The most crucial aspect of first person view (FPV) racing is arguably the analogue FPV video feed which allows pilots to observe a live view from the front of their craft. Basically, an FPV camera sends images to the video transmitter (VTX) which is then wirelessly transmitted to a set of FPV goggles and observed by the pilot. But, how does this analogue FPV video feed actually work? Well, glad you asked, this article will discuss just that and provide you with an overview of what is going on behind the scenes whilst you rip through the sky! And in regards to the Clearview’s magic, I will try to explain a portion of that too.
The Physics Behind it all
The FPV camera translates light hitting its sensor into a PAL or NTSC format analogue FPV video signal. More on these formats later. The analogue FPV video signal is then fed along a video wire to the video transmitter.
The job of the VTX is to take the electrical video signals from the camera and to translate them into a radio wave at the desired video channel frequency. Frequencies around the 5.8 gigahertz (GHz) band are the most common for analogue FPV video feeds. An example of a video frequency is Fatshark channel 1 which has a frequency of 5740 megahertz (MHz) or 5.74 gigahertz (GHz). The process of translating/encoding the analogue FPV video signals from the camera into a radio wave is known as modulation. The VTX transmits the analogue FPV video signals into the air as a radio wave by amplifying the encoded FPV video signals and passing them through the antenna. This radio wave can then be picked up by a set of FPV goggles where the live analogue FPV video feed can be viewed.
For the VTX to form a radio wave, a current must be run through a wire known as an antenna. The current flowing through the antenna produces an electromagnetic field or radio wave around it. By changing the current flowing into the antenna around 5,800,000,000 times per second, the radio waves will mimic these changes and propagate out at the chosen video frequency within the 5.8GHz band.
The video receiver (VRX) also has an antenna mounted to it. This antenna receives the transmitted radio waves. When the VRX antenna receives the radio waves containing the encoded analogue FPV video feed, the radio waves moving across the VRX antenna creates a changing current flow through it. This changing current flow closely mimics the changing current flow going through the VTX antenna. Because the current through the receiving antenna is changing, the voltage across the antenna will also be changing. This is because voltage and current are linked by the ‘voltage = current * resistance’ formula. The VRX measures the voltage across the antenna and decodes or demodulates it to recover the PAL or NTSC analogue FPV video feed that was sent to the VTX.
Antennae technically pick up every frequency transmitted but the size and shape of the antenna ‘tune it’ to be more sensitive to some frequencies over others. This is why a bent video antenna usually has worse signal than a brand new one. In the process of demodulating the analogue FPV video feed, the VRX filters out all received signals except those at the selected frequency. This is done using what is known as a ‘tuned circuit’.[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]
Segments of an Analogue FPV Video Signal
Now that the physics behind how analogue FPV video transmission has been discussed, I will move on to explaining the parts/segments of an analogue video signal. Analogue video signals vary quite a lot from digital FPV. The following diagram demonstrates what the video signal looks like and the following sections will explain the significant segments of the signal. This video signal diagram represents a single line of an entire analogue FPV video feed. The ‘height’ of a video signal is measured using its voltage. The voltage of a standard PAL or NTSC video signal will range from zero volts to one volt (0V-1V). These video formats were designed in the days of old televisions where the video signal would control an electron beam and help steer it across the display to draw the picture, line by line, on a glowing phosphor screen. In order to achieve this, the analogue video signal included segments known as horizontal sync pulses, vertical sync pulses and a color burst. These segments are also used to control aspects of the more modern analogue FPV video feeds which are also constructed line by line from the top to bottom of the screen. Each segment of an analogue FPV video signal is outlined in greater detail below.[/vc_column_text][vc_row_inner][vc_column_inner width=”1/6″][/vc_column_inner][vc_column_inner width=”2/3″][vc_single_image image=”1476″ img_size=”full” alignment=”center” onclick=”link_image”][/vc_column_inner][vc_column_inner width=”1/6″][/vc_column_inner][/vc_row_inner][vc_column_text]
Horizontal Sync Pulses
The video signal needs a method of informing the screen displaying the image to switch to the next line of video otherwise the entire FPV image would be displayed as a single line of video. This is done using a horizontal sync pulse. The sync pulse is a quick drop in voltage to 0V; below what the video screen knows to interpret as part of the image. When the video screen sees the horizontal sync pulse, it knows to move on to the next line of the image and carry on displaying the next line of video.[/vc_column_text][vc_column_text]
Vertical Sync Pulses
[/vc_column_text][vc_row_inner][vc_column_inner width=”1/6″][/vc_column_inner][vc_column_inner width=”2/3″][vc_single_image image=”1479″ img_size=”full” alignment=”center” onclick=”link_image”][/vc_column_inner][vc_column_inner width=”1/6″][/vc_column_inner][/vc_row_inner][vc_column_text]Once a single screen of video lines has been displayed, the screen moves from displaying the bottom line to the top line. This process is triggered when the screen receives a vertical sync pulse. The vertical sync pulse , as shown in the above diagram, consists of a series of quick pulses however these do not go all the way down to 0V like the horizontal sync pulses. Without this sync pulse, the analogue FPV video image would continue down off the screen.[/vc_column_text][vc_column_text]
Color
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The color information is encoded or ‘put into’ the analogue FPV video signal using sine waves which are an alternating waveform. The above diagram shows an example sine wave with three of its parameters labelled: the phase difference, amplitude and height. Adjusting these parameters of the sine wave can alter the color, contrast and brightness of the image. Without these sine wave segments in the analogue video signals, the image would be in black and white. Each pixel of an analogue FPV video feed will have a corresponding sine wave segment in the video signal which dictates its color. Two pixels next to each other could have completely different sine wave segments.
