Digital ATV Project

Digital ATV

For the latest DATV system you need to go here:

A new site dedicated to a much improved version of my DATV system

New board layouts and software are available.

OR you can continue the rest of my pages for the original system!!

A few test results Dec 2011

Here we have some tests done at 4Ms Digital vs analogue quality...

With 4Ms 1/2 i set the power level so i could access GB3KM with a small margin, i then switched over to Analogue and set the same power level as the digital was.

Here is the result: ____________Analogue___________________________________________Digital

______________

I also tested the difference in power level between a locked signal at 1/2 FEC and then 7/8 FEC, the result was ~4dB difference. According to the specifications for DVB-S the actual number is meant to be 4.4dB so that is close enough for me!

When going down to 1Ms at 1/2 the signal to noise will increase another 6dB compared with 4Ms 1/2, so if we drop the power from the above test by another 6dB on the analogue transmission we get this signal level at the repeater... quite weak!

.

It juist shows how well the lower symbol rates can get through compared with analogue!

The digital receiver at GB3KM is a Comag SL-35, the analogue receiver is a Standard Continental with IF Bandwidth at 16MHz i believe.
Both are driven with ~35dB gain Bob Platts type preamp tuned for a centre frequency of 1280MHz.

Back to the original information...

The story with my DATV system..

Since November 2007 I have been documenting my experiences with the Poor Man's Digital ATV Transmitter starting with F4DAY's 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 F1FAU showed his design for a fully functional 1024Ks/s DATV transmitter using similar hardware and also using Linux, I'm 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’m using some software called ffmpeg which takes the program stream from the capture card and splits it into separate Audio and Video elementary streams, these streams then get processed with the software tools in OpenCaster and then into transport stream format. I’m 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, that’s 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 F1HPR's 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 pot's to adjust I/Q level and I/Q voltage bias for canceling out the LO.

After that I’ve 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!