Video: Video Compression Basics

Video compression is used everywhere we look. So often is it not practical to use uncompressed video, that everything in the consumer space video is delivered compressed so it pays to understand how this works, particularly if part of your job involves using video formats such as AVC, also known as H.264 or HEVC, AKA H.265.

Gisle Sælensminde from Vizrt takes us on this journey of creating compressed video. He starts by explaining why we need uncompressed video and then talks about containers such as MPEG-2 Transport Streams, mp4, MOV and others. He explains that the container’s job is partly to hold metadata such as the framerate, resolution and timestamps among a long list of other things.

Gisle takes some time to look at the past timeline of codecs in order to understand where we’re going from what went before. As many use the same principles, Gisle looks at the different type of frames inside most compressed formats – I, P and B frames which are used in set patterns known as GOPs – Group(s) of Pictures. A GOP defines how long is between I frames. In the talk we learn that I frames are required for a decoder to be able to tune in part way through a feed and still start seeing some pictures. This is because it’s the I frame which holds a whole picture rather than the other types o frame which don’t.

Colours are important, so Gisle looks at the way that colours are represented. Many people know about defining colours by looking at the values of Red, Green and Blue, but fewer about YUV. This is all covered in the talk so we know about conversion between the two types.

Almost synonymous with codecs such as HEVC and AVC are Macroblocks. This is the name given to the parts of the raster which have been spit up into squares, each of which will be analysed independently. We’ll look at who these macro blocks are used, but Gisle also spends some time looking to the future as both HEVC, VP9 and now AV1 use variable-size macro block analysis.

A process which happens throughout broadcast is chroma subsampling. This topic, whereby we keep more of the luminance channel than colours, is explored ahead of looking at DCTs – Discrete Cosine Transforms – which are foundational to most video codecs. We see that by analysing these macro blocks with DCTs. we can express the image in a different way and even cut down on some of the detail we get from DCTs in order to reduce the bitrate.

Before some very useful demos looking at the result of varying quantisation across a picture, the difference signal between the source and encoded picture plus deblocking technology to hide some of the artefacts which can arise from DCT-based codecs when they are pushed for bandwidth.

Gisle finishes this talk at Media City Bergen by taking a number of questions from the floor.

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Speaker

Gisle Sælensminde Gisle Sælensminde
Senior Software Engineer,
Vizrt

Video: HTTP/2 – Abstraction, protocol design, and practical use

HTTP/2 is an evolution of what most people know as HTTP with the aim of increasing the speed of websites by streamlining the request and delivery of resources. Apple have mandated the use of HTTP/2 for their LL-HLS protocol. Within a typical web page there can easily be 100 requests to the web server so it’s easy to see how increased efficiency could be a benefit. For low latency streaming such as LL-HLS, there are many requests each second so again, even small gains in efficiency can add up.

Rolf W Rasmussen from VizRT explains in this talk the benefits of HTTP/2 taking us through the differences from HTTP. He starts simply by looking at HTTP/1.1 with the messages sent between the client and the server and shows how the requests and responses are sent. Rolf then looks at how the messages are sent at each of the layers of the OSI model. By doing this we discover that the messages are sent in binary.

Binary sending and header compression are ways in which the data to be sent is minimised. We see though that the HTTP/2 is a connection which multiplexes different streams on the same connection. Maintaining the same connection for multiple data streams again reduces the amount of negotiation needed. Multiplexing helps increase the efficient use of that connection. Unlike before, we now see that small requests are cheap whereas there has traditionally been a lot of work to reduce the number of small requests in HTTP/1.1.

Server Push is another key improvement where the server itself can push data into the open connection without a corresponding request. This was originally a requirement of the LL-HLS protocol but has been made optional since. For web pages, there are times when if a page needs resource A, the server knows that it will require resource B later. It’s in these situations that server push is used. Clearly for online streaming, it’s known when the client will need certain chunks or playlist files hence the potential use of server push.

Rolf concludes with questions from the flow and looking at some practical examples of debugging with curl, using proxies and Wireshark as well as dealing with encryption.

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Speakers

Rolf W. Ramussen Rolf W. Ramussen
Chief Software Architect,
VizRT

Video: Where can SMPTE 2110 and NDI co-exist?

When are two video formats better than one? Broadcasters have long sought ‘best of breed’ systems matching equipment as close as possible to your ideal workflow. In this talk, we look getting the best of both compressed, low-latency and uncompressed video. NDI, a lightly compressed, ultra-low latency codec, allows full productions in visually lossless video with a field of latency. SMPTE’s ST-2110 allows full productions with uncompressed video and almost zero latency.

Bringing together the EBU’s Willem Vermost who paints a picture from the perspective of public broadcasters who are planning their moves into the IP realm, Marc Risby from UK distributor and integrator Boxer brings a more general view of the market’s interest and Will Waters who spent many years in Newtek, the company that invented NDI we hear the two approaches of compressed and uncompressed complement each other.

This panel took place just after the announcement that Newtek had been bought by VizRT, the graphics vendor, who sees a lot of benefit in being able to work in both types of workflow, for clients large and small and who have made Newtek its own entity under the VizRT umbrella to ensure continued focus.

A key differentiator of NDI is its focus on 1 gigabit networking. Its aim has always to enable ‘normal’ companies to be able to deploy IP video easily so they can rapidly benefit from the benefits that IP workflows bring over SDI or other baseband video technologies. A keystone in this strategy is to enable everything to happen on normal, 1Gbit switches which are prevalent in most companies today. Other key elements to the codec are: free, software development kit, bi-directionality, resolution-independent, audio sample-rate agnostic, tally support, auto-discovery and more.

In the talk, we discuss the pros and cons of this approach where interoperability is assured as everyone has to use the same receive and transmit code, against having a standard such as SMPTE ST-2110. SMPTE ST-2110 has the benefit of being uncompressed, assuring the broadcaster that they have captured the best possible quality of video, promises better management at scale, tighter integration into complex workflows, lower latency and the ability to treat the many different essences separately. Whilst we discuss many of the benefits of SMPTE ST-2110, you can get a more detailed overview from this presentation from the IP Showcase.

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This panel was produced by IET Media, a technical network within the IET which runs events, talks and webinars for networking and education within the broadcast industry. More information

Speakers

Willem Vermost Willem Vermost
At the time, Senior IP Media Technology Architect, EBU
Now, Design and Engineering Manager, VRT
Marc Risby Marc Risby
CTO,
Boxer Group
Will Walters Will Waters
Formerly Vice President Of Worldwide Customer Success,
Now Head of Global Product Management,
VizRT
Russell Trafford-Jones Moderator: Russell Trafford-Jones
Exec Member, IET Media
Manager, Support & Services, Techex
Editor, The Broadcast Knowledge