Knowledge Base

Sample Rate Optimization

Repeat the steps above by selecting smaller sample rates. Once the data are plotted, there should be a range of sample rates for which the dependent variable value does not change much (plateau). This means that slight changes in the sample rate do not result in loss of information, or addition of redundant information (noise and movements like body wobble).

Low sample rates result in loss of useful information, because the sinuosity of the original path is removed. Therefore, the total distance moved is usually decreased.

High sample rates result in acquisition of redundant information. In the case of body wobbling, and assuming that the animal is moving along a straight line, the lateral shift of the body center causes the total distance moved to be longer than the real one. With Track smoothing you can filter out noise as a result of body wobble. See Smooth the Tracks.

Missed Samples

The actual sample rate may be lower than the maximum you set, because an image cannot be captured until the previous one is processed. If the sample rate you define is too high, EthoVision will miss samples (up to 1% is acceptable) and the processor load will be high. The percentage of missed samples is shown in the Detection Performance pane and in the Trial List as a System Variable. You can calculate the number of missed samples in acquired tracks with the Number statistic of continuous variables (e.g., velocity). If your processor load is larger than 100, and there are large amounts of missed values, you will have to lower the sample rate.

After acquisition you can see the proportion of missed samples in the Trial List as one of the System Variables.

When acquiring from video files, in the Playback Control window select the option DDS (Detection Determines Speed). This way, no samples will be missed. See DDS (Detection Determines Speed).

Factors Affecting the Occurrence of Missing Samples

The following factors may cause the processor load to be too high:

• Computer memory, processor speed and video graphics card capacity: See the system requirements. In general, using a computer with a dual-core CPU helps you to work with higher sample rates than normal computers do.
• Other programs installed: Do not install other video software (for example, video editing programs, DVD burning software), because this can interfere with EthoVision XT's video processing and cause a reduction in performance.
• Other programs are running: Make sure you shut down all other programs, including those running in the background such as e-mail programs and virus scanners. These are usually shown in the System Tray in the bottom-right corner of your screen.
• Image resolution: For live video tracking, in the Experiment Settings you can choose the resolution for your video image.
• Size of arenas: Make arenas as small as possible (but including the entire area the animal can be in).
• Number of arenas: If you track live and use more than four arenas in a trial, check first that no samples are missed. If the number of missed samples is too high, first make a video file, then track from that. More generally, if you track from video files the number of arenas is never a problem as long as you select DDS (Detection Determines Speed). When making detection settings, you could start with making an arena definition with only one area which speeds up the detection process. After you have finished configuring detection settings for one arena, you can add the others to the arena definition.
• Display options: You can decrease processor load by minimizing the number of Track Features to be displayed. See View the Detection Features on the Video Window.
• Detection method: If possible, use the Gray Scaling method which requires less processor load than Static Subtraction. Static Subtraction requires less processor load than Dynamic Subtraction and Differencing. See Advanced Detection Settings: Method.