NXV

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NXV is a container format used by the 2006 MP4 Player Watch manufactured by "Shenzhen Adragon Digitek" (China). The wrist watch is reported to support MP4, WMV and WMA, however this is achieved by first converting the files to the NXV format.

Container Format

The format consists of a sequence of 512-bytes packets.

Header packet:

 bytes  0-11   ASCIIZ  magic ("NXV File")
 bytes 12-19   ASCIIZ  version ("1.0.0", "3.0.1" or "3.0.2")
 byte     20   width (pixels)
 byte     21   height (pixels)
 byte     22   always 0
 byte  23-511  unknown but required for playback (see below)

The unknown sequence requires a couple of bytes set for playback to work on the watch. The official encoder sets different values for different files, but just using one known sequence seems to work fine. One known good sequence is byte 0x17 = 0x87 and byte 0x87 = 0x02, and 0 for the rest of the values in the block.

The second value in this sequence is the number of audio buffers between video buffers. The first value appears to be an offset to the second.

The a/v sequence commences at byte 512 with an audio packet:

 uint curSequence = 0
 while !eof
   u8[500]        audio payload
   u8[4]          unknown
   be32           sequence number
   le16           length (bytes)
   le16           unknown (pixels == bytes/2?)
   if (sequence == curSequence)
     curSequence += 1
     u8[length]     video payload

Note: This encoding is a superset of what the actual watch supports, but will correctly decode all known NXV files from the encoder. On the watch, the number of audio packets between video packets isn't determined by dynamically detecting a counter but rather by the number in the header. Values of 1, 2, and 4 have been tested (anything larger is too low of a framerate to be interesting for most videos)

Audio Payload

The audio payload is an MP3 stream, including RIFF WAVE headers.

Video Payload

Raw video - be16 in 565 RGB format. If the version is "1.0.0", the video is at full-resolution. If the version is "3.0.1", the video is at quarter-resolution; that is, the video data is 1/4 of the size indicated and is scaled in each direction by two on playback. Version "3.0.2" seems to indicate a different format - likely sixteenth-resolution.

Notes

  • Supported video resolutions are: 96x64, 96x80, 96x96, 128x96, 128x128, 160x128, 176x128. Despite the reduction in video resolution, the resulting NXV file size exceeds that of the input video file.
  • Intermediate files are used by the NxvConverter program to store the audio and video payloads (filename.mp3 and filename.tmp) prior to muxing to the NXV file.
  • All NXV files examined so far have one frame per video packet and the packets appear at regular intervals. Low and Mid quality have one frame per four audio packets; High quality has one frame for every two audio packets.
  • The content of the sequence and length values in an audio packet not followed by a video packet is unknown - probably nothing.
  • Example file is an encoding of public domain footage from Internet Archive: Moving Image Archive

External Links