Circuit Notebook 96 - A Baptism of Fire into the Digital Age of ATV

The digital switchover of broadcast TV channels has just begun and in a few years all channels will be digital. Recently, not wishing to be left out, I decided that it was time to experiment with digital amateur TV. The main attraction is that a digital ATV transmission can provide good picture quality and yet need only occupy a bandwidth of about 2MHz. This opens up the possibility of bringing ATV back to the 70cms band, thus giving increased range over difficult terrain. When moving into a new technology it is often difficult to get started on the learning curve. Once something, no matter how basic, is up and running then you can build on it and progress is much easier.

Recent History

A few years ago TV amateurs and those who are also professional engineers, in Germany and Holland, produced kits and complete digital encoder-modulators for amateur use. One such kit is that produced by the AGAF e.V. Group at Bergische University of Wuppertal. The BATC obtained several of these units for loan and/or sale to members for experimental use. A description of work carried out by Ian Waters G3KKD with one of these units is given in CQTV No. 208, pages 30-34 and CQTV No. 212, pages 6-11.

More recently, a trickle of professional ex-broadcast (NDS) digital encoder-modulator units have made their way into ATV home shacks. A description of the activities of Mike G8LES and Dave G8ADM using this kind of equipment is given in the ATV Column of the September 2007 RSGB RadCom, by Trevor G8CJS.

NDS Encoder-Modulator E5910 - Experience so far

After discussions with Dave G8ADM a large, heavy box arrived by carrier containing an NDS MPEG-2 Digital SNG Coder Model E5910. This unit forms the basis of a news gathering satellite ground station which is used for sending pictures and sound from a remote location back to the TV studio by satellite link.

By amateur standards it is big and it is heavy. It is '5U' (8 ¾") high by 19" deep and is suitable for mounting in a standard 19" rack. The front panel, shown in Fig.1, has a digital display screen (4 rows of 20 characters) for monitoring and diagnostic purposes, a numerical keypad and an X-Y keypad for setting the encoding and modulation parameters - no knobs. The back panel, shown in Fig.2, has a range of BNC connectors for video in and out and a range of XLR connectors for audio in and out and the usual IEC mains input connector.

The unit contains five very large PC boards, each mounted in a 1U tray. These comprise an Audio/Data/Teletext board, a video input board, a video encoder board which encodes to MPEG-2, a multiplexer and modulator which provide forward error correction and QPSK modulation and a signal IF modulator board which provides a modulated RF output in the range 50-90MHz and 100-180MHz. The final board is an RF receiver which can work with an external LNB to receive off-air signals or can loop back to check the output of the complete system.

Switching on was impressive. Six cooling fans burst into life, warning lights came up and bleepers sounded. I was fortunate in that I had been sent a CD of the operators manual by Noel G8GTZ. Over the next few days I worked my way through the 69 pages of the setting up procedure. However, all was not well, I kept getting a message 'FAIL - Receiver (IRD) Card'.

The news of my difficulty got around and in typical amateur radio fashion help came from all quarters. Paul G6MNJ, who has one of the NDS Encoders which is in working condition stepped through each of the settings on his machine and logged these as a file. He then sent me a copy of the file containing the 28 settings so that I could set my machine to exactly the same parameters. Paul has offered his list of settings for the benefit of anyone else who may inherit an NDS EN5910 machine. The list is given below.

I installed the settings according to the list but again the FAIL message 'Receiver (IRD) Card' appeared. I have now come to the conclusion that this is most likely to be a hardware fault, connector, component etc. which is feeding the IRD card. (IRD means Integrated Receiver Decoder by the way).

I went into the shack today 20th December and switched on the beast. Lights came on, bleepers sounded as usual but fans failed to start. Is it fan failure? fuse gone? software setting? No! The shack temperature was only +1 deg C - so no cooling required! Why does one always expect the worst?

