Hi All
There has been a lot of postings regarding Constant and Variable frame rates.
So which is best.
As far as I can tell Constant came first, then Variable was developed to allow us to burn more video to a disc without jeopardising quality. This varied the bit rate according to movement/action.
This saved on file size allowing more footage.
I only want to burn 60 minutes of video.
At Constant 8000 kbps my 60 minutes will fit on the disc at full quality.
If I use Variable 8000 kbps it should burn at 8000 kbps for the whole video because the file will fit the disc. But does it? If it does I might as well use constant.
If it varies the bit rate lower than 8000 kbps then those sections may suffer in quality.(may be not-I don’t know).
When I have a video 90 minutes long I use 6000 kbps constant the file will fit the disc.
If I were to use Variable what bit rate should I choose?
Selecting 6000kbps variable means that the average rate is lower than 6000 say 5000.
At the moment I use Constant Bit Rate for videos up to 90 minutes long.
Will I see any benefit by using Variable?
Regards
Trevor
Constant V variable bit rate
Moderator: Ken Berry
Dear Trevor,
I think you'll find that the "variable" part of the variable bitrate in VS is quite restricted. There was some discussion of this in previous postings, and you can actually enable more parameter settings by editing the .ini files. When you do this, however, there is a great confusion of options - so it's not surprising that Ulead kept things simple. It's a shame you don't have a variability slider control, though, to widen the range of variation.
I notice that commercial DVD's use maximum bitrates right up to the DVD-VR compliant 9800 kbps limit. In order for them to fit onto the discs, though, the average bitrate must be significantly lower than you would get by using a max 9800 VBR setting in VS.
In any case, from my own empirical tests, I believe variable bitrate does give better video quality, especially if you also opt for two-pass encoding. Intuitively, it makes sense that this would occur since the bitrate is increased for faster changing sections of video, and reduced in slower changing parts.
I've never worried about the reduced quality in those lower bitrate sections - after all, in the extreme, a video would become an image clip, with no change from one frame to the next, and could be compressed with little quality loss.
On the other hand, I've noticed aliasing of rapid moving sections of video (eg dogs' tails!) when using constant bitrate is reduced by using variable bitrate resulting in a similar file size. Two-pass VBR with max 9800 kbps is quite good!
I think you'll find that the "variable" part of the variable bitrate in VS is quite restricted. There was some discussion of this in previous postings, and you can actually enable more parameter settings by editing the .ini files. When you do this, however, there is a great confusion of options - so it's not surprising that Ulead kept things simple. It's a shame you don't have a variability slider control, though, to widen the range of variation.
I notice that commercial DVD's use maximum bitrates right up to the DVD-VR compliant 9800 kbps limit. In order for them to fit onto the discs, though, the average bitrate must be significantly lower than you would get by using a max 9800 VBR setting in VS.
In any case, from my own empirical tests, I believe variable bitrate does give better video quality, especially if you also opt for two-pass encoding. Intuitively, it makes sense that this would occur since the bitrate is increased for faster changing sections of video, and reduced in slower changing parts.
I've never worried about the reduced quality in those lower bitrate sections - after all, in the extreme, a video would become an image clip, with no change from one frame to the next, and could be compressed with little quality loss.
On the other hand, I've noticed aliasing of rapid moving sections of video (eg dogs' tails!) when using constant bitrate is reduced by using variable bitrate resulting in a similar file size. Two-pass VBR with max 9800 kbps is quite good!
JVC GR-DV3000u Panasonic FZ8 VS 7SE Basic - X2
Hi Trevor,
personally I'ver never noticed any difference in video quality between CBR & VBR. Granted, I've never dropped below the 6000 bit rate either. I've always subscribed to the thinking that 2Dogs was discussing, using VBR. As with VBR, the bit rate would increase where the video demands were higher, faster motion, etc.
Going to the old bit rate calculator at videohelp.com, using a DVD5 media which is the norm, single sideded and single layer disc, a 90 minute video using an audio bit rate of 224 kbit/s, the calculated bit rate is 6548 kbits.
