Recherche avancée

Sur d’autres sites (7066)

• 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 C++ encoder fails with hardware acceleration quick sync intel

  28 août 2022, par teals

  I am trying to debug an issue I have with a simple C++ encoder using FFMmpeg. The following code below works correctly on these other hardware acceleration systems/platforms :

  


  

  • Mac/VideoToolbox,

  


  • Cuda/NVEnc

  


  • Raspberry pi/h264_v4l2m2m.

  


  


  It fails on intel quick sync at avcodec_open2. I'm using Ubuntu 22.04. I built FFMpeg from source with the Intel Media SDK installed and configured. I verified that the QuickSync install works. I also have a corresponding decoder implementation that works correctly using intel quick sync. So I know everything properly installed. I am also running this on an 11th Gen Intel CPU.

  


  [hevc_qsv @ 0x564b0d34f0c0] Low power mode is unsupported (This sometimes shows up)
[hevc_qsv @ 0x563691234000] some encoding parameters are not supported by the QSV runtime. Please double check the input parameters.
Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height


  


  This is the only error that I get and I haven't found any helpful resources. I tried playing around with different parameters but nothing seems to work. Any help or insight would be extremely helpful.

  


  bool VideoEncoder::create() {&#xA;    char const* outfile = filePath.c_str();&#xA;&#xA;     // open output format context&#xA;    int ret = avformat_alloc_output_context2(&amp;ofctx, nullptr, nullptr, outfile);&#xA;    if (ret &lt; 0) {&#xA;        std::cerr &lt;&lt; "fail to avformat_alloc_output_context2(" &lt;&lt; outfile &lt;&lt; "): ret=" &lt;&lt; ret;&#xA;        return false;&#xA;    }&#xA;&#xA;    // create new video stream&#xA;    const AVCodec* codec = avcodec_find_encoder_by_name(encoderName.c_str());&#xA;    vstrm = avformat_new_stream(ofctx, codec);&#xA;    if (!vstrm) {&#xA;        std::cerr &lt;&lt; "fail to avformat_new_stream";&#xA;        return false;&#xA;    }&#xA;&#xA;    // open video encoder&#xA;    cctx = avcodec_alloc_context3(codec);&#xA;    if (!vstrm) {&#xA;        std::cerr &lt;&lt; "fail to avcodec_alloc_context3";&#xA;        return false;&#xA;    }&#xA;    const AVRational dst_fps = {fps, 1};&#xA;    cctx->width = width;&#xA;    cctx->height = height;&#xA;    if(pixel_format != AV_PIX_FMT_NONE) {&#xA;        cctx->pix_fmt = pixel_format;&#xA;    }&#xA;    else {&#xA;        cctx->pix_fmt = codec->pix_fmts[0];&#xA;    }&#xA;    cctx->time_base = av_inv_q(dst_fps);&#xA;    cctx->framerate = dst_fps;&#xA;    cctx->bit_rate = bitrate * 1000000;&#xA;&#xA;&#xA;    AVDictionary* options = nullptr;&#xA;    if(gpu_id >= 0) {&#xA;        av_dict_set_int(&amp;options, "gpu", gpu_id, 0);&#xA;    }&#xA;&#xA;    &#xA;    if (ofctx->oformat->flags &amp; AVFMT_GLOBALHEADER)&#xA;        cctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;&#xA;    ret = avcodec_open2(cctx, codec, &amp;options);&#xA;    if (ret &lt; 0) {&#xA;        std::cerr &lt;&lt; "fail to avcodec_open2: ret=" &lt;&lt; ret;&#xA;        return false;&#xA;    }&#xA;    avcodec_parameters_from_context(vstrm->codecpar, cctx);&#xA;&#xA;    //For mac/quicktime we need to add hvc1 tag.&#xA;    if (encoderName.find("hevc") != std::string::npos) {&#xA;        vstrm->codecpar->codec_tag = MKTAG(&#x27;h&#x27;, &#x27;v&#x27;, &#x27;c&#x27;, &#x27;1&#x27;);&#xA;    }&#xA;&#xA;    /*&#xA;    std::cout&#xA;        &lt;&lt; "outfile: " &lt;&lt; outfile &lt;&lt; "\n"&#xA;        &lt;&lt; "format:  " &lt;&lt; ofctx->oformat->name &lt;&lt; "\n"&#xA;        &lt;&lt; "vcodec:  " &lt;&lt; codec->name &lt;&lt; "\n"&#xA;        &lt;&lt; "size:    " &lt;&lt; width &lt;&lt; &#x27;x&#x27; &lt;&lt; height &lt;&lt; "\n"&#xA;        &lt;&lt; "fps:     " &lt;&lt; av_q2d(cctx->framerate) &lt;&lt; "\n"&#xA;        &lt;&lt; "pixfmt:  " &lt;&lt; av_get_pix_fmt_name(cctx->pix_fmt) &lt;&lt; "\n"&#xA;        &lt;&lt; std::flush;*/&#xA;&#xA;    // initialize sample scaler&#xA;    swsCtx = sws_getContext(&#xA;        width, height, AV_PIX_FMT_BGR24,&#xA;        width, height, cctx->pix_fmt,&#xA;        SWS_BILINEAR, nullptr, nullptr, nullptr);&#xA;    if (!swsCtx) {&#xA;        std::cerr &lt;&lt; "fail to sws_getContext";&#xA;        return false;&#xA;    }&#xA;&#xA;    // allocate frame buffer for encoding&#xA;    frame = av_frame_alloc();&#xA;    frame->width = width;&#xA;    frame->height = height;&#xA;    frame->format = static_cast<int>(cctx->pix_fmt);&#xA;    ret = av_frame_get_buffer(frame, 32);&#xA;    if (ret &lt; 0) {&#xA;        std::cerr &lt;&lt; "fail to av_frame_get_buffer: ret=" &lt;&lt; ret;&#xA;        return false;&#xA;    }&#xA;&#xA;    // allocate packet to retrive encoded frame&#xA;    pkt = av_packet_alloc();&#xA;    // open output IO context&#xA;    ret = avio_open2(&amp;ofctx->pb, outfile, AVIO_FLAG_WRITE, nullptr, nullptr);&#xA;    if (ret &lt; 0) {&#xA;        std::cerr &lt;&lt; "fail to avio_open2: ret=" &lt;&lt; ret;&#xA;        return false;&#xA;    }&#xA;&#xA;    // write media container header (if any)&#xA;    ret = avformat_write_header(ofctx, nullptr);&#xA;    if (ret &lt; 0) {&#xA;        std::cerr &lt;&lt; "fail to avformat_write_header: ret=" &lt;&lt; ret;&#xA;        return false;&#xA;    }&#xA;&#xA;    return true;&#xA;}&#xA;</int>

