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• Dreamcast Track Sizes

  1er mars 2015, par Multimedia Mike — Sega Dreamcast

  I’ve been playing around with Sega Dreamcast discs lately. Not playing the games on the DC discs, of course, just studying their structure. To review, the Sega Dreamcast game console used special optical discs named GD-ROMs, where the GD stands for “gigadisc”. They are capable of holding about 1 gigabyte of data.

  You know what’s weird about these discs ? Each one manages to actually store a gigabyte of data. Each disc has a CD portion and a GD portion. The CD portion occupies the first 45000 sectors and can be read in any standard CD drive. This area is divided between a brief data track and a brief (usually) audio track.

  The GD region starts at sector 45000. Sometimes, it’s just one humongous data track that consumes the entire GD region. More often, however, the data track is split between the first track and the last track in the region and there are 1 or more audio tracks in between. But the weird thing is, the GD region is always full. I made a study of it (click for a larger, interactive graph) :

  
  
  

  Some discs put special data or audio bonuses in the CD region for players to discover. But every disc manages to fill out the GD region. I checked up on a lot of those audio tracks that divide the GD data and they’re legitimate music tracks. So what’s the motivation ? Why would the data track be split in 2 pieces like that ?

  I eventually realized that I probably answered this question in this blog post from 4 years ago. The read speed from the outside of an optical disc is higher than the inside of the same disc. When I inspect the outer data tracks of some of these discs, sure enough, there seem to be timing-sensitive multimedia FMV files living on the outer stretches.

  One day, I’ll write a utility to take apart the split ISO-9660 filesystem offset from a weird sector.

• seeking mp4 with audiostream inaccurate in mobile safari

  23 février 2015, par dubbelj

  I have an issue with seeking an MP4 videofile on iOS (ipad). We use currentTime to skip through the video. This is working very well on most browsers, and also for h264 files without an audiostream on iOS. When I add an audiostream to the mp4 file however, the seeking becomes very inaccurate. This leads to all sorts of issues as the application bases a lot around timing.

  So we definitely concluded that a file that is encoded (using ffmpeg) with the exact same encoding settings but without audio is working well.

  These are the ffmpeg parameters I’m using when encoding with audio (added breaks for readability) :

  ffmpeg
  
  -i source.mov 
  
  -force_key_frames 00:00:04.840,00:00:04.880,00:00:04.920,00:00:04.960,00:00:05.000,00:00:05.040,00:00:05.080,00:00:14.760,00:00:14.800,00:00:14.840,00:00:14.880,00:00:14.920,00:00:14.960,00:00:15.000,00:00:24.760,00:00:24.800,00:00:24.840,00:00:24.880,00:00:24.920,00:00:24.960,00:00:25.000,00:00:31.720,00:00:31.760,00:00:31.800,00:00:31.840,00:00:31.880,00:00:31.920,00:00:31.960
  
  -c:a libvo_aacenc
  
  -b:a 128k 
  
  -ar 44100 
  
  -c:v libx264 
  
  -preset slow 
  
  -crf 21 
  
  -g 25 
  
  -r 25 
  
  -maxrate 2000k 
  
  -bufsize 2000k 
  
  -pix_fmt yuv420p 
  
  -movflags +faststart 
  
  -tune zerolatency 
  
  -vstats 
  
  encoded.mp4

  Things I’ve tried :

  • all available aac and mp3 codecs
  • different audio and video bitrates
  • different audio frequencies
  • with without zerolatency faststart flags
  • maximum amount of keyframes in file
  • settings keyframes only at (and around) the area where I need to skip to

  Any ideas anyone ?

• How can I do videos into a better format without losing resolution and quality ?

  3 juillet 2020, par Melissa

  I have the job of developing a video converter program. I started to do a lot of research and development on this subject. I currently have a problem like this. The videos of the institution where I developed the converter are in mxf 50 megabit 4 : 2 : 2 format.
They want me to turn these videos into a better format without losing resolution and quality. For example, they said I could try h.265. In my work, the resolution and the quality of the video change in direct proportion when the format changes, how can I overcome this with an algorithm ? As I have just learned about the FFMPEG library, my view may be limited in this area. I am waiting for your help.