Just a reminder that the (revised) Summer of Code application submission deadline is tomorrow, Monday, April 7. If you are a student and want to be considered for an FFmpeg Summer of Code project slot, you need to enter an application (one or more) into Google’s system by the end of the day on Monday. That is not, however, the deadline for qualification tasks. That comes next week.
Author Archives: Multimedia Mike
PDP-1 Multimedia
I got to see a demonstration of a restored, 45 year old DEC PDP-1 computer today at the Computer History Museum in Mountain View, CA, USA. Does that sound interesting in the context of multimedia hacking? The thing could be hooked up to some kind of dot-plotting video device, and it didn’t feature any sound audio. At least, no sound hardware out of the box. Thing is, the unit was highly mod-able.
The PDP-1 hosted what is widely believed to be the first video game ever– Spacewar!. I have already written up that aspect of the experience in my Gaming Pathology blog.
Sound, however, was possible through a hardware mod. The computer had an array of LEDs and one clever hacker thought to wire 4 of these up to square wave generators, thus producing 4-channel music. This was originally programmed in the early 1960s and was demoed today. The hacker who had originally written the music engine on a PDP-1 at MIT found himself on the restoration committee many decades later. It seems MIT had donated paper tape sequences that contained musical data that played on his music engine– but the engine code had been lost. Still, he was able to reverse engineer the audio format and reimplement the engine on the original PDP-1 hardware. Sounds familiar. He even made the same point that I like to make in my multimedia technology presentations — data is more important than code.
It almost made me feel young again. Here I am, studying multimedia formats that largely only date back about 15 years to around 1993.
Solid Snake Oggs
I was studying a file called vox.dat scavenged from the GameCube version of Metal Gear Solid: The Twin Snakes, the seminal, tactical, tip-toeing game starring Solid Snake. The file contains a lot of multi-lingual subtitle strings as well as the actual English speech recited along with the subtitle presentation. What format does this commercial game use? Would you believe Ogg Vorbis? The constituent audio streams are all tagged with the string “Xiph.Org libVorbis I 20020717”, which is quite old. The current version of the Xiph’s ogg123 playback tool does not decode a stream properly. Some of the data is audible, but a lot of audio chunks are skipped. FFmpeg fares a little bit better but still scrambles some audio.
Is this a case of poor backwards compatibility? Or perhaps the creators — Silicon Knights — deliberately monkeyed with the bitstream? I found that last situation a bit implausible as I assumed developers would treat this third party codec stuff as a black box. But as an experiment, let’s go back in time, courtesy of Xiph’s source control:
svn co -r {20020717} http://svn.xiph.org/trunk/ogg ogg-svn
svn co -r {20020717} http://svn.xiph.org/trunk/vorbis vorbis-svn
svn co -r {20020717} http://svn.xiph.org/trunk/vorbis-tools vorbis-tools-svn
I removed all of the related components on my system for good measure. With a little persistence and a lot of disabled options while building the tool set, I was finally able to get the components to build. Those old tools still have the same trouble with these Ogg Vorbis files:
$ oggdec mgs1-sample1.ogg
OggDec 1.0
Decoding "mgs1-sample1.ogg" to "mgs1-sample1.wav"
[ 1.5%]Warning: hole in data
[ 4.5%]Warning: hole in data
[ 6.5%]Warning: hole in data
[...]
[127.5%]Warning: hole in data
[130.5%]Warning: hole in data
[132.5%]Warning: hole in data
[134.5%]
Or maybe the tool is just extremely capable if it can decode more than 130% of the file.
I have placed three manually ripped samples in the archive; each is 512 KB. I would start ripping at offsets where I saw an ‘OggS’ marker that was followed soon after by the libVorbis ID string. If you care enough, have a look. And to what end? Isn’t it obvious? To create a “Learn English With Solid Snake And Friends” application.
Learn handy, everyday phrases like, “I’m no rookie!” and “Don’t think! Shoot!” English speakers will be able to learn the same phrases in other languages, though they won’t have the benefit of pronunciation.
I’m still working out the details of the vox.dat file format. I have some things sorted out. Perhaps readers who know German, French, Italian, or Spanish, and who understand non-ASCII character encodings can answer whether these schemes fit any well-known encodings (I know that the 0x0A is a line break in the subtitle):
53 70 72 69 63 68 20 6E 69 63 68 74 Sprich nicht
20 7A 75 20 6D 69 72 20 77 69 65 20 7A 75 0A 65 zu mir wie zu.e
69 6E 65 6D 20 41 6E 66 1F 0B 6E 67 65 72 21 inem Anf..nger!
4C 61 20 66 65 72 6D 65 2C 20 6C 1F La ferme, l.
09 2D 64 65 64 61 6E 73 20 21 .-dedans !
5A 69 74 74 6F 20 6C 1F 09 20 64 65 Zitto l.. de
6E 74 72 6F 21 ntro!
1F 42 1F 42 41 20 71 75 1F 0F 20 65 .B.BA qu.. e
73 74 1F 08 73 20 65 73 70 65 72 61 6E 64 6F 21 st..s esperando!
21 0A 1F 42 1F 42 44 69 73 70 61 72 61 21 21 !..B.BDispara!!
Empirical analysis simply implies that a character 0x1F is followed immediately by a character not in the standard ASCII set.
Novice Relations Department
The qualification tasks required of students in order to participate in FFmpeg’s Summer of Code program has finally afforded me the opportunity to create a “Small FFmpeg Tasks” page on the MultimediaWiki. Of course, the small tasks list was originally chartered to qualify students, but it need not be limited to that purpose. As I wrote on the page, it can be used for anyone who needs a good starting point for FFmpeg hacking. Further, it could be used for someone who has been away from the codebase and needs an exercise to motivate re-familiarization.
I have this — perhaps unfounded — vision that there are many lurkers out there, watching the FFmpeg project from a distance, on or off the notoriously abrasive mailing lists, hoping one day to get involved somehow, but not knowing exactly how to break in. The small tasks list is a great place to start. Maybe you don’t feel all that comfortable with what you have seen during your lurkings, perhaps because you can’t figure out what is meant with this whole “top-posting” thing for which n00bs are routinely savaged. If that’s you, you can always email me privately about getting started. I will (probably, depending on the day) be happy to tutor you on the basics for contributing some code. Email address is on the side bar.