As time moves on I anticipate that a few more NDS units will filter on to the amateur market and I hope to confirm my hardware fault by substituting boards and if possible replace the offending item.

By e-mail from Paul G6MNJ

E5910 DSNG IRD Setup

I have listed below the current settings of my NDS Modulator/Receiver for the purpose of a paper backup for myself and for others to use in order to know that with these parameters and a valid video input their unit should work.

The amateur 'standard' for digital TV is still emerging but it seems that the favourite FEC Rate has become ½ , unfortunately as yet I have not been able to persuade my unit to accept this and the lowest level achieved has been ¾. If as I suspect this is a limitation of the professional kit we may have to rethink our standards.

Setup of the NDS DSNG IRD, once powered up and booted you will be presented with a menu on the screen. Press the Setup button to the right of this and then the System, Modulator, Mux etc along the bottom of the screen for access to each of the sections documented below.

System
	Service Name			G6MNJ_Digital
	Set UTC Offset Hrs			0 or 1 depending on Summer/Winter
	Set Date & Time			key in the date and time!

Modulator
	IF O/P 				ON *
	IF Power				ON *
	Modulation				ON *
	Tx Bandwidth			2.560 Mhz (2.000 Msym/s) +
	Symbol Rate				2.000 Msym/s +
	FEC Rate				3 / 4
	IF					60.50 Mhz **
	Spectrum Inversion			OFF
	IF Preset Power			0.0   dBm (about 3mW)
	Satellite Frequency			Don't care!
	Satellite Position 			Don't care!
	Satellite Direction 			Don't care!
	Satellite Polarisation		Don't care!

* you may need to return to this menu last to get the I/F and modulation set to ON as the unit will refuse to produce RF unless all arameters are correct.

+ These items affect each other so no need to set both, I set the Symbol rate and it calculated the bandwidth itself.

** the frequency is dependant on what you need to multiply up to your final TX frequency. I will have a modified Microwave Modules 28Mhz to 430Mhz converter, this had a 101Mhz oscillator with multiplied up by 4 to 404Mhz and the 28Mhz was mixed in with that making 432Mhz. It has now a 94Mhz xtal in its oscillator so making the 4 times 376Mhz so to get on 436.50 Mhz the IF frequency will be set to 60.5Mhz on the modulator.

Mux
	Output Format			IF
	Scramble				OFF
	Scramble Code			0000000

Video
	Video Source			PAL
	Pixel Resolution			AUTO (1 / 2)
	Video Bit Rate			2.000 Mbit/s
	Coding Mode				IBP
	Low Delay Mode 			OFF
	O/P on Video Loss			Freeze Frame

Audio		
	Channel A				ON
	Coding Mode				MONO
	Type					20K ohms
	Bit Rate				96Kbit/s
	Language 				ENGLISH
	View Levels - useful display of incoming audio

Data
	Nothing to set here

Receiver
	Receiver Mode			TRACKING
	Input Interface			INTERNAL

All other settings will follow modulator with the TRACKING option set.

I hope this is of use to all who read it, and I welcome comments and feedback from others using this kit for DATV.

Paul G6MNJ - paul@andrews.eu.com

When progress with the NDS unit halted I moved to the AGAF Encoder PC Boards

AGAF PC Boards - Experience so far:

I obtained on loan from the BATC a set of (two) AGAF PC boards. These had previously been out on loan and were fitted in an open chassis which gave easy access to the links on the PC boards and to the input (phono) and output (SMA) connectors through holes in the side. Shown in Fig.3.

Inputs are (phono) 
Video PAL/NTSC, Y/C
Audio left & right.

Outputs are (SMA)
LO 478MHz -24dBm
IF 44MHz -24 dBm
RF 434MHz (QSPK) 0dBM.

