Interface Requirement Specification for
PXGF Streaming and File Format
COPYRIGHT
The content of this document is restricted and intended for reading
only by the addressee. All rights including Intellectual Property
Rights flowing from, incidental to or contained in this document
irrevocably vest in Grintek Ewation (Pty) Ltd (GEW) unless
otherwise agreed to in writing.
© GEW 2007
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TABLE OF CONTENTS
1 SCOPE................................................................................................................................................6
1.1 IDENTIFICATION......................................................................................................................6
1.2 OVERVIEW...............................................................................................................................6
1.3 DOCUMENT OVERVIEW.........................................................................................................6
2 REFERENCED DOCUMENTS............................................................................................................7
3 PXGF DESCRIPTION.........................................................................................................................8
3.1 BACKGROUND........................................................................................................................8
3.2 THE PXGF CHUNK STRUCTURE...........................................................................................8
3.3 APPLICATION NOTES.............................................................................................................9
3.4 DEFINITION OF CHUNKS......................................................................................................10
3.4.1 Single channel Short IQ time data – SSIQ chunk......................................................10
3.4.2 Single channel IQ Packing – SIQP chunk..................................................................10
3.4.3 Sample Rate – SR__ chunk.......................................................................................11
3.4.4 BandWidth – BW__ chunk.........................................................................................11
3.4.5 BandWidth Offset Frequency – BWOF chunk............................................................11
3.4.6 Centre Frequency – CF__ chunk...............................................................................11
3.4.7 dB Full Scale – dBFS chunk......................................................................................11
3.4.8 dBTotal Gain – dBTG chunk......................................................................................12
3.4.9 IQ DisContinuity – IQDC chunk.................................................................................12
3.4.10 Single channel Short Real data - SSR_ chunk.........................................................12
3.4.11 Group Short IQ time data – GSIQ chunk..................................................................12
3.4.12 Group IQ Packing - GIQP chunk.............................................................................13
3.4.13 Group Channel BandWidth – GCBW chunk.............................................................14
3.4.14 Group Centre Frequencies – GCF_ chunk...............................................................14
3.4.15 Start Of File Header – SOFH chunk.........................................................................14
3.4.16 End Of File Header – EOFH chunk..........................................................................14
3.4.17 TEXT string – TEXT chunk......................................................................................14
3.5 PROPOSED EXTENSIONS....................................................................................................15
3.5.1 UTF-8 string – UTF8 chunk........................................................................................15
3.5.2 IF frequency – IF__ chunk........................................................................................15
3.5.3 Proposed text chunks.................................................................................................15
3.5.4 Direction data chunk..................................................................................................15
3.6 DEPRECATED CHUNK TYPES.............................................................................................15
3.6.1 Start Of Header – SOF_ chunk. Use SOFH chunk instead........................................15
3.6.2 End Of Header – EOH_ chunk. Use EOFH instead...................................................16
3.7 SYNCHRONISATION.............................................................................................................16
4 ABBREVIATIONS..............................................................................................................................17
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LIST OF TABLES
TABLE 3.1: STRUCTURE OF PXGF CHUNKS.....................................................................................8
