Extracting event times from an audio record using Audacity
Events timed with a beeper box
You should find a shortcut to start Audacity on your star menu under Programs -> Audacity
After Audacity is started let's load the audio record [13] 070523_RodneyAudio.wav from Data/Lunar Grazes. To open the file use File -> Open from the Audacity menu. Once you have the file opened you should see something like what's shown on the screenshot below. Audacity is very easy to use. On the top you can find the rounded buttons to play, pause and stop the record and below them (highlighted in red on the screenshot) are buttons to zoom-in and zoom-out. Finally the most important part is the scale highlighted in orange that shows the time from the beginning of the record. You can see that this record goes for exactly one minute.
Let's play the audio and hear what it is all about. You will hear the beeps coming from the beeper box. The beeper box has a particular pattern for the time signals and if you have used one of these devices before you will know the pattern. It uses a Piezzo buzzer to output 'beeps' at one second intervals. The buzzer emits a 'beep' of approximately 50 milliseconds (msec) duration at one second intervals. Every tenth second the 'beep' is slightly longer. On the 55 – 58th seconds of each minute the 'beep' is very short. The short beeps start at 51 seconds on the fifth minute. The 59th second is silent and the minute is marked by an extra long beep (500msec). This very closely emulates the former VNG time signals.
So after the first play we know that the "gone" and "back" calls are happening shortly after the whole minute announcement. So let's zoom-in a little bit and see what happens. It is very important to associate in your mind the audio you hear with the shapes on the graphic:
So we can tell easily by looking at the graphic that the first disappearance happens between 8:22:12 and 8:22:13, then the first reappearance happens between 8:22:21 and 8:22:22 and finally the second disappearance happens between 8:22:22 and 8:22:23. To extract the time more precisely we could use two different approaches:
A) By making the Audacity time scale to display UTC seconds after the marker before the first event
We need to find the closest minute marker before the first event. In our case there is only one minute marker and it is for 8:22:00 UT. The idea of the first method is that if we delete (cut) the part of the audio before this minute marker then the zero of the Audacity time scale will be positioned exactly on the UT minute so the time scale will now show UTC (a sort of as we still have to know and remember which is the UT minute corresponding to this minute marker). We will cut the part before the minute marker in two steps.
First zoom-out to see the entire audio or at least the entire bit from the beginning to the minute marker. Then select roughly the area to cut before the minute marker by clicking and holding down the mouse and then moving the mouse to the left/right and select the part to cut.
Once you've done this - press the "Delete" key to delete the selection. Now we can zoom-in and perform a more precise cut of the bit before the minute marker. Using the same technique select and delete the selection as shown below:
That's it now the zero of the time scale in Audacity corresponds exactly to 8:22:00 UT so if the time scale shows 10.0 for example this will mean 8:22:10.0 UT.
Now let's measure the first disappearance time. If we zoom-in to the first "gone" call we can directly extract the time when "gone" was called to be 8:22:12.65 UT
B) By doing some simple calculations
We will need to zoom-in a bit more into the original uncut audio and will try to measure the first "gone":
We can see that the 12-th second beep happens roughly at the 44.73 second from the beginning of the record and the beginning of the "gone" call starts at the 45.38 second from the beginning of the file. A simple calculation shows that the "gone" call starts 45.38 - 44.73 = 0.65 seconds after the 8:22:12.00 beep. So we can conclude that the "gone" call was made at 8:22:12.0 + 0.65 = 8:22:12.65 UT
Now that we have done our measurement we also have to exclude the personal equation which is individual but is typically around 0.4 sec or more. So let's assume that Rodney's reaction time was 0.4 sec. In this case he has called "gone" 0.4 sec after he has seen the star had gone. With this the final timing of the first disappearance is 8:22:12.65 - 0.4 = 8:22:12.25 UT
We learned how to ...
- Load an audio file in Audacity
- Recognize the "position" of voice and sound samples by the "shape" of the graphical representation of the audio
- Determine the elapsed time since the beginning of the audio to any part of the record
- Recognize beeper box pattern and the absolute UT time of each "beep"
- Determine the time of a "gone"/"back" call by deleting the audio record before the minute marker right before the first event and then using the time scale directly
- Use simple calculations to determine the absolute time in UT when a "gone"/"back" call was made
- Apply the personal equation to the final timing of the Ds and Rs
End of "Extracting event times from an audio record using Audacity"
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