG version. Possible values are (members of the eMPEGVersion enumeration):

Value
Meaning

LAAC_MPEGVersion_MPEG4

[0] MPEG-4 AAC.

LAAC_MPEGVersion_MPEG2

[1] MPEG-2 AAC. With this version, only LC and MAIN profiles (object types) are allowed.

 

eBlockType

BlockType

The allowed block type; data window length.

Possible values are (members of the eBlockType enumeration):

Value
Meaning

LAAC_BlockType_NORMAL

[0] Both long and short blocks are allowed.

LAAC_BlockType_NOSHORT

[1] Only long blocks allowed. A long block is usually 1024 samples.

LAAC_BlockType_NOLONG

[2] Only short blocks allowed. A short block is usually 128 samples.

 

eObjectType

ObjectType

The AAC profile used. Possible values are (members of the eObjectType enumeration):

Value
Meaning

LAAC_ObjectType_LC

[0] Low complexity profile. Requires lower CPU and Memory resources.

LAAC_ObjectType_MAIN

[1] Main profile. Greater coding efficiency on the expense of complexity (more resources).

LAAC_ObjectType_LTP

[2] Long Term Prediction profile. Most suitable for speech and signals with clear pitch property. This profile can be used with MPEG4 AAC only; the MPEG4Version property must be LAAC_MPEGVersion_MPEG4.

 

long

QFactor

The quality factor used in compressing the audio stream; the higher the quality factor, the better the quality. Possible values range from 10 (worst) to 500 (best).

eRateControl

RateControl

The method used to control the output bitrate. Possible values are (members of the eRateControl enumeration):

 

Value
Meaning

LAAC_RC_CONST_QUALITY

[0] Constant quality, or variable bitrate. The output bitrate depends on the used quality factor and the nature of the input signal. If the encoder is connected to the AVI multiplexer, this option is disabled and the encoder will revert to the constant bitrate (LAAC_RC_CONST_BITRATE).

LAAC_RC_CONST_BITRATE

[1] Average bitrate. The output bitrate is defined by the OutputBitrate property.

 

long

Cutoff

The cutoff frequency, the frequency beyond which the audio signal is attenuated, in Hz. Possible values range from 3000 to 24000 Hz, and the value must be a multiple of 1000.

VARIANT_BOOL

UseCutoff

Determines whether the cutoff frequency specified by the Cutoff property will be used. Possible values are:

 

Value
Meaning

VARIANT_TRUE

The cutoff frequency specified by the Cutoff property is used.

VARIANT_FALSE

No cutoff frequency is used. The Cutoff property has no effect.

 

eStreamFormat

StreamFormat

The format of the output stream. Possible values are (members of the eStreamFormat enumeration):

Value
Meaning

LAAC_STREAMFORMAT_RAW

The encoder will output raw compressed data; raw data blocks. Only one header at the beginning of the stream is output. Using this value, the MPEGVersion property is restricted to LAAC_MPEGVersion_MPEG4.

LAAC_STREAMFORMAT_ADTS

The encoder output format is Audio Data Transport Stream, where each frame has its own header.

 

long

OutputSampleRate

The output sample rate, in samples per second.

Possible values range between 8000 and 96000 samples per second (Hz).

Note: Changing the sample rate causes a change in the quality resulting from audio data resampling.

VARIANT_BOOL

IgnoreTimeDiscontinuity

If set to VARIANT_TRUE, this property will cause the encoder to ignore discontinuities in the timestamps (time gaps) of the incoming audio samples. Sometimes this is needed to work around audio source timing problems. This property is not active (VARIANT_FALSE) by default.

VARIANT_BOOL

IgnoreTimeOverlapping

If set to VARIANT_TRUE, this property will cause the encoder to ignore overlapping in the timestamps of the incoming audio samples. Sometimes this is needed to work around audio source timing problems. This property is not active (VARIANT_FALSE) by default.