TABLE 3.2: THE SSIQ CHUNK............................................................................................................10
TABLE 3.3: THE SIQP CHUNK............................................................................................................10
TABLE 3.4: THE SR__ CHUNK............................................................................................................11
TABLE 3.5: THE BW__ CHUNK...........................................................................................................11
TABLE 3.6: THE BWOF CHUNK..........................................................................................................11
TABLE 3.7: THE CF__ CHUNK............................................................................................................11
TABLE 3.8: THE DBFS CHUNK...........................................................................................................11
TABLE 3.9: THE DBFS CHUNK...........................................................................................................12
TABLE 3.10: THE IQDC CHUNK..........................................................................................................12
TABLE 3.11: THE SSR_ CHUNK.........................................................................................................12
TABLE 3.12: THE GIQP CHUNK..........................................................................................................13
TABLE 3.13: THE GIQP CHUNK..........................................................................................................13
TABLE 3.14: THE GCBW CHUNK.......................................................................................................14
TABLE 3.15: THE GCF_ CHUNK.........................................................................................................14
TABLE 3.16: THE SOFH CHUNK.........................................................................................................14
TABLE 3.17: THE EOFH CHUNK.........................................................................................................14
TABLE 3.18: THE TEXT CHUNK.........................................................................................................15
TABLE 3.19: PROPOSED UTF8 CHUNK............................................................................................15
TABLE 3.20: PROPOSED IF__ CHUNK..............................................................................................15
TABLE 3.21: DEPRECATED SOF_ CHUNK........................................................................................16
TABLE 3.22: DEPRECATED EOH_ CHUNK.......................................................................................16
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LIST OF FIGURES
FIGURE 1: EXAMPLE CHUNK USAGE IN A PXGF STREAM..............................................................9
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AMENDMENT RECORD
ISSUE DESCRIPTION DATE NAME
0.01 Initial version 2005-02-11 Robert Crida
0.02 Update after first value management meeting with
Trevor, Adrian, Michael and Francois
2005-02-18 Robert Crida
0.03 Added chunk types after discussion with Trevor,
Michael, Francois and Richard - channels of int16 IQ
2005-02-18 Robert Crida
0.04 Refined chunk types to support single and
multichannel and added type names
2005-02-24 Robert Crida,
Michael McGrath
0.05 Revised all bandwidths to micro Hz using int64 2005-02-25 Robert Crida,
Michael McGrath
0.06 Added Group Descriptor chunks 2005-03-01 Michael McGrath
0.07 Changed Names of some chunks, changed some
types to allow Java compliance
2005-03-02 Michael McGrath
0.08 Changed Names of some chunks, changed definition
and name of GIQP chunk
2005-03-03 Robert Crida,
Michael McGrath
0.09 Added chunk for bandwidths with frequency offsets.
BWOF chunk
2005-03-16 Michael McGrath
0.10 Added "TEXT" chunk, moved "UTF8" and "IF__"
chunks to the proposed chunk section.
2005-03-22 Michael McGrath
0.11 Added Purpose of Document header 2005-04-09 Richard Sharrock
0.12 Clarification of Timestamping, and added Use Of
Format section
2005-06-21 Richard Sharrock
0.13 SOH_ chunk renamed to SOFH and EOH_ chunk
renamed to EOFH to match software library.
2005-06-22 Michael McGrath
0.14 The Type of each header is stored as an int32 rather
than a char[4] array, this change affects how the
endian swapping is performed.
2005-06-22 Michael McGrath
0.15 SOH_ and EOH_ chunks deprecated, use SOHF
and EOHF instead.
2005-06-22 Michael McGrath
0.16 Reformatted document into report format. 2005-10-10 Michael McGrath
0.17 Added chunk usage illustration. 2005-10-10 Richard Sharrock
0.18 Minor formatting changes 2005-10-14 Michael McGrath
0.19 Added real data chunk 2005-10-20 Michael McGrath,
Richard Sharrock
0.20 Modified real data chunk 2005-10-20 Michael McGrath
0.21 Clarification of the dBFS value. 2005-12-07 Mark Cammidge
0.22 Changed title and doc number as per GEW request 2007-07-13 David Durrett
1.23 Changed the document number again to a more
generic value. Changed the document file name.
Changed the document issue number to be based at
1 and not 0.
