Video transmission F.A.Q.

This Frequently Asked Questions section has been put up in response to many requests for information and advice on video transmission. The following material represents opinions based on experience and research. There may be ideas here that some people disagree with, and we welcome any such comments or suggestions. This FAQ is intended to serve you, the reader, so if you have specific questions on this subject, please visit the Contact page. That way we can make this FAQ really cover the material you want.

Last Update:  December 15, 2001


I'm thinking of transmitting live video from my model aircraft. What are the different types of transmitters, and which type is best for this type of work?

The consensus appears to be that a 2.4 Gigahertz FM system is ideal. We'll discuss some of the reasons for this. The information below just scratches the surface, but will give you a bit of a feel for what's going on. Two very important characteristics of a video transmitter are the modulation type (AM or FM), and the frequency it operates on.

Modulation Type

AM (amplitude modulation) is used by TV stations and picked up by your TV set. If you have one of those UHF "video senders" that can be picked up directly by a TV somewhere in the house, that will be AM, since the signal is being picked up directly by your TV, which has an AM receiver.

The weakness of AM is that the picture information is encoded in the amplitude of the signal, which is really just the strength of the signal at any given moment. If the overall strength diminishes, the ability of the receiver to decode the signal will be impaired.

FM (frequency modulation) is considered vastly superior, as it looks at variations in the frequency of the signal. It's a bit like digital - it does not care so much if the signal gets weak. It keeps working fine, unless the signal gets so low that it stops working altogether. You notice this difference when you compare AM and FM radio.

There is another effect that influences our choice of modulation type: the phenomenon called "multipathing". This occurs when the signal finds an additional route to the receiver by bouncing off an object such as a building or a car, causing constructive or destructive interference where the two paths meet at the receiving antenna, resulting in sudden high or low signal strength. For the same reason as before, FM copes better with this.

Another problem with AM occurs when the received signal strength fluctuates rapidly, such as from vibration of the transmitter, or from reflections off a rapidly moving object such as a helicopter's rotor. At the receiver, there will be rapid variations in signal strength. In effect, there is unwanted amplitude modulation occurring, distorting the original amplitude-modulated signal. With AM, the amplitude carries picture information, so this will damage the picture. With FM, the picture information is not carried this way, so the picture information is not corrupted.

Having said all this, it must be pointed out that FM can still be affected by multipathing, as well as loss of signal because of obstructions. But when these things happen, they tend to appear as sudden breakups in an otherwise excellent picture. You will have seen this, for example, in live coverage of bike races through public streets. Yes, it even happens with hundred-thousand-dollar equipment! But with AM it would have appeared as constantly varying picture quality.

So how do we minimize these breakups in this otherwise excellent FM picture? The secret is the antenna. By using a somewhat directional antenna (such as a patch) on the receiver, we can "ignore" the reflections coming off buildings. And in extreme cases, such as a model helicopter with carbon fibre rotor blades, the blades are so reflective that a directional transmitter antenna may be needed to send all the signal downward.

Frequency

Various frequency ranges have been set aside for license-free use, as well as amateur use. Going from lowest to highest frequency, the common ones you'll come across are around 440MHz (used by the UHF video senders just discussed), 900MHz, 1.2GHz and 2.4 GHz. By convention, 1 GHz or higher is defined as "Microwave".
Just for interest, your microwave oven generates 2.45 GHz to heat the food.

Note:
 MHz stands for MegaHertz (million cycles per second)
 GHz stands for GigaHertz (billion cycles per second).

The higher in frequency you go, the more you require line-of-sight. In our case, this is not a problem anyway, since our flying machine is up in the sky!

A big advantage of higher frequency systems is that they have more bandwidth available for the video signal. Here's an explanation:

If you drew a horizontal line with a scale along it representing frequency, you could show the commonly used frequency ranges (bands) by drawing short horizontal lines in the appropriate places. You could show the ranges allocated to absolutely everything, including well known things like CB radio, AM and FM radio, and VHF and UHF TV. But even a single single transmitting device will use up some width on this scale. This is called bandwidth.

At higher frequencies, there tends to be a wider frequency range available for a particular band. Consequently, devices can transmit a signal with a wider bandwidth without causing congestion in that band. Manufacturers of 2.4GHz equipment take advantage of this. The wider bandwidth actually means more information can be squeezed in. The result: sharper pictures and more accurate colours.

Another nice thing about extremely high frequencies like 2.4GHz is that the wavelength of the radiation is smaller. In fact, it's inversely proportional to frequency. Why is this good? Well, it means the antenna on our model can be very small and light!

