Bit-rate for burnt-in subtitles

[Anders M Olsson]

Let's say that I have an original DCP that's been encoded at 100 Mbps, and I want to add burnt-in subtitles.

Would it be a good idea to set the encoding bit-rate to, say, 150 or 200 Mbps? The logic being that any re-encoding will cause some degrading of image quality, and a higher bit-rate should minimize that degrading. Also, the subtitles will add some complexity to the images, possibly making a higher bit-rate desirable.

Will the difference in bit-rates within the new DCP be likely to cause any trouble at projection? The frames without subtitles are kept unmodified at the lower bit-rate, while any frames with subtitles will have been re-encoded at the higher bit-rate.

[scorpio81]

Well, DOM won't let you create OV DCP with differing bitrates. It will reencode the whole DCP with new bitrate. In the end you'll have two independent DCPs taking very much disk space.
If you want to keep original frames unmodified, then you need to create supplementary DCP (VF) where only subtitled frames are reencoded with new bitrate. But this way is much complicated because you'll have to extract frames to be subtitled and then inserted into original DCP as reels. So third party software will be needed. And I don't know if DOM can create VF with insert reels. As for bitrate for subtitled frames, I think 120-125 Mbit/s is enough and perhaps most servers will play this VF DCP as long as other settings (SMPTE or Interop standard, frame size, etc.) are the same as in the original DCP.

[Anders M Olsson]

Well, DOM won't let you create OV DCP with differing bitrates. It will reencode the whole DCP with new bitrate. In the end you'll have two independent DCPs taking very much disk space.

My experience is that only frames with subtitles will be re-encoded. Frames without subtitles will just be copied as is. I think this has been hinted at, or explained, in some other thread at this board, even though I can't find it right now.

Unless of course you change anything that makes it necessary to re-encode the entire film, for example crop it or change the resolution.

A clue might be in the caption: "JPEG2000 bandwidth for newly-encoded data".

[carl]

Will the difference in bit-rates within the new DCP be likely to cause any trouble at projection? The frames without subtitles are kept unmodified at the lower bit-rate, while any frames with subtitles will have been re-encoded at the higher bit-rate.

I can't see why it would cause any trouble at projection, but you never know. I'm not sure if anyone has tested it.

[carl]

Well, DOM won't let you create OV DCP with differing bitrates. It will reencode the whole DCP with new bitrate.

Not in my tests: it will re-use existing frames if nothing has changed, no matter what the bitrate. Are you seeing something different?

This does need addressing at some point: mostly it's useful, but it may be a little surprising if you make a new DCP from an old one and change the bitrate setting: the output will not have that changed bitrate.

[scorpio81]

Thanks for clarification, Carl.
Now I have another question, slightly off-topic. Why DOM doesn't use previously encoded and unencrypted frames to make encrypted DCP? I must assume encryption is done together with encoding. Am I right?