Why would the bits/pixel drop?

[capman21]

I took a raw dv file and converted it to xvid, and the bits/pixel were 0.660 and bitrate was 6458 kb. The video was dark and had a lot of noise in it so I took and used the brightness/contrast filter and a noise filter, but than it droped down to 0.375 bits/pixel and 3665 kb bitrate. Why? Was it because I was not using the raw file, but a file that had already been converted?

[guns1inger]

Noise needs a high bitrate because of the amount of change between frames. Remove the noise, and the bitrate required reduces, sometimes significantly.

[capman21]

Man, what dont you know This is a bad thing isnt it, the bitrate dropping? BTW, thanks for the info.

[guns1inger]

If you are doing quality based encoding and the bitrate drops because of noise reduction then it is usually not an issue. Quality based encoding (also called Constant Quality) automatically assigned bitrate based on the need to keep the image quality at the selected level. If you reduce image noise sufficiently then the encoder may decide it no longer needs such a high bitrate, and will drop it of it's own accord. This is not a bad thing if the image quality remains the same.

[capman21]

Yeah I am using 1 pass quantizer and quantizer 2. I got one more question and it may seem to be a stupid one. When my vids are dark or have a lot of noise in them I try to adjust them brightness/contrast, noise filter, etc using virtualdub filters. How can you tell when you have adjusted them the "right" way. Is it all about what you consider "right" when the vid is to your statisfaction?

[guns1inger]

Noise reduction is always difficult to judge. Leave too much and the video is messy. Remove too much and the video looks plasticy and artificial. I prefer to use avisynth and either Convolution3d or Peachsmoother. They are usually faster than virtualdub filters and a lot more configurable.

The best solution is to shoot with more light so you don't have the problem in the first place. The other thing to remember is that TV circuitry will remove/hide a certain amount of noise as well, so you don't have to take everything out with filters.

It is also worth having a look at Colormill for virtualdub for your brightness issues, as it does a lot.

[capman21]

You hit it exactly the way I was trying to describe noise filters " Leave too much and the video is messy. Remove too much and the video looks plasticy and artificial." I will take your advice and try those others that you recommened. Thanks again for the info.

[guns1inger]

https://forum.videohelp.com/viewtopic.php?t=309181 is a great discussion on the pros and cons of filtering, with lots of examples, suggestions, and methods using different filters for virtualdub and avisynth. If you are serious about noise removal, this is a great read to get you started.

[capman21]

Thanks I will definitely read it.

[ronnylov]

I took a raw dv file and converted it to xvid, and the bits/pixel were 0.660 and bitrate was 6458 kb. The video was dark and had a lot of noise in it so I took and used the brightness/contrast filter and a noise filter, but than it droped down to 0.375 bits/pixel and 3665 kb bitrate. Why? Was it because I was not using the raw file, but a file that had already been converted?

It is always better using the original source if possible compared to reencode an already converted file. Open your raw file again and add the filters you want before saving it to xvid. As already mentioned noise is difficult for the encoder so a noisy clip requires higher bitrate to keep the same quality. The encoding to a lossy format like xvid is by iself removing noise so that is also a part of the explanation of why the reencoded clip requires less bitrate to keep the quality of an already lossy source video.