VBR with WMV9 Advanced Profile

[tominator]

Hi
Im currently encoding CBR WMV9 Main profile 1500kbit/s (192kbit/s audio) and 800kbit/s (192kbit/s audio). I use 3000ms buffer.

Im thinking about changing to VBR WMV9 Advanced Profile.

What kind of settings should I be using to get the same results as with what Im doing now?

Maybe constraint VBR? Then I need to know how to work out what peak bitrate I should be using and what maximum buffer time to use.

All help is appreciated. Thanks.

[zambelli]

Im currently encoding CBR WMV9 Main profile 1500kbit/s (192kbit/s audio) and 800kbit/s (192kbit/s audio). I use 3000ms buffer.

Im thinking about changing to VBR WMV9 Advanced Profile.

What kind of settings should I be using to get the same results as with what Im doing now?

Maybe constraint VBR? Then I need to know how to work out what peak bitrate I should be using and what maximum buffer time to use.

Is this for streaming over a network or playing from an optical disc media? Do you have bandwidth constraints?

At 1500kbps and a 3000ms buffer, your buffer size never exceeds 4500 kbits. To meet the same type of buffer constraint in VBR mode, you'd have to set your peak bitrate * peak buffer to a combination that doesn't exceed 4500 kbits.

Rule of thumb is: if you are only constrained by bandwidth but not by file size, use CBR mode. If you are constrained by both bandwidth and file size, use peak limited VBR mode.

[tominator]

Is this for streaming over a network or playing from an optical disc media? Do you have bandwidth constraints?

At 1500kbps and a 3000ms buffer, your buffer size never exceeds 4500 kbits. To meet the same type of buffer constraint in VBR mode, you'd have to set your peak bitrate * peak buffer to a combination that doesn't exceed 4500 kbits.

Rule of thumb is: if you are only constrained by bandwidth but not by file size, use CBR mode. If you are constrained by both bandwidth and file size, use peak limited VBR mode.

Okay thanks.

Im encoding for a video on demand-service so both bandwidth and file size are of essence.

Why is it that I dont want to use any other type of VBR? Should I use VBR for the audio too?

Ive also read a few specifications from a few rather large companies saying they want either VBR VC-1 1600kbit/s with 4000kbit/s peak or 2500kbit/s CBR.

Could you please explain the logics of that above? From what you said in your last post I find that specification very strange.

[zambelli]

Why is it that I dont want to use any other type of VBR? Should I use VBR for the audio too?
Regular 2pass VBR doesn't allow you to set buffer/bitrate constraints. While that's OK for local file playback, it's not well suited for streaming or VoD over IPTV.

It's best to use the same mode for audio that you do for video, but make sure the total (peak) bitrate doesn't exceed your bandwidth requirements.

Ive also read a few specifications from a few rather large companies saying they want either VBR VC-1 1600kbit/s with 4000kbit/s peak or 2500kbit/s CBR.
Could you please explain the logics of that above? From what you said in your last post I find that specification very strange.
That is somewhat strange. With CBR and a short buffer (i.e. 3 secs) the bitrate is not likely to exceed the target bitrate at any point in time by more than 20% or so. The two recommendations they have don't seem to apply to the same scenario.

The first thing to look at is your peak bitrate limitation. If the bandwidth (network, optical disc read rate, etc.) cannot exceed a certain bitrate, than the peak bitrate should be set slightly below that number. The second thing to look at is the maximum total file size permitted. This determines your average bitrate needed. If the average bitrate and peak bitrate are close, then CBR makes most sense. If they're more than, say, 25% apart, it might make more sense to do 2p constrained VBR.

[tominator]

Thanks a whole lot. Youve really helped me.

Two more things though:

1. The only real difference, or the main difference, between CBR and VBR is that VBR-files use less disk space. Am I correct?

2. Do you know a good irc-channel where I can talk to people about encoding WM? #doom9@efnet isnt all that good for WM.

Thanks again.

[zambelli]

1. The only real difference, or the main difference, between CBR and VBR is that VBR-files use less disk space. Am I correct?
No, the difference between CBR and VBR is in the variation of the bitrate over time. CBR tends to stay around the target bitrate, at least when averaged over the buffer used. In VBR the bitrate can go up and down as much as it needs to.

[tominator]

No, the difference between CBR and VBR is in the variation of the bitrate over time. CBR tends to stay around the target bitrate, at least when averaged over the buffer used. In VBR the bitrate can go up and down as much as it needs to.

Well yes. But that would mean that CBR uses more disk space than VBR, right?

What Im trying to work out really is if there is a big difference in quality. Would you say there is?

[check]

in VBR, you define an average bitrate which the video will equal out to if you take the whole lot. So the video might be 200kbits in one part and 4200kbits in another, but overall it comes out to 1000kbits.
In CBR, the average is computed only for the length of the buffer. So for every 3 second slice, the encoder has 3000kbits to allocate as it chooses.

[tominator]

in VBR, you define an average bitrate which the video will equal out to if you take the whole lot. So the video might be 200kbits in one part and 4200kbits in another, but overall it comes out to 1000kbits.
In CBR, the average is computed only for the length of the buffer. So for every 3 second slice, the encoder has 3000kbits to allocate as it chooses.

In other words if a user with a bandwidth of 2000kbit/s was to watch the stream the part with the 4200kbit/s stream would start buffering. Or am I wrong?

[check]

yes, unless they had enough in their buffer they would need to wait for that segment to finish downloading.
If you are encoding for a video stream, I'd suggest you pick a peak based VBR, set the max bitrate to the maximum stream headroom you want to give, and set the quality reasonably high. That will give you a stream that sticks to the max bitrate most of the time, but will go lower in lower complexity parts.

[tominator]

yes, unless they had enough in their buffer they would need to wait for that segment to finish downloading.
If you are encoding for a video stream, I'd suggest you pick a peak based VBR, set the max bitrate to the maximum stream headroom you want to give, and set the quality reasonably high. That will give you a stream that sticks to the max bitrate most of the time, but will go lower in lower complexity parts.

Okay thanks.

Im sorry about asking a lot of questions but just to be sure..

When switching from CBR to VBR, do I still keep the same maximum bitrate? How do I calculate the maximum buffer?

What encoder and decoder complexity suits constraint vbr?