Latency Explained

From Multirotor Pilot Issue: 8
by Erick Royer

One of the primary reasons that people buy a multirotor is for video and photo capture from the air. Both hobbyists and professionals alike find countless uses for capturing this bird’s-eye footage. From aerial selfies to Hollywood Cinema, multirotors are changing the world. However, what is good for a hobbyist is not always going to work for a professional. In fact, professional videographers have a much more discerning eye for “the right shot” and the directors and producers put a lot of pressure (to back up the money they are spending) on pilots and camera operators to capture what they need. It can be a very high-stress job and because of that, pilots and camera operators tend to look for the best possible equipment to keep in their arsenal. As a professional camera operator, the number one thing I look for in systems that we fl y is zero latency in the video transmission. While there are certainly a lot of other things that we look for, I wanted to discuss latency and why it is so important to eliminate it at all costs for professional aerial work.

Can you live with the delay?

WHAT IS LATENCY?

Latency, for our purposes, is a time interval between when a video signal is transmitted and received. When you are looking at your monitor while holding the copter in your hand and you quickly spin the copter, you should see the video move on the screen along with the copter’s movement. Any lag in this video is referred to as latency. One easy way to test this is to activate the stopwatch on your phone and record it using the FPV camera. Then place the FPV monitor behind the phone so your FPV camera is now taking video of both the phone and the FPV monitor (see photo). If you freeze a frame on that video, you will notice that there is a time diff erence between the actual stop watch and what is being received on the FPV monitor.

WHY IS LATENCY BAD?

For general hobbyists recording family events or aerial selfi es, this will not be a problem. Milliseconds are not going to be that noticeable, nor will they be detrimental to the project.

If you fly FPV race quads for fun as a hobbyist, however, it is a whole different story. Imagine that you are racing around a course at 50mph and there are a lot of turns with trees along the outside of the course. If you have even a couple of milliseconds of latency in your video, that could make you react to taking the turn later than you needed to and by the time your video shows you where to turn, the copter has already had a meeting with a tree. Most pilots run FPV quads with video goggles so you are totally immersed in the video that is being received from the FPV camera and you have no reference to the outside world. Because of this, you need to rely 100% on the video that you are seeing – it is a very fly-by-wire experience. Any latency can lead to a less than stellar experience.

Large professional rigs rely on the best of everything. Professional camera operators requires zero latency HD to frame the best shots for big budget productions.

Professional pilots and camera operators fall into a class much like the FPV race pilots. When you are trying to get the perfect shot for a client, producer or big-budget director, it is imperative that what you see on your monitor is what you are shooting.

There are some cases where this is not as critical. For instance, if you are shooting a house or some other inanimate object, slight latency will not kill your shot as long as you are smooth with the sticks controlling your There are some cases where this is not as critical. For instance, if you are shooting a house or some other inanimate object, slight latency will not kill your shot as long as you are smooth with the sticks controlling your tilt, pan and yaw. However, if you are tracking a moving object like a racecar, a surfer or some other subject that you must keep in frame, then it will be virtually impossible to run a system with latency.

TYPES OF SYSTEMS AND WHAT TO EXPECT

There are a couple different types of FPV transmission systems that are common on the market for both hobbyists and professionals. They range from systems that use tablets or smartphones to establish a Wi-Fi link from the camera to commercial HD transmission systems with zero latency.

GoPro was one of the first camera platforms to incorporate a Wi-Fi feature that allowed your smartphone to link to the camera via an app and deliver a live video feed. The concept is great and it does work well for a lot of applications. However, the Wi-Fi runs at 2.4GHz, which is the same band that the transmitters that we fly our copters with run on. While they are not the same channel, there have been concerns and speculation that it is possible for the Wi-Fi to interfere with the operation of the copter which makes many people leery of using this as an FPV method. I can’t say one way or another, but I always err on the side of minimizing interference with the flight control of my machines, so I am not one to use the Wi-Fi on a GoPro for FPV.

Companies like Blade with the 350 QX2 and QX3 as well as DJI with the Phantom Vision and Vision 2+ use an integrated Wi-Fi system that is designed not to interfere with the operation of the copter. Much like the GoPro, they use an app and a Wi-Fi link to your phone or tablet to establish a connection. Depending on your location and the distance that you are from the copter in flight will vary the amount of latency that you have. I have had very good luck with both the above-mentioned machines with low latency (never zero), but they are certainly useable for hobbyists filming of inanimate objects. For tracking subjects in motion, the latency is too great, especially the further away you get from the copter. But, again, if you can remain close, then tracking moving subjects could be possible.

