Corrupted start time in tdms file

[rizardus]

So I get my data as AnalogWaveformy<double>[] with a regular sample intervall. In this case there shouldnt be any need for timing information for each sample. But the problem ist when I start a measurement and stream the data into a TDMS file, the field "wf_start_time" (in Excel) doesnt show the actual start time. It shows the timestamp from the last samples when I stop the measurement.

If I load the TDMS file into my Programm and get the DateTime[] info from it, the start time is also incorrect. The sampling data is right but every timestamp ist rightTimeStamp+measurementDuraction. There is a offset. If i measure till 6 AM the DateTime[] info starts at 6 AM.

The shown time is exact and I could reprocess it to get the real start time. But this doesnt seem like the way to go.

Right now I am setting up my file like this:

public void _setUpTdmsFile(string _path, string[] _channelNames)
{
    int _numberOfChannels = _channelNames.Length;       ///< Anzahl der Kanäle welche in die Datei geschrieben werden sollen.
    // Öffne eventuell vorhandene Datai.
    if (File.Exists(_path))
    {
        TdmsFile.Delete(_path);
    }
    // Erstelle eine neue Datei.
    //_file = new TdmsFile(_path, new TdmsFileOptions(TdmsFileFormat.Version20));
    _file = new TdmsFile(_path, new TdmsFileOptions());
    _file.AutoSave = true;
    // Erstelle ChannelGroup.
    TdmsChannelGroupCollection _channelGroups = _file.GetChannelGroups();
    _channelGroup = new TdmsChannelGroup("Main Group");
    _channelGroups.Add(_channelGroup);
    // Setze Datei Layout.
    TdmsChannelCollection _tdmsChannels = _channelGroup.GetChannels();
    _waveformChannels = new TdmsChannel[_numberOfChannels];
    _channelGroup.WaveformLayout = TdmsWaveformLayout.NoTimeChannel; 
    // Setze Kanäle welche für das Schreiben von Daten benötigt werden.
    for (int i = 0; i < _numberOfChannels; i++)
    {
        string _channelName = _channelNames[i];
        _waveformChannels[i] = new TdmsChannel(_channelName, TdmsDataType.Double);
        _tdmsChannels.Add(_waveformChannels[i]);
    }
}

and append data to it like this:

Snippet

public void _appendTdmsFile(AnalogWaveform<double>[] _waveform)
{
    _channelGroup.AppendAnalogWaveforms<double>(_waveformChannels, _waveform);
}

Snippet

public void _stopStreaming()
{
    _file.Close();
}

Currently my theory is, that the start time field gets overwritten every time I append to the file. Is my code wrong or could this be a bug? I checked the documentian and the forum and cant find anything about this. If needed I would be happy to provide more information.

It would also be an option to write just a single time collum but in my understanding this isnt supported right now.

[rizardus · Accepted solution]

I found a workaround and will post it here in case anyone else encounters this problem. The first time I get an Analogwaveform I get the start time and save it for later use.

Snippet

if (first)
{
    _handler._setStartTime(data[0].Samples[0].TimeStamp);
    first = false;
}

At the end I overwrite the start time property in the TDMS file manually.

Snippet

public void _stopStreaming()
{
    for (int i = 0; i < _waveformChannels.Length; i++)
    {
        _waveformChannels[i].GetProperty("wf_start_time").SetValue<DateTime>(realStartTime);
    }
    _file.Close();
}

 This is probably not the best solution but it works.