Combining a short video and long audio file efficiently using Vegas or FFMPEG I have a need to repeatedly create videos that have a short 15 second intro that fades to black and have really long audio tracks like 8 hours. I generate the video in Vegas and the audio in Cubase. The desired result is to have the video and audio start together and the video's last frame (completely black) be repeated to match the audio length. I can do this easily in Magix Vegas 16 Pro but the performance is awful. It takes 2+ hours to say render 15 seconds of actual changing video and then another 8 hours of black frames. This is on a 10850K with an RTX 2070 rendering using the GPU as it is faster in Vegas than using the Intel CPU renderer. I thought it was somehow choking on the audio so I eliminated it and just the video rendering of all of the black frames is still 2 hours. Any thoughts on how to speed this up in Vegas would be greatly appreciated. I feel like FFMPEG could be a viable alternative. I had some promising results with render time using ffmpeg -i Input_Video.mp4 -i Input_audio.mp3 -c:v libx265 -c:a copy -map 0:v:0 -map 1:a:0 Combined_Output.mp4 .  This completes the task in under a minute BUT the resulting video is not as it seems. I can play the video from beginning to end in a media player and it looks fine. However, if I watch for a second and fast forward beyond those first 15 seconds, I don't see the black screen. It still shows whatever frame I went from. I examined the video and it sort of appears like the final video result it just the frames of the input video for 15 seconds with no encoded video after that. I need to have just black video after the first 15 seconds playing along with the audio. The black frames need to be encoded as video all the way until the audio ends. Any thoughts on this?
Have you tried using the ffmpeg playlist feature dubbed concatenate? The fade to black with ffmpeg has been described here.