Video: Maintaining Colour Spaces

Getting colours right is tricky. Many of us get away without considering colour spaces both in our professional and personal life. But if you’ve ever wanted to print a logo which is exactly the right colour, you may have found out the hard way that the colour in your JPEG doesn’t always match the CMYK of the printer. Here, we’re talking, of course about colour in video. With SD’s 601 and HD’s 709 colour space, how do we keep colours correct?

Rec. ITU-R BT.601-7 also known as REC 601 is the colour space standardised for SD video, Rec. ITU-R T.709-6 also known as Rec. 709 is typically used for HD video. Now for anyone who wants to brush up on what a colour space is, check out this excellent talk from Vimeo’s Vittorio Giovara. A great communicator, we have a number of other talks from him.

In this talk starting 28 minutes into the Twitch feed, Matt Szatmary exposes a number of problems. The first is the inconsistent, and sometimes wrong, way that browsers interpret colours in videos. Second is that FFmpeg only maintains colour space information in certain circumstances and, lastly, he exposes the colour changes that can occur when you’re not careful about maintaining the ‘chain of custody’ of colour space information.

Matt starts by explaining that the ‘VUI’ information, the Video Usability Information, found in AVC and HEVC conveys colour space information among other things such as aspect ratio. This was new to AVC and are not used by the encoder but indicate to decoders things to consider during the decoder process. We then see a live demonstration of Matt using FFmpeg to move videos through different colour spaces and the immediate results in different browsers.

This is an illuminating talk for anyone who cares about actually displaying the correct colours and brightnesses, particularly given there are many processes based on FFmpeg. Matt demonstrates how to ensure FFmpeg is maintaining the correct information.

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Matt Szatmary Matt Szatmary
Senior Video Encoding Engineer,

Video: Pervasive video deep-links

Google have launched a new initiative allowing publishers to highlight key moments in a video so that search results can jump straight to that moment. Whether you have a video that looks at 3 topics, one which poses questions and provides answers or one which has a big reveal and reaction shots, this could help increase engagement.

The plan is the content creators tell Google about these moments so Paul Smith from takes to the stage at San Francisco Video Tech to explain how. After looking at a live demo, Paul takes a dive into the webpage code that makes it happen. Hidden in the tag, he shows the script which has its type set to application/ld+json. This holds the metadata for the video as a whole such as the thumbnail URL and the content URL. However it also then defines the highlighted ‘parts’ of the video with URLs for those.

Whiles the programme is currently limited to a small set of content publishers, everyone can benefit from these insights on google video search. It will also look at YouTube descriptions in which some people give links to specific times such as different tracks in a music mix, and bring those into the search results.

Paul looks at what this means for website and player writers. On suggestion is the need to scroll the page to the correct video and make the different videos on a page clearly signposted. Paul also looks towards the future at what could be done to better integrate with this feature. For example updating the player UI to see and create moments or improve the ability to seek to sub-second accuracy. Intriguingly he suggests that it may be advantageous to synchronise segment timings with the beginning of moments for popular video. Certainly food for thought.

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Paul Smith Paul Smith

Video: 2019 What did I miss? – SRT

We’re looking at the most popular posts of 2019 now as The Broadcast Knowledge takes a break over the holiday season. Twitch’s Alex Converse had one of the most visited posts of the year in his video detailing how SRT works. It’s a great technical resource for developers and engineers wanting to understand more than just the highlights of SRT. Did it do well because it was Alex? Because the San Francisco’s Video Tech meet up is a well known part of Demuxed’s community for ‘engineers working with video’ or because its title? Any or all of these could be true and it wouldn’t invalidate it’s usefulness or its popularity. So if you haven’t already, read more about it here, or click play below.

Another SRT talk of interest this year you may want to catch up on was the IBC SRT Open Source Technical panel which looked at the general SRT features and looked at the pros and cons against SRT. The panel looked at a case study with Red Bee Media and South American broadcaster Globo and the use of RTP and SRT together. Read more detail here or click here to watch for free


Alex Converse Alex Converse
Streaming Video Software Engineer,

Video: SRT – How the hot new UDP video protocol actually works under the hood

In the west, RTMP is seen as a dying protocol so the hunt is on for a replacement which can be as widely adopted but keep some of it’s best parts including relatively low latency. SRT is a protocol for Secure, Reliable Transport of streams over the internet so does this have a role to play and how does it work?

Alex Converse from Twitch picks up the gauntlet to dive deep into the workings of SRT to show how it compares to RTMP and specifically how it improves upon it.

RTMP fails in many ways, two to focus on are that the spec has stopped moving forward and it doesn’t work well over problematic networks. So Alex takes a few minutes to explain where SRT has come from, the importance of t being open source and how to get hold of the code and more information.

Now, Alex starts his dive into the detail reminding us about UDP, TS Packets and Ethernet MTUs has he goes down. We look at how SRT data packets are formed which helps explain some of the features and sets us up for a more focussed look.

SRT, as with other, similar protocols which create their resilience by retransmitting missing packets, need to use buffers in order to have chance to send the missing data before it’s needed at the decoder. Alex takes us through how the sender and receiver buffers work to understand the behaviour in different situations.

Fundamental to the whole protocol is packet the packet acknowledgement and negative acknowledgements which feature heavily before we discuss handshaking as we start our ascent from the depths of the protocol. As much as acknowledgements provide the reliability, encryption provides the ‘secure’ in Secure Reliable Transport. We look at the approach taken to encryption and how it relates to current encryption for websites.

Finally Alex answers a number of questions from the audience as he concludes this talk from the San Francisco Video Tech meet up.

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Alex Converse Alex Converse
Streaming Video Software Engineer,