XHA1 HDV 24p to H.264: Best bitrate?

[Micky Hulse]

Hi,

Here is the info from one of my clips:

File Path: /Volumes/SCRATCH/springs/t2.gallery.mpeg
Type: MPEG Movie
File Size: 9.9 GB
Image Size: 1440 x 1080
Pixel Depth: 1440
Frame Rate: 23.976
Source Audio Format: 48000 Hz - compressed - Stereo
Project Audio Format: 48000 Hz - 32 bit floating point - Stereo
Total Duration: 00:52:30:00
Average Data Rate: 3.2 MB / second
Pixel Aspect Ratio: 1.333

I was using Adobe Media Encoder to encode a high-resolution edit to H.264 VBR, but I was wondering what would be the optimal target bitrate and maximum bitrate?

For example, should my target bitrate be 3.2 (to match the Average Data Rate of the source footage)... and what should be my maximum? AME will let me go up to 24mbs, but is that extreme and/or superfluous?

Any tips for a noob to H.264?

Also, what is the best keyframe distance?

I would love some help! :)

Have a great night!
Cheers,
Micky

Thanks a billion in advance!

[Tripp Woelfel]

Maybe it's because I haven't had enough coffee yet, but I cannot figure out what you are trying to do.

Picking the correct codec and settings depends upon your delivery media. Are you going to BD, a kiosk playback, or the Web? Without knowing where you are going, it's hard to give you salient advice. Give us your use and delivery case information and we'll try to help you out.

[Micky Hulse]

Hi Tripp! Thanks for the reply, I really appreciate it.

Sorry that I did not provide enough info.

I am eventually going to web.

The streaming web service that my company uses allows for "HD" playback, and they suggest users upload H.264. This service creates several encoded versions of the uploaded video... The highest resolution version being 1.5mbps. The uploaded source version is not included in the allowed versions for playback.

I was told that knowing the original encoding rate of my source material is useful for determining the target variable bit rate that I am exporting.

Basically, I want to make sure that the uploaded version has the optimal settings for down sampling on their end.

I have output some vids at 5mbps/7mbps VBR and it looks pretty good (when viewed locally and when viewing the 1.5mbps version on their end).

I have never really used H.264 before, and thought I would ask for some help here. :)

Questions:

• I guess I am just curious about how the H.264 VBR works... Let's say I am going for an optimal HQ/HD encoding... If my source footage has an "Average Data Rate: 3.2 MB / second", then should my target H.264 bitrate be 3.2mbps? Or, is the data rate different from the VBR settings? In other words, am I comparing apples to oranges here?
• Is there a formula for determining the "maximum" vbr setting?
• In general, what are keyframes used for? I assume that they help for scenes where there is a lot of action (the more keyframes, the better the action looks). Are there any drawbacks to setting a small/large keyframe distance?

Please let me know if you need more info... Sorry if I sound like a complete noob.

Thanks Tripp!

Have a great day!
Cheers,
Micky

[Bill Busby]

Google "bit rate calculator" to help with optimal bit rates, etc.

Also, a source's data rate (3.2 MBytes in your case) is not the same as an encoders bit rate (3.2 mbps would be megaBITS per sec, not megabytes).

[Micky Hulse]

Hi Bill!

Thanks for the help! I really appreciate it. :)

I am googling now.

Also, thanks for the clarification on the data rate.

If ya'll don not mind, I may be back with more questions. :D

Have a great day!
Cheers,
Micky

[Micky Hulse]

This seems to answer my question about keyframes:

In video compression, a key frame, also known as an Intra Frame, is a frame in which a complete image is stored in the data stream. In video compression, only changes that occur from one frame to the next are stored in the data stream, in order to greatly reduce the amount of information that must be stored. This technique capitalizes on the fact that most video sources (such as a typical movie) have only small changes in the image from one frame to the next. Whenever a drastic change to the image occurs, such as when switching from one camera shot to another, or at a scene change, a key frame must be created. The entire image for the frame must be output because the visual difference between the two frames is so great that the new image cannot be reproduced incrementally from the previous frame.

Because video compression only stores incremental changes between frames (except for key frames), it is not possible to fast forward or rewind to any arbitrary spot in the video stream. That is because the data for a given frame only represents how that frame was different from the preceding frame. For that reason it is beneficial to include key frames at arbitrary intervals while encoding video. For example, a key frame may be output once for each 10 seconds of video, even though the video image does not change enough visually to warrant the automatic creation of the key frame. That would allow seeking within the video stream at a minimum of 10 second intervals. The down side is that the resulting video stream will be larger in size because many key frames were added when they were not necessary for the visual representation of the frame.

-- Key frame - Wikipedia, the free encyclopedia

[Micky Hulse]

These settings worked pretty well for me.

Here is the preset.

Here is sample footage.

Brightcove will create "renditions" based on the uploaded mp4. The highest one being 1.5mbps.

My company has an account that does not stream, only progressive downloads, so I have just been uploading one "rendition"/version with a 1 - 1.5mbps CBR (with all the same settings as settings above)... Seems to work pretty well for my needs.

Not sure if that will help anyone.

Cheers,
Micky

[Ian Wright]

I've been going as low as 0.8 Mb/s for HD 720 on the web. The logic being that the file sizes are much smaller and viewers don't need a fast internet connection. The down side is that viewers' pc's need to be reasonably modern for the cpu to be able to decode the file in real-time.

Here's an example of 0.8 MB/s: Nick Pynn at the Brighton Fringe Launch Party Festival Previews Video Blog It's 190 seconds long and is just over 20 MBytes in size.

Ian
Festival Video and Audio Previews - Festival Previews Ltd