Specifications

Format of Time-Tag Data Files 55
Format of Time-Tag Data Files
In most of the operation modes the DPC-230 allows the user to record time-tag data of the
individual photons. The format of the time-tag data files is described in this section.
Time-tag data files consists of a sequence of 32 bit records. The first record is a descriptor that
identifies the file as a DPC-230 time-tag file and contains the time-channel width of the re-
cording. The subsequent records contains photon data. Each record contains four bytes as
shown in Table 1.
Byte 0 Bits [7:0]
Byte 1 Bits [15:8]
Byte 2 Bits [23:16]
Byte 3
1st Record = Descriptor
Bits [31:24]
Byte 0 Bits [7:0]
Byte 1 Bits [15:8]
Byte 2 Bits [23:16]
Byte 3
2nd Record
Bits [31:24]
Table 1: Byte order in the .spc files
Descriptor
The structure of the descriptor is shown in Table 2.
Bit 31 Bit 30 Bit 29 Bit 28 Bit 27 Bit 26 Bit 25 Bit 24
1 1 0 0 0 RAW 0 1
Bit 23 Bit 22 Bit 21 Bit 20 Bit 19 Bit 18 Bit 17 Bit 16
TPB[23] TPB[22] TPB[21] TPB[20] TPB[19] TPB[18] TPB[17] TPB[16]
Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8
TPB[15] TPB[14] TPB[13] TPB[12] TPB[11] TPB[10] TPB[9] TPB[8]
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
TPB[7] TPB[6] TPB[5] TPB[4] TPB[3] TPB[2] TPB[1] TPB[0]
Table 2: Format of the descriptor
RAW Bit
The RAW bit identifies whether or not the data in the file are pre-processed. ‘Raw = 1’ identi-
fies data as they are read from the incoming data stream. Such data are difficult to use for sev-
eral reasons: The TDC channels of the DPC board have individual FIFOs. The software may
therefore read the individual photons not in the order they were detected. Moreover, the inter-
nal clock of the TDC channels overflows in regular intervals. The clock overflows have to be
marked in the data stream read from the DPC module and be taken into regard by the SPCM
software. The raw data read from the DPC module are therefore difficult to interpret. Never-
theless, raw data can be stored by the SPCM software. Such data should, however, be used for
test purpose only.
To facilitate users to analyse time-tag data the SPCM software generates files in which all
these effects are already taken into regard. Please note that pre-processing does not cause any