2007-07-27 Trevor Bartleet
1.24 Updated table of contents and list of tables 2009-01-13 Philip Bliss
SOURCE DOCUMENT
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ISSUE FILE NAME
0.00 FileFormat_SPEQ.html
0.16 PXGF-Streaming Format.odt
1.23 svn://iserver.ct/raid/SVN/repos/EWDev/trunk/com/peralex/Framework/pxgf/documents/
design/GSY-0D8-SE_IRS-PXGF-Streaming-Format.odt
1.24 svn://iserver.ct/raid/SVN/repos/EWDev/trunk/com/peralex/Framework/pxgf/documents/
design/GSY-0D8-SE_IRS-PXGF-Streaming-Format.odt SVN 36470
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1 SCOPE
1.1 IDENTIFICATION
This is the functional specification of the PXGF Streaming and File Format.
1.2 OVERVIEW
The PXGF streaming and file format provides a framework for the streaming and storage of sampled
data along with the meta data required to process the sampled data. It is a streaming format in that
synchronisation can be regained if lost.
A file using the PXGF format contains a PXGF stream with a prepended header. The header was
designed to allow an application to identify a file without processing the file. The capability to identify
files becomes more important as file sizes get bigger. The PXGF file format supports large file sizes.
1.3 DOCUMENT OVERVIEW
This functional specification comprises the following sections:
Section 1: Scope
This section identifies the functional specification.
Section 2: Referenced Documents
This section lists all referenced documents.
Section 3.1: PXGF Description
This section introduces the PXGF streaming and file format. It gives an overview
of the framework and its functions.
Section 4: Abbreviations
This section contains the list of the abbreviations that are used in this document.
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2 REFERENCED DOCUMENTS
The following documents are referenced in this manual. Their applicability is limited to the referenced
aspects only.
Ref. Title Document No. Date and / or
Issue
Source
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3 PXGF DESCRIPTION
The PXGF format was designed to represent sampled data with additional information pertaining to
the way in which the data was sampled.
3.1 BACKGROUND
The PXGF format is loosely based on the Microsoft RIFF file format. The RIFF format is based on
the concept of a chunk. Chunks are blocks that contain specific application defined data. In the RIFF
format the complete file is a single RIFF chunk. RIFF chunks and LIST chunks are currently the only
two types of chunks that may contain sub-chunks. All the remaining chunks in the file are children of
the global RIFF chunk.
The RIFF format is unsuitable for our purposes for two primary reasons:
1. The global RIFF chunk is limited in size to 4GB, thereby effectively restricting the file size to
4GB.
2. The RIFF format is unsuitable for streaming applications as one needs to read the whole file
sequentially to be able to parse it. There is no synchronisation mechanism available.
For these reasons a new file and streaming format was proposed and developed, namely the PXGF
format.
3.2 THE PXGF CHUNK STRUCTURE
The PXGF format puts data into chunks. Different types of chunks are defined to store different
information. The type of a chunk is specified by an int32 field in the chunk as shown in table 3.1. An
application that requires data from a particular chunk will register to receive data from that type of
chunk. Chunks that are not recognised are simply skipped over. The size field in the chunk allows
unrecognised chunks to be skipped over. Each chunk starts with the sync number 0xa1b2c3d4.
Element Type Description
sync int32 Synchronisation number 0xa1b2c3d4
type int32 Derived from the chunk name, e.g. "SOFH", " EOFH", "SSIQ".
size int32 The number of data bytes in the remainder of the chunk The
value of size must be a multiple of 4.
data byte[size] The chunk data in a format specific to the type.
Table 3.1: Structure of PXGF chunks
• The maximum amount of data in a chunk is limited to 65536 bytes. This limits the separation
between sync patterns.
• The length of each chunk must contain an integral number of 32 bit words even though the
size element in the chunk header is specified as a number of bytes.
• The PXGF format supports both little and big endian byte ordering, although it may be
necessary to provide the stream reader with the endian used depending on its
implementation. The endian format for a file or stream may be determined by reading the sync
pattern. It is not permissible to mix chunks of different endian format within a stream or file.
• When the PXGF format is used to store information in a file, there must be a global header at
the beginning of the file to aid identification of the file format and the data stored in the file.
This is necessary due to the potentially large size of files.
