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• Grand Unified Theory of Compact Disc

  1er février 2013, par Multimedia Mike — General

  This is something I started writing about a decade ago (and I almost certainly have some of it wrong), back when compact discs still had a fair amount of relevance. Back around 2002, after a few years investigating multimedia technology, I took an interest in compact discs of all sorts. Even though there may seem to be a wide range of CD types, I generally found that they’re all fundamentally the same. I thought I would finally publishing something, incomplete though it may be.

  Physical Perspective
  There are a lot of ways to look at a compact disc. First, there’s the physical format, where a laser detects where pits/grooves have disturbed the smooth surface (a.k.a. lands). A lot of technical descriptions claim that these lands and pits on a CD correspond to ones and zeros. That’s not actually true, but you have to decide what level of abstraction you care about, and that abstraction is good enough if you only care about the discs from a software perspective.

  Grand Unified Theory (Software Perspective)
  Looking at a disc from a software perspective, I have generally found it useful to view a CD as a combination of a 2 main components :

  • table of contents (TOC)
  • a long string of sectors, each of which is 2352 bytes long

  I like to believe that’s pretty much all there is to it. All of the information on a CD is stored as a string of sectors that might be chopped up into a series of anywhere from 1-99 individual tracks. The exact sector locations where these individual tracks begin are defined in the TOC.

  Audio CDs (CD-DA / Red Book)
  The initial purpose for the compact disc was to store digital audio. The strange sector size of 2352 bytes is an artifact of this original charter. “CD quality audio”, as any multimedia nerd knows, is formally defined as stereo PCM samples that are each 16 bits wide and played at a frequency of 44100 Hz.
  

  (44100 audio frames / 1 second) * (2 samples / audio frame) * 
    (16 bits / 1 sample) * (1 byte / 8 bits) = 176,400 bytes / second
  (176,400 bytes / 1 second) / (2352 bytes / 1 sector) = 75
  

  75 is the number of sectors required to store a single second of CD-quality audio. A single sector stores 1/75th of a second, or a ‘frame’ of audio (though I think ‘frame’ gets tossed around at all levels when describing CD formats).

  The term “red book” is thrown around in relation to audio CDs. There is a series of rainbow books that define various optical disc standards and the red book describes audio CDs.

  Basic Data CD-ROMs (Mode 1 / Yellow Book)
  Somewhere along the line, someone decided that general digital information could be stored on these discs. Hence, the CD-ROM was born. The standard model above still applies– TOC and string of 2352-byte sectors. However, it’s generally only useful to have a single track on a CD-ROM. Thus, the TOC only lists a single track. That single track can easily span the entire disc (something that would be unusual for a typical audio CD).

  While the model is mostly the same, the most notable difference between and audio CD and a plain CD-ROM is that, while each sector is 2352 bytes long, only 2048 bytes are used to store actual data payload. The remaining bytes are used for synchronization and additional error detection/correction.

  At least, the foregoing is true for mode 1 / form 1 CD-ROMs (which are the most common). “Mode 1″ CD-ROMs are defined by a publication called the yellow book. There is also mode 1 / form 2. This forgoes the additional error detection and correction afforded by form 1 and dedicates 2336 of the 2352 sector bytes to the data payload.

  CD-ROM XA (Mode 2 / Green Book)
  From a software perspective, these are similar to mode 1 CD-ROMs. There are also 2 forms here. The first form gives a 2048-byte data payload while the second form yields a 2324-byte data payload.

  Video CD (VCD / White Book)
  These are CD-ROM XA discs that carry MPEG-1 video and audio data.

  Photo CD (Beige Book)
  This is something I have never personally dealt with. But it’s supposed to conform to the CD-ROM XA standard and probably fits into my model. It seems to date back to early in the CD-ROM era when CDs were particularly cost prohibitive.

  Multisession CDs (Blue Book)
  Okay, I admit that this confuses me a bit. Multisession discs allow a user to burn multiple sessions to a single recordable disc. I.e., burn a lump of data, then burn another lump at a later time, and the final result will look like all the lumps were recorded as the same big lump. I remember this being incredibly useful and cost effective back when recordable CDs cost around US$10 each (vs. being able to buy a spindle of 100 CD-Rs for US$10 or less now). Studying the cdrom.h file for the Linux OS, I found a system call named CDROMMULTISESSION that returns the sector address of the start of the last session. If I were to hypothesize about how to make this fit into my model, I might guess that the TOC has some hint that the disc was recorded in multisession (which needs to be decided up front) and the CDROMMULTISESSION call is made to find the last session. Or it could be that a disc read initialization operation always leads off with the CDROMMULTISESSION query in order to determine this.

