The Filter CHOP smooths or sharpens the input channels. It filters by combining each sample and a range of its neighbor samples to set the new value of that sample. Each filter type uses its own weighting factors for the neighboring samples. The Filter Width determines the number of neighbors to use.
For a similar, more-abrupt effect, see the Lag CHOP.
Type - There are seven types of filters:
- Gaussian - This filter has a Gaussian (normal or "bell" curve) shape that smooths the channel. It acts as a low pass filter. The wider the filter, the lower the cutoff frequency, resulting in smoother data.
- Left Half Gaussian - This produces a lag on the channel. If the input channels represent values over time, this filter is seen as only using samples back in time from the current sample. For time-data, this is more realistic as you can't look ahead in time. (Maybe some day.) It has a half-bell shape.
- Box - This filter is box-shaped, meaning that each neighbor sample it uses has the same weighting factor. It can produce unwanted steps in the output channel because the effect of the samples at the extremes of the filter don't fade out as the window slides over the samples. It low-pass filters data, similar to the Gaussian filter.
- NOTE: When using Gaussian or Box filtering, the channel is delayed by half the filter size (i.e. a Filter Size of 30 samples will delay the output by 15 samples). To eliminate this delay, use either a Left Half Gaussian or a Left Half Box filter. Applying a Sharpen or Edge Detect filter always delays the output by half the filter size. Applying a Despike filter will delay the output by the full filter size.
- Left Half Box - This filter produces a lag on the data, uses only samples back in time, and otherwise acts like a box filter.
- Edge Detect - This filter detects "edges", sharp changes in the input channels. It acts as a high pass filter. As the filter width is increased, more low frequencies are added.
- Sharpen - This filter sharpens all high frequencies. It is the sum of the edge detect result and the original data.
- De-spike - This filter removes "spikes" (samples more than `Spike Tolerance' above or below the expected sample value). The filter width allows you to eliminate spikes that are several samples long. Wide filters will remove wide spikes (spikes of several samples) and small filters will only remove narrow spikes (one or two samples in length).
Effect - The extent to which the filter affects the channel (0 - not at all, 1 - maximum effect).
Filter Width - The amount of surrounding samples used in the calculation of the current sample. It is expressed in the Units.
Spike Tolerance - For the De-spike filter type, this is the amount that a sample can differ from its neighbours without being considered a spike.
Number of Passes - The number of times the filter is applied to the channel.
Standard Options and Local Variables