What is Video Latency? The Definitive Guide for Broadcasters
Video latency is a major aspect of live streaming over the internet. It is a technical measure that affects the viewing experience of your stream.
There are several technical components that contribute to video latency, so the latency of your steam will depend on the strength of your streaming setup.
In this post, we’re going to answer all of your questions about video latency. We’ll start by discussing what video latency is in the context of live streaming before we take a look at some more technical aspects. We will determine what affects video latency and how to control the latency of your streams.
Table of Contents
- What is Video Latency?
- Why Does Latency Matter?
- What Affects Video Latency?
- What is an Average Video Latency?
- How to Calculate Video Latency
- How to Reduce Latency While Streaming
- How to Reduce Latency at the Encoder
- Related Streaming Metrics
- Video Latency on Dacast
- Final Thoughts
What is Video Latency?
Video latency is the measure of time it takes for a video signal to travel from the source that is capturing the video to the user-facing video player. Latency depends on several technical factors, so it varies depending on the website streaming setup you have.
This is not to be confused with “delay,” which is the intentional lapse in time between the capture and airing of a video. This is used to sync up streaming sources and give producers time to implement cinematographic elements.
Video latency is less intentional and more of a byproduct of the stream’s technical setup.
Why Does Latency Matter?
Latency is very important for broadcasters because, in many cases, it affects the user experience. This is especially true for live streaming events that benefit from feeling lifelike.
Let’s say you are streaming a graduation ceremony. The moment that each student walks across the stage marks a major milestone for that student and their family. The loved ones that are watching the stream from home believe that they are experiencing that special moment in real-time, or at least close to it. If your audience knows that your stream is running behind, the connection feels less real.
Many professionals in the streaming space argue that high latency is detrimental to the outcome of a stream since the more latency you have, the less lifelike your stream will be.
Latency is also an important consideration for video conferencing. Platforms like Zoom and Google Meets support real-time latency which allows people to have digital conversations to mimic face-to-face interactions. Without real-time latency, video chatting would not be possible.
Remember how we mentioned that delay and latency are not the same things? In some situations where broadcasters want a bit of delay, streaming with a slightly higher latency is okay. One of the main reasons for streaming with a delay or higher latency would be to give producers the chance to censor profanity or other inappropriate content.
What Affects Video Latency?
Different parts of a live streaming setup contribute to latency. Everything from your internet network to your live streaming video host causes latency.
That said, let’s review some things that contribute to video latency.
Fast and reliable internet is a must for live streaming. When your internet connection is not up to par, your video latency will increase. The speed, or throughput rate, of your internet network, will directly affect the latency of your stream.
The suggested internet speed is double the bandwidth you intend to use for your video stream, so higher resolution videos need faster internet in order to keep the latency low.
You can test your internet speed by searching “internet speed test” on Google and clicking “Run Speed Test” on the first result.
We recommend using an Ethernet connection for the fastest, most reliable internet. WiFi and cellular data can be used in backup options, but Ethernet is the preferred choice. No matter which option you choose, you should run a speed test before your stream to make sure it is good to go.
With online streaming, a video needs to be encoded, transported, and decoded. This is where a large portion of latency is caused, so it is important that your encoder settings are properly set up and configured to streamline this process.
While there is not a magic combination of encoder settings that work for low latency streaming across the board, it is important to check with your chosen streaming video host to see what settings they require or recommend.
Dacast, for example, requires that broadcasters use an H.264 or x264 codec, a frame rate of 25 or 30, a 2-second frame rate interval, and progressive scanning. To get a better idea of the optimal encoder settings for reducing your video latency, we recommend checking out our complete guide to encoder setting configurations.
Video protocols play a major role in latency. Different streaming protocols are capable of streaming with different amounts of latency.
Currently, the optimal protocol choice for low-latency streaming is HTTPS Live Streaming (HLS) for delivery and Real-Time Messaging Protocol (RTMP) for ingest. HLS is the most widely supported protocol for streaming since it is compatible with the HTML5 video player. However, the RTMP ingest aspect significantly lowers the latency.
HLS delivery with HLS ingest is possible, but it does not support low latency streaming like the HLS delivery/RTMP ingest combination does.
Secure Reliable Transport (SRT) is an innovative video streaming protocol from Haivision. This protocol is known for secure, low-latency streaming. Unfortunately, SRT is relatively new, so most modern live streaming platforms and related technology are yet to get onboard with SRT streaming. However, once this protocol is more widely supported, it will reduce video latency across the board.
Another newer technology that was created to reduce video latency is WebRTC. WebRTC is an open-source streaming project that was designed to enable streaming with real-time latency. It was specifically designed with video conferencing in mind. This project is currently used by major video conferencing platforms, but it could help live video streaming platforms reduce their latency, as well.