  


• Trolls in trouble

  6 juin 2013, par Mans — Law and liberty

  Life as a patent troll is hopefully set to get more difficult. In a memo describing patent trolls as a “drain on the American economy,” the White House this week outlined a number of steps it is taking to stem this evil tide. Chiming in, the Chief Judge of the Court of Appeals for the Federal Circuit (where patent cases are heard) in a New York Times op-ed laments the toll patent trolling is taking on the industry, and urges judges to use powers already at their disposal to make the practice less attractive. However, while certainly a step in the right direction, these measures all fail to address the more fundamental properties of the patent system allowing trolls to exist in the first place.

  System and method for patent trolling

  Most patent trolling operations comprise the same basic elements :

  1. One or more patents with broad claims.
  2. The patents of (1) acquired by an otherwise non-practising entity (troll).
  3. The entity of (2) filing numerous lawsuits alleging infringement of the patents of (1).
  4. The lawsuits of (3) targeting end users or retailers.
  5. The lawsuits of (3) listing as plaintiffs difficult to trace shell companies.

  The recent legislative actions all take aim at the latter entries in this list. In so doing, they will no doubt cripple the trolls, but the trolls will remain alive, ready to resume their wicked ways once a new loophole is found in the system.

  To kill a patent troll

  As Judge Rader and his co-authors point out in the New York Times, “the problem stems largely from the fact that, [...] trolls have an important strategic advantage over their adversaries : they don’t make anything.” This is the heart of the troll, and this is where the blow should be struck. Our weapon shall be the mightiest judicial sword of all, the Constitution.

  The United States Constitution contains (in Article I, Section 8) the foundation for the patent system (emphasis mine) :

  The Congress shall have Power [...] To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.

  Patent trolls are typically not inventors. They are merely hoarders of other people’s discarded inventions, and that allowing others to reap the benefits of an inventor’s work would somehow promote progress should be a tough argument. Indeed, it is the dissociation between investment and reward which has allowed the patent trolls to rise and prosper.

  In light of the above, the solution to the troll menace is actually strikingly simple : make patents non-transferable.

  Having the inventor retain the rights to his or her inventions (works for hire still being recognised), would render the establishment of non-practising entities, which most trolls are, virtually impossible. The original purpose of patents, to protect the investment of inventors, would remain unaffected, if not strengthened, by such a change.

  Links

  • Taking on Patent Trolls to Protect American Innovation
  • Fact Sheet : White House Task Force on High-Tech Patent Issues
  • Make Patent Trolls Pay in Court
  • Constitution of the United States