No announcement yet.

Sub-sonic explorer

  • Filter
  • Time
  • Show
Clear All
new posts

  • Sub-sonic explorer

    Subsonic Explorer

    (Forensics Version Only)
    Click image for larger version  Name:	dataurl585031.png Views:	0 Size:	370 Bytes ID:	55670
    Forensics audio recordings often contain very low frequency or DC offset information that can be useful in an investigation. These signals are called subsonic, because they fall below the ability of most people to be able to hear them (and/or for sound systems to be able to reproduce them). The Diamond Cut Subsonic Explorer function allows some analysis of signals that fall below 30 Hz. Some examples of subsonic signals can be distant explosions, arms-fire, earthquakes, slamming doors, vehicle collisions (located far away from the recorder) and stop-start edits produced by analog tape recorders (most commonly – microcassettes). It can sometimes be useful in detecting certain types of digital edits recorded at high signal levels due to DC Offset shifts between two different recorders quiescent operating points. The system works in conjunction with the time domain display in the “classic edit” mode. The top (source) display shows the full-bandwidth audio signal while the bottom (destination) display shows only the subsonic signals derived after processing has been completed by the routine. The horizontal time axis of both displays are in sync for ease of correlation between the two signal portions. When invoking the Subsonic Explorer, the system will automatically revert to “classic edit” mode with the source and destination placed in a “sync files” configuration. Thus, one can observe a subsonic event in the destination display and correlate it to the sonic source file directly above it. Subsonic events generally look like small “blips” or “spikes” on the destination portion of the screen when using this feature. There is usually some form of discontinuity in the Source File that can be correlated with the subsonic event seen in the destination file. In the case of a micro-cassette recording edit point, you may see an abrupt change in the signal level or wave-shape at the edit point. Often, this appears as a “blip” or “spike” in the Subsonic Explorer display. Both files can be played via the playback controls of the Diamond Cut program. Generally, little or nothing will be heard when playing the subsonic destination file (although, something might be “felt” when a high quality sub-woofer is employed). The Subsonic Explorer functionality is nicely augmented when used in conjunction with the Time Display function (found under the View menu) coupled with Markers (the “M” key on your keyboard). Additionally, numerical values are provided for the DC levels of the subsonic signals found by the system. The system is very easy to operate. Simply bring up the file that you want to analyze for subsonic signals. Then go to the Forensics menu and click on the Subsonic Explorer. Click on the run button, and the system will perform some calculations and display the results in the Destination display. Note that the Subsonic Explorer only works in Classic Edit mode. If you had been working in Fast Edit mode, the system will automatically switch over to Classic Edit mode if the “Automatically Switch to Classic Edit Mode” checkbox is checked. An amplitude vs time graph will be created showing the subsonic events, along with numerical data including Max DC Level, Average DC Level, and Ratio. The ratio measurement is a crest factor calculation of the Max DC Level divided by the Average DC Level and is expressed in linear terms (not dB). A “Enhance Gain” checkbox is provided which imposes a non-linear transfer function to the display which is sometimes useful when dealing with highly compressed files.
    Click image for larger version  Name:	dataurl585035.png Views:	0 Size:	252.7 KB ID:	55672

    Subsonic Explorer Example: Micro-Cassette Recording with Analog Edits

    The screen-shot shown above is that of a Micro-Cassette recording having 9 analog edits. The edits can’t be seen in the regular, full bandwidth signal shown in the upper Source display. However, they become clear in the Destination display after running the Subsonic Explorer. To hear where the potential edits are, just look and listen to the Source display proximal to each blip or spike in the lower display (study the time period 2 seconds before and after each blip or spike looking for anything that may be suspicious).

    Note 1: The Subsonic Explorer responds to the channel selected. When both channels are selected, it responds to the sum of the two.

    Note 2: An analog edited recording is included with this software so that you can experiment with the Subsonic Explorer. It is called:

    Microcassette Recording with Analog Edits.wav.

    Click image for larger version  Name:	dataurl585033.png Views:	0 Size:	27.4 KB ID:	55671

    The Subsonic Explorer Dialog Box

    "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

  • #2
    This works especially well on old analog recording edit detection.
    "Who put orange juice in my orange juice?" - - - William Claude Dukenfield