murali
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convert image to rtty

Mon Feb 25, 2013 9:25 am

Hi all,
I am working on a project for which I need to interface Raspberry Pi with NTX2 Radio transmitter.
As part of the above, I would like to know how the image is converted to RTTY?

I learnt that NTX2 is a frequency modulator (Voltage controlled osciallator). I am unable to understand how the image data can vary the voltage input for NTX2. Please explain. Your help is greatly appreciated.

Thanks!

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rurwin
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Re: convert image to rtty

Mon Feb 25, 2013 10:07 am

Do you mean like this: http://www.rtty.com/gallery/rttyeasy.htm
Or do you have a more modern greyscale pixel-based thing in mind?

murali
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Re: convert image to rtty

Mon Feb 25, 2013 11:14 am

ok..Let me be very clear. I want the images taken by camera (webcam ) connected to USB interface of Raspberry Pi to be transmitted to ground station. Those images shall be transmitted over radiolink.
I have seen some of the high altitude ballooning hobbyists who got succeeded with the same experiment. But, I did not find any information which can clearly explain how exactly the interfacing has to be done.

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rurwin
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Re: convert image to rtty

Mon Feb 25, 2013 12:18 pm

Generally you will find those people are licensed radio amateurs, or at least have one on their staff. I doubt that a license-exempt transmitter such as those you linked to would have sufficient power to send from an extended distance. It sounds like you are talking about a weather balloon, so the range will generally be several miles. For something like an R/C quadcopter or aircraft, where range will be no more than 100m or so, those modules would work well.

There are three ways to transmit images:
  1. Slow-scan TV. An image takes a minute or two to transmit, but it can be done over an audio link.
  2. TV. Full 625-line analogue TV, but it needs a bandwidth of around 1MHz.
  3. Packet, such as AX25 or TCP/IP over PPP, where you send images as files. The bandwidth and hence frame-rate will be between the above two. It requires a bi-directional link, which wont work with those modules.

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daveake
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Re: convert image to rtty

Mon Feb 25, 2013 12:36 pm

rurwin wrote:Generally you will find those people are licensed radio amateurs
In the UK and some other countries radio amateurs are not allowed to use their licence from an airborne device, so we are restricted to using licence-free transmitters in some small, low-power bands where airborne use is permitted. The NTX2 is a popular choice for high altitude ballooning, and both the 434.075 and 434.650MHz variants are allowed. Maximum power is 10mW ERP which, surprisingly can reach over some very long distances (the record is over 800km) when paired with a good receiving aerial and a sensitive (amateur radio) receiver.

Baud rates are generally 50 baud but up to 1200 has been used albeit with a much reduced range. Images have been sent over these links, usually at 300 baud, using an encoding system that splits the original jpeg image into a number of self-contained 256-byte packets each of which includes FEC. I've used this on 3 flights so far (and am doing another one this week) and it works very well.

To the original poster, if you're planning on flying a balloon, I suggest (quite strongly actually) getting just a basic tracker running first, transmitting some simple telemetry containing the balloon's GPS position, before progressing further.

Dave

murali
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Re: convert image to rtty

Mon Feb 25, 2013 1:41 pm

Thanks rurwin

murali
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Re: convert image to rtty

Mon Feb 25, 2013 2:56 pm

Thanks Dave!

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