AMV video origin and history

The AMV is a proprietary file format (container format) created for the S1MP3 players to allow video playback without having to rise the costs of manufacturing. The video playback could have been another, like AVI for example, but often these formats need special hardware codecs and they have expensive license royalties. Also, the AMV format uses less compression and in this way there is no need for high power processors. This also leads to higher playback time.

The compression

When playing AMV format, there is clearly some image de/compression taking place, as can be demonstrated by a simple calculation (AMV video format). The image compression is, however, low by modern standards (around 4 pixels/byte, compared with over 10 pixels/byte for MPEG2/DVD). With a resolution of 128 × 96, and a frame rate of 12 fps, a 30 minute file will be around 80MBytes in size.

The AMV file format

Container

The AMV format is in fact a modified version of the AVI container, the former Microsoft video format (now replaced by the more capable WMV). Like the AVI format, the header contains two strings, "avih" and "amvh" ("AVI" and "avih" for AVI). The section where codec, frame size, and other information would normally be found is filled with zeroes (because most of that information is hard-coded into the player's firmware). Tthe string "AMV_END" is found at the end of the file. The modified avi header contains several 'garbage' values because many of the parameters are hardcoded. It contains a RIFF packed stream of video data and ADPCM encoded audio. There is an open source decoder (written in Perl) for the format.

Video

According to its author, the video codec is a modified version of Motion JPEG but missing quantization tables which are fixed (from jpeg standard).

Audio

Audio is a variant of IMA ADPCM where the first 8 bytes of each frame are:

origin (16bits)
index (16bits)
number of encoded 16-bit samples (32bits).

There is also apologize that trellis quantizaton is used.

Formal AMV movie format Specification

RIFF('AVI '
LIST(
'hdrl'
'avih'
LIST(
'strl'(video stream info)
'strh'
'strf'
)
LIST(
'strl'(audio stream info)
'strh'
'strf' )
LIST(
'movi'(data)
.
00dc(video frame data)
01wb(audio data)
00dc(video frame data)
01wb(audio data)
...
00dc(video frame data)
01wb(audio data)
'AMV_END_'
)

typedef struct _amvmainheader {
FOURCC fcc; // 'amvh'
DWORD cb;
DWORD dwMicroSecPerFrame;
BYTE reserve[28];
DWORD dwWidth;
DWORD dwHeight;
DWORD dwSpeed;
DWORD reserve0;
DWORD reserve1;
BYTE bTimeSec;
BYTE bTimeMin;
WORD wTimeHour;
} AMVMAINHEADER;

typedef struct _amvstreamheader {
FOURCC fcc; // 'strh'
DWORD cb;
BYTE reserve[56];
} AMVSTREAMHEADER;

typedef struct {
WORD wFormatTag;       //(Fixme: this is equal to PCM's 0x01 format code)
WORD nChannels;        //(Fixme: this is always 1)
DWORD nSamplesPerSec;  //(Fixme: for all known sample files this is equal to 22050)
DWORD nAvgBytesPerSec; //(Fixme: for all known sample files this is equal to 44100)
WORD nBlockAlign;      //(Fixme: this seems to be 2 in AMV files, is this correct ?)
WORD wBitsPerSample;   //(Fixme: this seems to be 16 in AMV files instead of the expected 4)
WORD cbSize;           //(Fixme: this seems to be 0 in AMV files)
WORD reserved;
} WAVEFORMATEX;

Video format is jpeg (without DQT,SOF,SOS)
Audio format is IMA-ADPCM

Open source decoder

The decoder extracts each frame into a JPEG file and then uses ffmpeg to encode the video in another format. Here is a simple test to understand what's in the AMV video file.

make an AVI video of total 10 frames using RAW DATA format, at 12fps sized 160x120 without sound
save eche frame to BMP (do not use jpeg now, name them as 01.bmp ~ 10.bmp)
convert the BMP files to JPEG, be careful with the JPEG compress level
convert the AVI file to 160x120 pixels, 12 fps AMV, again be careful about the Image quality setting which is the JPEG compress level, and set Image Zoom to zero
compare the AMV and JPEG files using a hex editor

You will find jpeg headers inside the AMV file. According to a comment in a blog post, AMV and MTV are both AVI based (ffmpeg demuxer has proven to be able to handle it, with a few hacks), some chunks are removed, some are modified. The audio stream is a variant of IMA ADPCM encoding. First 8 bytes of each frame are used os origin and index (two shorts, 4 bytes) for ADPCM decoder and for length of audio frame (long, 4 bytes). Video stream is composed of mjpeg frames (similar to jpeg images, but without quantization tables which can be obtained in jpeg standard).

The Apex issue

There is a big possibility that the AMV movie format used by S1mp3 is a rip of 'Apex Media Video' for Nintendo Game Boy (Color). Older info on it can be found at the Apex Designs page. Apex claim pretty good compression ratios and it is capable of being decoded on GB. It is possible that AMV is a rip-off of that codec, mainly considering both devices work with a Z80-based instruction set. The company hasn't replied to the email sent about the issue.

AMV Video format isn't Apex Media Video format, it is simple motion JPEG where each frame is saved as JPEG.

Software players that support AMV playback

FFmpeg (since SVN r10623)
MPlayer (1.0rc2 and later)
Proprietary converter (and the attached player) that comes on the original CD sold with the unit

Software players that support MTV playback

FFmpeg (seeking is broken).
Proprietary converter (and the attached player) that comes on the original CD sold with the unit.

Hardware players that support AMV and MTV playback

Chinese-made portable MP3 players, also known as S1 MP3 Players.

Anime Music Video

The AMV term can be used also to designate Anime Music Videos.

Controversies

The players that use this format are wrongly named "MP4 Players". In fact, the AMV (and the MTV) formats are not belonging to the MP4 family. The proper term that would define these players is "S1MP3" as named by their creators.