TM2X

• FOURCCs: TM2A, TM2X
• Company: On2 (formerly Duck)
• Sample: http://samples.mplayerhq.hu/V-codecs/TM2a.avi
• Sample: http://samples.mplayerhq.hu/V-codecs/TM2x.avi

Duck TrueMotion 2X is believed to be related to Duck TrueMotion 2.

Frame structure

Each frame consists of chunks with the following structure:

 0-2 always 0xA0 0x00 0x01
 3      chunk identifier
 4-7 chunk size (big-endian)
 8-... chunk payload

Analyzed frames have chunks in the following order:

• 0x06 (usually takes more than a half of coded frame)
• 0x15- small, variable size
• 0x09- 3 bytes long
• 0x02- two chunks of variable size
• 0x0B- there are always 16 of them with size=4. Maybe for some Huffman tables?
• 0x0A- variable size

In order to ease reverse-engineering Duck seemed to used pseudo-encryption on data. Pseudo-encryption means XORing some chunks with parts of LFSR which may be updated before some chunks. Register data is stored as big-endian number and looks like first 4 bytes of 0x06 chunk are used to initialize it (it also seems to be that chunk size minus four bytes).

LSFR update algorithm:

 calculate sum by modulo 2 of bits 31, 21, 3 and inverted bit 0
 shift register contents left by one bit and store new sum in LSB
 repeat 4 times