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FrameBodyASPI.javaAPI DocJaudiotagger 2.0.45839Wed Mar 30 16:12:02 BST 2011org.jaudiotagger.tag.id3.framebody

FrameBodyASPI

public class FrameBodyASPI extends AbstractID3v2FrameBody implements ID3v24FrameBody
Audio files with variable bit rates are intrinsically difficult to deal with in the case of seeking within the file. The ASPI frame makes seeking easier by providing a list a seek points within the audio file. The seek points are a fractional offset within the audio data, providing a starting point from which to find an appropriate point to start decoding. The presence of an ASPI frame requires the existence of a TLEN frame, indicating the duration of the file in milliseconds. There may only be one 'audio seek point index' frame in a tag.

Indexed data start (S) $xx xx xx xx Indexed data length (L) $xx xx xx xx Number of index points (N) $xx xx Bits per index point (b) $xx

Then for every index point the following data is included;

Fraction at index (Fi) $xx (xx)

'Indexed data start' is a byte offset from the beginning of the file. 'Indexed data length' is the byte length of the audio data being indexed. 'Number of index points' is the number of index points, as the name implies. The recommended number is 100. 'Bits per index point' is 8 or 16, depending on the chosen precision. 8 bits works well for short files (less than 5 minutes of audio), while 16 bits is advantageous for long files. 'Fraction at index' is the numerator of the fraction representing a relative position in the data. The denominator is 2 to the power of b.

Here are the algorithms to be used in the calculation. The known data must be the offset of the start of the indexed data (S), the offset of the end of the indexed data (E), the number of index points (N), the offset at index i (Oi). We calculate the fraction at index i (Fi).

Oi is the offset of the frame whose start is soonest after the point for which the time offset is (i/N * duration).

The frame data should be calculated as follows:

Fi = Oi/L * 2^b (rounded down to the nearest integer)

Offset calculation should be calculated as follows from data in the frame:

Oi = (Fi/2^b)*L (rounded up to the nearest integer)

author
: Paul Taylor
author
: Eric Farng
version
$Id: FrameBodyASPI.java 832 2009-11-12 13:25:38Z paultaylor $

Fields Summary
private static final int
DATA_START_FIELD_SIZE
private static final int
DATA_LENGTH_FIELD_SIZE
private static final int
NO_OF_INDEX_POINTS_FIELD_SIZE
private static final int
BITS_PER_INDEX_POINTS_FIELD_SIZE
private static final int
FRACTION_AT_INDEX_MINIMUM_FIELD_SIZE
private static final String
INDEXED_DATA_START
private static final String
INDEXED_DATA_LENGTH
private static final String
NUMBER_OF_INDEX_POINTS
private static final String
BITS_PER_INDEX_POINT
private static final String
FRACTION_AT_INDEX
Constructors Summary
public FrameBodyASPI()
Creates a new FrameBodyASPI datatype.
public FrameBodyASPI(FrameBodyASPI copyObject)
Creates a new FrameBodyASPI from another FrameBodyASPI

param
copyObject

        super(copyObject);
public FrameBodyASPI(ByteBuffer byteBuffer, int frameSize)
Creates a new FrameBodyASPI datatype.

param
byteBuffer
param
frameSize
throws
InvalidTagException

        super(byteBuffer, frameSize);
Methods Summary
public java.lang.StringgetIdentifier()
The ID3v2 frame identifier

return
the ID3v2 frame identifier for this frame type

        return ID3v24Frames.FRAME_ID_AUDIO_SEEK_POINT_INDEX;
protected voidsetupObjectList()

        objectList.add(new NumberFixedLength(INDEXED_DATA_START, this, DATA_START_FIELD_SIZE));
        objectList.add(new NumberFixedLength(INDEXED_DATA_LENGTH, this, DATA_LENGTH_FIELD_SIZE));
        objectList.add(new NumberFixedLength(NUMBER_OF_INDEX_POINTS, this, NO_OF_INDEX_POINTS_FIELD_SIZE));
        objectList.add(new NumberFixedLength(BITS_PER_INDEX_POINT, this, BITS_PER_INDEX_POINTS_FIELD_SIZE));
        objectList.add(new NumberVariableLength(FRACTION_AT_INDEX, this, FRACTION_AT_INDEX_MINIMUM_FIELD_SIZE));