• Nested sub-chunks are not supported as this would unnecessarily complicate synchronisation.
• The implication of the previous point is that all chunks are at root level and are interpreted
entirely sequentially. The parser must know which chunks need to be identified before it can
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use other chunks. The only constraint here is that files must start with a “SOFH chunk”. Due to
the sequential nature of parsing and the inability to nest chunks, a separate global chunk is
needed to identify the end of the file header, namely the "EOFH" chunk.
3.3 APPLICATION NOTES
Above is a graphical illustration showing how PXGF chunks of different type are ordered in a
stream/file. Below are some notes to the developer to keep in mind when using the PXGF format in an
application.
1. The PXGF framework for streaming and storage is designed to be extensible. Different
applications require different information and if this information is not available in a stream,
then that application will not be able to process that stream successfully. Just because an
application uses the PXGF format doesn't mean that it will be able to process all PXGF
streams or files. For a particular project care should be taken to ensure that all necessary
chunks are included.
2. It is recommended that meta data like the sample rate and packing description be sent every
second. This allows state information to be recovered if synchronisation is lost and makes it
possible to process large files from the middle of the file.
3. Only data from one data source and of one format must be included in each stream or file.
Current formats include "SSIQ" for single channel data and "GSIQ" for multi-channel data. The
format used in files should be indicated using the SOFH chunk. The format name may also be
used for the file extension to allow visual discrimination of different files.
4. State information is accumulated by an application by reading different chunks sequentially. If
synchronisation is lost, state information needs to be reset. This is why it is essential to
resend meta data every second.
5. It is necessary to be able to distinguish between continuous data and block data where only
part of the time data is available. Data chunks contain timestamps to enable detection of
discontinuities. A chunk has also been defined to indicate discontinuities in the time data,
namely the "IQDC" chunk.
6. Playback control is essential for the off-line analysis of files, however due to the stream based
design of the PXGF format, playback control is not easily supported. The PXGF format uses
data chunks supported by a number of meta chunks that describe the state of the data stream.
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Figure 1: Example chunk usage in a PXGF stream
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Before processing data chunks it is necessary to obtain sufficient state information, like the
sample rate, by processing the necessary chunks in the data stream.
The use of an index file has been proposed as a possible solution to the problem of playback
control. By reading an index file an application could determine over what period the recording
was made and determine where to start processing the stream to play back a particular
section.
7. C++ and Java libraries have been developed for the writing and reading of PXGF streams.
The libraries take care of synchronisation and formatting issues; they do not provide or dictate
the communication medium.
8. The PXGF streaming format does not provide any mechanism for communication between the
source of the data stream and the application receiving the data stream. The PXGF stream
therefore represents a unidirectional flow of information from the source to the sink of the
stream.
9. Applications that process PXGF input streams should not make assumptions about the data.
For example, if the sample data were being sent using the SSIQ chunk the application should
wait for a SIQP chunk to determine the packing of the data rather than assuming a particular
packing.
3.4 DEFINITION OF CHUNKS
3.4.1 Single channel Short IQ time data – SSIQ chunk
Data is assumed to be continuous when using this data format, if the data is blocky, an IQDC chunk
must be sent after every block of continuous data.
Element Type Description
lTimestamp int64 The timestamp is stored as a 64 bit signed number,
representative of the time of capture of the first sample in the
chunk block, in microsecond resolution. It is stored as the
number of microseconds since beginning of the epoch (i.e. 1st
January 1970 midnight).
awIQData int16[length of IQ
data array]
IQ pairs of signed int16 short numbers. Note that regardless of
the number of valid bits, the most significant bits in each short
should be used. This allows us to specify the full-scale level
without needing to specify the number of bits.
Table 3.2: The SSIQ chunk
3.4.2 Single channel IQ Packing – SIQP chunk
The information in this chunk is required to parse the data in the SSIQ chunk.
