The story with my OLD DATV system...
I don't use this system at all now but it documents the beginning of my DATV Experience!
Since November 2007 I have been documenting my experiences with the Poor Mans Digital ATV Transmitter starting with F4DAYs design and then trying to bring it up to date to make it easier for the rest of us to use, it has been a long climb to get to where we are now with a working system to generate Live DVB-S transmissions.
My system as of January 2010
My current DATV transmitter system connects to the USB port of your computer and enables live video transmission at 1 or 2Ms/s on 70/23/13cm bands using Linux (ubuntu) operating system and a few pieces of software.
It is possible to do some live video using windows but it is very far from perfect so for now I have stopped work on it. If there is any future advancement with windows software I may try and get it operational but for now it just does not seem possible without some software expert to come up with an answer! There is only my original information at the bottom of the Live DATV page if you want to experiment.
With this current setup pre-recorded transmissions are fully operational on both Linux and Windows; live transmissions are only possible using Linux.
In December 2009 F1FAU showed his design for a fully functional 1024Ks/s DATV transmitter using similar hardware and also using Linux, I am currently awaiting boards to test this setup and try to understand his design...
How Does It Work?
A Digital Television Transmitter is built of multiple stages, these are: MPEG-2 encoder, multiplexer, Error correction and Convolutional coding, Serialiser, Nyquist filter and then to an IQ modulator.
The MPEG-2 encoder takes the analogue Composite video and audio signals in and converts them into a digital signal, it then gets compressed into the standard MPEG-2 format. For this I am now using a Hardware MPEG-2 encoder, the Hauppauge PVR-150 (or 250/350/500) pci card in the PC.
The multiplexer takes the MPEG-2 audio, Video and service information and puts them into one single stream called a Transport Stream which is made of 188 byte long packets of data.
For this I am using some software called ffmpeg which takes the program stream from the capture card and splits it into separate Audio and Video elementry streams, these streams then get processed with the software tools in OpenCaster and then into transport stream format. I am also generating the Service Information tables (PAT, PMT, SDT, EIT etc.) with OpenCaster which are added into the transport stream.
Then there is two types of error correction added to the stream, one is Reed Solomon and this adds 16 bytes of coded to each packet so that at the receiving end some corrupted data can be recovered using some error checking routine - probably like a crc check..
Then the data is passed through a convolutional encoder, this sends multiple bits out that represent a single bit of data, thats where the 1/2, 2/3, 3/4, 5/6, 7/8 system comes from.
For 1/2 there are two bits of data transmitted for every one bit of data input, a kind of lookup table is used to recover the coding at the receiver end I think.
For 1/2 rate only half of the possible data rate is actually utilized by the AV stream and the other half is for error correction.
This is also done in software for me using a clever program called gbDVB.
The serialiser takes in the parallel data stream and converts it to a serial stream in IQ format ready for the modulator.
This is done in a microprocessor ATTiny2313 from Atmel thanks to Cesco HB9TLK for the code, maybe he will be interested in writing a new version with selectable coding for different FEC sometime.. hint hint ;-)
The Nyquist filter is a special Low Pass Filter that removes the unwanted sidebands from the I/Q signals and retains the impulse response of the data signal very well. This is just an LC filter designed specially for the purpose in software - the same as used in F4DAY system.
The filtered IQ outputs are fed into a modulator, I?m using F1HPRs design using an Analog Devices AD8345 IC for this as it directly modulates the LO for the band you are operating on. This makes it very easy to change between different bands by simply supplying a different LO frequency and some minor adjustments of IQ bias and level.
The modulator has some pots to adjust I/Q level and I/Q voltage bias for canceling out the LO.
After that I have added a couple of MMIC amplifiers to get the signal up to a level for driving a low power amplifier like the ones seen on ebay (MHW2723 for 70cm) or directly into one of the Nokia TTRX 70cm amplifiers also found on ebay!
