compression settings for high quality web streaming

[Ed Dooley]

Streaming doesn’t take any time to load. Do you mean progressive download?
If you don’t care how long it takes to load then you can make it as big in pixels and data rate as you want. The bigger the file, the longer it takes to download. If you actually have some maximums (frame rate, pixel size, data-rate), let us know and we can suggest something. I use 512×384 or 480×270 at 700kbps and 15fps for 16×9 material all the time for example. But if you really don’t care how long it takes to download why not make it 1280×720 at 5mbps at 30fps? It will look awesome. 🙂
Ed

[Sam Pope]

What are you setting your key frames to?

[Ed Dooley]

It varies. I often compress 30P to 15FPS, so I set keyframes to 75 a lot. If I leave it at 30FPS I set keyframes to 150. But that’s for my average stuff. If bandwidth, download time, and computer’s horsepower, etc. are no object, then the more the merrier. The fewer keyframes you use, the harder it is for a viewer to scrub to the exact point they want.
Ed

[Daniel Low]

There is never a need to use fractional frame rates these days. Lower frame rates destroy motion in your clip and can actually make the job much harder for the codec. Even low powered cell phones can play back full frames rates.

The keyframe setting you apply is only saying tot he encoder that you want to force a keyframe every ‘n‘ seconds or frames. All modern codecs insert keyframes automatically when they need to, at scene changes or when the difference in frames passes a certain threshold based on calculations which take other factors of your encoding parameters in to account.

However Ed is correct in saying that the fewer keyframes there are the harder it is to scrub through a clip, accuratley but it’s more noticeable with certain formats, like Flash

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[Ed Dooley]

I disagree. Of course a cell phone or computer can play full frame rate, but the image quality is not as good as using fractional frame rates when you have limited bandwidth (under 500-700kbs for example). While faster internet has made it easier to have higher bandwidth video, compressing 30 frame video to 15fps doubles the quality of the image when comparing it to a 30fps image. Depending on the subject, the viewer is unlikely to see any difference between 15 and 30 frames, but will definitely see the difference in image quality. If you have the bandwidth by all means do it at full frames per second. If you’re looking to save file size or increase the image quality, lowering the frame rate is a good solution.
As for keyframes, I *sort of* agree and *sort of* disagree. It’s true that encoders set “natural” keyframes at hard cuts and other noticable video differences, but manually setting keyframes not only helps in scrubbing/searching, it also adds keyframes in video that may go on for a while without a distinct cut or major change (a long talking head for example). Setting more keyframes manually doesn’t interfere with the automatic keyframes, but it forces additional ones as new reference points in those non-changing clips. For most video I leave keyframes on automatic, but it has helped the quality of a number of compressions to play with adding more manually.
Ed

[Daniel Low] “There is never a need to use fractional frame rates these days. Lower frame rates destroy motion in your clip and can actually make the job much harder for the codec. Even low powered cell phones can play back full frames rates.

The keyframe setting you apply is only saying tot he encoder that you want to force a keyframe every ‘n’ seconds or frames. All modern codecs insert keyframes automatically when they need to, at scene changes or when the difference in frames passes a certain threshold based on calculations which take other factors of your encoding parameters in to account.

However Ed is correct in saying that the fewer keyframes there are the harder it is to scrub through a clip, accuratley but it’s more noticeable with certain formats, like Flash”

[Daniel Low]

but the image quality is not as good as using fractional frame rates when you have limited bandwidth (under 500-700kbs for example)

I was delivering live streams of news and sports at QCIF resolution at 100kb/s, 25fps to 1st generation 3G phones over 4 years ago. Picture quality needed to pass a number of different QA levels before being signed off and all fraction frame rates were presented. There was no need to reduce temporal resolution then and I personally haven’t had the need to do it since.

Your milage obviously varies.

When dealing with HTTP (Progressive download) delivered content, halving the frame rate rarely doubles the image quality, by doing so you are simply making the job a lot harder for the codec as the differences between frames is that much greater, this does not apply to static or near stationary content like talking heads of course.

As the title of the this thread asks for high quality, I consider that to be high quality in both the spatial and temporal domains. Fractional frames rates are instantly noticeable to me and reduce the quality of the viewing experience more so than the odd bit of blocking.

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Please post back saying what solved your problem. It could help others, and saying ‘thanks’ is free!

[Sam Pope]

So is there any consensus here? Is it better to halve the frame rate – even if there’s a lot of action/motion?
And is it necessary to manually put in keyframes, or can you just leave that on auto?
And what about data rate? How high can you go?

[Daniel Low]

Well you say your audience is on a ‘fast connection’ and that could be anything. Fast to one person might be 1Mb/s, fast to me is over 8Mb/s but fast to people in other parts of Europe is over 20Mb/s.

If I were you I’d be delivering H.264 at a full frame rate with manually forced keyframes set at between 5 and 10 seconds, obviously alongside the automatic keyframes the codec will insert.

Neither I or Ed can give you a perfect profile, it’s up to you to experiment with your footage and your typical connection but we’re here to try and help tweak it after you’ve run a few tests.

__________________________________________________________________
Please post back saying what solved your problem. It could help others, and saying ‘thanks’ is free!

[Ed Dooley]

And going back to your first post, you say you don’t care how long it takes to download. That opens up a whole rnage of possibilities. As Daniel said, do some testing. But at a high data rate you should be able to easily go full frame rate, with some forced keyframes at a data rate that will give you quality to rival Hollywood movie trailers. Do you want it at HD pixel size, or is SD size-768 x 432- big enough (or even smaller than that- 512×288)?
If it’s SD size you could go 1.5-2Mbps and have it look awesome.
Ed