To change the color of an image segment or pixel, the corresponding sine wave segment has its horizontal position moved horizontally ‘out of phase’ to the reference sine wave as shown in the above diagram. This is represented as the ‘phase difference’ which is the difference (in degrees) between two peaks of the sine waves. The phase difference can vary from 0 to 360 degrees. For example, the blue range of colors centers at ~0o phase difference, green at ~240o phase difference and red at ~120o phase difference.
The amplitude or height of a sine wave segment controls the saturation or contrast of the respective color. A high saturation image will appear quite vibrant whereas a low contrast image will look dull and grey. This is demonstrated in the diagram below.[/vc_column_text][vc_row_inner][vc_column_inner width=”1/6″][/vc_column_inner][vc_column_inner width=”2/3″][vc_single_image image=”1463″ img_size=”full” alignment=”center” onclick=”link_image”][/vc_column_inner][vc_column_inner width=”1/6″][/vc_column_inner][/vc_row_inner][vc_column_text]The height of the sine wave segment’s midpoint relative to the the reference voltage controls the intensity or brightness of the color. The reference voltage is the lowest voltage of a video signal which a screen knows to interpret as a ‘visible image’ portion of the signal. This is usually around 0.3V. Low brightness analogue FPV video feeds will have a dark tint whereas high brightness analogue FPV video feeds will appear washed out. This is shown in the diagram below.[/vc_column_text][vc_row_inner][vc_column_inner width=”1/6″][/vc_column_inner][vc_column_inner width=”2/3″][vc_single_image image=”1462″ img_size=”full” alignment=”center” onclick=”link_image”][/vc_column_inner][vc_column_inner width=”1/6″][/vc_column_inner][/vc_row_inner][vc_column_text]When setting up an FPV camera, it is quite important to set the brightness and contrast values correctly. This can be done through the camera’s on screen display (OSD) menu. More information on setting up an FPV camera can be found here. The brightness and contrast can also be modified using the settings on most FPV goggles and screens.[/vc_column_text][vc_column_text]
Color Burst
Earlier I talked about how color is adjusted by moving a sine wave segment out of phase to a reference sine wave segment. The color burst is this sine wave segment used for reference. Simply put, the color burst keep the colors for a video line lined up or ‘in phase’. Observing the ‘segments of a video signal’ diagram, the color burst will proceed after the horizontal sync pulse for every video line. The color burst is a brief sine wave segment with a constant 180o phase difference. The color burst’s constant phase difference allows the video screen to reference the phase of the video signal’s color waves relative to the color burst to ensure that they are the correct and intended color. In high radio noise environments however, the receiver may not detect the color burst or the phase of the signal may be altered. A signal phase alteration is apparent by an oddly colored image such as the one shown below. If the color burst is lost completely due to video noise, the signal tends to drop to a black and white image.[/vc_column_text][vc_row_inner][vc_column_inner width=”1/6″][/vc_column_inner][vc_column_inner width=”2/3″][vc_single_image image=”1465″ img_size=”full” alignment=”center” onclick=”link_image”][/vc_column_inner][vc_column_inner width=”1/6″][/vc_column_inner][/vc_row_inner][vc_column_text]
Formats: NTSC vs PAL
The two most common formats for analogue video are NTSC and PAL. America tends to use NTSC where the rest of the world tends to use PAL. NTSC has a refresh rate of ~30Hz meaning that it can display roughly 30 frames per second. PAL in comparison refreshes at ~25Hz with a rough frame-rate of 25 frames per second. Although PAL has a lower frame rate, it has a higher resolution of 720×576 versus NTSC which has a resolution of 720×480 pixels. PAL is a newer format which had been specifically designed for color television whereas NTSC was adapted for color after initially being used for black and white television. Personally, I use PAL on all of my quad-copters however the choice is yours to make.[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]
The Clearview’s Magic
Now onto the most exciting part of the article! As a quick disclaimer, this information is only speculation based upon logic and personal reasoning. The Clearview may not employ this technology however it is quite likely that it does. To further improve the video quality, it is very likely that the Clearview also employs other technology (magic).
