File format:Sigrok/v3

This page describes the proposed file/stream format (v3) for storing and transmitting sigrok related data.

NOTE: This is work in progress and has not yet been implemented!

Motivation

The previous sigrok session file format (version 2) is a ZIP file containing multiple files (some metadata files and data files containing the actual samples). This works fine, but it also has some issues:

• In order to get to the data you want, you need to decompress the whole file.
• Appending to a file is not possible easily (and it's not efficient).
• ...

Goals

The following list highlights some of the goals of the new file format (v3):

• It must be able to store
  • arbitrary data (logic samples, and/or analog samples, and/or protocol decoder data, and more), as well as
  • arbitrary meta-/config-data and other extra information that may be useful to frontends (UI state data, user-configured probe colors, names, positions, and so on).
• It must support and facilitate stream-oriented processing (save, load, transmission, compression/decompression, and so on).
• It must support compression of the payload data.
• It must be usable independent of hardware architecture (x86, ARM, PowerPC, MIPS, and so on), operating system, endianness, float representation, and so on. All data fields must be properly specified (endianness, signedness, size, format).
• It must allow for sufficiently good performance for the common operations a frontend needs to perform on the data/file/stream (save, load, compress/uncompress, append, and so on) so that it doesn't become the bottleneck. This is especially important for stream-oriented devices which could otherwise lose samples if the processing on the host side is not sufficiently fast (Saleae Logic, Saleae Logic16, IKALOGIC ScanaPLUS, others).
• It should be able to handle run-time changes in the data streams (via meta packets on the session bus), e.g. changing samplerates, changing probes, etc. etc.
• It should have better compression properties than ZIP (e.g. using LZO or other algorithms, this is to be evaluated). What we ideally want out of the compression algorithm is:
  • Good and relatively fast compression results at only moderate CPU usage.
  • Very fast decompression (LZO is probably the best one here, as it's specifically designed for this).
  • Ideally, support for appending further data to already compressed data chunks (though this could be also implemented outside of the compression algorithm per se).
  • Open-source license and OS portability. There should be an open-source library or code chunk for compression/uncompression and it should be widely available in Linux distros, and portable to Windows, Mac OS X, FreeBSD, Android, and so on.

UUIDs

The format uses random UUIDs (version 4) as per RFC4122 in various places. These UUIDs are always 16 bytes long.

A simple way to generate a random (version 4) UUID (ASCII and hex representation):

$ python3 -c 'import uuid; u = uuid.uuid4(); print(u); print(u.hex)'
14c49f22-f08a-4ef2-b3d7-82ee16c3d531
14c49f22f08a4ef2b3d782ee16c3d531

File/stream format

The format consists entirely of a stream of packets of various types.

These packets can be either written to or read from a file, buffer, pipe, socket, or any other source/destination.

Packet format

Every packet consists of three fields:

Field	Length	Description
Short-UUID	2	An ID (2 bytes, little-endian) that maps to a previously defined 16-byte UUID. The Short-UUID values can range from 0x0002 to 0xffff, which allows for 65535 different packet types in a single file/stream. The Short-UUID 0x0000 is special and cannot be used for "normal" packets, see below. The reason for using a (Short-)UUID here instead of some simple index number is to allow for clients to define and use their own special-purpose packet types as they see fit, without having to fear any conflicts with existing packet types (or packet types that someone else might add later).
Length	4	The length of the data in this packet (in number of bytes). The length does not include the length of the Short-UUID field or the Length field itself, only the length of the Data field. The length is given as an uint32_t number (little-endian).
Data	0..n	The actual payload data, max. 2^32 bytes (4GiB). For some packet types the Data field is optional (in that case it is completely omitted and the Length field is set to 0). The contents of the Data field are entirely dependent on (and vary with) the type of packet.

Using the common Short-UUID/Length/Data triplet (type-length-value idom) for each packet allows clients to easily skip over (ignore) any packets they do not know how to handle, and instead continue on to checking/handling the next packet.

Example packet with a 7-byte data field (Short-UUID is 0x55aa):

Short-UUID	Length	Data
55 aa	00 00 00 07	11 22 33 44 55 66 77

Example packet without a data field (Short-UUID is 0x55aa):

Short-UUID	Length
55 aa	00 00 00 00

PACKET_MAP_UUIDS packet

This is a special packet that is used to map 16-byte UUIDs to 2-byte Short-UUIDs.