Element Type Description
iIsIQPacked int32 Value 1 for IQ ordering and
value 0 for QI ordering. For
example: a value of 1 will
indicate that the first sample in
the element awIQData of a SSIQ
chunk is an “I” sample.
Table 3.3: The SIQP chunk
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3.4.3 Sample Rate – SR__ chunk
Element Type Description
lSampleRate_uHz int64 The number of samples per second that are being
recorded by this channel
Table 3.4: The SR__ chunk
3.4.4 BandWidth – BW__ chunk
The bandwidth centred about the centre frequency. If the bandwidth is not centred about the centre
frequency use the BWOF chunk instead.
Element Type Description
lBandwidth_uHz int64 The bandwidth of the signal in micro Hertz
Table 3.5: The BW__ chunk
3.4.5 BandWidth Offset Frequency – BWOF chunk
In some cases the signal bandwidth will not be centred about the centre frequency. Such cases may
occur when demodulating SSB signals.
Element Type Description
lBandwidth_uHz int64 The bandwidth of the signal in micro Hertz
lOffsetFrequency _uHz int64 The offset frequency of the signal band from the centre
frequency in micro Hertz
Table 3.6: The BWOF chunk
3.4.6 Centre Frequency – CF__ chunk
Element Type Description
lCentreFrequency_uHz int64 The centre frequency of the signal in micro Hertz
Table 3.7: The CF__ chunk
3.4.7 dB Full Scale – dBFS chunk
Element Type Description
fFullScaleLevel_dBm float32 The analogue input level to the ADC in dBm, which will
produce maximum full scale digital samples for the
current IQ time data chunk integer type. eg. If we are
using SSIQ chunks, then a dBFS chunk will indicate
the analogue input level that will yield a maximum
digital sample swing of +-(215-1). Note that this value
may be different from the full scale value of the ADC.
Table 3.8: The dBFS chunk
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3.4.8 dBTotal Gain – dBTG chunk
Element Type Description
fGain_dB float32 The total analogue gain from the input of the receiver
(usually an antenna) to the input of the ADC.
Table 3.9: The dBTG chunk
3.4.9 IQ DisContinuity – IQDC chunk
This chunk should be sent as indicator to the reading application to reset it's history. Discontinuities
may be caused by samples being dropped, changes in sample rate, changes in bandwidth, changes in
centre frequency or changes in receiver gain. In systems where continuous data is expected from a
stream an IQDC chunk should not be expected unless a parameter change has forced a discontinuity.
In some applications it may be necessary to send an IQDC chunk if the writing application doesn't
support the necessary chunk to identify an obvious discontinuity, for instance if CF__ chunks were not
supported but it was known that the centre frequency had changed an IQDC chunk could be send.
This chunk has zero size.
Element Type Description
N/A N/A N/A
Table 3.10: The IQDC chunk
3.4.10 Single channel Short Real data - SSR_ chunk
Data is assumed to be continuous when using this data format, if the data is blocky, an IQDC chunk
should be sent after every block of continuous data. This chunk can be used to send audio data.
Element Type Description
lTimestamp int64 The timestamp is stored as a 64 bit signed number,
representative of the time of capture of the first sample in the
chunk block, in microsecond resolution. It is stored as the
number of microseconds since beginning of the epoch (i.e. 1st
January 1970 midnight).
awRealData int16[length of
real data array].
The length of the
array must be a
multiple of 2.
Real signed int16 short numbers. Note that regardless of the
number of valid bits, the most significant bits in each short
should be used. This allows us to specify the full-scale level
without needing to specify the number of bits. The number of
real int16 shorts in the array must be a multiple of 2.
Table 3.11: The SSR_ chunk
3.4.11 Group Short IQ time data – GSIQ chunk
The Group IQ chunk came out of the need to send multiple channels worth of time data sampled from
several adjacent channels in the frequency domain. These channels are often slightly overlapped in
the frequency domain and can be used to create FFT information of a wider bandwidth than what is
contained in a single channel.