DC Supply +12Volts

I down-loaded technical information from the AGAF website www.datv-agaf.de/DATV_Boards_V1.pdf This gave information on setting various links on the PCBs to define the required modulation parameters I set the links to the recommended positions to provide:

Input				PAL
Input				CVBS (Composite Video Blanking & Sync)
Data rate elementary stream 	6.0 MBits/sec
Data rate transport stream	6.75 MBits/sec
Forward error correction	⅞

The AGAF information states that these settings result in a QSPK symbol rate of 4.167 MSymbols/sec. The 434MHz output has a occupied bandwidth of 4 MHz and so is more suitable for up-conversion to other microwave bands than for direct use on 70cms.

I connected a 12V supply, making sure the polarity was correct and switched on. It states that it is protected against reverse polarity but I didn't wish to try it. A red LED flashed ten times as stated in the instructions and then a green LED came on indicating all was well.

My present requirement is to transmit DATV on 23cms. To make initial tests I put together a double balanced mixer, Figs 4 & 5, using a Mini-Circuits SBL-1 to up-convert to 23cms. Ideally the SBL-1X should be used as it has a higher frequency rating, but the SBL-1 was to hand. This method is described by F4DAY in CQ-TV 219 page 20. I connected the output of a signal generator, which had been set to 846MHz, to the LO input of the mixer and the 434MHz output from the AGAF board to the IF input. One of the output signals is 1280 MHz (846+434 MHz). The circuit is shown in Fig.6. The intention is to replace the signal generator with a crystal based oscillator and frequency multiplier stages up to the desired frequency.

To receive signals, I required a satellite receive which would tune to 1280MHz. I purchased an excellent little 12V receiver, the Comag SL65/12 from Maplin (A94FJ) for £49.99, shown in Fig.7. It will be ideal for both shack and portable use. It comes complete with a plug-top power supply and remote control. It has signal search facilities and automatically sets its decoding to suit the incoming signal. Brilliant! The normal LNB supply (via the aerial connector) is switchable from the remote control, so I set this to OFF and checked that it was in fact off before connecting anything.

I connected a string of co-ax attenuators (around 30db) between the output of the mixer and the receiver input so as to avoid overloading the receiver. I set the receiver to 'Search' and waited. After about 30 seconds the receiver detected the signal and locked on. Video and sound signals applied to the AGAF board appeared at the receiver output SCART connector. The coding delay was immediately noticeable. Waving my hand in front of the camera was visible on the receiver monitor many milliseconds later. Keying 'INFO' on the remote control brings up a screen giving details of the received transmission, shown in Fig.8.

I decided to test the quality of transmission by transmitting my station digital test card through the system. The results were excellent. I have a video burst generator which transmits frequency bursts at 1, 2, 3, 4, 5 & 6 MHz. I tried transmitting this, but to my surprise the results were very poor. Only the 1 & 2 MHz bursts were received at full amplitude. I then transmitted a 'pulse & bar' signal from a professional generator and this was fine. I concluded that the problem with the burst generator test may be due to the frequency bursts being from a free-running oscillator and not synchronized to line frequency or to each other. A cautionary tale, which requires further investigation.

I removed the attenuators and fixed a little ¼ wave aerial (5.9cms of wire) to the output of the mixer and a similar aerial to the input of the receiver and 'over the air' communication was established immediately. This may not be a great step for mankind but it is a point to which I can always return as I continue developing other parts of my DATV system.

My sincere thanks, for most helpful discussions and first hand DATV information, go to Dave G8ADM, Noel G8GTZ, Roy G8CKN, Mike G8LES, Mike G8ASI, Paul G6MNJ, Ian G3KKD, Peter G3PYB & Brian G8GQS.

Figures

Fig.1. Front of NDS MPEG-2 Encoder-Modulator E5910

Fig.2. Rear of NDS Encoder-Modulator E5910

Fig.3. AGAF DATV Boards

Fig.4. Double-balanced Mixer

Fig.5. Double-balanced Mixer - Internal

Fig.6. Mixer Test Circuit

Fig.7. Comag SL65/12 Satellite Receiver

Fig.8. Receiver displaying Signal Information