It’s been a while since my last post and that’s not because I abandoned silkopter, but because I’ve been very busy working on it.
During my latest test 3 weeks ago I noticed that the video link is not as stable as I wanted. I’m using a system similar to wifibroadcast – wifi cards in monitor mode doing packet injection. They are working on 2.4Ghz and the area where I’m flying seems to have this band a bit crowded, causing the video link to be glitchy.
On the other hand my 433Mhz RC link has been pretty solid. So inspired by this I started thinking if I could do the same thing: send video through my RC link.
I already had the RF4463F30 transceivers which are using the si4463 chip and I knew them well (and hated them a lot) so I thought to give it a try.
Now – the max bandwidth the si4463 supports is limited to 1mbps which might seem like enough for 640×480 resolution but there is a lot of overhead that effectively limit this to around 755kbps. Since I want some FEC – say 5/7 coding – I end up with 500kbps target video bitrate.
I made an xls to ease the calculations. It allows me to specify the air bitrate, packet sizes, packet overhead, video bitrate, fps, preamble/sync sized, header sizes etc and calculates all kind of nice info, including if the link is viable or not.
Here is a screenshot with my current calculations:
For the video link, 500kbps seems like enough as there’s not a lot of movement in a quad, specially with a gimbal. To further improve quality and decrease latency I’m experimenting with the x264 software compressor library instead of the GPU compressor in the PI. It seems to be able to compress real-time a 640×480 video with good quality, zero latency setting and 2 cores only.
I still have to compare the quality between the 2 (GPU and x264) though.