NMRPipe Processing Functions
HT: Hilbert Transform

Flag Argument Default Description
 -ps90-180 Mirror Image Mode (For Data with Half-Point Delay, First Order Phase=180).
 -zf Temporary Zero Fill for Speed.
 -td Set Time-Domain Size to SIZE/2.
 -auto Auto Mode (Selects HT and ZF mode).
Controls for Auto Mode:
 -ps0-0 Force Ordinary Hilbert Transform.
 -nozf No Temporary Zero Fill.

HT applies a Hilbert transform (HT) to the vectors in the data. Its usual purpose is to reconstruct the imaginary part of real-only frequency-domain data in cases where the imaginary part was deleted during processing. This in turn allows spectra to be re-phased or inverse-transformed whenever needed, even if imaginary data was previously deleted.

The result of an HT will be complex. If the input vectors already have an imaginary part, this part will be replaced by the HT.

In particular cases, the HT can reconstruct imaginary data exactly. The requirements for this are:

  1. During the original processing, the data size was at least doubled by zero filling.
  2. The required phase correction was either P1 = 0 (no acquisition delay), P1 = 180 (1/2 point acquisition delay), or P1 = 360 (1 point acquisition delay).
  3. The complete spectral range is included; no subset of the spectrum was extracted.
In other cases, the HT might cause small distortions in the data, especially at the edges. These are usually of no consequence, but can be more serious for data with limited numbers of points.


This option enables mirror-image HT mode, suitable for data which have a first order phase correction P1 = 180 (1/2 point acquisition delay). This option works by building a temporary mirror image of the original spectrum which has the same behavior as a spectrum with no acquisition delay.

This option forces the HT not to use mirror-image mode.

This option enables use of temporary zero-filling for speed. Zero-filling is disabled by default.

This option disables use of temporary zero-filling for speed. This is the default.

This option will attempt to automatically choose whether or not a mirror image HT is needed (only when the recorded P1 is 180 and data have not been extracted to a smaller region), and whether temporary zero-filling can be used (all other cases). -td
If this option is included, HT will update the header value NDAPOD recording the number of valid time-domain points.


The following is a typical recipe for inverse processing and reprocessing with LP for a given dimension:

   | nmrPipe  -fn HT  -auto             \
   | nmrPipe  -fn PS  -inv -hdr         \
   | nmrPipe  -fn FT  -inv              \
   | nmrPipe  -fn ZF  -inv              \
   | nmrPipe  -fn SP  -inv -hdr         \
   | nmrPipe  -fn LP  -fb               \
   | nmrPipe  -fn SP  -hdr              \
   | nmrPipe  -fn ZF  -auto             \
   | nmrPipe  -fn FT                    \
   | nmrPipe  -fn PS  -hdr -di          \
The following scheme transforms a 2D spectrum back into a 2D hypercomplex FID:
   nmrPipe -in test.ft2             \
   | nmrPipe -fn TP                 \
   | nmrPipe -fn HT -auto -verb     \
   | nmrPipe -fn PS -inv -hdr       \
   | nmrPipe -fn FT -inv            \
   | nmrPipe -fn ZF -inv            \
   | nmrPipe -fn APOD -inv -hdr -ad \
   | nmrPipe -fn TP -hyper          \
   | nmrPipe -fn HT -auto -verb     \
   | nmrPipe -fn PS -inv -hdr       \
   | nmrPipe -fn FT -inv            \
   | nmrPipe -fn ZF -inv            \
   | nmrPipe -fn APOD -inv -hdr     \
      -out test.fid -ov

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