  I suppose I could figure out how to create a multisession disc with modern software, or possibly dig up a multisession disc from 15+ years ago, and then figure out how it should be read.

  CD-i
  This type puzzles my as well. I do have some CD-i discs and I thought that I could read them just fine (the last time I looked, which was many years ago). But my research for this blog post has me thinking that I might not have been seeing the entire picture when I first studied my CD-i samples. I was able to see some of the data, but sources indicate that only proper CD-i hardware is able to see all of the data on the disc (apparently, the TOC doesn’t show all of the sectors on disc).

  Hybrid CDs (Data + Audio)
  At some point, it became a notable selling point for an audio CD to have a data track with bonus features. Even more common (particularly in the early era of CD-ROMs) were computer and console games that used the first track of a disc for all the game code and assets and the remaining tracks for beautifully rendered game audio that could also be enjoyed outside the game. Same model : TOC points to the various tracks and also makes notes about which ones are data and which are audio.

  There seems to be 2 distinct things described above. One type is the mixed mode CD which generally has the data in the first track and the audio in tracks 2..n. Then there is the enhanced CD, which apparently used multisession recording and put the data at the end. I think that the reasoning for this is that most audio CD player hardware would only read tracks from the first session and would have no way to see the data track. This was a positive thing. By contrast, when placing a mixed-mode CD into an audio player, the data track would be rendered as nonsense noise.

  Subchannels
  There’s at least one small detail that my model ignores : subchannels. CDs can encode bits of data in subchannels in sectors. This is used for things like CD-Text and CD-G. I may need to revisit this.

  In Summary
  There’s still a lot of ground to cover, like how those sectors might be formatted to show something useful (e.g., filesystems), and how the model applies to other types of optical discs. Sounds like something for another post.

• FFmpeg and video4linux2 parameters - how to capture still images faster ?

  13 août 2021, par mcgregor94086

  Problem Summary

  


  I have built an 18-camera array of USB webcams, attached to a Raspberry Pi 400 as the controller. My Python 3.8 code for capturing an image from each webcam is slow, and I am trying to find ways to speed it up.

  


  The FFMPEG and video4linux2 command line options are confusing to me, so I'm not sure if the delays are due to my poor choice of parameters, and a better set of options would solve the problem.

  


  The Goal

  


  I am trying to capture one image from each camera as quickly as possible.

  


  I am using FFMPEG and video4linux2 command line options to capture each image within a loop of all the cameras as shown below.

  


  Expected results

  


  I just want a single frame from each camera. The frame rate is 30 fps, so I was expecting that capture time would be on the order of 1/30th to 1/10th of a second worst case. But the performance timer is telling me that each capture is taking 2-3 seconds.

  


  Additionally, I don't really understand the ffmpeg output, but this output worries me :

  


  frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    1 fps=0.5 q=8.3 Lsize=N/A time=00:00:00.06 bitrate=N/A speed=0.0318x    
video:149kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing  


  


  I don't understand why the "frame=" line is repeated 4 times. And in the 4th repitition, the fps says 0.5, which I would interpret as one frame every 2 seconds not the 30FPS that I specified.

  


  Specific Questions :

  


  Can anyone explain to me what this ffmpeg output means, and why it is taking 2 seconds per image captured, and not closer to 1/30th of a second ?

  


  Can anyone explain to me how to capture the images in less time per capture ?

  


  should I be spawning a separate thread for each ffmpeg call, so they run asynchronously, instead of serially ? Or would that not really save time in practice ?

  


  Actual results

  


    Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 6004.168748, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj422p(pc, bt470bg/unknown/unknown), 1920x1080, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> mjpeg (native))
Press [q] to stop, [?] for help
Output #0, image2, to '/tmp/video1.jpg':
  Metadata:
    encoder         : Lavf58.20.100
    Stream #0:0: Video: mjpeg, yuvj422p(pc), 1920x1080, q=2-31, 200 kb/s, 30 fps, 30 tbn, 30 tbc
    Metadata:
      encoder         : Lavc58.35.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    1 fps=0.5 q=8.3 Lsize=N/A time=00:00:00.06 bitrate=N/A speed=0.0318x    
video:149kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

Captured /dev/video0 image in: 3 seconds
Input #0, video4linux2,v4l2, from '/dev/video2':
  Duration: N/A, start: 6007.240871, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj422p(pc, bt470bg/unknown/unknown), 1920x1080, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> mjpeg (native))
Press [q] to stop, [?] for help
Output #0, image2, to '/tmp/video2.jpg':
  Metadata:
    encoder         : Lavf58.20.100
    Stream #0:0: Video: mjpeg, yuvj422p(pc), 1920x1080, q=2-31, 200 kb/s, 30 fps, 30 tbn, 30 tbc
    Metadata:
      encoder         : Lavc58.35.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    0 fps=0.0 q=0.0 size=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    1 fps=0.5 q=8.3 Lsize=N/A time=00:00:00.06 bitrate=N/A speed=0.0318x    
video:133kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

Captured /dev/video2 image in: 3 seconds
...


