Expression Encoder 2 Service Pack 1 – Intro and Multibitrate Encoding
- Posted: Jan 17, 2009 at 1:17 AM
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Oh, I’m a bad, bad blogger. The Expression Encoder Service Pack 1 has been out for MONTHS, and and it’s cool enough that I haven’t had the time to do it full justice. Oh well, better short than nothing; I’ll just start off with the biggest new feature: multibitrate encoding, including support for Smooth Streaming.
This first post I’m going to focus on Smooth Streaming and multibitrate streaming in general.
When doing multibitrate encoding, EE does multiple simultaneous encodes from the same source file. While the big driver for that feature is Smooth Streaming (another overdue blog topic), we also support encoding a single multiplexed “Intelligent Steaming” WMV file or individual WMV files for each bitrate.
SmoothHD.com in action. Those bars in the lower right show which data rate band you’re getting; mouse over for more details. Click on it to bring up a diagnostic menu and play around with scaling. And boy, that 20 Mbps DSL upgrade I’ve got on order can’t come soon enough!
So, you want to play around with Smooth Streaming, but don’t want to have to upload your files to Akamai every time you want to do a quality check? Fair enough, but we don’t have media players that support the Fragmented MPEG-4 format used in .ismv files yet. Fortunately, EEv2 SP1 also includes a little localhost Smooth Streaming web server built into it. If you encode to a Silverlight 2 template and make sure that “Preview in Browser” is checked, you’ll get Smooth Streaming working, heuristics and all! It’s only on the local machine, and only when EE is running, but it’s a start.
To preview the video quality of Smooth Streaming, you can also make individual WMV files and watch them as normal. The output bitstreams are identical no matter what the file is wrapped in.
WMSnoop remains a great (and free) way to check out the GOP structure of WMV files. Below (click to zoom) you can see how Smooth Streaming produces a rock-solid cadence of a keyframe (in yellow) every 60 frames, while still being able to insert extra keyframes as needed for high quality scene changes.
Unlike the old FSDK-based MBR encoder, each stream is encoded in its own thread, which means a whole lot of cores can be saturated; my 8-core typically is running at nearly 100%, and if HD is involved, a 16-core would as well. The limiting factor can wind up being the speed of the source decoder past a certain point. Here’s a pretty typical load I’ll see when running from a Lagarith source file.
Most of the parameters must be identical per stream; only frame size and bitrate can vary each. Other parameters like frame rate, audio, and codec settings are fixed. This is to facilitate Smooth Streaming by making sure that I-frames appear at the same point in each output stream. For technical reasons with the current VC-1 SDK, this also means only 1-pass CBR is supported for multibitrate encoding.
Only one audio option can be selected when doing Adaptive Streaming encoding. This isn’t the limitation it would have been in the WMA days, as WMA 10 Pro’s low bitrate audio quality is so good. We find 64 Kbps provides great quality for most content, and fits into most web delivery bitrates.
There are three container choices
There’s a few required settings to get optimal Smooth Streaming compatible content in EEv2SP1. Smooth Streaming requires that all bitrates start have Closed GOPs (Group of Pictures – a keyframe and all the frames that reference it) starting on the same frame every few (typically 2) seconds. This means that the decoder gets a continuous sequence of frames to decode without any overlaps or missing frames, and without having to run two simultaneous decoders. These are compatible with (but not needed for) ASF, and using them gives the option to remux to Smooth Streaming at a future date.
These are the changes I made from the Adaptive Streaming default, implementing the typical “make it look good” settings I recommend for EE in general also apply to multibitrate compression. Just don’t mess with the settings up above!
These settings won’t have much impact on encode time, but can help quality significantly, particularly with
These are settings I use when I’m more worried about optimum quality than the speed of encode.
Expression Encoder defaults to letterboxing to be used when the output frame size doesn’t match the source aspect ratio. However, even a slight variation in output aspect ratio from the source’s (typically to get the encoded height and width divisible by 16, the most efficient option) can result in a thin black line of letterboxing (top and bottom) or pillarboxing (left and right). If Resize Mode=Stretch and Mode=Profile Adaptive, Profile Adaptive will get the output frame size in the right ballpark while Stretch adjusts the video slightly to avoid those darn black lines. To make sure the output aspect ratio really is perfect, you can set the Video Aspect to that of the source, and on playback it’ll stretch the image pack to the exact original frame size and shape. Leaving it at the default of Square Pixel generally looks fine as well.
Note that you may need to reapply the Profile Adaptive setting if you apply a job preset to multiple files at once. That can reset to Mode to Custom.
By default, Adaptive Streaming profiles gives a keyframe every 2 seconds. However, that’s a precise 2.0000 seconds. So, if your source is a typical NTSC frame rate like 29.97 or 23.976, you’re 0.1% short of 30 and 24 frames a second, and so wind up getting a keyframe every 47 or 59 frames, not the 48 or 60 you might expect. If you set the frame rate to the whole number equivalent, the encode will actually maintain the source frame rate correctly (the value serves as an upper bound, but EE won’t insert duplicate frames when set higher than the source). But it will round up the keyframe cadence to the whole number, so you’ll get the keyframe every 48 or 60 frames like you’d expect.
Not really a big deal in practice I suppose, but I’ve long since internalized the multiples of 48 and 60, which makes it easier to me to figure out where my keyframes are.
Picking the optimum single frame size and data rate combination for a particular piece of content can already be a challenge. Having to determine optimal sizes for a bunch of different data rates. Fortunately, Alex Zambelli is as always ready with a handy utility for us – the Smooth Streaming Multi-Bitrate Calculator!
All you need to know is
There’s actually some pretty deep math in there that knows how VC-1 bit-per-pixel requirements change as frame size changes, etcetera.