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Old Dec 12, 2009, 11:00 AM
MelihK's Avatar
United Kingdom, England, Canterbury
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New Audio Modem Protocol For FPV Telemetry

Hi Everyone,

I'm designing new audio modem protocol for FPV telemetry over PC.
It started for my SimpleOSD XL but i think every OSD can use it.

Protocol will very simple and i will share protocol and software with all OSD brands/users, i hope they will use same protocol and someone will write new softwares like googleearth connection or telemetry screens like voltage etc.

I just started the modem software over soundcard's microphone input.
I can write my own googleearth and antenna tracking codes into the software but this is not usefull for everyone, i'm planning giving outputs for another softwares like Happykillmore's softwares. this is best way for new softwares.



Modem software working like an oscilliscope for reading the bytes. baudrate is 50 byte per second.

What you thinking about this?

Thanks
Melih
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Old Dec 12, 2009, 11:20 AM
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Brunswick, OH
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Excellent! That's a great idea!

Are you thinking about using a text based protocol (liek NMEA) or machine code (like uBlox hex characters)?

I personally like the easier debug of the text protocol, but in actual use, the machine code is about 1/2 the size (or less).
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Old Dec 12, 2009, 11:52 AM
MelihK's Avatar
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Actually i'm searcing about simplest protocol and i found a way.

hardware protocol
signal looking like you saw on the software (green line is signal)
it starting with a start bit and stop bit, then it sending 8 bit value and silence area bigger than this signal totals. software will learn bit timing from starter bit's width. and it will find the start bit from big silence area.

This sample's value 3, you can see start+stop+10100000

I'm using 0.8ms for bit timing, this mean maximum freqency (1010101010) is 1200hz end it is compatible with all audio systems. and i'm syncronising bytes with camera frames. and it is enough for 50 byte per second.

software protocol
I'm not sure but, we can use compressed protocol for data.
1 byte starter byte(0x00) for finding line
4 bytes(32 bit) enough for GPS position values x 2 = 8 Byte,
1 bytes from ADC's (8 bit) like voltage, current, temp etc x 4 = 4 Byte,
2 byte Altitude (from sea level)
1 byte CRC for data correction

total 16 byte package. You can send it 3 times in a one second.

What is your comment?
is it useful or useless?
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Old Dec 12, 2009, 01:30 PM
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I think it's useful... you're talking 50 baud? Any chance it can go faster?

I like the idea. Is the hardware going to be inexpensive to do this? For about $100 users can add X-Bee...
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Old Dec 12, 2009, 02:19 PM
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50 byte = 50 x8 = 400 baud.
It is fast enough because you need 1200hz beeps for sending this data, higher speeds possible but it depends your videotx's audio output quality. useless for everyone.


You dont need any hardware for that.
It will free, because it is just a software that connected your video receivers audio output. And you can write a code to your OSD for sending signals over videoTX's audio output.

I will write this modem software that reads audio signals. What is best way for output from my firmware? i can give the bytes over TCP-IP, or application APIs, or DLL plugs but i'm not sure about that. did you used any way for a arduino softwares.
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Old Dec 12, 2009, 02:36 PM
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So the audio channel will get connected to the microphone input on my laptop? Or on a USB to serial channel? If it's actually sending using a serial protocol, then it makes more sense to use a "standard" baud rate like 4800...

All the stuff I've been doing lately is serial using a COM port.
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Old Dec 12, 2009, 03:49 PM
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Yes, audio signals coming from microphone input of laptop. Then my software will convert it to bytes.
Then i will use it for tracking plane over google earth and other telemetry values.
Normally i can integrate everything on same software and it can work only my OSD and software.
But i want to share it with other software developers like you or ardupilot users etc.
This is why i need a "plug" on modem software. If i design a plug(tcp or API) you can read values from my modem software and design your interface for this values.
And all OSD developers can use same protocol for sending telemetry to ground. this is why i start this thread.
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Old Dec 12, 2009, 04:10 PM
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I'm wondering, what makes it hard to use a standard modulation?
e.g. v34 comes to mind: http://www.itu.int/rec/T-REC-V.34-199802-I/en
(28.8k and 33.6 with v34bis)
its the fastest "simple, analog" standard (v.92 uses pcm and its not that simple afaik)
or the chips just aren't quick enough?
advantage would be that you don't need to write anything on the receiver side and it works on any system
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Old Dec 12, 2009, 05:03 PM
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Quote:
Originally Posted by MelihK View Post
What is best way for output from my firmware?.
Firstly, a virtual com port running at one of the common baud rates - many could use it for applications that are ready to use NMEA sentances over com ports.i.e. Google earth or a PC based antenna tracker that already exists.

Secondly dll etc for developers.
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Old Dec 12, 2009, 05:45 PM
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Quote:
Originally Posted by Shikra View Post
a PC based antenna tracker that already exists.

Which one is this?

Dimitris
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Old Dec 12, 2009, 06:09 PM
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This is the best! It would be so nice to have a antenna tracker that works with all major osd.
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Old Dec 12, 2009, 06:24 PM
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So what modem does the OSD use to encode onto the audio channel?

The info in the packets I would assume would include:

Lat & Long, Speed, Alt, Heading and maybe some other items like battery voltage and amps. I'm not sure the baud rate will be fast enough for all this in one second....
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Old Dec 12, 2009, 07:07 PM
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Best news this month
This will be sweeeet
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Old Dec 13, 2009, 12:37 AM
MelihK's Avatar
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Quote:
Originally Posted by jimbob00 View Post
I'm wondering, what makes it hard to use a standard modulation?
It is impossible because this is not telephone wire and you need fast DAC chips for sending full analog encoded standart modem signals. My modem protocol working like old CB modems for radio amateurs. Low speed is a rule for supporting all video transmitters etc.

Quote:
Originally Posted by Shikra View Post
Firstly, a virtual com port running at one of the common baud rates - many could use it for applications that are ready to use NMEA sentances over com ports.i.e. Google earth or a PC based antenna tracker that already exists.

Secondly dll etc for developers.
I was write it on top, but sorry, i guess my english not enough for that.
This is not serial modem device, nor NEMA transmitter etc. It is a software for translating audible signals to strings for OSD telemetry. And it cant give out Serial Port data, because it is alread inside the computer, dll or PI connections enough. I will connect it on google earth over APIs.


Quote:
Originally Posted by Dimitris76 View Post
Which one is this?

Dimitris
I will show it after modem protocol clear and everything is ready. basicaly 1 analog input, 1 button, 1 led and 2 servo out for tracking

Quote:
Originally Posted by bracky72 View Post
This is the best! It would be so nice to have a antenna tracker that works with all major osd.
It depends OSD developers, but i hope they will use.

Quote:
Originally Posted by HappyKillmore View Post
So what modem does the OSD use to encode onto the audio channel?

The info in the packets I would assume would include:

Lat & Long, Speed, Alt, Heading and maybe some other items like battery voltage and amps. I'm not sure the baud rate will be fast enough for all this in one second....
OSD's porcessor will create a string and it will send over audio.
I was calculate it on top of the page, 16 byte enough for most of values and mean 3 refresh per second.

Quote:
Originally Posted by KristianS View Post
Best news this month
This will be sweeeet
Thanks
I hope it will
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Old Dec 13, 2009, 12:55 AM
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Very nice idea!

Hope this will come useful for all of us.

Thanks.
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