Changes between Version 5 and Version 6 of Help/AudioMapping/Calibration


Ignore:
Timestamp:
2008-03-02T19:12:21+01:00 (18 years ago)
Author:
David Earl <david@…>
Comment:

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  • Help/AudioMapping/Calibration

    v5 v6  
    55An error of 5 seconds per hour could mean you are 100m or more out after four hours surveying on a bike, and more in a car, if you are relying on the clock in the audio device to indicate position on a GPS track.
    66
    7 To calibrate, you need to enter a number close to but not exactly 1.0 in the calibration setting of JOSM's [wiki:Help/Preferences/Audio Audio Preferences]. This number is the ratio of the sound recorder clock's notion of how long a recording is to the accurate time. For example, if the audio recording of a 3 hour interval turns out to be 3 hours and 15 seconds long, your calibration number is (3 x 60 x 60 + 15)/(3 x 60 x 60), which works out at 1.00139 (five decimal places is ample). If your recorder's clock runs fast, the number will be slightly less than 1.0. If it is very much different from 1.0, throw away your voice recorder!
     7To calibrate, you need to enter a number close to but not exactly 1.0 in the calibration setting of JOSM's [wiki:Help/Preferences/Audio Audio Preferences]. This number is the ratio of the sound recorder's notion of how long a recording is to the accurate length. For example, if the audio recording of a 3 hour interval turns out to be 3 hours and 15 seconds long, your calibration number is (3 x 60 x 60 + 15)/(3 x 60 x 60), which works out at 1.00139 (five decimal places is ample). If your recorder's clock runs fast, the number will be slightly less than 1.0. If it is very much different from 1.0, throw away your voice recorder!
    88
    99== procedure ==
    … …  
    1111 1. Make a recording of a precise interval, of the same order of length as a surveying session, say one to four hours: say "NOW!" or some such at a known time, or in the UK you could record the Greenwich Time Signal (the "pips"; use an FM, not DAB, radio) and again at the end.
    1212
    13  2. measure how long your recorder thinks this is. You can't do this by playing back the recording an timing it, because that will be using the same clock for playback as it used to record. You need to take the digital file and determine this according to the purported sampling rate. You can do this in JOSM or more accurately using a sound editor:
     13 2. Measure how long your recorder thinks this is. You '''can't''' do this by playing back the recording on the recorder and timing it, because that will be using the same clock for playback as it used to record. You need to take the digital file and determine this according by comparing the purported sampling rate to the actual sampling rate. You can do this in JOSM or more accurately using a sound editor:
    1414
    1515   * Using a sound editor such as [http://audacity.sourceforge.net/ Audacity], open the clip and delete the portions before and after your two marks. The pips are easy to edit visually. Read the length from the duration display (in Audacity this runs along the top of the soundtrack waveform).
    … …  
    1919     b. [wiki:Help/Action/ImportAudio Import Audio] on the GPX layer's context menu to make an audio marker layer,
    2020     c. play the audio until you hear your starting cue,
    21      d. [wiki:Help/Action/MakeAudioMarkerAtPlayHead Make make an Audio Marker at the play head]
     21     d. [wiki:Help/Action/MakeAudioMarkerAtPlayHead Make an Audio Marker at the play head]
    2222     e. repeat for the end point.
    2323     f. look at the labels for the two points, which are in GPS time offset from the start of the track; the difference is the measured sound track duration.