245 vs 250 MB/s encoding rate @ 25fps

[macuser]

Hello everyone !

At the moment I am working at the pre-process of a film festival, checking and inspecting all sorts of DCP's.

A few days ago I had a discussion with a company that produces and delivers DCP's. The topic is about encoding rates, some DCP's the company delivers gave hints with 'dcp_inspect' saying that the bit rate is above the recommandations 250 MB/s instead of 245 MB/s @25fps.

I confronted them with that and their answer was that the filmmaker can see the difference between 250 MB/sand 245 MB/s so they deliver the 25fps always with 250 MB/s only to avoid discussions with the filmmakers....

I would like to read your comments about this - is there any visible difference ? What are your points to this ? What would you answer to the producer ? What would you say to the filmmaker ?

Kind regards
Max

[carl]

Others will have more direct experience of this than me, but I would be extremely surprised if anybody can see the difference between 245MB/s and 250MB/s.

[Carsten]

No one can see that difference, not with JPEG2000 codec, and certainly not with any other codec.

A still very popular DCI compliant server crashes towards 250 MBit/s, even well below, depending on content. It is NOT a good idea to approach that limit. I have never seen commercial DCPs challenging it, and for a good reason. Explicit HFR DCPs are another story.


- Carsten

[tfordmcc]

Hello and thank you in advance for your help! I have submitted a DCP file to a film festival with a Mbit/s rate of 250; they emailed and said there are a couple of glitches in viewing the file, and could I slow the rate to somewhere in 230-240 range.

If I change it in the existing file, can I then resend with an update Mbit rate, or do I need to re-do the file completely as a new file?

Thank you!

Tim

[macuser]

Hello Tim,

because you do an encoding with a slower bitrate you need to send a complete new DCP. The encoding with a slower bitrate will create a new and smaller (in size) video. There is no other way than this.

Kind regards
Max

[tfordmcc]

Thanks so much!! One other question, I know there have been some conversations about differences in viewing between 250 and 245. I have reset my Mbit/s to be 235 to be safe in my mind for the screening. Do you think there will be a significant change of quality in the viewing? Thanks in advance!

Tim

[Carsten]

No. Even in a direct A/B comparison, you will not notice the difference.

I have seen glitches even at 220MBits with some Doremi servers. Commercial mastering studios avoid to approach these limits.




- Carsten

[tfordmcc]

Thank you for the additional information, greatly appreciate it!