Lately I’ve been trying to solve a few specific problems with Smooth Streaming, and I noticed as I go through forums, blogs, and Microsoft web sites, the only thing clear is that this topic is vague, and there are lots of unanswered questions out there. In addition, the way Microsoft uses the term “Live Smooth Streaming”, it almost seems intentionally misleading. So, with that in mind, I’d like to clear up a few things.
How does Smooth Streaming work? (the quick and straightforward version)
Smooth Streaming (or, as I will call it, On-Demand Smooth Streaming) relies on having the same stream encoded at varying bitrates, with one complete streaming file per bitrate. So you will typically have individual files that look like this in a web server directory:
These files all contain special timecode marks in them, and are all tied together by two “manifest” files, for example:
These files simply contain a mapping of which bitrates correspond with each stream (for example, the 230k stream = MyStream_230.ismv, 708k stream = MyStream_708.ismv).
The Smooth Streaming player makes tiny HTTP requests requesting a second or two of video at a certain bitrate. For example, “give me 230k @ 00:01:06, give me 708k @ 00:01:08”, etc. These requests are no different than how your browser downloads images on a web page. The “smooth” part comes in because with this architecture, it’s extremely easy for your computer to switch bitrates and adjust to your network connection.
IIS Media Services
An IIS7 feature called IIS Media Services is what makes it possible for the server to understand these player requests, timecode, and a few special URL constructs.
An example of this is the Manifest request; if you had a smooth stream (with MyStream.ism) in the root folder of your IIS web server, you can open up your browser and go to:
IIS Media Services will generate the XML manifest file that the player would consume, and you can view it in your browser.
What is “Live Smooth Streaming”?
Live Smooth Streaming uses the same technique of on-demand Smooth Streaming, except encoded in real-time for live events by hardware encoders. Although it is similar in player concept, it is quite different in implementation.
IIS Features: Smooth Streaming Presentations vs. Live Smooth Streaming Publishing Points
Once you install IIS Media Services, and navigate to a folder under an IIS web site, you will see these options:
- Live Smooth Streaming Publishing Points – Similar to Windows Media Services, you can create publishing points that live hardware encoders can use.
- Smooth Streaming Presentations – This is the feature that detects ISM and ISMC files (smooth streaming manifests) within a web site directory and interprets them as Smooth Streams.
How can you encode content for On-Demand Smooth Streaming?
Expression Encoder, with the IIS Live Smooth Streaming option.
Clarification Point: Expression Encoder is offered in 2 versions, a free version without the ability to do this, and a paid version with IIS Smooth Streaming encoding capabilities.
The IIS Smooth Streaming version will:
- Encode your content into separate bitrate files, and….
- Create the manifest files (.ISM, .ISMC) automatically
How can you encode content for Live Smooth Streaming?
Let me be clear, since this is the most confusing and vague part of Smooth Streaming to date: As of this writing, there is no software encoder that can do Live Smooth Streaming. You will have to be prepared to drop anywhere between $5,000 to $20,000 on a hardware IP video encoder (or a series of hardware encoders). For example, the Inlet Spinnaker Encoder.
The reason for this is the amount of sheer processing power that it requires to encode a smooth stream – it’s fairly intensive for a regular computer to encode one stream in realtime of acceptable quality; for a normal smooth streaming event, it’s eight realtime streams, ranging from low-quality to extremely high quality.
Again, Expression Encoder does NOT do Live Smooth Streaming. Expression Encoder only prepares content for On-Demand Smooth Streaming.
How do you play a Smooth Stream with Silverlight?
Just as you would play another media file in a Silverlight media player, but with a twist or two.
- Set the MediaSource property of your playlist item to the HTTP path of your .ISM file, appended with /Manifest - for example:
- Make sure to also set the playlist item property IsAdaptiveStreaming to true. Without this, Silverlight won’t understand that it’s a smooth streaming source, and will be confused.
- If your Silverlight player is not located on the same host / relative path of your Smooth Stream, make sure that the root directory of your IIS site has a CrossDomain.xml file present, with contents that look like this:
<!DOCTYPE cross-domain-policy SYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">
<allow-http-request-headers-from domain="*" headers="*"/>
This simply tells Silverlight that it’s okay to contact this domain for content.
- If you’re making your own Silverlight application with a player, Silverlight somehow needs to reference the AdaptiveStreaming.dll assembly. The easiest way to do this is to take the stock boilerplate player that Expression Encoder creates for you, and look for the SmoothStreaming.xap file in your output directory. Rename it to SmoothStreaming.zip (XAP files are really Zip files), and extract it. Inside are the assemblies you need to reference in your custom player (including AdaptiveStreaming.dll).
Hopefully this will clear up some understanding when working with Smooth Streaming, or at least making the total picture a little easier. Good luck!
Oh, and by the way – if you don’t want to deal with any of this junk, try NimbusHD – it’s Smooth Streaming as an affordable cloud service with a lot more features.