2-Pass VBR H.264 Encoding Question

rtbond wrote on 4/18/2006, 5:24 AM

Folks,

I'm new to using Vegas for 2-pass VBR encoding.

When I transcode Windows Media Video v9 HD (1440x1080) material to H.264/AVC (1280x720) using a 2-pass encoding operation the resulting file size of the transcoded media is effectively double that of the single pass encoding, with all other encoding parameters (e.g., Ave/Max. bit rates, frame size, frame rates) the same. I tested this twice to verify reproducibility.

Any ideas what is going on here? I assumed that a 2-pass encoding would produce either a more efficiently encoded video stream for approximately the same video quality or a slightly improved video quality at around the same encoding efficiency (file size). This, however, is not what is happening.

Thanks!

--Rob

Comments

Former user wrote on 4/18/2006, 6:12 AM

2 pass encoding will not affect the file size. That only affects the quality of the rendering by better anticipating the amount of compression needed for each frame.

Bitrate is the biggest factor in file size. IF you are using VBR, then the bitrate will vary, so you cannot be sure you are achieving the same bitrate as the WMV file. Do you know what your MAX and MIN bitrate settings are?

Dave T2

rtbond wrote on 4/19/2006, 5:00 AM

Dave,

>2 pass encoding will not affect the file size.

Do you mean to say it "should not" rather than "will not", as I have an existance proof that in Vegas the 2-pass VBR encoding using my custom template is doubling the file size.

I'm doing 1280x720 VBR encoding with an averge bit rate of 10 Mbps, Max of 14 Mbps. As stated on my original post, making only a single change to the customer template (i.e., checking the 2-pass box) doubles the size of the final rendered file.

I'm going to submit this as a bug to Tech Support.

Thanks for your input Dave.

--Rob