Open discussion about tools and MXF in general
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Joined: Thu Nov 09, 2006 5:51 am
Posts: 3

Post Posted: Wed Nov 29, 2006 3:02 am 
I'm having trouble reconciling what I'm reading about UUID and UL's in 5.5.8 of the SMPTE eg41-2004 MXF Engineering Guide and what has been implemented in MXFLib (types.h), with what I'm seeing in an MXF file that was produced by an FS100 DTE.

A specific example being their ProductID. Here's a snippet of MFXDump run against a file:

Dump an MXF file using MXFLib
- using dictionary "T:\MXF data for Richard\dict.xml"

Partition at 0x00000000 is for BodySID 0x0000
0x00000000 : ClosedCompleteHeader(PartitionMetadata)
0x00000014 : MajorVersion(UInt16) = 1
0x00000016 : MinorVersion(UInt16) = 2
0x00000018 : KAGSize(UInt32) = 1
0x0000001c : ThisPartition(UInt64) = 0
0x00000024 : PreviousPartition(UInt64) = 0
0x0000002c : FooterPartition(UInt64) = 1575968768
0x00000034 : HeaderByteCount(UInt64) = 4656
0x0000003c : IndexByteCount(UInt64) = 27840
0x00000044 : IndexSID(UInt32) = 1
0x00000048 : BodyOffset(UInt64) = 0
0x00000050 : BodySID(UInt32) = 0
0x00000054 : OperationalPattern(Label) = [060e2b34.0401.0102.0d010201.10020000]
0x00000064 : EssenceContainers(EssenceContainers)
0x0000006c : EssenceContainer(Label) = [060e2b34.0401.0101.0d010301.02026102]

Header Metadata:
0x00000584 : Preface(InterchangeObject)
0x00000598 : InstanceUID(UUID) = {570a7c14-d8ca-4af8-91db-c03cd5c660b8}
0x000005ac : LastModifiedDate(Timestamp) = 2006-10-24 9:28:11.000
0x000005b8 : Version(VersionType) = 258
0x000005be : Identifications(Identifications)
0x000005ca : Identification(UUID) = {cce2ed58-9001-05a4-93fe-9d7ede3afb35}
0x000005ca : Identification(UUID) -> Strong Reference to Identification
0x0000062a : Identification(InterchangeObject)
0x0000063e : InstanceUID(UUID) = {cce2ed58-9001-05a4-93fe-9d7ede3afb35}
0x00000652 : ThisGenerationUID(UUID) = {51e9bfe1-eac4-867e-c323-fca15df7d6a1}
0x00000666 : CompanyName(UTF16String) = Focus Enhancements, Inc.
0x0000069a : ProductName(UTF16String) = FireStore
0x000006b0 : VersionString(UTF16String) = v1.4
0x000006bc : ProductUID(UUID) = [65000000.0000.0000.46697265.53746f72]
0x000006d0 : ModificationDate(Timestamp) = 2006-10-24 9:28:11.000

From the square brackets and the fact that the element has a type of UUID, this should be a UL. Right ?

If so, then if I'm reading 5.5.8 right, the UL shown above is illegal, right ? - not documented in SMPTE spreadsheets.

5.5.8 Storing ULs and UUIDs in the same property
Some data fields, such as the UID property of the LocalTagEntry batch in the primer, can contain
either a UL or a UUID. In this case there is an advantage to using a particular byte order for the
UUIDs. All UUIDs have a 1 in the most significant bit of the “clk_seq_hi_res” word (byte 9),
whereas all ULs have a 0 in the most significant bit of the first byte. If UUIDs are stored with a
byte order that places the “clk_seq_hi_res” word first, then it is always possible to tell if the value
is a UL or a UUID by examining the MSB of the first byte. This byte order also prevents the
remote possibility of a UUID being stored that matches a registered UL. For these reasons, it is
recommended that when any UUID is published for inclusion in a data field that can also
contain ULs, the byte order specified for that UUID be the same as the ISO/IEC 11578 default
order, but with the upper and lower eight bytes swapped.

The section above gives rise to the following guidelines:
• A UUID may be stored in a data field of type UL by swapping the top and bottom 8 bytes of the
UUID (the most significant bit of the first byte of such a swapped UUID is always 1);
• MXF decoders should accept a swapped UUID in a place where a UL is expected.
NOTE – AAF uses a compatible byte-swap method for storing ULs and UUIDs in the same
properties, which it defines as AUIDs.



Joined: Thu Apr 15, 2004 10:39 am
Posts: 198
Location: Scotland

Post Posted: Thu Nov 30, 2006 9:42 am 
If so, then if I'm reading 5.5.8 right, the UL shown above is illegal, right ? - not documented in SMPTE spreadsheets.


For some reason there is a common misconception that you can put whatever you want in a UUID field. This is not true - the format is defined by RFC 4122.

MXF (and for that matter AAF) use a trick to allow UUIDs and ULs to be used interchangeably; all valid UUIDs have the MSB of the 9th byte set and all ULs start with 0x06. This means that if "end-swap" a UL (you re-order a UL so that the first eight bytes are stored last and the last eight first) you can store it in a UUID field and distinguish it from a genuine UUID. This process is described in EG41 section 5.5.8.

So, we have the situation that MXF UUID fields can either store a UUID as defined by RFC 4122 (9th byte has its MSB set) or an end-swapped UL (9th byte is 0x06).

What I frequently find is someone has simply put ASCII text in the field instead, such as "MyFirstDevice". This is not a UUID, nor is it an end-swapped UL.

If you look at the bytes that are stored in the ProductUID given you will see that they are actually "FireStore".

When MXFLib displays a UUID it checks the MSB of the 9th byte; if it is set it displays it as a UUID (with curly brackets), otherwise it end-swaps it and displays it as a UL (with square brackets). If the data is neither a UUID or a UL you will just get garbage!

Note: These mixed UUID/ULs are generally called AUIDs. If you do the same trick in reverse (store an end-swapped UUID in a UL field) it seems to be increasingly common to call that an IDAU (you see what they have done? They have end-swapped AUID)

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