Digital FPV systems have been hype lingering around since the hobby began. As technologies progress more and more, manufacturers are bringing out their own digital FPV systems. The question is however, are digital FPV systems the future of the hobby? In this article, I will discuss the various issues and points regarding digital FPV to form an opinion on the topic.
What is Digital FPV and the Difference Between Analogue
There is one main difference between digital FPV signals and analogue signals. Analogue signals are continuous and digital is discrete. Continuous signals can take any value within a set range however a discrete signal can only take a set quantity of values.[/vc_column_text]
By signals, I refer to the signal being received by the screen of the FPV goggles, not the signal being transmitted. This is important because digital and analogue signals are sent through the air for in an analogue format. For digital FPV systems, this is known as digital modulation. Digital modulation is where an analogue RF (radio frequency) signal is modulated with digital data.
To learn more on analogue video, you can read my article on decoding the Clearview’s magic. Digital FPV signals vary quite a bit from analogue FPV signals. A digital signal at its core sends similar information, just in a different manner. A digital signal takes the information from the camera, an analogue device, and encodes it into a digital format. For the digital FPV signal to be displayed, it must first be decoded into a standard display format. An analogue signal however does not need to be decoded as it is already in its standard display format (PAL or NTSC). Common digital formats are JPG or GIF for compressed images and AVI or MP4 for compressed video.
Both analogue and digital FPV systems have their advantages and drawbacks although overall, they serve a similar purpose.
The most significant advantage of digital FPV systems is the image quality, or at least the potential for it. A digital FPV image can be much sharper compared to the characteristically softer image quality of analogue. Notice how the digital feed from a DJI aircraft looks incomparably better than that of an analogue system. A better looking FPV feed improves pilot immersion and experience. It also helps to identify the dreaded ghost branches.
Analogue Image Quality
An analogue camera produces a video signal for each horizontal line followed by a pulse indicating that the next horizontal line’s signal has begun. The goggles interpret these signals into information regarding what colour, brightness and contrast each pixel should be. Due to the continuous nature of analogue signals, a signal segment intended to be displayed on a single pixel can ‘bleed’ into the neighbouring pixels. For example, when transitioning from a black pixel to a white pixel in a video line, an analogue signal cannot instantaneously ‘snap’ to the new colour. Instead it essentially merges to the new colour. In some cases, it will essentially average the colours to introduce a grey pixel in between the black and white transition which has the visual effect of lowering the sharpness. You can witness this effect for yourself by printing out a camera focusing chart and pointing your FPV camera at it. You will notice when the blacks and whites become close enough, the camera will display grey. This effect will also occur for digital images however it will be to a lesser extent. Although certain top tier analogue cameras produce quite sharp images, a digital image in comparison is still miles ahead.
Digital Image Quality
In comparison to analogue, a digital FPV system assigns each pixel information regarding colour, brightness and contrast. This separates the information going to each pixel rather than merging them together as is the case with analogue. The discrete nature of digital signals is responsible for this separation.
Digital FPV systems also have the capability to run with higher resolution which further improves the sharpness and clarity of the video. Analogue video in the PAL or NTSC format is locked to a maximum resolution of 720×576 pixels or 720×480 respectively. Digital FPV systems can look better through the goggles however these advantages diminish if the digital FPV system is forced to transmit within the same bandwidth as a conventional analogue signal with low latency.
Compression & Bandwidth
As previously discussed, a digital FPV system can produce a better-quality image. There is a major ‘but’ to this: compression and bandwidth. Bandwidth is a selected frequency range for an FPV system to transmit on. The Shannon-Hartley theorem states that a certain quantity of information can be transmitted over a given bandwidth in the presence of noise. Increasing bandwidth increases the quantity of information which can be transmitted over a given time. Standard analogue FPV signals have an allocated channel maximum bandwidth of 20MHz as they transmit up to 10MHz above and below their set frequency. Note that there should be unused sections of this 20MHz on each side to reduce interference in neighbouring video channels.
