Channel Phase vs. Time (forensics only)

[Craig Maier]

View Channel Phase vs Time

(Forensics version only)

A useful “cousin” plot of the X-Y averaging display is the “View Channel Phase vs Time” function found under the Forensics menu. We recommend reading the section on the X-Y plotter to help gain some insight into all channel phase related phenomenon before using this feature. The channel phase vs time routine plots the average phase angle between the two audio channels vs time which can be useful in Forensics authentication applications. The vertical axis (Y) represents the average phase between two audio channels (with a span from 0 to 90 degrees) while the horizontal (X) axis represents time. The middle of the Y axis represents an average phase angle of 45 degrees. The intensity (Z Axis) of the green graph line is intensity modulated as a function of signal magnitude. This system requires a two–channel signal to perform its function and will not function with single channel signals. This plot overlies (on top of) the spectrogram and is shown via a green line. Sometimes, it may be found to be useful to bring up the X-Y plotter (View Menu) while using this tool since it also shows the instantaneous phase of the two audio channels thereby giving you additional information about the file being investigated.



Average Channel Phase vs Time Graph (green line)
Discontinuities in the phase vs time plot may be worthy of further inspection. The graph may relate to the coherence of the two signals or they could indicate a possible edit in the stereo file or people changing position in the venue or even a movement of the position of the recorder. When using this in Classic Edit mode, you can see the time domain signal directly above the phase vs time plot and correlate discontinuities directly to the time domain. You can zoom-in onto the time domain graph or the Channel Phase Graph where these anomalies occur and study those areas of the file for potential authentication related issues. The two views will track each other via the synchronization function, so you can zoom-in or zoom-out via either display.

Procedure:

Note: As with most Diamond Cut Productions Forensics Audio Tools, the best results will be had on files that are 44.1 kHz, 16 bits. If your file is not of that format, change it before proceeding. The file sample rate and bit depth converter are found under the Edit Menu/Change Sample Rate/Resolution.

1. Place the system in Classic Edit* mode (via the FE icon on the system toolbar near the top of the software program). Also, place the system in Source and Destination Sync Files mode (View Menu or Sync Icon).

2. Bring up a stereo (or 2 channel) file of interest in the Source Workspace. Do not bring up a single channel file. This algorithm will only work with two channel files. If only one channel is found, no phase vs time graph will be plotted.

3. You should see two signal lines in the Source Window display and nothing in the Destination display Window.

4. Go to the Edit/Preferences function and then click on the Spectrogram tab.

5. For the best contrast and visibility of the green phase graph, select the Gray-scale Color Palette for the Spectrogram. Other palettes are possible however with poorer visibility of the Channel Phase vs Time graph.

6. Adjust all of the remaining Spectrogram parameters to your liking.

7. Go to the Forensics Menu and click on the "View Channel Phase vs Time" function (near the bottom of the menu listing)

8. After a few moments, the system will construct both the spectrogram and the phase line (in green) in the Destination display.

9. Look for discontinuities in the phase graph. If you see something suspicious, zoom in on that area of the file in the time domain (source) display. To turn off the phase vs time display, click on the spectrogram icon (button with red lines running through it) in the upper right corner of the Source Display window, or click on the View Channel Phase vs Time function (under the Forensics Menu) again. It is essentially a “toggle-type” selector function.

*Note 1: It is also possible to use this feature in Fast Edit mode if that is preferable by the user.
Note 2: Two demo files are provided to help in understanding this feature. “Phase vs Time speech demo with edits” & “60 Hz Phase Shift with 4 Phase Discontinuities”

Summary of Basic Average Phase vs Time Waveforms

0 degrees – phase vs time line at bottom of the graph (probably one channel only is modulated)

90 degrees – phase vs time line at top of the graph (probably one channel only is modulated)

45 degrees – coherent signals (both channels modulated)

45 degrees with variance thereof – normal stereo signal

0 to 90 degrees – high variance of signal coherence

0 to 90 degrees with discontinuities or step functions – suspicious signals.

The following are descriptions both of the phase vs. time demo files:

1. Phase vs time Speech Demo

This file was made with a binaural (two-channel) microphone in a 36 foot long venue. The microphone was located on a stand at the half-way point (18 feet). Two people were located in the room, one at each end. The female voice remains physically stationary at the left side of the room, while the male (voice) slowly walks from the right side of the room and ends up on the left side of the room by the end of the recording (both persons being on the left side of the room by the end of the recording). The recording is of Lincoln’s Gettysburg address, with sections of the speech alternately being spoken by the male and female starting out with the male voice. If you bring up this file in classic edit mode and file sync mode and then click on the “View Channel Phase vs Time” feature (found at the bottom of the Forensics Menu), a green line will be displayed atop the spectrogram view. Note how the line has a slight positive slope (up and to the right) indicating the movement of the male voice in the room. Also, note that there are 4 edits points in the file, with two located amidst the speech portion. These are areas of interest of potential file tampering and need to be further explored via the software’s zoom function to evaluate what is going on at those positions.

1. 60 Hz Phase Shift Demo

This file was created using the software. A 60 Hz, two channel signal was created using the Make Waves Generator (Edit Menu). It was then fed through the filter sweeper one channel at a time using the HP and LP functions set an octave away from the 60 Hz signal thus creating a variable phase shift of one 60 Hz signal compared to the other. This is intended to simulate an acoustical environment in which some 60 Hz acoustical noise is present in the venue (from things like HVAC systems, pumps, or large transformers) and is picked up by the stereo recorder. In practice, a pre-conditioning step would be included to low pass filter everything below 100 Hz so as to only allow the 60 Hz room noise to be recorded. Bring up this demo file along with the “View Channel Phase vs Time” feature. Note that a non-linear line is painted on the screen with discontinuities thereon. These discontinuities are areas of the file that have been modified and the edits are clearly shown in the phase vs time graph.