Overview of VOX file format

This topic presents an overview of a VOX file format and describes:

Index structure
Frame sizes

A VOX file consists of a 32 byte fixed size header section, followed by an array of 10 byte segment descriptors called indices. The number of segments is specified at file creation and can range from 48 to 6500. Segment sizes are in units of frames. The number of bytes in a frame depends on the encoding.

The following table illustrates the VOX file format:

Encoding	VTYPE	2 bytes	Header (32 bytes)
Total number of indices	TOTLIDX	2
Number of indices in use	USEDIDX	2
Number of indices in messages	ACTVIDX	2
Number of indices in free pool	FREEIDX	2
Highest message number in use	HIGHMSG	2
Total number of bytes	TOTLBYTE	4
Total number of frames	TOTLFRM	4
Reserved		12
Active indices 10 x ACTVIDX bytes			Indices [10 x TOTLIDX bytes] (see Index structure)
Freed indices 10 x (USEDIDX - ACTVIDX) bytes
Unused indices 10 x FREEIDX bytes
Actual voice frames in segments of various length where an index describes each segment in either the Active Index list or the Freed Index list.			Frames [TOTLFRM x framesize] (see Summary of frame sizes)

Index structure

The following table details the index structure:

Message number

MSGNO

2 bytes

Index structure

[10 bytes]

Starting byte in file

STRTBYTE

Number of frames in segment

NFRAMES

Summary of frame sizes

The following table details the frame sizes of common encodings:

Encoding	Description	msec/frame	bytes/frame
1	NMS ADPCM 16 kbit/s	20	42
2	NMS ADPCM 24 kbit/s	20	62
3	NMS ADPCM 32 kbit/s	20	82
4	Framed PCM 64 kbit/s	20	162
10	mu-law 64 kbit/s	10	80
11	A-law 64 kbit/s	10	80
13	PCM 8 kss mono 16-bit	10	160
14	OKI ADPCM 24 kbit/s	10	30
15	OKI ADPCM 32 kbit/s	10	40
16	PCM 11 kss 8-bit	10	110
17	PCM 11 kss 16-bit	10	220
20	G.726 ADPCM 32 kbit/s	10	40
22	IMA ADPCM 24 kbit/s	10	36
23	IMA ADPCM 32 kbit/s	10	46
24	MS-GSM 13 kbit/s	80	130
26	G.723 5.3 kbit/s	30	20
27	G.723 6.3 kbit/s	30	24
28	G.729A 8 kbit/s	10	10
46	mu-law 64 kbit/s formatted in EDTX frames	variable typically 10	variable Max = 82
47	A-law 64 kbit/s formatted in EDTX frames	variable typically 10	variable Max = 82
48	CCITT G.726 ADPCM 32 kbit/s formatted in EDTX frames	variable typically 10	variable Max = 42
49	G.729A 8 kbit/s formatted in EDTX frames	variable typically 10	variable Max = 12
50	G.723 frames formatted with EDTX headers	variable typically 30	variable Max = 26
51	G.723 5.3 kbit/s frames formatted with EDTX headers	variable typically 30	variable Max = 22
52	G.723, 6.4 kbit/s frames formatted with EDTX headers	variable typically 30	variable Max = 26

Note: kss = kilo-samples per second.

Use the voxinfo utility to display information about a VOX file.