DigiLite

.................................DigiLite System Overview ...........by G4EWJ ..(edited by G8AJN)

DigiLite is a system designed by radio amateurs to provide a lower cost method of home building a digital modulator for transmitting live digital TV pictures (DATV). It is a derivative of the “Poor Man's DATV System” which many amateurs have contributed to over several years. The system has 6 main parts:

 A Windows PC with a video capture card that uses an MPEG-2 compression chip.

 A recording program on the PC to save the data from the capture card onto hard disk.

 A program running on the PC to read the recorded data from hard disk, convert it to a form suitable for transmission and send it to a serialiser.

 A serialiser, which is a device that connects between a PC USB port and a modulator.

 A QPSK modulator.

 A program to configure the transmission parameters.

DigiLite System Outline Details

A Windows PC with a video capture card that uses an MPEG-2 compression chip. The current project assumes a WinTV PVR card. A recording program on the PC (either WINTV v 7 or GBPVR v1.4.7 )to save the data from the capture card onto hard disk.
A program running on the PC to read the recorded data from hard disk, convert it to a form suitable for transmission and send it to a serialiser. A serialiser, which is a device that connects between a PC USB port and a QPSK modulator. A program to configure the transmission parameters.
If an older PC is available, which can be dedicated to DigiLite, this is ideal. A single core 1.8GHz processor with USB2.0 should be sufficient on a 'quiet' system. On a non-dedicated PC, if Windows gets busy doing other things, the conversion program may not get all the processing time it needs and that can cause an occasional glitch in transmission. The more powerful the PC, the less chance there is of this happening.

Recording program:
DigiLite has been designed to use the Hauppauge PVRx50 PCI video capture cards. There is also a Hauppauge PVR USB2 external device that can be used with laptops. Other cards are being evaluated, but they need to do the mpeg2 encoding in hardware (in an IC not in software).There are often WinTV cards cards available cheaply on Ebay etc.It should be possible to get a card and disk (a disk is needed to download WinTV v 6 free). If you dont get a disk you will need any version of an Original Hauppage Install disk in the PC to get a free download of the WinTV 6 programme.

If you cannot get WinTV you can use GB-PVR.
GBPVR is a free PVR (personal video recorder) program. Version 1.4.7 has been used sucessfully.Later version may not work. GBPVR requires a certain standard of video graphics card to run, even if you do not actually want to watch the video on that PC. Older graphics card may have problems. More card details on hardware pages.

Conversion program: DigiLite Transmit is the program that reads the recorded data from hard disk immediately it has been written.
The data is in Program Stream format, which is very similar to the format used on DVDs. The video and audio are extracted and converted into DVB-S transport stream format, which is the format used by European broadcasters to deliver standard definition programs via satellite. This means that a cheap FTA (free to air) satellite box may be used to receive the transmission. Other data is placed in the transport stream such as date and time, channel name, program name and Electronic Program Guide info. The transport stream data is sent to the serialiser via a USB2 port.

Serialiser:
This is the device that connects to a high speed (480Mbps) USB 2.0 port and accepts the transport stream data from the PC. Two bits at a time (a symbol) are sent to the modulator at the symbol rate. The serialiser may optionally apply the FEC (aka Viterbi) forward error correction processing to the transport stream data, or this can be done on the PC. Doing the FEC on the serialiser is prefferred as it reduces the data rate over the USB connection. The serialiser has a second communication channel which is used to receive control information from DigiLite Transmit to set the required symbol rate and FEC. The serialiser can also be put into one of several test modes, to help set up the modulator. The serialiser has been used successfully at symbol rates between 1250k and 6250k. Several symbol rates are pre-programmed into the system.
The serialiser has an SD card socket for playing pre-recorded transport stream files. This function is still in development and is not available in the initial release. The serialiser CPU chip dsPIC333 will need to be re-programmed to add new functions such as this.

Modulator:
QPSK stands for Quadrature Phase Shift Keying. The modulator must be provided with a signal source at the required transmission frequency. It filters the two digital signals (I and Q) from the serialiser and produces an output at the transmission frequency which can then be amplified and transmitted. The type of filtering used is known as 'Nyquist'.

Configuration:
DigiLite Config is the program that is used to set the symbol rate and FEC and also the channel name and program details.Settings are stored in the Windows registry, which DigiLite Transmit monitors continually to look for any changes in transmission parameters.DigiLite originally began life as the 'Poor Mans Digital encoder'. Credits must go to those with the original concept.(including F4DAY and F1HPR ). Brian G4EWJ has worked on the software and developed the serialiser and USB version. Using the MPEG2 encoder chip on the WIN TV capture card with a PVR program to record on to a PC's hard drive whilst simultaneously reading from the file to create a serial USB stream.
This goes to the serialiser board where the stream is buffered and extra code is added to the packets before being sent to an IQ modulator as a Transport Stream.
The modulator uses a local oscillator set to the required output frequency,e.g. 1249MHz. Rob(M0DTS) worked on getting the PC to serialiser and modulator hardware running up to the required 4Ms/s and using the more convenient USB input. Brian(G4EWJ) worked on the hard drive reading software for the serialiser and USB and moved it from the original Linux software to the current Windows version. Test boards were built by Rob and Malcolm G0UHY to confirm that it can be replicated. Symbol rates were gradually stepped up from 2Ms/s to 4Ms/s and above.

Here is a description of the digital system adapted from the M0DTS website...
A Digital ATV Transmitter is built of 3 main 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. Currently 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.Software reads the transport stream from the HDD and splits it into separate Audio and Video elementary streams, Service Information tables (PAT, PMT, SDT, EIT etc.) are added also Reed Solomon correction is added into the transport stream.
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 dsPIC333.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, the same as used in the F4DAY system.
The filtered IQ outputs are fed into a modulator, using F1HPR's design, an Analog Devices AD8346 IC 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 (and perhaps the Nyquist filter components if using lower symbol rate). The modulator has preset pot's to adjust balance of I/Q level and I/Q voltage bias for canceling out the LO. After that a a MMIC amplifier gets the signal up to a level required for input to an external amplifier to raise the level to that required by the PA. Remember that the bandwidth of the 4000ks/s is roughly 4Mhz, so the power required to cover as well as the 12Mhz analog one does is much lower. This allows you to lower the drive to your PA and thus get into the best linearity that you can without any loss of coverage.