Ripbot Cq18 bitrate low

[silverfoxxxx]

Hi,
I have been using ripbot to back up some of my blu rays, i have been using the CQ 18 setting and have recently noticed on my last few rips that the bitarate is showing as under 5000 Kbps for the video stream, i would think this is far to low for a 1080 rip but was under the impression that using the CQ option it would select the best bitrate for the content.

Example Resident Evil Afterlife 4528 Kbps @ 1920 x 800, file size is nice at 3.56 gb but i dont want to be losing out on to much quality when streaming it round the house.

Am i missing something in the settings, i haven't changed any other settings that i know of, or has x264 been updated to deliver better compression and i didn't notice the update.

Thanks

[aedipuss]

cq is short for constant quality. vbr as needed per scene. 18 sounds low enough but 5k for blu-ray isn't. you need around 8k for normal movies in 1080p. it also depends on the source material, with low quality source video there isn't enough bitrate to start with.

try 2 pass encoding at a bitrate around 8k and see if it works better.

[redwudz]

What codec are you using, audio and video? And how does the quality look compared to the original?

CQ 18 would be a bit of overkill on some videos. I use 19.5 as a compromise, but 20 should give you very close to the original quality.

But most times for BD backups I use MKV two pass with 640Kbps AC3 and set the output size to 8150MB to fit on DL media for backups. But every video compresses differently. Some Anime using a CQ of 19.5 will end up ~1GB, but it looks very close to the BD original. Other videos with a lot of action and lots of contrast may take a lot more bitrate to achieve the same quality, along with a larger file size.

But others here may be able to give you better information.

And welcome to our forums.

[silverfoxxxx]

I am using ripbot which only uses x264 as far as i know, and sound is the same as you 640Kbps ac3. The two recent backups are resident evil as above and the new harry potter which came out at a bitrate for the video of 5676Kbps for the 1920 x 800 rip, using CQ 18, which again to me seemed to low a bit rate for the movie.

I dont know that much about encoding but a quick google seemed to have blu ray rips around 9000Kbps as an average. I know i can set my own bitrate but thought the CQ option was supposed to be the best way to go, it may be choosing the correct bitrate and that is all these films need but google is currently telling me this may not be correct.

[wulf109]

Dumb question,using CQ encoding is the length of the movie a factor? Resident Evil is about 1:30 and the HP is about 2:15.

[silverfoxxxx]

The length of the movie will effect the overall size of the output file but I wouldn't think it would effect the bitrate at all.

[jagabo]

Every video is different and requires different amounts of bitrate to maintain quality. CQ encodes encode everything at the same quality. If the bitrate turns out very low that's because higher bitrates weren't needed to achieve that quality.

Look at the examples in this post:
https://forum.videohelp.com/threads/295672-A-problem-for-video-experts?p=1811057&viewfu...=1#post1811057
One video requires 20 times the bitrate of the other. Yes, they're Xvid, not h.264, but the idea is the same.

[timekills]

You're all (mostly) correct except CQ does not mean Constant "Quality"; it is Constant "Quantizer". It refers to P-frame compression, i.e. the amount of them instead of I and B frames. P frames have some compression as they are based on previous frames (yes, B frames are also based on adjacent I frames but they are forward based as well). The CQ refers as to just how adjacent they are h.264 can use multiple preceding frames to creat a P-frame). One of the reasons a rule of thumb for DVD (MPEG2) is to use no lower than CQ19 because then you are actually replacing P-Frames. I haven't seen a similar rule for blu-ray.

I personally prefer CRF for x264 encoding systems as it is more advanced thar CQ, striking a balance between complex and less complex frames.

[jagabo]

CQ can mean constant quality or constant quantizer depending on the context. In x264 CRF and QP are both constant quality modes. QP is constant quality in a mathematical sense. CRF is constant in a visual sense (taking into account what's visible or not to the human eye). In QP mode the requested quantizer is used for all P frames. I frames use a lower quantizer (higher quality) and B frame use a higher quantizer (lower quality), depending on the ipratio and pbratio settings. Neither has any effect on the relative frequency of I, P, and B frames or the number of reference frames used for B and P frames.

One of the reasons a rule of thumb for DVD (MPEG2) is to use no lower than CQ19 because then you are actually replacing P-Frames.

I have no idea what you mean by that. When you are converting MPEG 2 to x264 the type of the source frame has nothing to do with the type of frame used by x264. The encoder simply sees a stream of uncompressed video frames.