What is an Average Video Latency?
The average latency for an online video stream is six seconds, which is considered low latency.
That said, here’s a breakdown of some common categories of latency.
|Type of Latency||Amount of Latency|
|Standard Broadcast Latency||5-18 seconds|
|Low Latency||1-5 seconds|
|Ultra-Low Latency||Less than 1 second|
|Real-Time Latency||Unperceivable to users|
Both traditional television and some OTT streaming fall into the standard broadcast latency category. Traditional television is often closer to 18 seconds, and OTT streaming. Some OTT streaming falls into the low latency category.
How to Measure Video Latency
Since latency affects the viewers’ experience, it is important to know what your streaming setup is capable of.
Measuring your latency can be a bit difficult. The most accurate way to measure video latency is to add a timestamp to your video and have someone watch the live stream. Instruct them to report the exact time that the time-stamped frame appears on their screen. Subtract the time on the timestamp from the time that the viewer saw the frame, and that is your latency.
If you don’t know how to timestamp your stream, you can also have someone watch your stream and tell them to record when a specific cue comes through. Take the time when the cue was performed and subtract it from the time the cue was viewed, and that will give you the latency.
The second method is less accurate since there is more room for error, but it will give you a good enough idea of the latency of your setup.
How to Reduce Latency While Streaming
There are a few different ways that broadcasters can reduce latency. Since the latency is determined by several components, broadcasters must take a holistic approach to reduce the latency of their streams.
First, you’re going to want to make sure that you’re streaming with a fast internet connection. As we mentioned, it’s important to have a consistent internet speed of double the bandwidth you plan to use in your stream.
As you work to reduce your latency, make sure that you’re not damaging the quality of your stream. Of course, there might be a bit of a tradeoff between quality and latency, but make sure that your encoder configurations will still produce a sharp video image.
How to Reduce Latency at the Encoder
So how can latency be reduced at the encoder level? Video is compressed by using frame-based techniques or sub-frame techniques, usually “slices”. A video stream is made up of individual picture frames. A frame is an entire picture. See diagram 1.
A-frame is made up of multiple slices so if you can encode by the slice you can reduce the latency of the encoder since you don’t have to wait for the entire frame before transmitting the information.
These encoders have latencies less than a frame, some as low as 10 – 30 milliseconds. A frame-based encoder typically has a latency of around 100 – 200 milliseconds.
It is also possible to reduce the latency by removing Bi-directional frames. B frames have to wait for a future frame to compress the video. This increases the efficiency of the encoder but also adds to latency.
If you remove B frames then you have to increase the bit rate to achieve the same video quality level, a price many are willing to pay for reducing latency.
Related Streaming Metrics
Throughput and bandwidth are two technical components of live streaming that go hand in hand with latency. We’ve briefly touched upon each of these metrics throughout this post, but to understand their relationship on a more technical level, it’s best to compare them all side by side.
As a refresher, bandwidth is the measure of data transferred in your video stream, and throughput is the amount of data that is transferred in a specific amount of time.
When you are considering the relationship between latency, throughput, and bandwidth, you can picture cars traveling through a tunnel. Only so many cars can go through at a time without causing backed-up traffic.
In this scenario, bandwidth is the width of the tunnel, throughput is the number of cars traveling through, and latency is the amount of time it takes for the cars to get through. To reduce your latency, your streaming setup needs to have the appropriate power for the size of the file you are pushing through.
Video Latency on Dacast
As we covered, video latency has a lot to do with the video streaming host you use. Streams on Dacast have a latency of only 12 to 15 seconds, which is considered low latency streaming.
This level of latency is achieved through the combination of HLS delivery and RTMP ingest. Dacast users can also use HLS ingest, but the latency is not as low with this approach.
Dacast also supports web conference streaming through a Zoom integration for even lower latency streaming.
Head over to our Knowledgebase to check out our dedicated guide to setting up a low latency streaming channel on Dacast.
As a broadcaster, it’s important to have a solid understanding of latency. Knowing how latency works and how you can control the latency of your streaming setup will give you more power over the outcome of your stream.
Are you looking for a low latency live streaming platform for professional streaming? Dacast might be the option for you. Our online video platform includes a variety of features that support streaming at the professional level, including an HTML5 all-device video player, advanced analytics, a video API, a white-label streaming platform, and RTMP ingest and playback, and 24/7 customer support.
You can try Dacast risk-free for 14 days with our free trial. Sign up today to start streaming. No credit card or binding contracts are required.
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Please note that this post was originally written by Mike Galli, CEO of Niagara Video. It was revised in 2021 by Emily Krings to include the most up-to-date information. Emily is a strategic content writer and storyteller. She specializes in helping businesses create blog content that connects with their audience.