  


  The code :

  


  list_of_camera_ids = ["/dev/video1","/dev/video2", "/dev/video3", "/dev/video4",
                      "/dev/video5","/dev/video6", "/dev/video7", "/dev/video8",
                      "/dev/video9","/dev/video10", "/dev/video11", "/dev/video12",
                      "/dev/video13","/dev/video14", "/dev/video15", "/dev/video16",
                      "/dev/video17","/dev/video18"
                     ]
for this_camera_id in list_of_camera_ids:
    full_image_file_name = '/tmp/' + os.path.basename(this_camera_id) + 'jpg'
    image_capture_tic = time.perf_counter()
    
    run_cmd = subprocess.run([
                              '/usr/bin/ffmpeg', '-y', '-hide_banner',
                              '-f', 'video4linux2',
                              '-input_format',  'mjpeg',
                              '-framerate', '30',
                              '-i', this_camera_id,
                              '-frames', '1',
                              '-f', 'image2',
                              full_image_file_name
                             ],
                             universal_newlines=True,
                             stdout=subprocess.PIPE,
                             stderr=subprocess.PIPE
                            )  
         print(run_cmd.stderr)
         image_capture_toc = time.perf_counter()       
         print(f"Captured {camera_id} image in: {image_capture_toc - image_capture_tic:0.0f} seconds")


  


  ADDITIONAL DATA :
In response to an answer by Mark Setchell that said more information is needed to answer this question, I now elaborate the requested information here :

  


  cameras : Cameras are USB-3 cameras that identify themselves as :

  


  idVendor           0x0bda Realtek Semiconductor Corp.
idProduct          0x5829 


  


  I tried to add the lengthy lsusb dump for one of the cameras but then this post exceeds the 30000 character limit

  


  How the cameras are attached : USB 3 port of Pi to a master USB-3 7-port hub, with 3 spur 7 port hubs (not all ports in the spur hubs are occupied).

  


  Camera resolution : HD Format 1920x1080

  


  Why am I setting a frame rate if I only want 1 image ?

  


  I set a frame rate which seems odd given that that specifies the time between frames, but you only want a single frame. I did that because I don't know how to get a single image from FFMPEG. This was the one example of FFMPEG command options that I found discussed on the web that I could get to capture a single image successfully. I've discovered innumerable sets of options that don't work ! I wrote this post because my web searches did not yield an example that works for me. I am hoping that someone much better informed than I am will show me a way that works !

  


  Why am I scanning the cameras sequentially rather than in parallel ?

  


  I did this just to keep things simple first and a loop over the list seemed easy and pythonic. It was clear to me that I might later be able to spawn a separate thread for each FFMPEG call, and maybe get a parallel speed up that way. Indeed, I would welcome an example of how to do that.

  


  But in any case the single image capture taking 3 seconds seems way too long anyway.

  


  Why am I only using a single 1 of the 4 cores on your Raspberry Pi ?

  


  The sample code I posted is just a snippet from my entire program. Image capturing takes place in a child thread at present, while a Window GUI with an event loop is running in the main thread, so that user input isn't blocked during imaging.

  


  I am not knowledgeable enough about the cores of the Raspberry Pi 400, nor about how the Raspberry Pi OS (aka Raspbian) manages allocation of threads to cores, nor whether Python can or should be explicitly directing threads to be running in specific cores.
  
I would welcome the suggestions of Mark Setchell (or anyone else knowledgeable about these issues) to recommend a best practice and include example code.

  


• Detect rotation angle and rotate final video using ffmpeg version 2.8

  9 novembre 2017, par Hemant Kumar

  I used to work on ffmpeg 2.2 until now and was detecting rotation angle of video uploaded from android / iPhone mobiles and rotate the resulting video so that it run perfectly on a correct angle.

  But since I have updated the ffmpeg to version 2.8 I am getting this rotation problem. My queries are not rotating the videos as they were earlier.

