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Zero Filling, etc

Zero Filling, etc

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Zero-filling, shrinking and shifting data

ZF — Zero Fill

This command doubles the 1D data size by adding zeros to the end of the current data set. If the number of data points collected is not a power of 2, ZF zero-fills to the next higher power of 2, rather than doubling the number of points. This is useful for resolution enhancement and in processing 2D experiments where few data points were collected. (Note that if the data size is not a power of 2, a zero-fill operation is automatically performed when an FT is executed.)

If the FID has not decayed to zero, executing ZF will introduce a discontinuity, resulting in distortion of the spectrum.

When the FID shown above is zero-filled, a discontinuity is created, as seen below.

After FT, the peak has "sinc wiggles" resulting from FT of the discontinuity.

To avoid this, a window function should be applied (usually exponential multiplication) to bring the end of the FID smoothly to zero before zero-filling.  

Normally, ZF is used on time-domain data.  If applied to frequency-domain data, the values of number of data points, sweep width and offset frequency are updated to be correct for the new data size.

See also: Linear Prediction (LN)

SH — SHrink data

Reduces the number of data points in a 1D data set. By default, the size is reduced by half the number of points, and the last half of the data is lost. This is (sort of) the reverse of zero-filling. The data can be reduced by a different number of points, as determined by the S@ parameter.

S@ — Shrink by

Sets the number of data points by which the current data set will be reduced on execution of the SH command. If S@ is set to zero or a negative number, SH will reduce the data size by a factor of 2, its default operation. This default can be altered by setting the S@ parameter to a positive number.

The particular situation for which this was added was 2D data consisting of a number of slices not equal to a power of 2, which had been automatically zero-filled out to the next power of 2 by the spectrometer, introducing truncation artifacts. SH (with non-zero value of S@) can be used to discard the zeroed points, allowing Linear Prediction (LN) of the data to avoid truncation.

  /2 — Reduce data by factor of 2 

The number of data points is halved, by discarding 25% of the points at each end of the spectrum.  This obviously only makes sense for frequency-domain data.  It is common to acquire data with a larger spectral window than needed, to reduce baseline artifacts.  This command allows the excess baseline to be discarded and the file size reduced.  The reported value for spectral width is adjusted.  The operation can be un-done with the following command.

In the non-2-letter command mode, a parameter can be used to specify a different number of points to be discarded from each end, rather than using the default.  

The command brickwall is equivalent to /2.

  *2 — Expand data by factor of 2

The number of data points is doubled by adding zeroes at each end.  This obviously only makes sense for frequency-domain data.  This is the inverse of the previous command.

In the non-2-letter command mode, a parameter can be used to specify a different number of zeroed data points to be added to each end, rather than using the default.  

RD — Rotate Data

Performs a circular shift of data points a specified number of times. A circular shift means that a point is removed from one end of the spectrum, the data are shifted by one data point, and the point is added to the other end of the data. (Do not confuse RD with left shift (LS) and right shift (RS)).

This is normally used for Bruker data from the Avance series of spectrometers (DnX models) which have been digitally filtered and "decimated" prior to saving the FID. The initial points of the FID are zero. A circular left shift must be performed before the FT. The number of points to shift is calculated from the Decimation Number found in the Bruker file header, but the user can change the number of points.

When RD is executed directly, a dialog box appears allowing the user to adjust parameters. When RD is included in a Link or Macro, the default or last entered value is used, without prompting for user input.

Note that any zero-filling or apodization must be performed before the RD. Show me how to process data with RD.

See also: Special considerations for Bruker data

LS — Left Shift data

Shifts the real and imaginary data left one point and assigns the last point the value of zero.

RS — Right Shift data

Shifts the real and imaginary data right one point and assigns the first point the value of zero.

 

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Last updated: 3/12/04.