Two pass or one pass render in Vegas? at DVinfo.net

Nick Stone · November 26th, 2008, 12:54 PM

I have a project that is 40 mins and will fit on a standard 4gig DVD. I'm wondering if I would get better quality doing one pass @ constant bit rate 8.5 vrs two pass with variable bit rate render down to Mpeg-2.
If storage on the disk was an issue two pass is better.... am I correct or does two pass extract more that one pass? I'm a bit confused

Does anyone have more knowledge on this subject than myself.

Thanks
Nick

John Cline · November 26th, 2008, 02:54 PM

For a 40-minute program, rendering 8Mbit CBR will produce the highest quality you can get. In fact, if you're feeling lucky, you could go higher than this. The total bitrate of the audio and video added together can go as high as 9.8Mbps and remain within DVD specs. You could concievably go as high as 9.6Mbps video and 192Kbps .AC3 audio but that may cause some compatibility issues with certain blank DVD media and some players.

2-pass is only valid with VBR and is only useful when you're trying to fit more than about 72 minutes of material on a single 4.7GB DVD and you have to use a lower bitrate to fit the entire program on the disc.

Let's say you're trying to fit 2 hours on a disc, this would require a bitrate of about 4.7Mbits/sec. If you used a CBR of 4.7Mbps, then the high motion areas would be starved for bits and display compression artifacts and the low motion areas would have too many bits and would be wasted. VBR intelligently allocates the bits to maintain video quality. The first pass of a 2-pass VBR reads the program and calculates how to allocate the available bits during the second pass. It allocates more bits to high motion sections and fewer bits to low motion areas in order to maintain the average bitrate which you have set. In the case of the two-hour video, you could set the max to 8Mbps, the average to 4.7Mbps and the lower limit to 2Mbps. High motion sections could get as high as 8Mbps and low motion areas would get as little as 2Mbps thus making the video look much better than it would if you had just used a CBR of 4.7Mbps.

Sam Houchins II · November 26th, 2008, 06:37 PM

excellent explanation... thanks, I've wondered too. I'll be able to properly use it to my advantage now.

John Cline · November 26th, 2008, 10:44 PM

Here is a link to a really great DVD bitrate calculator application:

www.johncline.com/bitcalc110.zip

Nick Stone · November 27th, 2008, 02:43 PM

Thanks for your help John you nailed it for me.
I'm rendering one of my last projects in Vegas (moving to Final cut) and it's giving me a hard time for the first time ever, rendering keeps stopping at 45%. Maybe Vegas knows i'm jummping ship?
I've had nothing but the best times with Vegas apart from this last render.

Thanks

November 26th, 2008, 12:54 PM	#1
Nick Stone Regular Crew Join Date: May 2008 Location: Australia Posts: 133	Two pass or one pass render in Vegas? I have a project that is 40 mins and will fit on a standard 4gig DVD. I'm wondering if I would get better quality doing one pass @ constant bit rate 8.5 vrs two pass with variable bit rate render down to Mpeg-2. If storage on the disk was an issue two pass is better.... am I correct or does two pass extract more that one pass? I'm a bit confused Does anyone have more knowledge on this subject than myself. Thanks Nick

November 26th, 2008, 02:54 PM	#2
John Cline Major Player Join Date: Oct 2002 Location: Albuquerque, New Mexico USA Posts: 333	For a 40-minute program, rendering 8Mbit CBR will produce the highest quality you can get. In fact, if you're feeling lucky, you could go higher than this. The total bitrate of the audio and video added together can go as high as 9.8Mbps and remain within DVD specs. You could concievably go as high as 9.6Mbps video and 192Kbps .AC3 audio but that may cause some compatibility issues with certain blank DVD media and some players. 2-pass is only valid with VBR and is only useful when you're trying to fit more than about 72 minutes of material on a single 4.7GB DVD and you have to use a lower bitrate to fit the entire program on the disc. Let's say you're trying to fit 2 hours on a disc, this would require a bitrate of about 4.7Mbits/sec. If you used a CBR of 4.7Mbps, then the high motion areas would be starved for bits and display compression artifacts and the low motion areas would have too many bits and would be wasted. VBR intelligently allocates the bits to maintain video quality. The first pass of a 2-pass VBR reads the program and calculates how to allocate the available bits during the second pass. It allocates more bits to high motion sections and fewer bits to low motion areas in order to maintain the average bitrate which you have set. In the case of the two-hour video, you could set the max to 8Mbps, the average to 4.7Mbps and the lower limit to 2Mbps. High motion sections could get as high as 8Mbps and low motion areas would get as little as 2Mbps thus making the video look much better than it would if you had just used a CBR of 4.7Mbps. Last edited by John Cline; November 26th, 2008 at 10:19 PM.

November 26th, 2008, 06:37 PM	#3
Sam Houchins II Major Player Join Date: Jun 2003 Location: Fredericksburg, VA Posts: 282	excellent explanation... thanks, I've wondered too. I'll be able to properly use it to my advantage now.

November 26th, 2008, 10:44 PM	#4
John Cline Major Player Join Date: Oct 2002 Location: Albuquerque, New Mexico USA Posts: 333	Here is a link to a really great DVD bitrate calculator application: www.johncline.com/bitcalc110.zip

November 27th, 2008, 02:43 PM	#5
Nick Stone Regular Crew Join Date: May 2008 Location: Australia Posts: 133	Thanks for your help John you nailed it for me. I'm rendering one of my last projects in Vegas (moving to Final cut) and it's giving me a hard time for the first time ever, rendering keeps stopping at 45%. Maybe Vegas knows i'm jummping ship? I've had nothing but the best times with Vegas apart from this last render. Thanks