GlobalM Overcomes Challenging Live Conditions with SRT

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Live streaming over the internet has long offered tantalising savings for broadcast contribution over satellite and dedicated connectivity such as SDH/SONET or leased lines. Practically, however, available bandwidth has been constrained compared to video requirements and connectivity has been unreliable. The open source protocol SRT exists to make any lossy network deliver media reliably, transforming spotty internet links into broadcast-contribution capable connectivity.

SRT stands for Secure, Reliable Transport because it combines encryption with data recovery to ensure lossless data delivery over lossy networks. Buoyed by rising available bandwidth, SRT adoption has increased significantly over the past few years. The SRT Alliance was formed in 2017 to coincide with SRT's release as an open source project and was an acknowledgement that the industry isn't looking for closed solutions for basic connectivity. From just two partners in 2017, the alliance has grown quickly up to over 270 members.

One member of the SRT Alliance is GlobalM, a startup that sees a massive opportunity in SRT-enabled contribution. GlobalM recently proved the significant benefits of SRT for contribution at the European Film awards in December. "For us, SRT is critical" explains CTO Paul Calleja, "to allow streams of live news and sports on our platform to be delivered safely anywhere in the world." GlobalM operates a platform matching up media outlets with professional and citizen reporters worldwide allowing news organisations a simple platform to solicit, receive, and manage news reports and other media. The GlobalM platform is also intended for contribution into live events, so when they were approached to deliver a live feed of award winner Antonio Banderas into the European Film Awards in Berlin, GlobalM were eager to prove their technology stack was up to the task. Their brief was to allow Antonio Banderas to participate live in the awards from Malaga, Spain, and for documentary film directors Waad el-Kateab and Edward Watts to take part from Los Angeles. 

The "Secure" in SRT refers to encryption of the media. The protocol uses the same TLS encryption that websites use to ensure no eavesdroppers can decode the media; vitally important for live sports and similar content. "Reliable" comes from SRT's ability to recover lost data by adding a receive buffer on the decoder. Measured in milliseconds, this buffer gives the receiver a chance to spot when it's missing a packet and to shout out back to the sender for it to be resent. This differs from regular web file transfers, which use TCP and acknowledge receipt of every single packet. Although TCP guarantees all the packets are received eventually, TCP's approach increases overhead and can easily push latency way beyond what's usable for live feeds.

SRT allows live streams to traverse badly performing networks without the content being sniffed on the way. The size of the receive buffer needs to increase to match the latency of your network. If you are a long way from your sender, the laws of physics dictate your traffic will take longer to get from A to B. Furthermore, congested networks may be queuing data which will add further delay. SRT receivers need enough buffer to cope with all of this. GlobalM succeeded in an overall latency of approximately 1 second and used their own private cloud network for the long-haul SRT delivery.   

The SRT open source project on GitHub provides the code for senders and listeners. Having this same code in all implementations means that there's interoperability between all products. GlobalM capitalised on this interoperability by using their own smartphone-based app to send the live feeds to a third-party SRT-enabled decoder in the outside broadcast truck at the awards. As SRT is bi-directional, they also sent a reverse vision feed, generated by a third-party encoder back to Antonio Banderas so he could see the ceremony unfold. Being able to see the other party, event, or TV channel is invaluable to help bring your correspondent fully into the programme and, because SRT is bidirectional, it doesn't require a second connection to be established.

So how well did it work? The LA connectivity was rock-solid, losing only two packets during the event. The video was delivered 6Mbps AVC and both lost packets were recovered by SRT. The Malaga connectivity, however, was a different story. Anthony Banderas sat in the centre of a cinema with no 4G reception and a fair distance from the Wi-Fi access point, so the 4.5Mbps video stream had a tough time getting out. The reports show that 70% of the packets were re-requested at least once by the SRT algorithm. However, with SRT working overtime behind the scenes, Antonio made it into the live programme and accepted his award. 

"SRT made the difference between getting on air and having to abandon the feed from an award winner. That's why all our live feeds use SRT on the platform." concluded Calleja. Looking to the next event, Calleja says he’d love to know in advance the network conditions at the remote locations, but this is exactly what the smartphone app is built to deal with. Wi-Fi or cellular, you can't predict network conditions, particularly in a breaking news or live sports situation which is exactly why protocols like SRT exist.

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