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Old Nov 17, 2011, 01:51 PM
sharkforce is offline
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Question

how does using a RF modem for video work


for a project that I am working on we are planning on using a 2.4 or 5.8 ghz modem for sending video to a ground station, but we dont know anything about the architecture of using rf modems. does anyone have experience with transmitting video digitally vs analog? how can we find the amount of bandwith required for video.
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Old Nov 18, 2011, 02:22 AM
radialmind is offline
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Quote:
Originally Posted by sharkforce View Post
for a project that I am working on we are planning on using a 2.4 or 5.8 ghz modem for sending video to a ground station, but we dont know anything about the architecture of using rf modems. does anyone have experience with transmitting video digitally vs analog? how can we find the amount of bandwith required for video.
Digital video transmission is easiest just using wifi. There are loads of projects for outdoor wifi transmissions (antennas, stations, boosters, etc.). Ubiquiti and Mikrotik come to mind. The bandwidth for wifi is 21-24 MHz depending on your band. Analog video typically uses around 23MHz, but only actually requires 17 Mhz. You can't achieve HD for analog, because it's completely full. The only improvements that can be made is to exchange color for luminance or make more use of the entire time window between vsyncs and after hsyncs. Digital (wifi) uses clever modulation techniques, which is why the information transfer is much, much higher.

WiFi is subject to issues over very long distance (>2-5km). ACK timeout windows need to be adapted for such long distances. So specialty outdoor wifi equipment becomes a necessity.

WiFi bitrates are not necessarily constant. If there are other stations around you, the station will typically reduce the bitrate to allow other stations to transfer information. So urban areas may give you much less actual bitrates than 'middle-of-nowhere' setups. Other questions that may be relevant are here:

http://www.wlanantennas.com/faq.php

MJPEG digital requires high bitrates that I don't think fit into a 54 Mbps wifi link (typical). But H.264 does, as this requires somewhere around 10 Mbps. I need to do more accurate measurements there, but it should be possible to contain up to 4 streaming HD video links, assuming there is no congestion on your band. A spectrum analyzer helps there.

A good and bad thing about analog is that you have gradual degradation of the signal. Digital hits a cliff where error correction algorithms suddenly can't deal with the errors any longer and absolutely nothing gets through. Digital however allows you to get picture perfect images from far distances if the link is good and equipment should typically have indications of signal strength and signal quality (the amount of error corrections being done).

Does that answer your question?
Old Nov 18, 2011, 04:43 AM
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Quote:
Originally Posted by radialmind View Post
Digital video transmission is easiest just using wifi. There are loads of projects for outdoor wifi transmissions (antennas, stations, boosters, etc.). Ubiquiti and Mikrotik come to mind. The bandwidth for wifi is 21-24 MHz depending on your band. Analog video typically uses around 23MHz, but only actually requires 17 Mhz. You can't achieve HD for analog, because it's completely full. The only improvements that can be made is to exchange color for luminance or make more use of the entire time window between vsyncs and after hsyncs. Digital (wifi) uses clever modulation techniques, which is why the information transfer is much, much higher.

WiFi is subject to issues over very long distance (>2-5km). ACK timeout windows need to be adapted for such long distances. So specialty outdoor wifi equipment becomes a necessity.

WiFi bitrates are not necessarily constant. If there are other stations around you, the station will typically reduce the bitrate to allow other stations to transfer information. So urban areas may give you much less actual bitrates than 'middle-of-nowhere' setups. Other questions that may be relevant are here:

http://www.wlanantennas.com/faq.php

MJPEG digital requires high bitrates that I don't think fit into a 54 Mbps wifi link (typical). But H.264 does, as this requires somewhere around 10 Mbps. I need to do more accurate measurements there, but it should be possible to contain up to 4 streaming HD video links, assuming there is no congestion on your band. A spectrum analyzer helps there.

A good and bad thing about analog is that you have gradual degradation of the signal. Digital hits a cliff where error correction algorithms suddenly can't deal with the errors any longer and absolutely nothing gets through. Digital however allows you to get picture perfect images from far distances if the link is good and equipment should typically have indications of signal strength and signal quality (the amount of error corrections being done).

Does that answer your question?
thanks, I understand what you're talking about with how digital works vs analog and I actually learned about the differences between analog and digital RF in my spacecraft systems engineering class today in a lecture on TT&C.
But what I want to figure out how to do is a video data link for a small uav. say using something like this, http://www.microhardcorp.com/n2420.php, the idea is to have a video data link similar to what aeroviroment has done with their SUAS DDL in upgrading their MAV's.

another question I have, is the compression of video (say h.264 for example) typically done on the camera unit itself or on the transmitter or somewhere inbetween like with an onboard computer..
Old Nov 21, 2011, 06:28 AM
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Quote:
Originally Posted by sharkforce View Post
thanks, I understand what you're talking about with how digital works vs analog and I actually learned about the differences between analog and digital RF in my spacecraft systems engineering class today in a lecture on TT&C.
But what I want to figure out how to do is a video data link for a small uav. say using something like this, http://www.microhardcorp.com/n2420.php, the idea is to have a video data link similar to what aeroviroment has done with their SUAS DDL in upgrading their MAV's.

another question I have, is the compression of video (say h.264 for example) typically done on the camera unit itself or on the transmitter or somewhere inbetween like with an onboard computer..
Generally the encoding is done on a separate dedicated device or by an on board computer.This is a $12,000.00 stand alone video modem you are talking about duplicating. Please note it is a two way active link system. Those are some mighty big pants to try and fill if you just learned about analog vs.digital transmission just the other day.

Everything you are asking about is going to introduce lag into your video system. You will not likely be able to reliably use the video for piloting purposes unless you manage to duplicate AeroVironment's work.

Good Luck,

Rob
Last edited by camship; Nov 21, 2011 at 06:43 AM.
Old Nov 21, 2011, 07:41 AM
Don Moretti is offline
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Is there a link to his work?
Old Nov 21, 2011, 09:16 AM
camship is offline
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If you are wanting AeroVironment, Inc.


Rob
Old Nov 21, 2011, 09:32 AM
Don Moretti is offline
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Superb, thanks
Old Nov 22, 2011, 02:31 PM
sharkforce is offline
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let me ask another question guys, I am looking simply for a system to compare with, what are the existing systems that anyone knows about that uses a digital video data link.

and specifically what I really need to know, what is the throughput requried for whatever resolution and framereate of the camera that they are using. the raven uav and other aeroviroment stuff does not list those parameters that I need to know about their camera specs and video link.
Old Nov 26, 2011, 03:10 AM
marhar is offline
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Mark Harrison
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Originally Posted by Don Moretti View Post
Superb, thanks
BTW, if his name sounds familiar... he's the guy that did the Gossamer Albatross back in 79, the first human-powered flight over the English Channel.

It's great to see he hasn't rested on his laurels the past 30 years!


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