IMA_ADPCM Class Reference
[Audio Codec - IMA ADPCM]

A class which implements the IMA ADPCM audio coding algorithm. More...

#include <IMA_ADPCM.h>

List of all members.

Public Member Functions
void	EncodeInit (int16_t sample1, int16_t sample2)
unsigned	Encode (int16_t pcm16)
int	Decode (unsigned adpcm)
unsigned	Encode (uint8_t *dst, int dstOffset, const int16_t *src, size_t srcSize)
unsigned	Decode (int16_t *dst, const uint8_t *src, int srcOffset, unsigned srcSize)
Public Attributes
int16_t	PredictedValue
uint8_t	StepIndex

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Detailed Description

A class which implements the IMA ADPCM audio coding algorithm.

Typically, IMA ADPCM data is stored as blocks of data with the PredictedValue and StepIndex values held in a header to each block. When decoding, these values should be writen to this class to initialise the decoding of a block. When encoding, these values should be writen out to a block's header prior to encoding the samples in the block.

Note, when IMA ADPCM data is stored in Microsoft WAV files, the PredictedValue found in the block's header is used as the first sample for that block. The first ADPCM value in the block's data then represents the second sample.

Version:

2006-05-20

• Changed code to use standard typedefs, e.g. replaced uint8 with uint8_t, and made use of size_t.

Definition at line 56 of file IMA_ADPCM.h.

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Member Function Documentation

void IMA_ADPCM::EncodeInit	(	int16_t	sample1,
		int16_t	sample2
	)

Initialise the PredictedValue and StepIndex members to the optimum values for encoding an audio stream whoes first two PCM samples have the values given. Use of this method at the start of audio stream encoding gives improved accuracy over a naive initialisation which sets PredictedValue and StepIndex to predetermined constant values.

Parameters:

	sample1	The first PCM sample in the audio stream.
	sample2	The second PCM sample in the audio stream.

Definition at line 59 of file IMA_ADPCM.cpp.

unsigned IMA_ADPCM::Encode

(

int16_t

pcm16

)

Encode a single linear PCM sample as an ADPCM value.

Parameters:

pcm16

The PCM value to encode.

Returns:

The 4 least significan bits represent the encoded ADPCM value. Other bits are zero.

Postcondition:

PredictedValue and StepIndex are updated ready for encoding the next sample.

Definition at line 74 of file IMA_ADPCM.cpp.

int IMA_ADPCM::Decode

(

unsigned

adpcm

)

Decode a single ADPCM value into a 16bit linear PCM value.

Parameters:

adpcm

The 4 least significan bits represent ADPCM value to encode. Other bits are ignored.

Returns:

The decoded 16 bit PCM value sign extended to an int.

Postcondition:

PredictedValue and StepIndex are updated ready for decoding the next sample.

Definition at line 132 of file IMA_ADPCM.cpp.

unsigned IMA_ADPCM::Encode	(	uint8_t *	dst,
		int	dstOffset,
		const int16_t *	src,
		size_t	srcSize
	)

Encode a buffer of 16 bit uniform PCM values into ADPCM values.

Two ADPCM values are stored in each byte. The value stored in bits 0-3 corresponds to the sample preceding that stored in bits 4-7. Note, if the last encoded ADPCM value is stored in bits 0-3, then bits 4-7 will be cleared to zero.

Parameters:

	dst	Pointer to location to store ADPCM values.
	dstOffset	Offset from dst, in number-of-bits, at which the decoded values will be stored. I.e. the least significant bit of the first ADPCM value will be stored in byte dst[dstOffset>>3] at bit position dstOffset&7 Where the bit 0 is the least significant bit in a byte and bit 7 is the most significant bit. The value of dstOffset must be a multiple of 4.
	src	Pointer to the buffer of PCM values to be converted.
	srcSize	The size, in bytes, of the buffer at src. Must be a multiple of 2.

Returns:

The number of bits which were stored at dst.

Definition at line 167 of file IMA_ADPCM.cpp.

unsigned IMA_ADPCM::Decode	(	int16_t *	dst,
		const uint8_t *	src,
		int	srcOffset,
		unsigned	srcSize
	)

Decode a buffer of ADPCM values into 16 bit uniform PCM values.

Two ADPCM values are stored in each byte. The value stored in bits 0-3 corresponds to the sample preceding that stored in bits 4-7.

Parameters:

	dst	Pointer to location to store PCM values.
	src	Pointer to the buffer of ADPCM values to be converted.
	srcOffset	Offset from src, in number-of-bits, from which the ADPCM values will be read. I.e. the least significant bit of the first ADPCM value will be read from byte src[srcOffset>>3] at bit position srcOffset&7 Where the bit 0 is the least significant bit in a byte and bit 7 is the most significant bit. The value of srcOffset must be a multiple of 4.
	srcSize	The number of bits to be read from the buffer at src. Must be a multiple of the size of 4.

Returns:

The number of bytes which were stored at dst.

Definition at line 204 of file IMA_ADPCM.cpp.

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Member Data Documentation

int16_t IMA_ADPCM::PredictedValue

The predicted value of the next sample. Typically, this value is read from the header, or written to the header, of a block of ADPCM values.

Definition at line 152 of file IMA_ADPCM.h.

uint8_t IMA_ADPCM::StepIndex

The step index used for the next ADPCM value Typically, this value is read from the header, or written to the header, of a block of ADPCM values.

Definition at line 159 of file IMA_ADPCM.h.

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The documentation for this class was generated from the following files:

• audio/IMA_ADPCM.h
• audio/IMA_ADPCM.cpp