In the case of digital FPV systems, the quantity of usable bandwidth essentially dictates the image quality and latency. If using a low bandwidth, a digital FPV system can either send a delayed high definition feed or drop the video quality to decrease the latency. The quality of the video can be reduced by dropping the resolution or compressing the image. Video compression however is quite processor intensive to carry out in real time hence why existing digital FPV systems cannot match the latency of analogue. Typical video compression often blocks video frames together, often waiting for 8 or more before even starting the compression, hence the increased latency. Compressing digital video drops the sharpness and overall video quality. If a digital FPV system displaying pictures as frames was forced to use 20MHz of bandwidth (the same amount as an analogue system), a 25 frames per second feed would require each frame to be compressed to a size of about 100Kb (this was a back of the envelope calculation for a typical noisy RF environment seen in drone racing). The result of this is a loss of detail due to compression as shown in the 640×480 image below.
Because digital systems require a large quantity of bandwidth to perform optimally, innovators such as DJI and Aminon have implemented frequency hopping into their digital FPV systems. This allows their digital transmitters to switch to unused frequencies to maximise the available bandwidth. (This is what your home wifi also does).
A New Hope for Digital FPV Systems
If more bandwidth becomes available, digital FPV systems will be able to transmit in higher resolutions with lower latency. More pilots will also be able to fly simultaneously. Higher frequencies in the 10-20GHz range would be suitable and would have a bandwidth abundancy. For example, in the 100MHz frequency range, 10MHz of bandwidth is quite significant however in the 10GHz range, 10MHz of bandwidth is miniscule. The downside to these higher frequencies however is their lack of penetration. The higher the frequency, the worse the penetration through solid objects is. This is why the TBS crossfire for long range is 915MHz compared to conventional 2.4GHz radio links. Consequently, the range of a 10GHz digital FPV system would be lower than a 5GHz system despite the bandwidth being higher. As technology improves with time and demand for bandwidth increases, the higher frequencies (such as 10GHz and 20GHz) and corresponding bandwidths are reallocated to shared ISM bands. When this happens digital FPV systems will become a lot more viable.
Video breakup is significantly different between analogue and digital FPV systems. Analogue FPV systems can deal with breakup quite well and with a system such as the Clearview or Rapidfire, a video feed can still be flyable when ~90% of the picture is noise.
Digital break up is significantly different. With current systems, any breakup in a frame results in the loss of the entire frame or frame segment (depending how the digital protocol works). For racing, this type of breakup would be detrimental. Often in video breakup, multiple frames are lost due the video compression algorithm. Ficu’s video below demonstrates digital FPV breakup.
Cost is also another factor. It is highly unlikely that manufacturing will become advanced enough to bring the cost of a digital VTX down to the current cost of an analogue VTX. Take 20-40% off the retail price of a video transmitter, half that again and that most likely gives you a ballpark figure as to how much each unit costs to manufacture & research. An analogue VTX most likely costs around $5-$10 to manufacture (a complete guess). Compare this to a digital transmitter where the internal computer would have a wholesale price greater than that.
With all this potential future digital technology, I feel the need to make a prediction. My prediction is that analogue FPV will remain the standard for racing for the next five years whilst a standard format of digital FPV is developed. Creation of a digital FPV standard will be essential for the sport for the purposes of standardisation and live streaming. Analogue FPV may not even be phased out as the progression in cameras and VTX technology has made the image quality quite stunning. There may even be the possibility of manufacturers creating their own analogue protocol superior to NTSC or PAL. For digital systems in the next few years, manufacturers will most likely continue to innovate bringing out new digital FPV systems with proprietary protocols. Freestyle and long-range pilots will most likely be the earliest adopters of these systems.
So, is digital FPV the future? My educated guess is a solid ‘yes, but not entirely’. I would love to be flying around my HD digital FPV sub 300-gram, sub $600 racing craft however the means of achieving all the above is simply not viable. On the other hand, flying around a sub $800 long range/freestyle quad with digital FPV is already possible with the technology likely to develop further. Digital FPV systems really seem to be a triangle of cost, image quality and latency where it is impossible to have all three.