Since every packet has a 2-byte Short-UUID, PACKET_MAP_UUIDS must be the first packet in a file/stream, otherwise the client will not be able to interpret any other packets.

However, PACKET_MAP_UUIDS can occur multiple times in a stream. Every time PACKET_MAP_UUIDS is seen, mappings that were not yet defined are added to the list of mappings, and mappings that already existed will be overwritten with the respective new mapping.

Since PACKET_MAP_UUIDS is a packet itself, it also consists of the three common fields Short-UUID/Length/Data. The Short-UUID of PACKET_MAP_UUIDS is always 0x0000.

The Data field has the following contents:

Field	Length	Description
Special Short-UUID for magic marker	2	A reserved special Short-UUID (2 bytes, little-endian) for the magic marker. Value: 0x0001.
Special UUID for magic marker	16	This is a special marker that can be used by the file utility (and other tools) to detect the file format easily. Contents: $sIgRoK$$sIgRoK$.
Short-UUID 1	2	The 2-byte Short-UUID with index 1 (valid values: 0x0002 to 0xffff) that will, from now on, map to the UUID specified below.
UUID 1	16	The UUID with index 1 (binary representation, 16 bytes, little-endian) which identifies the type of packet (globally unique).
Short-UUID 2	2	The 2-byte Short-UUID with index 2 (valid values: 0x0002 to 0xffff) that will, from now on, map to the UUID specified below.
UUID 2	16	The UUID with index 2 (binary representation, 16 bytes, little-endian) which identifies the type of packet (globally unique).
...	...	...

Important notes:

• The Data field contains a list of Short-UUID to UUID mappings. Since every such pair is 18 bytes in size, the Length field of PACKET_MAP_UUIDS can be used to deduce how many such mappings are contained in the Data field.
• The special "magic marker" fields (2 + 16 bytes) are required to be in every PACKET_MAP_UUIDS and are required to always be the first entries of PACKET_MAP_UUIDS. The file format can thus easily be detected by looking at the unique bytes 6-23 in the file (additionally, the file also always starts with the two bytes 0x00 0x00).
• The special Short-UUID 0x0000 must not be used in any mapping, it is reserved for PACKET_MAP_UUIDS itself.
• The special Short-UUID 0x0001 must not be used in any mapping, it is reserved for the special "magic marker", see above.
• There is no guarantee of any kind about which Short-UUIDs will be mapped (and to what). Specifically, a client can not assume that Short-UUIDs start at 0x0002, and it can not assume that Short-UUIDs are ordered in any way. The Short-UUIDs can have a completely random order and they can also have gaps.
• Mappings are generally not static in nature. Every additional PACKET_MAP_UUIDS that occurs can dynamically add or overwrite/change mappings, for example.

Example packet:

Short-UUID	Length	Data
00 00	00 00 00 48	00 01 24 73 49 67 52 6f 4b 24 24 73 49 67 52 6f 4b 24 77 a1 5a 17 72 eb 28 54 48 a8 a4 1c 73 97 d7 e9 22 3d 00 06 59 de f3 30 53 6a 46 b1 8e dd 62 f2 19 5d 1c 95 a3 9f ec 6b d7 63 c8 79 4a a7 a9 7a 7e df 0e 68 af c7

The above PACKET_MAP_UUIDS maps three different UUIDs to the Short-UUIDs 0x77a1, 0x0006 and 0xa39f.

sigrok packets

The following packets are currently defined for use in projects hosted on sigrok.org.

The "names" (e.g. "SIGROK_PACKET_LOGIC") are for documentation purposes only, the (Short-)UUIDs are what actually matters. The names are prefixed with SIGROK_ to make it clear that other 3rd-party software may define their own additional packet types with arbitrary contents and for arbitrary purposes.

SIGROK_PACKET_LOGIC

This is a packet type used to store/transmit (only) digital samples, usually from a logic analyzer.

This packet uses the fixed UUID 2236202e-9ee7-4bc6-81f6-56b4e6e029ba.