Other machines like the new Yuneec Q500 Typhoon and the soon-to-be-released Hubsan X4 Pro use a 5.8GHz video downlink, which is still based on a Wi-Fi-link signal, however they have integrated FPV video monitors and receivers in the transmitters. These systems off er very little latency, which make them usable for a wide variety of applications. The main downside for professional productions with these machines is also one of their upsides; they are designed for a single operator. There is no provision for a camera operator to control the camera and with the video being integrated into the transmitter it makes it hard for a second person to watch it. However, as a single operator machine, these are very good and viable solutions, especially if you are shooting real estate, golf courses or the like.

A common system for standard definition FPV video signal is using a true 5.8GHz (or other lower frequencies for long range applications) transmitter and receiver. Companies like Immersion RC, Fat Shark, Flysight and DJI are very common in the FPV world. These systems provide a standard definition video link that can cover a relatively long range with no latency. As you fl y out of range these systems will typically display static on the screen, much like tuning an old TV antenna back in the day. However, the video remains with no latency. These systems are preferred for FPV quad racing and are common among many professionals. Even though the video is SD, a camera operator can still see what they are filming and manipulate the camera in real time. These systems require you to attach your camera system to the video input on the transmitter to supply a live feed. A cloverleaf-type antenna is most common on the transmitter to send the best possible signal to the ground. On the ground you can typically use a single receiver with a matching cloverleaf antenna or you can use a diversity setup where you have two receivers each with a different antenna and the system will switch back and forth between the two to grab the best signal to deliver to your FPV monitor or goggles. I typically use a cloverleaf and a patch antenna so I have the most diverse reception fi eld.

DJI introduced the Lightbridge over a year ago, which promised to change the world when it came to FPV. The Lightbridge delivers a full HD signal from the copter/ camera to the receiver over a 2.4GHz band. The idea was excellent, but we have used one for over a year and were sure to install all the latest updates, but still had problems with Latency. While it did get better over time with each update, there was still a noticeable lag in the video, especially as you flew farther away.

DJI just released the Inspire One, which has a new version of the Lightbridge built-in and we noticed the same latency in this system until we installed the latest update. Then the latency all but disappeared. If you use a tablet as your FPV monitor, you will notice some significant delay, but if you use the HDMI output and run it to a monitor, there was very little, almost unnoticeable latency. I have been rooting for this product for some time because it is one of the most inexpensive HD transmitter solutions on the market and I would love to see it with Zero Latency.

On the high end of the market are systems from companies like Paralinx and Teradeck who make professional HD video transmission systems. These units have absolutely ZERO latency and deliver a full HD picture that is crystal clear. They have different models from each company that off er different ranges, which they work within. They tend to be the preferred solutions for cinema work where directors require an HD monitor or for solutions where camera controls like adjustable focus in flight is necessary. For these applications you need full HD with no latency. However, for this quality of video, you pay a higher price tag. Most of their systems start in excess of $3,000 and can range as high as $10,000.

A new system was just released at the NAB show from a company called Amimon, that is proven to redefine HD FPV solutions. While we have not had a chance to use one yet, I have been in touch with the manufacturer and have seen sample video and demos online. Their Connex Solution offers zero latency, long ranges and crisp HD video. Their system will start at $1599, which will be a game changer if they can live up to their specs. We are hoping to have a system for a full review in the next issue.

THE FINAL WORD

There are certainly a large number of options when it comes to FPV video solutions and while there are some applications where latency is ok, there are many others where it is simply unacceptable. It is important to understand what your applications are going to require when you set out to make your aerial video system purchase. Many of the ready-to-fly systems like the Blade 350 QX3, Hubsan X4 Pro and DJI Phantom are excellent for the majority of users, but if you are looking to race FPV machines or have visions of making a name for yourself the high-end production world, then solutions that off er zero latency should be your only option.

CONTACTS

AMIMON connex.amimon.com
BLADE  bladehelis
DJI dubro.com
FATSHARK  fatshark.com
HUBSAN hubsan.com
IMMERSION RC  immersionrc.com
PARALINX paralinx.com
TERADEK teradek.com