Data modes

Overview

The data source field of the entry table (see Data table) indicates how the other 15 bytes of the entry are to be interpreted. Values of 0 through 4 are defined. The various data table formats are defined below.

Although there are various schemes, note that the entries in the various schemes are the same size, 16 bytes long.

No-op data mode

The data table entry has the format for no-op mode shown in the following table:

Field descriptions

Data source = 0

A value of 0 indicates that this data table entry is to be ignored.

Immediate data mode

The data table entry has the format for immediate mode shown in the following table:

Field descriptions

Data source = 1

A value of 1 indicates that the data is to be immediately taken from the bytes of data that follow. = term Immediate length: An 8-bit integer indicating the number of bytes to take from the data that follows. Legal values range from 0 to 14.

Immediate data

14 bytes of data to place into the payload portion of the packet. Only the first number of bytes indicated by the immediate length field is used.

Sample mode

The data table entry has the format for sample mode shown in the following table:

Field descriptions

Data source = 2

A value of 2 indicates that the data is to be taken from a track’s sample data.

Track ref index

A value that indicates which track the sample data will come from. A value of 0 means that there is exactly one media track referenced, so use that. Values from 1 to 127 are indexes into the hint track reference atom entries, indicating which original media track the sample is to be read from. A value of -1 means the hint track itself, that is, get the sample from the same track as the hint sample you are currently parsing.

Length

A 16-bit integer specifying the number of bytes in the sample to copy.

Sample number

A 32-bit integer specifying sample number of the track.

Offset

A 32-bit integer specifying the offset from the start of the sample from which to start copying. If you are referencing samples in the hint track, this will generally points into the Additional Data area.

Bytes per compression block

A 16-bit unsigned integer specifying the number of bytes that results from compressing the number of samples in the Samples per compression block field. A value of 0 is equivalent to a value of 1.

Samples per compression block

A 16-bit unsigned integer specifying the uncompressed samples per compression block. A value of 0 is equivalent to a value of 1.

If the bytes per compression block and/or the samples per compression block is greater than 1, than this ratio is used to translate a sample number into an actual byte offset.

This ratio mode is typically used for compressed sound tracks. Note that for QuickTime sound tracks, the bytes per compression block also factors in the number of sound channels in that stream, so a QuickTime stereo sound stream’s BPCB would be twice that of a mono stream of the same sound format.

(CB = NS \* BPCB / SPCB)

where CB = compressed bytes, NS = number of samples, BPCB = bytes per compression block, and SPCB = samples per compression block.

An example:

A GSM compression block is typically 160 samples packed into 33 bytes.

So, BPCB = 33 and SPCB = 160.

The hint sample requests 33 bytes of data starting at the 161st media sample. Assume that the first QuickTime chunk contains at least 320 samples. So after determining that this data will come from chunk 1, and knowing where chunk 1 starts, you must use this ratio to adjust the offset into the file where the requested samples will be found:

chunk_number = 1; /* calculated by walking the sample-to-chunk atom  */
first_sample_in_this_chunk = 1; /* also calculated from that atom  */
chunk_offset = chunk_offsets[chunk_number]; /* from the stco atom  */
data_offset = (sample_number - first_sample_in_this_chunk) * BPCB  / SPCB;
read_from_file(chunk_offset + data_offset, length); /* read our  data */

Sample description mode

The data table entry has the format for sample description mode shown in the following table.

Field descriptions

Data source = 3

A value of 3 indicates that the data is to be taken from the media track’s sample description table.

Track ref index

A value that indicates which track the sample description will come from. A value of 0 means that there is exactly one hint track reference, so use that. Values from 1 to 127 are indexes into the hint track reference atom entries, indicating which original media track the sample is to be read from. A value of -1 means the hint track itself, that is, get the sample description from the same track as the hint sample you are currently parsing.

Length

A 16-bit integer specifying the number of bytes to copy.

Sample description index

A 32-bit integer specifying the index into the media’s sample description table.

Offset

A 32-bit integer specifying the offset from the start of the sample description from which to start copying.

Reserved

Four bytes that must be set to 0.

Additional data

A variable length field containing data pointed to by hint track sample mode entries in the data table.