8 years after buying a Raspberry Pi Model 1B, I finally bit the bullet and invested in an upgrade, this time to the 8GB model of the Raspberry Pi 4 Model B (at the time on sale from Okdo for £62). This was largely because I wanted it to do video transcoding using ffmpeg. More specifically, I wanted to concatenate some H.264 files together and compress them at the same time.
Having installed DietPi as my distro of choice, my first attempt at this went something like:
ffmpeg -f concat -i /root/tapo/batch.txt -c:v h264 -c:a aac output.mp4
The problem with this is that it’s very slow (even when using -profile ultrafast). The CPU in the 4B, while much better than the 1B, is just not capable enough to encode H264 video very quickly. In particular, I needed it to transcode at a rate greater than 1x (i.e. to transcode more than one second of video per second) – a target it was missing by some way. I was just about to despair when I realised that the 4B supports hardware acceleration for H264, as set out in this excellent blog post by Will Usher.
Suitably enthused, but expecting something bright green, I switched to h264_v4l2m2m:
ffmpeg -f concat -i /root/tapo/batch.txt -c:v h264_v4l2m2m -c:a aac output.mp4
and… it didn’t work:
[h264_v4l2m2m @ 0x55913ffa00] Could not find a valid device
To cut a (very) long story short, the probem I was having was with DietPi and its reduced driver set. In particular, bcm2835-v4l2 is hidden behind a config option (“RPI Camera” under “Display options”). Without bcm2835-v4l2, h264_v4l2m2m can’t access the necessary video devices. To fix this problem, simply enable RPI Camera and reboot.
Oh, and I still had to build my own version of ffmpeg, as Will did, but he’d done the hard detective work there.