I guess another factor would be how comfortable you are with allowing the encoder make the decision to apply bits as needed. With VBR the encoder is deciding what frames are high demand and add bits to those frames accordingly, borrowing bits from low or no demand frames.
Regrads,
Erock
personally I'ver never noticed any difference in video quality between CBR & VBR. Granted, I've never dropped below the 6000 bit rate either. I've always subscribed to the thinking that 2Dogs was discussing, using VBR. As with VBR, the bit rate would increase where the video demands were higher, faster motion, etc.
Going to the old bit rate calculator at videohelp.com, using a DVD5 media which is the norm, single sideded and single layer disc, a 90 minute video using an audio bit rate of 224 kbit/s, the calculated bit rate is 6548 kbits.
I guess another factor would be how comfortable you are with allowing the encoder make the decision to apply bits as needed. With VBR the encoder is deciding what frames are high demand and add bits to those frames accordingly, borrowing bits from low or no demand frames.
Regrads,
Erock
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This is from http://www.aidinc.com/features/dvdfaq.asp
CBR stands for "Constant Bit Rate", while VBR stands for "Variable Bit Rate". Bit rate is a measurement of how much your MPEG-2 video is compressed. DVD video (not including audio or overhead) can be anywhere from 1.5Mbps (Megabits per second) to 9.2Mbps. When performing a CBR encode, all video is encoded to MPEG-2 at the same (constant) bitrate. When performing a VBR encode, the encoder intelligently varies the bitrate up and down as appropriate to the given video segment. VBR is more efficient, because different video segments are more or less complex than other segments. By using a low bitrate on some, the encoder can then using a higher bitrate on others. The idea is to only lower the bitrate when the video is easy enough to encode that you won’t notice the higher compression. As the compressionist, you define how much the encoder is allowed to vary the bitrate (typically by entering a minimum, maximum, and average bitrate).
In theory, you might think you should always use VBR because of its higher efficiency. In practice, however, encoders can make mistakes when they determine how much bitrate is needed for a given scene. If they guess to low, they can create ugly artifacts. For this reason, professional encoders offer features such as 2-pass VBR encoding and segment re-encoding to help reduce these errors. But professional encoders are expensive and 2-passes takes twice as long.
A good encoder using CBR will look great on shorter programs (up to about an hour). At some point, you will decide it is worth it to use VBR. There is no precise threshold for when to invoke VBR, but most programs that runs more than 70 to 90 minutes will benefit greatly from it.
Hope this helps, despite some repetitiveness from the previous posts.
Ron[/url]
CBR stands for "Constant Bit Rate", while VBR stands for "Variable Bit Rate". Bit rate is a measurement of how much your MPEG-2 video is compressed. DVD video (not including audio or overhead) can be anywhere from 1.5Mbps (Megabits per second) to 9.2Mbps. When performing a CBR encode, all video is encoded to MPEG-2 at the same (constant) bitrate. When performing a VBR encode, the encoder intelligently varies the bitrate up and down as appropriate to the given video segment. VBR is more efficient, because different video segments are more or less complex than other segments. By using a low bitrate on some, the encoder can then using a higher bitrate on others. The idea is to only lower the bitrate when the video is easy enough to encode that you won’t notice the higher compression. As the compressionist, you define how much the encoder is allowed to vary the bitrate (typically by entering a minimum, maximum, and average bitrate).
In theory, you might think you should always use VBR because of its higher efficiency. In practice, however, encoders can make mistakes when they determine how much bitrate is needed for a given scene. If they guess to low, they can create ugly artifacts. For this reason, professional encoders offer features such as 2-pass VBR encoding and segment re-encoding to help reduce these errors. But professional encoders are expensive and 2-passes takes twice as long.
A good encoder using CBR will look great on shorter programs (up to about an hour). At some point, you will decide it is worth it to use VBR. There is no precise threshold for when to invoke VBR, but most programs that runs more than 70 to 90 minutes will benefit greatly from it.
Hope this helps, despite some repetitiveness from the previous posts.
Ron[/url]