# Processing data stored in the VLBI Data Interchange Format (VDIF)

**Important Note:**
Currently, `dspsr` can process only a VDIF Data Stream that
consists of a single Data Thread carrying multi-channel Data Frames.
Support for a VDIF Data Stream that consists of multiple single-channel
Data Threads has not yet been implemented.

---

To process data stored in the VLBI Data Interchange Format (VDIF) format,
you'll need to

1. check out the `vdif-devel` development branch; e.g.

   ```
   cd dspsr/
   git pull
   git checkout vdif-devel
   ```
2. compile dspsr with the string `vdif` included in
   the `backends.list` file.

The VDIF format does not completely describe the data in each packet.
Therefore, for each VDIF binary data file, it is necessary to provide
a separate ASCII header file that further describes the data.
**This ASCII header file is passed on the command line** to `dspsr`,
which then looks for the binary data file in one of two ways:

1. If the DATAFILE keyword/value pair is specified in the ASCII header file,
   then the specified path to the binary data file is used
2. If DATAFILE is not specified in the ASCII header file,
   then `dspsr` looks for a binary data file with the same name
   as the ASCII header file after stripping off the ".hdr" extension
   (e.g. `data04_wa_no0009.00000049.hdr` becomes `data04_wa_no0009.00000049`).

## Header parameters

When parsing ASCII header parameters, the minimum information required by
DSPSR is as follows:

* INSTRUMENT VDIF
* TELESCOPE: the name of the observatory as recognized by tempo/tempo2
* SOURCE: the name of the source
* FREQ: centre frequency of the observed band in MHz
* BW: total width of the observed band (summed over all contiguous frequency channels) in MHz
* NPOL: number of polarizations
* NCHAN: number of contiguous frequency channels into which the band is divided

Here is an example:

```
INSTRUMENT VDIF
NCHAN 8
NPOL 2
TELESCOPE wark30m
SOURCE J0835-4510
FREQ 6476
BW 128
```