Miriam Ruiz
random thoughts on technology and life

{December 27, 2007}   SMV Video Format

Some small portable MP3 players also allow reproducing images and videos (does anyone know why they call them MP4 players?), but they need the latter to be in some funny format called SMV (for devices with SigmaTel chipset. Other devices use different formats, such as AMV for devices with Actions chipset, MVS for those with Sunplus chipsets, MTV, DMV or ASF).

SMV seems to stand for “SigmaTel Motion Video”, and was developed by SigmaTel Inc to allow users to transform common PC video files and watch them on their portable multimedia players (PMP). I’ve been trying to find out how to convert videos to this format for a while, to be able to watch them in a small gadget I bought some time ago, but haven’t been able to do so from Linux yet. All the converters seem to be privative, and just for Windows, although they seem to work with wine according to some reports I’ve read on the net. I’m not putting any closed-source software in my lappy anyway, so that’s not an option for me.

I recently found a web page of someone developing a conversion tool for SMV called smvconv, based on mplayer and ffmpeg and released under a MIT/X11 license. It’s just a small C program and a shell script, and seems to be in a very preliminary stage, but the most important part is that he has also published the structure of SMV files, which seem to be, in fact, quite a simple format composed of some headers, a WAV audio file and some JPEG frames:

An SMV file has three sections, concatenated together:

  1. An IMA ADPCM .wav file, with header.
  2. An SMV header.
  3. A number of .jpeg files, with headers, each prepended by a 24-bit (3-byte) little-endian value representing the size, and buffered at the end with nulls, so that they are at predictable locations (the jpeg modulo). SMV is a constant-bit-rate format, but the underlying format (jpeg) is not. Proprietary encoders -according to smvconv’s author- probably make up for this by encoding at various qualities until a jpeg of the proper size is made.

The SMV header is a magic string in ASCII (“SMV002000″) followed by a number of 24-bit (3-byte) little-endian values representing the width, height, header length (in 3-byte words, starting with some magic “002″), some apparently constant value (1), the jpeg modulo, fps, number of frames. some apparently constant values (a 1 and a 0), the number of frames per jpeg, and 4 padding values (65793 == 0×010101). The default qualities seem to be 35kBps (“High”), 25kBps (“Medium”) and 16kBps (“Low”).

Lets hope to have some working converter soon, so we can use this feature in those cheap players. I don’t remember where I put the USB connector for mine, but as soon as I find it -it seems to be non-standard, so no way to replace it with a new one, AFAIK- I’ll start experimenting with it.


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