Data is assumed to be continuous when using this data format, if the data is blocky, an IQDC chunk
should be sent after every block of continuous data.
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Element Type Description
lTimestamp int64 The timestamp of the first sample in microsecond
resolution, this is the number of microseconds since
beginning of the epoch
awIQData int16[(length of the
group IQ data
array]
IQ pairs of signed int16 short numbers. Note that
regardless of the number of valid bits, the most
significant bits in each short should be used. This
allows us to specify the full-scale level without needing
to specify the number of bits.
The packing used is described by the GIQP chunk.
Table 3.12: The GSIQ chunk
3.4.12 Group IQ Packing - GIQP chunk
Since the GSIQ chunk supports many variations in contents, the content specific information is
supplied by the GIQP chunk, and is required to parse the chunk correctly.
Element Type Description
iNumChannels int32 The number of channels in the group
iIsIQPacked int32 Value 1 for IQ ordering and value 0 for QI ordering. The
first int 16 in the awIQData array mentioned above is
an “I” sample if the value is 1.
iIncrement int32 The number of samples to increment to read the next
sample for a particular channel. The value of
iIncrement will be 1 or iNumChannels.
aiChannelOffset int32[iNumChannels] The channel offset to the start of each channel given in
samples where a sample is an IQ pair.
Table 3.13: The GIQP chunk
Examples:
Take a stream which contains 4 channels of IQ data A,B,C and D. If the data were packed as follows
(where N = number of samples per channel):
• A[0] A[1] A[2] ... A[N-1] B[0] B[1] B[2] ... B[N-1] C[0] C[1] C[2] ... C[N-1] D[0] D[1] D[2] ... D[N-1]
For this packing scheme: iIncrement = 1 and aiChannelOffset = [0 N 2N 3N]
• A[0] A[1] A[2] ... A[N-1] B[0] B[1] B[2] ... B[N-1] D[0] D[1] D[2] ... D[N-1] C[0] C[1] C[2] ... C[N-1]
For this packing scheme: iIncrement = 1 and aiChannelOffset = [0 N 3N 2N]
• A[0] B[0] C[0] D[0] A[1] B[1] C[1] D[1] A[2] B[2] C[2] D[2] ... A[N-1] B[N-1] C[N-1] D[N-1]
For this packing scheme: iIncrement = 4 and aiChannelOffset = [0 1 2 3]
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3.4.13 Group Channel BandWidth – GCBW chunk
Element Type Description
iChannelBandwidth_uHz int64 The bandwidth of each channel contained in a GSIQ
chunk. Value stored in micro Hertz. It is assumed that
all of the channels are sampled at the same sample
rate, and therefore the bandwidth of all channels in the
group is equal.
Table 3.14: The GCBW chunk
3.4.14 Group Centre Frequencies – GCF_ chunk
Element Type Description
iNumChannels int32 The number of channels in the group
alCentreFrequencies_uHz int64[iNumChannels] The centre frequency of each channel in micro
Hertz
Table 3.15: The GCF_ chunk
3.4.15 Start Of File Header – SOFH chunk
All files must be started with an instance of the SOFH chunk. The presence of this chunk can be used
to identify the file format as a PXGF file. For more information about what type of data is stored, the
remaining chunks before the EOFH chunk should be evaluated. This chunk must only appear once at
the start of a file.
Element Type Description
iFormat int32 Identifier for the format used in a file. It is recommended that the
numeric value of the data chunk name used in the file be used for this,
e.g. SSIQ or GSIQ.
Table 3.16: The SOFH chunk
3.4.16 End Of File Header – EOFH chunk
This chunk must contain an empty data block, i.e. the size must be 0. It is used to indicate the end of
the header at the start of a file.
