[FFmpeg-user] Causes of color bars?

[Simon Roberts]

Hi all,

I've been working with a Decklink Quad HDMI card under Ubuntu 20.04. The
kernel driver is loaded and the Blackmagic software "mediaexpress" displays
the input signal perfectly. However, when I try to use ffplay, it seems
like it knows which of the four channels on the card has a signal, because
it drops out with "can't determine format" on the other three, but when I
ask to view the valid input signal, I simply get color bars.

I just built this version of ffmpeg with the decklink support, and it ran
without any evident errors. I've done this before on another machine that
didn't have a strong enough CPU for the concurrent capture I want to do,
and everything worked properly.

So, my question is, what possible causes of color bars are there? I know
that one is "if the input signal is lost", but it's claiming that it has a
signal and it correctly identifies it as 1080 60p. I'm wondering if the
bars are from the Decklink drivers, and not actually anything to do with
ffmpeg/ffplay. How could I determine that?

This is the log output:

$ ffplay -f decklink -i 'DeckLink Quad HDMI Recorder (1)'
ffplay version N-98470-g7772666 Copyright (c) 2003-2020 the FFmpeg
developers
  built with gcc 9 (Ubuntu 9.3.0-10ubuntu2)
  configuration: --prefix=/home/simon/ffmpeg_build
--pkg-config-flags=--static
--extra-cflags='-I/home/simon/ffmpeg_build/include
-I/home/simon/ffmpeg_sources/BMD_SDK/include'
--extra-ldflags=-L/home/simon/ffmpeg_build/lib --extra-libs='-lpthread -lm'
--bindir=/home/simon/bin --enable-gpl --enable-libass --enable-libfdk-aac
--enable-libfreetype --enable-libmp3lame --enable-libopus
--enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265
--enable-nonfree --enable-decklink
  libavutil      56. 55.100 / 56. 55.100
  libavcodec     58. 95.100 / 58. 95.100
  libavformat    58. 48.100 / 58. 48.100
  libavdevice    58. 11.101 / 58. 11.101
  libavfilter     7. 87.100 /  7. 87.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
[decklink @ 0x7fb628000bc0] Autodetected the input mode  0B f=0/0
[decklink @ 0x7fb628000bc0] Found Decklink mode 1920 x 1080 with rate 59.94
[decklink @ 0x7fb628000bc0] Frame received (#1) - No input signal detected
- Frames dropped 1
Input #0, decklink, from 'DeckLink Quad HDMI Recorder (1)': f=0/0
  Duration: N/A, start: 0.000000, bitrate: 1990203 kb/s
    Stream #0:0: Audio: pcm_s16le, 48000 Hz, 2 channels, s16, 1536 kb/s
    Stream #0:1: Video: rawvideo (UYVY / 0x59565955), uyvy422(progressive),
1920x1080, 1988667 kb/s, 59.94 tbr, 1000k tbn, 1000k tbc
   1.23 A-V: -1.453 fd=  62 aq=  261KB vq=16200KB sq=    0B f=0/0

Thanks for any help!