To sum up, 2.4 GHz systems are ideal for our kind of work. A decade or two ago, they were very expensive. This is because it is very hard to design a circuit to work properly at a couple of billion cycles per second. Unwanted effects occur: tracks on the circuit board can act like inductors and capacitors! With recent advances including computer-aided circuit design, they are now easier to manufacture, making them much more affordable. As a bonus, 2.4 GHz systems are usually FM.


How do I pick up the signal?  Can I just tune into it with my TV, or do I need a special receiver?

You can only pick up the signal using your TV's tuner if the transmitter is designed for this. These "Video Senders" are intended for home use, and generally don't work well for aerial applications. They will probably be UHF, although there are some VHF ones still around. They will also be AM.

In contrast, a 2.4 GHz FM system will require a special receiver to match the transmitter. This receiver has video and audio connectors which plug into the A/V inputs on your TV, VCR or camcorder. This means that it's a good idea to get a TV that has A/V inputs. Alternatively you could use your VCR's A/V inputs, and then connect your VCR to your TV. Another method is to use a camcorder, as many of these have A/V inputs.


There are lots of micro video cameras around, with different costs and specifications. How do I choose what type to get?

You'll come across two distinct types: CCD and CMOS. These names refer to the two fundamental types of image sensors used in these cameras: CCD (charge-coupled device) and CMOS (complimentary metal-oxide semiconductor).

CMOS sensors tend to be very cheap, because they use the same technology as silicon chips, and can be manufactured on any standard silicon production line. However, they suffer from picture noise and lack of sensitivity to low light. In comparison, CCDs use a special manufacturing process, which lets them transport charge across the chip without distortion. The result is high-quality, low-noise images. A further advantage of CCDs is that they've been around longer, and the technology is more mature, resulting in further refinements in quality and higher numbers of pixels.

The verdict: unless you are on a shoestring budget, avoid CMOS and get a CCD camera. If you are unsure, talk to someone who has used both and see what they say!

The other consideration is resolution, which for these cameras is specified as the number of lines in a test image that the camera can successfully "see". So if the camera has more lines of resolution, the picture will be sharper. The camera will also cost more. Resolutions range from around 250 lines in the cheap CMOS cameras, up to over 500 for very expensive CCD cameras. As a comparison, have a look at the resolution you get with these recording formats:

 VHS & 8mm - 240 lines
 Super-VHS & Hi-8 - 400 lines
 Mini DV camcorders & DVD players - 500 lines

So, what resolution is good for a micro video camera? Well, the smallest cameras tend to have low resolution, so if you are after something small and light, you may have to compromise in regard to resolution. You can still get something very small in the superior CCD format rather than the CMOS format (which is common in small ones). People with small planes or helicopters will need to choose a very small, light camera. People who are using the system as a "video assist" to help frame up their photos will probably also want to keep things light, considering all the extra equipment on board. If you're recording the received picture on VHS tape, a camera of anything over 300 lines will be adequate, as you are only recording at 240 lines!

If you have a Mini DV recorder on the ground to record the picture, then the greater cost and weight of a high-res camera will have a noticeable benefit. However there is always some loss of quality associated with a downlink, even with the most expensive systems used by TV networks. The only way to get the ultimate quality is to record the picture up on your flying machine with your DV camcorder - and make sure it is insured for this!


Obviously I need an on-board battery system on my aircraft to power the camera and transmitter. What type of battery should I use?

Unless you're prepared to keep buying batteries for every "video session", a rechargeable battery pack will be needed. There are two main types: Ni-Cad (Nickel Cadmium) and Ni-MH (Nickel Metal Hydride). Either way the pack must have the right number of cells to give the voltage needed by the camera and transmitter. (Each cell makes roughly 1.2 Volts.)

Both types do the job well, but there are differences. Ni-Cads can suffer from memory effect, resulting in a loss of capacity. This happens when use them for a bit (without fully discharging), then charge them for a bit, and so on. This can sometimes be undone by deep cycling (full charge and full discharge).

Ni-MH batteries don't have this problem. You can partially discharge them or partially charge them without fear of long-term damage.

The big advantage of Ni-MHs is that they have a much greater capacity for a given size. This is significant for model aircraft. It means that you can use a smaller (and lighter) size and still have a satisfactory battery life. For example, you can use a pack of "AAA" cells (much lighter than "AA") and still have 650 to 700mAh of capacity... very impressive.

Another factor in favour of Ni-MHs is that they're environmentally friendly. They don't contain the extremely toxic Cadmium, which is why Ni-Cads have strict laws about how you must dispose them.

So, what's the catch, you ask? Well, Ni-MH batteries are more expensive than Ni-Cads. In our opinion they're well worth it, but ultimately it's a personal decision.


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