Element Type Description
Table 3.17: The EOFH chunk
3.4.17 TEXT string – TEXT chunk
Text chunk using ISO-8859-1 encoding. Each character is stored as a byte. This chunk can be used to
store meta data. It is suggested that this information appear in the header section of files. Text chunks
with different names could be created or meta data could be encoded in a single text chunk using
XML. See section 3.5.3 for a list of proposed text chunks.
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Element Type Description
iTextLength int32 The number of characters in the text message
ayMessage byte[iTextLength] Text encoded using ISO-8859-1.
byte[] Zero padding to ensure word alignment of chunk.
Table 3.18: The TEXT chunk
3.5 PROPOSED EXTENSIONS
3.5.1 UTF-8 string – UTF8 chunk
Text chunk using UTF-8 encoding. It may be desirable to be able to store text messages using
different formats, eg UTF-8 etc.
Element Type Description
ayUTF8 byte[] Text encoded using UTF-8. The byte array is to be
padded to ensure word alignment.
Table 3.19: Proposed UTF8 chunk
3.5.2 IF frequency – IF__ chunk
Some systems require data to be played back at the original IF frequency used by the receiver. As the
sampled data is already at baseband the IF frequency doesn't provide any information about the signal
useful for analysis. This chunk could also cause confusion if the data were not at baseband, but rather
offset to some frequency as occurs when using some demodulators in SSB mode. This special case is
currently dealt with using the BWOF chunk.
Element Type Description
lIfFrequency_uHz int64 The IF frequency of the signal in micro Hertz
Table 3.20: Proposed IF__ chunk
3.5.3 Proposed text chunks
• Location of recording - TLOC.
• Equipment used to generate file - TREQ.
• Name of operator – TNOP.
3.5.4 Direction data chunk
1. Timestamp for start of first sample
2. Frequency of the first bin
3. Bin resolution
4. Number of bins
5. Azimuth/Elevation pairs encoded as shorts
3.6 DEPRECATED CHUNK TYPES
3.6.1 Start Of Header – SOF_ chunk. Use SOFH chunk instead
Used to denote the start of the header in a PXGF file. The presence of this chunk can be used to
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identify the file format as a PXGF file. For more information about what type of data is stored, the
remaining chunks before the EOH_ chunk should be evaluated. This chunk uses the same chunk type
number as the SOFH chunk that replaces it.
Element Type Description
iFormat int32 Identifier for the format used in a file. It is recommended that the
numeric value of the data chunk name used in the file be used for this,
e.g. SSIQ or GSIQ.
Table 3.21: Deprecated SOF_ chunk
3.6.2 End Of Header – EOH_ chunk. Use EOFH instead
This chunk must contain an empty data block, i.e. the size must be 0. It is used to indicate the end of
the header at the start of a file. This chunk uses the same chunk type number as the EOFH chunk.
Element Type Description
Table 3.22: Deprecated EOH_ chunk
3.7 SYNCHRONISATION
If an application is reading a file from the beginning there will be no trouble synchronising unless the
file has become corrupted. However, if the application is connecting to an output stream in this format
which has been running since prior to the connection, then it is necessary to ensure that the
application becomes synchronised with the stream. Synchronisation is largely hidden from the
developer as it is handled by the libraries for reading and writing PXGF streams.
The procedure to obtain synchronisation is as follows:
Step 1. Obtain synchronisation
1. Read the stream until the 4 byte sync pattern is recognised.
2. Reset state information in the application.
3. Move on to step 2.1.
Step 2. Attempt to process a chunk
1. Read the type and length of the chunk.
2. If the length is more than 65536 bytes return to step 1.1.
3. If there is a registered handler for the type then process it otherwise skip over the data of the
chunk.
4. Move on to step 3.1.
Step 3. Check synchronisation
1. Read the sync pattern from the stream. If sync pattern matches move to step 2.1 otherwise
move back to step1.1.
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4 ABBREVIATIONS
Abbreviation Meaning
RIFF Resource Interchange File Format
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