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• avcodec_decode_audio4 decoded data different on device and simulator

  5 décembre 2013, par Jona

  I'm having issues where the decoded audio sounds punchy/garbled simply can't tell what is being heard when decoded on an iPhone/iPad device. However, when decoding the audio on simulator the audio is perfect.

  I do realize emulator runs on a different CPU architecture i386 while actual device is ARMv7 so the issue could be something around that.

  I have printed the compressed data on device and simulator before being decoded and it matches. Then I printed the decoded data for device and simulator and they are different. That leads me to think something is happening with the decoder. I can see that it is not a byte swap issue since the data is just completely different.

  Any ideas on what I could or look into ? I don't have this issue on Android...

  First decoded audio frame.

  DEVICE :

  0x00, 0x00, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x70, 0x08, 0x00, 0x00, 0x00, 0x00, 0x03, 0x00, 0xda, 0x0a, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,

  EMULATOR :

  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,

  ---

  UPDATE 1
  I was able to resolve the issue by disabling asm with --disable-asm. Something must not be working correctly with the ARM asm files.

• Add a 2nd audio stream using ffmpeg so you can switch audio streams on iOS ?

  26 décembre 2019, par Tony M

  I have an m.mp4 with English audio and want to add a second audio stream in Italian so that I can play either language using the iPhone. When I used this command :

  ffmpeg -i m.mp4 -i ita.mp3 -c copy -map 0 -map 1 out.mp4 

  The output from ffmpeg -i out.mp4 (see below) shows that a new audio stream is added, and I can switch between audio streams using the VLC player. Yet when I transfer the video to the TV app and play it on either MacOS or iOS it only plays the original English and I see no options to play other languages.

  How do I get it to be able to switch between either audio on iOS ?

  Here is the output from ffmpeg -i out.mp4:

      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 400x300 [SAR 1:1 DAR 4:3], q=2-31, 482 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 24k tbc (default)
  
      Metadata:
  
        handler_name    : VideoHandler
  
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 102 kb/s (default)
  
      Metadata:
  
        handler_name    : SoundHandler
  
      Stream #0:2: Audio: mp3 (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 152 kb/s
  
      Metadata:
  
        encoder         : LAME3.100
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (copy)
  
    Stream #0:1 -> #0:1 (copy)
  
    Stream #1:0 -> #0:2 (copy)

• How do I calculate optimal dimensions and bitrate for displaying a video on an iPhone ?

  6 février 2020, par wachutu

  I’m currently developing a mobile app that will have a library of 2-5 minute videos (approx 100 in total) and going through the process of determining which versions of the videos to have ready to serve to different mobile devices. In my research, I have noticed that there is a lot of room to play with video settings such as dimensions and bitrate.

  As a first test, I am attempting to find the minimum video size I can deliver to an iPhone XS with dimensions 1125x2436 without losing any noticeable quality. I started by scaling the video to 1125x2436 and creating versions with 5 different bitrates ranging from 500kbps-4400kbps. I noticed that at 1500kbps, the video looks great and the size is cut 1/3 so that was a good start.

  Then after doing some reading, I saw that in adaptive bitrate scenarios Apple recommends delivering video of lower bitrate AND lower resolution. So in my next test I just cut both in half - scaled to 562x1218 and bitrate at 750kbps and noticed the video also looked great on the iPhone. So 1125x2436 at 750kbps looks bad, but 562x1218 at 750kbps looks great on the same device. To some extent this makes sense to me as you need less bits to fill a smaller screen but what I’m not understanding is how the scaling plays a factor. Shouldn’t it essentially pixelate because the resolution is 1/2 of the iPhone dimensions ? And at a higher level, is there a somewhat concrete way to figure out this optimal resolution / bitrate balance given the dimensions of a device ? We want to most modern smartphones (iPhone 6 and later, Samsung Galaxy, etc.) so we need to be prepared for a range of dimensions (aspect ratios 9:16 or 6:13).