  Here’r the commands I was using :

  To check rotation angle :

  ffprobe -of json -show_streams {$input} | grep rotate

  below is my final command to convert a video to mp4

  "ffmpeg -i {$input} -strict -2 -vcodec libx264 -preset slow -vb 500k -maxrate 500k -bufsize 1000k -vf 'scale=-1:480 ".fix_video_orientation($input)."' -threads 0 -ab 64k -s {$resolution}  -movflags faststart -metadata:s:v:0 rotate=0 {$output}";

  "fix_video_orientation" function is given below. It detect the angle of rotation of the initial video and output optimal option for rotating the final video.

  function fix_video_orientation($input){
  
  
  
  $return= ", transpose=1 ";
  
  
  
  $dd= exec("ffprobe -of json -show_streams  {$input}   | grep rotate");
  
  
  
  if(!empty($dd)){
  
  
  
  $dd=explode(":",$dd);
  
  $rotate=str_replace(",","",str_replace('"',"",$dd[1]));
  
  
  
  if($rotate=="90")return $return;
  
  
  
  else if ($rotate=="180") return ", transpose=2,transpose=2 ";
  
  
  
  else if($rotate == "270") return ", transpose=2 ";
  
  }

  Currently above script is supporting "flv","avi","mp4","mkv","mpg","wmv","asf","webm","mov","3gp","3gpp" extensions, also the script is supporting the resulting .mp4 file to play on all browsers and devices.

  Now the ffprobe command is not returning rotation angle and so a portrait video if uploaded from iphone is showing as landscape on the website.

  Output console of ffprobe command :

  ffprobe version N-77455-g4707497 Copyright (c) 2007-2015 the FFmpeg developers
  
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
  
  configuration: --extra-libs=-ldl --prefix=/opt/ffmpeg --mandir=/usr/share/man --enable-avresample --disable-debug --enable-nonfree --enable-gpl --enable-version3 --enable-libopencore-amrnb --enable-libopencore-amrwb --disable-decoder=amrnb --disable-decoder=amrwb --enable-libpulse --enable-libdcadec --enable-libfreetype --enable-libx264 --enable-libx265 --enable-libfdk-aac --enable-libvorbis --enable-libmp3lame --enable-libopus --enable-libvpx --enable-libspeex --enable-libass --enable-avisynth --enable-libsoxr --enable-libxvid --enable-libvo-aacenc --enable-libvidstab
  
  libavutil 55. 11.100 / 55. 11.100
  
  libavcodec 57. 20.100 / 57. 20.100
  
  libavformat 57. 20.100 / 57. 20.100
  
  libavdevice 57. 0.100 / 57. 0.100
  
  libavfilter 6. 21.101 / 6. 21.101
  
  libavresample 3. 0. 0 / 3. 0. 0
  
  libswscale 4. 0.100 / 4. 0.100
  
  libswresample 2. 0.101 / 2. 0.101
  
  libpostproc 54. 0.100 / 54. 0.100
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/standard/PORTRAIT.m4v':
  
  Metadata:
  
  major_brand : qt
  
  
  
  minor_version : 0
  
  compatible_brands: qt
  
  
  
  creation_time : 2016-02-03 05:25:18
  
  com.apple.quicktime.make: Apple
  
  com.apple.quicktime.model: iPhone 4S
  
  com.apple.quicktime.software: 9.2.1
  
  com.apple.quicktime.creationdate: 2016-02-03T10:52:11+0530
  
  Duration: 00:00:03.34, start: 0.000000, bitrate: 7910 kb/s
  
  Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 63 kb/s (default)
  
  Metadata:
  
  creation_time : 2016-02-03 05:25:18
  
  handler_name : Core Media Data Handler
  
  Stream #0:1(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, bt709), 720x1280 [SAR 1:1 DAR 9:16], 7832 kb/s, 29.97 fps, 29.97 tbr, 600 tbn, 50 tbc (default)
  
  Metadata:
  
  creation_time : 2016-02-03 05:25:18
  
  handler_name : Core Media Data Handler
  
  encoder : H.264
  
  Stream #0:2(und): Data: none (mebx / 0x7862656D), 0 kb/s (default)
  
  Metadata:
  
  creation_time : 2016-02-03 05:25:18
  
  handler_name : Core Media Data Handler
  
  Stream #0:3(und): Data: none (mebx / 0x7862656D), 0 kb/s (default)
  
  Metadata:
  
  creation_time : 2016-02-03 05:25:18
  
  handler_name : Core Media Data Handler
  
  Unsupported codec with id 0 for input stream 2
  
  Unsupported codec with id 0 for input stream 3

  If latest version of ffmpeg (2.8) is used to auto rotate the video, can you please suggest me what option I need to add or remove from my final command.