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Waveform data file format

This page describes the binary and JSON data formats produced by the audiowaveform (1) program.

Binary data format (.dat)

audiowaveform expects binary waveform data files to use the .dat file extension. This format consists of a header block followed by the actual waveform data. All values are little-endian.

The header block starts with a version field that identifies how the rest of the data in the file is structured.

Byte offset Type Field
0-3 int32_t Version

The version 1 header is structured as follows:

Byte offset Type Field
4-7 uint32_t Flags
8-11 int32_t Sample rate
12-15 int32_t Samples per pixel
16-19 uint32_t Length

The version 2 header is structured as follows:

Byte offset Type Field
4-7 uint32_t Flags
8-11 int32_t Sample rate
12-15 int32_t Samples per pixel
16-19 uint32_t Length
20-23 int32_t Channels

Each of these fields is described in detail below.

Version

This field indicates the version number of the waveform data format. The version 1 and 2 data formats are described here. If the format changes in future, the Version field will be incremented.

Flags

The Flags field describes the format of the waveform data that follows the header.

Bit Description
0 (lsb) 0: 16-bit resolution, 1: 8-bit resolution
1-31 Unused

Sample rate

Sample rate of the original audio file (Hz).

Samples per pixel

Number of audio samples per waveform minimum/maximum pair.

Length

Length of waveform data (number of minimum and maximum value pairs per channel).

Channels

The number of waveform channels present (version 2 only).

Waveform data

Waveform data follows the header block and consists of pairs of minimum and maximum values that each represent a range of samples of the original audio (the "samples per pixel" header field).

The version 1 data format supports only a single audio channel; the audiowaveform program converts stereo audio to mono when generating waveform data. The version 2 data format supports multiple channels, where the data from each channel is interleaved.

For 8-bit data, the waveform data is represented as follows. Each value lies in the range -128 to +127. The example shows a two channel waveform data file.

Byte offset Type Value
20 int8_t Minimum sample value, index 0, channel 0
21 int8_t Maximum sample value, index 0, channel 0
22 int8_t Minimum sample value, index 0, channel 1
23 int8_t Maximum sample value, index 0, channel 1
24 int8_t Minimum sample value, index 1, channel 0
25 int8_t Maximum sample value, index 1, channel 0
26 int8_t Minimum sample value, index 1, channel 1
27 int8_t Maximum sample value, index 1, channel 1
etc ... ...

Pairs of minimum and maximum values repeat to the end of the file.

For 16-bit data, the waveform data is represented as follows. Each value lies in the range -32768 to +32767. The example shows a two channel waveform data file.

Byte offset Type Value
20-21 int16_t Minimum sample value, index 0, channel 0
22-23 int16_t Maximum sample value, index 0, channel 0
24-25 int16_t Minimum sample value, index 0, channel 1
25-26 int16_t Maximum sample value, index 0, channel 1
27-28 int16_t Minimum sample value, index 1, channel 0
29-30 int16_t Maximum sample value, index 1, channel 0
31-32 int16_t Minimum sample value, index 1, channel 1
33-34 int16_t Maximum sample value, index 1, channel 1
etc ... ...

Pairs of minimum and maximum values repeat to the end of the file.

JSON data format (.json)

The JSON data format contains the same information as the binary format. audiowaveform expects JSON data files to use the .json file extension. The format consists of a single JSON object containing fields as described below.

version

The version number of the waveform data format. The version 1 and 2 data formats are described here. If the format changes in future, the version field will be incremented.

channels

The number of waveform channels present (version 2 only).

sample_rate

Sample rate of original audio file (Hz).

samples_per_pixel

Number of audio samples per waveform minimum/maximum pair.

bits

Resolution of waveform data. May be either 8 or 16.

length

Length of waveform data (number of minimum and maximum value pairs per channel).

data

Array of minimum and maximum waveform data points, interleaved. The example shows a two channel waveform data file. Depending on bits, each value may be in the range -128 to +127 or -32768 to +32767.

Array offset Value
0 Minimum sample value, index 0, channel 0
1 Maximum sample value, index 0, channel 0
2 Minimum sample value, index 0, channel 1
3 Maximum sample value, index 0, channel 1
4 Minimum sample value, index 1, channel 0
5 Maximum sample value, index 1, channel 0
6 Minimum sample value, index 1, channel 1
7 Maximum sample value, index 1, channel 1
etc ...

Example

The following is an example of a (very short) waveform data file in JSON format.

{
  "version": 2,
  "channels": 2,
  "sample_rate": 48000,
  "samples_per_pixel": 512,
  "bits": 8,
  "length": 3,
  "data": [-65,63,-66,64,-40,41,-39,45,-55,43,-55,44]
}