DIDSON data files

DIDSON (.ddf)

DDF_03

The DDF_03 version of the DIDSON file format is supported in Echoview.

This file format consists of a master file header followed by one or more frames of data. Both snap shot (master header and a single frame) and movie format files (master header and multiple frames) are supported. Data is displayed in a multibeam echogram where frames correspond to pings.

Each 8-bit underlying data sample stores a number from 0 to 255 representing 0 to 90 dB when processed - these can be exported as underlying data. No phase information is available.

The range (see also note) of each datapoint is calculated as follows:

Start Range = WindowStart * DelayPeriod * C / 2.0;
Stop Range = Start Range + SamplesPerChannel * C / (2.0 * SampleRate);

Where:

WindowStart is determined from the data file and is 0, 1, 2, or 3
SampleRate is determined from the data file (Hz)
SamplesPerChannel is determined from the data file, typically 512 (integer)
C is the sound speed entered on the Calibration page of the Variable properties dialog box (m/s)
DelayPeriod is determined from the data file using the following lookup table:

HighResolution

SN
No

Yes

<=18 0.001024 0.000512

>18 0.001144 0.000572

Where:

HighResolution is determined from the data file (yes or no)
SN is determined from the data file (integer)

The range of any one sample then is calculated as:

Range = C / 2.0 * (WindowStart * DelayPeriod + (n + 0.5) / SampleRate)

Where n = 0 to SamplesPerChannel-1

DDF_04

The DDF_04 version of the DIDSON file format is supported in Echoview. This data format accommodates additional information in the frame headers. The major difference from the DDF_03 format defined above is that the master file header has been expanded from 512 bytes to 1024 bytes, and the frame headers have been expanded from 256 bytes to 1024 bytes.

Range calculations are the same as those for the DDF_03 data file format.

Echoview derives position GPS fixes from the frame header fields m_dlatitude and m_dlongitude. These values represent the data from an installed AUX GPS sensor.

When the first ping contains data in the frame header fields m_fSonarPan, m_fSonarRoll, and m_fSonarTilt; Echoview will derive raw variables for heading, roll, and pitch. Heading data affects the platform. Roll data and pitch data can affect variables.

Notes

A discrepancy between DIDSON times in the file and the ping times that Echoview displays can occur when the Sonar time is set incorrectly. Both Sonar time and PC time are stored in the file. Echoview reads the Sonar time which is invariant with location. PC time is stored as a UTC time with no time zone information; as a result the original logged time can't be determined when you change time zones.
The workaround for incorrectly setup Sonar time is to use a Time Offset on the Fileset Properties dialog box.
Raw DIDSON data files that contain multiple pings with the same recorded time are processed so that each (co-incident) consecutive ping has 1ms added to its time in a compound fashion.
From Echoview 4.50 onwards, Beam spread which indicates the lens type used, is automatically deduced by Echoview from the file header.
From Echoview 4.50 onwards, the correct beam widths and sector angles are calculated (based on information supplied by Sound Metrics Corp.) and applied by Echoview. Prior to Echoview 4.50, Echoview used the same beam width for all beams in the swath. This was found to be an incorrect assumption. Incorrect beam widths affect the display of DIDSON data and calculations involving DIDSON data. Refer to About sector plots and beams for more information.
DIDSON data files can contain one or more changes in range and window size. Echoview supports the DIDSON algorithms for such changes and (Echoview) defaults to the DIDSON specified sound speed of 1457m/s. Changing the sound speed changes the normalizing multiplier for the range and window size algorithm.
Files recorded with Cluster Size Over Thresholding (CSOT) applied, replay better when the Pings per second setting is selected on the Echogram page of the EV File Properties dialog box.
Improve your workflow by assigning familiar keys to quickly change the multibeam replay direction and multibeam replay rate. Refer to Multibeam echogram replay controls and User-defined keyboard shortcuts for information on how to assign the following shortcut names and keys:
MULTIBEAM_REPLAY_SPEEDUP = UPARROW

MULTIBEAM_REPLAY_SLOWDOWN = DOWNARROW

MULTIBEAM_REPLAY_FORWARD = RIGHTARROW

MULTIBEAM_REPLAY_REVERSE = LEFTARROW
Where multiple pings have the same time, Echoview processes the data so that each (co-incident) consecutive ping has 1ms added to its time in a compound fashion.

	HighResolution
SN	No	Yes
<=18	0.001024	0.000512
>18	0.001144	0.000572

DIDSON data files

DIDSON (.ddf)

DDF_03

DDF_04

See also