Nothing is more frustrating – or suspicious – than a company claiming to have made technological progress without sharing how it was achieved. Zattoo’s recent announcement that it had cut live sports latency by up to 25 seconds just in time for the Euros soccer tournament this summer was an impressive feat lacking in any credible detail.
Luckily, Faultline was quick to get to the bottom of exactly how this two-pronged attack on live latency was achieved, thanks to some exclusively meticulous guidance from the company’s CTO, Stefan Lietsch. These developments took over a year of R&D and required significant updates to both Zattoo’s internal delivery chain, as well as on the client-side devices.
A bit of context is always helpful. Zattoo’s B2C business ingests video from satellite and broadcast services, before encoding, portioning into short chunks (chunking), storing, and preparing video for delivery to the end client. Each of those stages marks a different link in the chain, and Lietsch explained that it is at these checkpoints where latency occurs.
Until last year, the chunks of linear feed that Zattoo sent to its end clients were all exactly four seconds in length, and it is the reduction of this that was one of the key drivers in slimming down latency on the service. Lietsch says that four seconds made sense for years, but the problem is that each link in the chain would add another four seconds to overall latency.
This process has some benefits. “The buffer isn’t bad per say, as it allows for network interruption,” Liestch explains. Most clients would request their feed at least a couple of chunks ahead of their playback so that they could maintain the stream to their end user.
Equally, Lietsch noted that for most OTT content segments – news, movies, drama – a 40-second delay is hardly a priority compared to a steady connection. But when it comes to sport, he argues the extra investment is worthwhile.
Two years ago, during the FIFA World Cup, Zattoo’s customers were experiencing 30 to 40 seconds of delay depending on the client. For any sports event where the entire population is watching, this just will not do. “We said that for the next major tournament we will have to improve, otherwise our viewers would hear their neighbors shout and yell before they have seen the goal. It can really ruin the viewer experience,” Lietsch argued.
Lietsch told us that Zattoo’s primary focus is working out the optimum size for the video chunks that move between the links of the delivery chain. Larger chunks are more reliable but increase latency as every link in the delivery chain marks another checkpoint for processing. On the other hand, smaller chunks reduce the efficiency of the encoding process because each new chunk requires full information for the first frame delivered, and more chunks mean more data-guzzling first frames.
A long process of calculation and compromise led to the new chunk length – 1.6 seconds – which aside from being significantly shorter than the previous four-second size, has some other handy bonuses. This length is easy to match to the 50 Hz to 60 Hz framerates that are used in North American and European television and is also better shaped for synchronizing audio tracks with the feed.
“Just this step alone allowed us to cut latency by half,” Lietsch explained.
But there was still more work to be done on the client side. How many chunks a client device requires in advance depends on the stability of the network connection, so Zattoo spent time tuning and reimplementing an algorithm that decides how many buffering chunks are required to maintain a steady stream over each user’s connection.
The latency reductions on this side of the process are much more variable as it depends on the client device and the connection in that home. Lietsch points out that Zattoo is now mostly watched on smart TVs and set tops, which are often connected via ethernet, and therefore require less of a safety margin. “Some connections we get down to 10 seconds latency, but for others it’s 15 seconds,” he explained.
These two developments took Zattoo over a year of R&D as it required changes across the entire chain of video delivery, using open-source components where possible. Trials begun on channels with low viewership, before gradually scaling up to hugely popular sports fixtures.
Despite Zattoo’s long history in P2P technologies, Lietsch explained that these were not suitable for live sports. While video conferencing services and YouTube offer ultra-low latency streams, these lack the buffering safety which is needed for scale. “A few seconds latency is compensated for by the improved user experience,” he argues.
The company’s ingest stream from broadcasters remains unchanged for now, but Zattoo is already looking to this side of the workflow to potentially cut latency further in the future. It already has some direct links to public broadcasters’ source signals – Sky is one – but Lietsch says that this does not yet manifest in shorter latency. Besides, he feels that the public broadcasters in Germany and Switzerland are usually pretty reliable with live sports latency, noting that ZTF has got its own lag down to a handful of seconds.
While many argue that low-latency is what keeps live sports away from the pirates, Lietsch argues that the move to shorten the delay was led by other factors – mainly being able to replicate the linear TV experience over IP. “The huge sports tournaments that we are thinking about – the Euros, the World Cup – they are not behind big pay walls. There’s no huge risk of piracy here,” he argued.
Lietsch reminded us of why Zattoo has been pursuing this latency-crunching. Since its beginnings in 2008, Zattoo has been largely set on one task – replicating the TV experience over digital delivery. This started off on PCs, expanding to mobile, smart TVs and set tops as the years went by, while its B2B operations have seen it assist operators with white label offerings to create highly tailored TV streaming platforms.
“In the early days, we were niche, but now we are tapping into almost 1 million concurrent users, and we are the main TV service for many of them,” Lietsch told us. “Our mission is freeing TV from its broadcast restrictions.”
Despite opening an office in Singapore in 2018, Zattoo has been incredibly silent in the region. Lietsch was not letting up any details on how that outpost is going, instead arguing that the company is first and foremost focused on being the best in class TV service in Europe, and does not want to spread itself thin. That said, Lietsch shared that Zattoo is preparing to launch its first customer in Africa.