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• Understanding the VP8 Token Tree

  7 juin 2010, par Multimedia Mike — VP8

  I got tripped up on another part of the VP8 decoding process today. So I drew a picture to help myself understand it. Then I went back and read David Conrad’s comment on my last post regarding my difficulty understanding the VP8 spec and saw that he ran into the same problem. Since we both experienced the same hindrance in trying to sort out this matter, I thought I may as well publish the picture I drew.

  VP8 defines various trees for decoding different syntax elements. There is one tree for decoding the tokens and it is expressed in the VP8 spec as such :

  PLAIN TEXT

  C :

  1. const tree_index coef_tree [2 * (num_dct_tokens - 1)] =
  2. {
  3.  -dct_eob, 2,        /* eob = "0"  */
  4.   -DCT_0, 4,        /* 0  = "10" */
  5.   -DCT_1, 6,        /* 1  = "110" */
  6.    8, 12,
  7.    -DCT_2, 10,      /* 2  = "11100" */
  8.     -DCT_3, -DCT_4,    /* 3  = "111010", 4 = "111011" */
  9.    14, 16,
  10.     -dct_cat1, -dct_cat2, /* cat1 = "111100", cat2 = "111101" */
  11.    18, 20,
  12.     -dct_cat3, -dct_cat4, /* cat3 = "1111100", cat4 = "1111101" */
  13.     -dct_cat5, -dct_cat6 /* cat4 = "1111110", cat4 = "1111111" */
  14. } ;

  Here is what the table looks like when you make a tree out of it (click for full size image) :

  
  
  

  The catch is that it makes no sense for an end-of-block (EOB) token to follow a 0 token since EOB already indicates that the remainder of the coefficients should be 0 anyway. Thus, the spec states that, "decoding of certain DCT coefficients may skip the first branch, whose preceding coefficient is a DCT_0." I confess, I didn’t understand what "skip the first branch" meant until I drew the tree.

  
  
  

  For those wondering why it might be sub-optimal (clarity-wise) for a spec to simply regurgitate vast chunks of C code, this makes a decent case. As you can see, the spec makes certain assumptions about how a binary tree should be organized in a static array (node n points to elements n*2 and n*2+1 as its branches ; leaves are either negative or 0). This is the second method I have seen ; another piece of code (not the VP8 spec) had the nodes in the first half of the array and pointed to leaves in the second half. There must be other arrangements.

• apedec : do not buffer decoded samples over AVPackets

  27 août 2013, par Rafaël Carré

  apedec : do not buffer decoded samples over AVPackets
  Only consume an AVPacket when all the samples have been read.
  When the rate of samples output is limited (by the default value
  
  of max_samples), consuming the first packet immediately will cause
  
  timing problems :
   The first packet with PTS 0 will output 4608 samples and be
  
  consumed entirely
  
   The second packet with PTS 64 will output the remaining samples
  
  (typically, a lot, that’s why max_samples exist) until the decoded
  
  samples of the first packet have been exhausted, at which point the
  
  samples of the second packet will be decoded and output when
  
  av_decode_frame is called with the next packet).
  That means there’s a PTS jump since the first packet is ’decoded’
  
  immediately, which can be seen with avplay or mplayer : the timing
  
  jumps immediately to 6.2s (which is the size of a packet).
  Sample : http://streams.videolan.org/issues/6348/Goldwave-MAClib.ape
  
  Signed-off-by : Justin Ruggles <justin.ruggles@gmail.com>
  

  • [DBH] libavcodec/apedec.c

• How do I make audio work with pulseaudio in systemd ? [migrated]

  1er avril 2022, par user5827693

  I'm trying to live stream my website to the rtmp server via pulseaudio, ffmpeg and chromium in ubuntu 20.04. Everything works great if I execute the following commands in shell :

  


  pulseaudio&
ffmpeg -y -f x11grab -draw_mouse 0 -s 1280x720 -i :1.0+0,0 -f pulse -ac 2 -i default -c:v libx264 -c:a aac -b:a 128k -f flv MY_SERVER &> /dev/null < /dev/null &
DISPLAY=:1.0 chromium-browser --autoplay-policy=no-user-gesture-required --disable-gpu --user-data-dir=/tmp/test --window-position=0,0 --window-size=1280,720 --app=MY_URL &


  


  However if I try to setup the same logic via systemd - everything works fine, except there is no audio. Below are my systemd files.

  


  pulseaudio.service

  


  [Unit]
Description=PulseAudio system server

[Service]
User=ubuntu
Type=notify
ExecStart=/usr/bin/pulseaudio --daemonize=no
Restart=always

[Install]
WantedBy=multi-user.target


  


  chromium.service

  


  [Unit]
Description=start chromium browser
After=pulseaudio.service

[Service]
User=ubuntu
Type=simple
Environment=DISPLAY=:1.0
ExecStart=/usr/bin/chromium-browser --autoplay-policy=no-user-gesture-required --disable-gpu --user-data-dir=/tmp/test --window-position=0,0 --window-size=1280,720 --app=MY_URL
Restart=always

[Install]
WantedBy=multi-user.target


  


  ffmpeg.service

  


  [Unit]
Description=start ffmpeg
Requires=display.service pulseaudio.service
After=pulseaudio.service

[Service]
User=ubuntu
Type=simple
ExecStart=/usr/bin/ffmpeg -y -f x11grab -draw_mouse 0 -s 1280x720 -i :1.0+0,0 -f pulse -ac 2 -i default -c:v libx264 -c:a aac -b:a 128k -f flv MY_SERVER
Restart=always

[Install]
WantedBy=multi-user.target


  


  I do not see any errors in my syslog regarding the audio. I can't figure out why it would work from shell but not from systemd.

  


  I tried the following :

  


  

  • Ran pulseaudio systemwide

  


  • Tried to setup systemd under root user

  


  • Tried to setup systemd under non root user

  


  • Tried all possible pulseaudio params