STA/LTA stands for Short Term Averaging / Long Term Averaging. More information about STA/LTA triggering can be found here Tuning of Seismic Networks. This is an excellent description on how STA/LTA works and what range of numbers should be used for the different channel menu items used for triggering event files.
To enable this mode of triggering go to the Alarm Settings dialog box and select the STA/LTA Triggering Mode for each channel you would like to use for triggering event files. The following menu items are used for STA/LTA triggering:
STA/LTA Threshold:
When the STA/LTA ratio goes above this number the channel will go into a pre-trigger or trigger mode depending on the Trigger Number. Input is a floating point number usually in the range of 3.0 to 50.0. Default for this field is 4.0.
Trigger Number:
For a channel to go into an alarm or trigger state the STA/LTA threshold must happen Trigger Number of times within the Pre-Trigger Time. Normal numbers for this field are 1 to 4. If set to 1, the channel will trigger whenever the threshold is reached. Default for this field is 2.
Pre-Trigger Time:
The time in seconds the channel stays in the pre-trigger condition. Default for this field is 10 seconds.
Trigger Time:
The time in seconds the channel stays in the trigger or alarm state. The default for this field is 60 seconds.
STA Time:
STA average window time in seconds. The number entered may be a floating point. Example, entering 1.5 will create a 1.5 second STA window time. The actual window time depends on the system sample rate. At 100 samples per second the resolution is 10 milliseconds, at 50 sps the resolution is 20 ms etc. Default for this field is 2 seconds.
LTA Time:
LTA average window time in seconds. The number entered may be a floating point. Like the STA Time above, the actual time depends on the system sample rate. Default for this field is 20 seconds.
LTA Minimum:
This field sets the minimum value the LTA can go down to. This sets the overall trigger sensitivity of the channel. The higher the number the less sensitive the channel will be to noise or a real event. Default for this field is 0 or no LTA minimum limit.
LTA Maximum:
This field sets the maximum value the LTA can go up to. Default for this field is 65000 or no LTA maximum limit.
The data supplied to the alarm / event detection process can be filter using a lowpass and / or highpass Butterworth digital filter. The user can view the filtered data by using the View / Display Filtered Data menu items. This image shows the difference between filtering the data and not filtering the data. The first 4 minutes is with the View / Display Filtered Data enabled and the last 6 minutes disabled. The data is from a 1 Hz vertical geophone and the filter was set to a 2 Hz 6 pole lowpass filter. The settings can be found in the Event Detection tab of the Channel Settings dialog box. Please note that the digital filters that can be applied in this tab does not effect the data saved to the daily record files or the event files produced by WinSDR. The filters in the Integrate / Filter Control tab of the Channels Settings dialog box can be used to permanently filter the income ADC data.
Lowpass Check Box:
Enables lowpass filtering of the data going to the alarm / event detection process.
Lowpass Frequency Edit Box:
Sets the lowpass filter cutoff frequency.
Lowpass Poles:
Sets the number of poles, or the steepness of the filter, used to attenuate the incoming data.
Highpass Check Box:
Enables highpass filtering of the data going to the alarm / event detection process.
Highpass Frequency Edit Box:
Sets the highpass filter cutoff frequency.
Highpass Poles:
Sets the number of poles, or the steepness of the filter, used to attenuate the incoming data.
To monitor the STA/LTA process go to the single channel display screen by pressing the 1 through 8 key or use the Channel menu item. Pressing the 'S' key, or using the View / Display STA/LTA menu items, will toggle between the normal data high / low display and the STA/LTA status display. The status numbers are:
Input Avg: The A/D data averaged over one second. Number is in A/D counts.
STA: The current STA number.
LTA: The current LTA number.
Ratio: The ratio of the STA and LTA numbers.
Max: The maximum STA/LTA ratio number. Use the 'C' key to clear this field.
WinQuake (versions 2.75 or higher) can display the STA/LTA ratios within an event file. This feature can be used to help you select the best STA/LTA parameters in WinSDR. The STA/LTA Ratio dialog box can be opened using the Calculate / STA/LTA Ratio menu items.
A command line (DOS Box) utility program called StaLta (StaLta.zip) has been written to generate a STA/LTA ratio report similar to the WinQuake feature above. Please see the readme.txt file for more information.
