Bitrate for HD slideshow

[cheerful]

1920x1080, 60i

Each picture is about 5 seconds and there is 30-frame transition b/w pictures.

What would the appropriate bitrate for this? There is some slideshow software chose to use 7 Mbps. Is that sufficient?

Thanks!

[jagabo]

Use a constant quality encoder then you won't have to worry about what bitrate is appropriate.

[cheerful]

What's the bitrate? 5 Mbps?

[jagabo]

When you use constant quality encoding the encoder uses whatever bitrate is necessary to achieve the quality you ask for.

Use bitrate base encoding when you need a file of a specific size. Use quality based encoding when you want a file of a specific quality (relative to the source -- it won't improve the quality of a poor source).

[racer-x]

I do all my HD slideshows in 1920 x 1080p @ 23.976 fps. HCenc works great at 7 Mb/s average. X264 works great @ CRF = 22. I mostly compile the slideshows in Vegas and frameserve out to encoder.

[cheerful]

Ah, I am doing 60i. Certainly should switch for 24p. What's the difference b/w 23.976 and 24?

Thanks!

[jagabo]

24p is usually used as shorthand for 23.976p. It can literally mean 24p at times though. Switching to progressive encoding will get you better use of bitrate and sharper colors, but your transitions will be a little jerkier.

[cheerful]

Thanks, will try 1920x1080 @ 24p.

Does VBR 1/2 pass matter in this case?

Should I use MPEG or H.264?

How did people arrive at 7 Mbps?

Thanks!

[jagabo]

Single pass bitrate based encoding is usually constant bitrate. It will use just as much bitrate during the static portions of your slideshow as it does during the transitions. If you use a low bitrate the static portions may look fine but the transitions will look poor. If you use a high bitrate the transitions will look fine but a lot of bitrate will be wasted on the static portions.

Multipass bitrate based encoding uses the first pass to examine the video to see which portions need more bitrate and which need less. During the second pass it uses that information to allocate bitrate as necessary, finally delivering the overall average bitrate you requested. During subsequent passes (if supported and requested) it refines the video even more.

There are single pass variable bitrate encoders but the results aren't as good as with multipass. Since they can't know what's coming later in the video they can't be very accurate at allocating bitrate throughout the video. Either the file size or the quality will be unpredictable.

CRF mode (constant quality but sensitive to what is visible to the human eye) in x264 works in a single pass. The encoder uses whatever bitrate is necessary at each frame (as it's encoding it) to deliver the quality you request.

x264 also has QP mode (constant quality in a mathematical sense) which works similarly. QP mode requires more bitrate than CRF mode.

If someone told you 7 Mb/s is the right amount that is something they found with a particular video. It doesn't mean it's the right amount for your video. Depending on the nature of your shots, how long the static portions are vs. how long and complex the transitions are, and your particular tolerance for defects, a higher or lower bitrate may be necessary.

Again, you use bitrate based encoding when you need a file of a particular size but don't care exactly what quality the resulting video will be. You know a higher bitrate will look better than a lower bitrate but you don't know exactly what the quality will be for that paricular video until after you've encoded it. Use quality based encoding when you want a file of a particular quality. You know that the quality will be what you asked for but you don't know exactly how big the file will be.

Experiment with a few short videos to determine what quality is acceptable to you. Any video encoded at that CRF value will have the same visual quality. I usually use CRF 18. Note that lower CRF values deliver higher quality, higher CRF values deliver lower quality.

Generally, MPEG 2 requires more bitrate than h.264 to achieve the same quality. Two or three times more.

[cheerful]

Thanks. Trying with H264, 1080p24. I am using Adobe Premiere and is struggling with its bitrate control. It does not seem to honor the low bitrate target.