The Data field has the following contents:

Field	Length	Description
Version	2	The version of the SIGROK_PACKET_LOGIC format in binary format (little-endian). Current version: 0x0001.
Reserved	2	Reserved field. Reads should ignore this field, writes should keep this field's value unchanged (if it was read before), otherwise set it to 0x0000. Current value: 0x0000.
Payload format Short-UUID	2	A Short-UUID (2 bytes, little-endian) which identifies a certain payload format.
Compression scheme Short-UUID	2	A Short-UUID (2 bytes, little-endian) which identifies a certain compression scheme that is applied to the payload data.
Payload length	4	The length of the actual payload data in this SIGROK_PACKET_LOGIC packet (in number of bytes). The length only includes the Payload field. The length is given as an uint32_t number (little-endian).
Payload	0..n	The actual payload data, i.e. logic analyzer samples in the specified payload format, using the specified compression scheme.

Example packet:

(Packet type SIGROK_PACKET_LOGIC Short-UUID 0xuuuu, 0x30 bytes packet data, SIGROK_PACKET_LOGIC version 0x0001, SIGROK_PAYLOAD_FORMAT_LOGIC_V1 payload format Short-UUID 0xvvvv, SIGROK_COMPRESSION_NONE compression scheme Short-UUID 0xwwww, 8 bytes of logic analyzer payload (uncompressed))

UUID	Length	Data
uu uu	00 00 00 30	00 01 00 00 vv vv ww ww 00 00 00 08 11 22 33 44 5 66 77 88

SIGROK_PACKET_ANALOG

This is a packet type used to store/transmit (only) analog samples, e.g. from a multimeter, oscilloscope, sound level meter, or any other source for analog data.

This packet uses the fixed UUID 59def330-536a-46b1-8edd-62f2195d1c95.

Details yet to be defined.

SIGROK_PACKET_CHANNEL_NAMES

This is a packet type used to assign channel names.

This packet uses the fixed UUID 1325b595-0d5e-40a4-ac4d-36e89224dcb9.

Details yet to be defined.

List of known packet types

This is a short overview of known packet types that are in use. This includes the packet types used in projects hosted at sigrok.org, as well as pointers to packet types that other (3rd-party) software is known to use.

UUID	Packet type	Description
5a1772eb-2854-48a8-a41c-7397d7e9223d	SIGROK_PACKET_LOGIC	See above.
59def330-536a-46b1-8edd-62f2195d1c95	SIGROK_PACKET_ANALOG	See above.
1325b595-0d5e-40a4-ac4d-36e89224dcb9	SIGROK_PACKET_CHANNEL_NAMES	See above.

List of known payload formats

This is a short overview of known payload formats that are in use. This includes the payload formats used in projects hosted at sigrok.org, as well as pointers to payload formats that other (3rd-party) software is known to use.

UUID	Payload format	Description
d2964f38-8b13-4570-9add-add5678a0394	SIGROK_PAYLOAD_FORMAT_LOGIC_V1	This payload format can only store digital samples from a logic analyzer (0/1 values for a certain channel/probe/pin). It is basically identical to the format that was used in the previous ZIP-based file format versions. Details are yet to be defined.
79e7cfd1-0f56-4d5e-968a-b66fdbdff624	SIGROK_PAYLOAD_FORMAT_ANALOG_V1	A certain type of payload format that can store (only) analog samples of a certain number of analog channels. Details are yet to be defined.

List of known compression schemes

This is a short overview of known compression schemes that are in use. This includes the schemes used in projects hosted at sigrok.org, as well as pointers to schemes that other (3rd-party) software is known to use.

UUID	Compression scheme	Description
ec6bd763-c879-4aa7-a97a-7edf0e68afc7	SIGROK_COMPRESSION_NONE	No compression whatsoever is used.
acd2e249-5c4d-426d-96ae-ded5b6020e6f	SIGROK_COMPRESSION_RLE_V1	A certain type of RLE-based compression is used. Details are yet to be defined.

Futher notes and ideas to consider

• Data should be encoded in a data aware way. This would give greater compression:
  • Logic Data is most efficient stored in RLE+Huffman or Golomb coding. e.g. a clock signal may compress to one bit per edge.
  • FLAC (libflac) or a FLAC inspired codec (linear predicition) is probably as good as it gets for lossless analog data encoding.
• If data is stored in a format specific way, it would be best to store it as a series of stream-blocks, similar to how video containers work. Would it be possible to simply leverage a video container such as OGG? IIRC this contains headers to declare metadata about each stream, then a series of timestamped stream blocks interleaved together. The time stamp is a format specific number... for audio: the sample number, for video: the frame number, so sigrok formats can easily leverage this.
  • Similarly RTP is a rather natural protocol for sigrok network streaming.