Transcoder drops significantly during subtitles

[FordGuy61]

Burning in subtitles involves the CPU, even when using hardware accelerated transcoding.

See quote below from a post in the Synology section of the forum. Synology NAS also use VAAPI for hardware accelerated transcoding.

Other posts mention that burning subtitles is single-threaded, which would also impact overall transcoding speed.

@ChuckPa

Intel never gave us the ability to burn in the images with the hardware. Theoretically, it can do it but no software exists to do it yet. Maybe someday? It will take someone at Intel or like that to write such software.

@trumpy81

Unsupported subtitles must be burned into the video stream and that requires the CPU to process the subtitle in software, regardless of whether or not the video is being processed in hardware.

Subtitle handling is also a single threaded process, meaning only a small portion of the CPU’s processing power can be utilised for this process, approx. 25% to be precise.