As previously discussed, an analogue FPV video signal contains horizontal sync pulses and vertical sync pulses. In an ideal video signal scenario, the VRX will receive a perfect signal, the position of the sync pulses will remain constant, and the video screen will be able to display a perfect FPV feed. Realistically, the video signal received by the VRX will not be perfect as the signal is subject to multipathing, interference from other radio transmitters, and general background radio noise. The effect of interference is video noise, a pilots worst enemy. This noise may corrupt individual pixels, lines or frames. In the presence of noise, the video screen may not be able to detect the sync pulses and will therefore display an un-ideal video signal where individual lines do not start on the left hand edge of the screen and appear to be ‘torn’ or missing.
What the Clearview does is to take note of the position of the sync pulses in the analogue FPV video signal and to overlay these pulses with identical generated pulses. This prevents the screen from losing the position of these pulses. The effect of this is that is prevents a noisy video segment from being displayed as misaligned, torn, or entirely static. The image below is a freeze frame of Joshua Bardwells video performing a Clearview range test compared to a freeze frame of my race day DVR.
When partial signal loss or noise occurs, a conventional receiver (such as the True-D or Laforge) that does not use sync pulses will lose or tear an entire line or video segment. With the generated horizontal and vertical sync pulses, the Clearview only loses the segments of the video line which were not received due to signal loss. Notice in the image below how the Clearview’s breakup is only the odd pixel versus the conventional receiver where a series of lines were lost. In regards to the comparative images below, the Clearview will not often break up this amount in a regular race scenario whereas the level of static from the conventional receiver is quite common during a race. Interestingly, even with a video image full of noise, as long as the noisy image has all the lines still in sync, the human brain can still process the image allowing the pilot to continue flying.[/vc_column_text][vc_row_inner][vc_column_inner width=”1/6″][/vc_column_inner][vc_column_inner width=”2/3″][vc_single_image image=”1664″ img_size=”full” alignment=”center” onclick=”link_image”][/vc_column_inner][vc_column_inner width=”1/6″][/vc_column_inner][/vc_row_inner][/vc_column][/vc_row][vc_row][/vc_row][vc_column][/vc_column][vc_column_text]
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thats super neato
Thank you for all the info. This is awesome!
fascinating! definitely learned something with this article!
Great info, glad to see an easy to access resource for all your fpv info
Actually, it is 5.740 Ghz or 5740 Mhz
If the final number is zero you can drop it. instead of saying 1.34000 GHz you drop the zeros because they have no meaning making it 1.34GHz
Yea so what going on with DIGITAL FPV these days
We have the technology where the Digital FPV market
The only ones I’m aware of are the Connex systems.
Oddly enough, the camera for that is apparently “discontinued” on GetFPV, but the RX and TX are still available.. Odd.
Fortunately, it looks like companies are working on digital systems. It will most likely be a while though before they replace conventional analogue systems, especially considering the image quality of a predator combined with the Rapidfire or CV. The future of FPV certainly looks exciting!
Real informative, thanks.
Clearview is the best
Thanks for the info!
Wait a sec…
So If I set up my fpv camera on NTSC which has an update rate of 30Hz, am I getting that same update rate on my eyes, with a little extra delay from the analog to radiowave to analog conversion?
If so, there we are having about 33ms latency at the least on video. Plus our reaction time, plus the RcLink Latency, and plus the QuadCopter PID & Physics Latency.
Just please confirm, that whatever image the Sensor captured on the quad, will be at least 33ms after, in my eyes :O