DATV / Digital Amateur Television Primer

Introduction

Work on this page began a few days ago. Please help to fill in the gaps or provide references/images if you can. Please send any contributions/corrections to:

The aim of this web page is to document some of the information that I have recently gathered, providing a single point of reference for newcomers seeking a basic introduction to the digital aspects of the ATV hobby. I don't hold any claim on technical authority, the sole purpose of this web page is to act as a single, convenient, repository for DATV related research. Please contribute corrections or new material as you see fit.

Much of this document is written from a European perspective. Please drop me a note if you can help to explain the DATV modes that are in use in your part of the world.

DATV Primer

What is DATV?
Why use DATV?
Which bands can I use for DATV?
What modes can I use for DATV?
What is the difference between the DVB modes such as DVB-S and DVB-T?
What do parameters like Symbol Rate and FEC mean?
Broadcast implementation of DVB technology
Amateur implementation of DVB technology
Where can I see DATV today?
How can I get on air with DATV?
How much of my existing ATV equipment can I use for DATV?
How can I scan the band to find DATV signals?
How do I grade a DATV contact?
What are does the future hold for DATV?
Further Reading

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Why Use DATV?

Which bands can I use for DATV?

70 cm, 436 MHz Preferred

The narrow bandwidth of DATV, relative to analogue ATV, facilitates the use of full colour ATV in the UHF 70 cm band. UK Stations are transmitting 2 MHz bandwidth DATV around 436 and 437 MHz. QPSK and GMSK modes are in use although GMSK receivers are not generally available at this time.

23 cm, 1285 MHz Preferred

After the IARU region 1 conference 2005 in Davos, the 1.3 GHz bandplan now shows the 1272 to 1291 MHz segment usage to include DATV. It has been proposed that DATV activity be concentrated between 1281 and 1289 MHz with the centre of activity on 1285 MHz. This allocation has been chosen to allow 8 MHz bandwidth signals to be exchanged without impact on EME and SSB segments.

13 cm and Above

The availability of spectrum for DATV above from 13 cm upwards is generous and it is anticipated that these bands will be used to provide broader bandwidth, higher symbol rate, DATV transmissions with multiplexed signals containing many video and data signal sources. A typical application for multiplexed DATV video is to allow many repeater inputs to be rebroadcast simultaneously on one RF carrier.

What modes can I use for DATV?

What is the difference between the DVB modes such as DVB-S and DVB-T?

What do parameters like Symbol Rate and FEC mean?

Typical broadcast implementation of DVB technology

Typical amateur implementation of DATV technology

Click the thumbnail image below to view a high level block diagram of a typical Digital Amateur TV station. This configuration is based upon the SR-Systems modules.

Where can I see DATV today?

In the UK many of the Home Counties Repeater Group systems support DATV. GB3HV was the first UK repeater to be DVB-S DATV enabled using ex-broadcast hardware. If you are not within range of the Home Counties Repeaters you can still observe proceedings in real-time by using the GB3HV Streaming facility.

Active UK DATV Repeaters

Callsign	RX Frequency	TX Frequency	Symbol	Video	Audio	FEC	Mode	Comments
GB3HV	1252MHz	1308MHz TX	4M S/sec			1/2	QPSK	Can receive DATV upon command. See GB3HV web site.
GB3KM	2328-2388MHz	2440MHz						No details of digital mode on web site. Can you help?
GB3RV	10425MHz	10065MHz TX	6M S/sec			1/2	QPSK	Source: G8DHE's web page
GB3SQ	1280MHz Proposed 3cm	1311MHz TX	4M S/sec			1/2	QPSK	Proposed Repeater: See G8AJN's site and BATG's site for details. Repeater has been used for digital tests during 2007.
GB3TZ	2388MHz	2326MHz						No web site to confirm digital access details or frequencies

Please contact G7LWT if you have exact symbol rates and details for DATV access to the repeaters listed above or if any additional repeaters should be listed.

Simplex Contacts on 70 cm

Most UK simplex activity seems to be on 70 cm at the moment. The table below shows the preferred settings:

Frequency	436.0 MHz
Symbol Rate	2 M S/sec
Video bit rate	~1.5 Mbit/s
1 Audio	64 K bits / sec
FEC	3/4 or 1/2
Mode	DVB-S QPSK (not C-OFDM)
RF Bandwidth	2 MHz (435 to 437 MHz only)
Talkback	144.750 MHz FM

Simplex Contacts on 23 cm

The table below shows the preferred settings for 23 cm simplex working:

Frequency	1285 MHz
Symbol Rate	4 M S/sec
FEC	1/2
Mode	DVB-S QPSK
RF Bandwidth	8 MHz (1281 to 1289 MHz only)
Talkback	144.750 MHz FM

How can I get on air with DATV?

How much of my existing ATV equipment can I use for DATV?

Although many analogue ATV station components can be reused with DATV, certain components either have to be swapped out or used in subtly different ways.

The following analogue ATV items can usually be reused in a DATV station:

Analogue Video Sources: VCR/DVD/Test Card Generator. Some older devices may need to be run through a TBC before a modern MPEG encoder will lock on to the video signal. Both CVBS and Y/C signals can be converted to an MPEG video stream using the SR-Systems and AGAF DATV systems. Professional broadcast systems can sometimes cope with CVBS if an appropriate option is fitted to the MPEG encoder.
Analogue Video Camera: Both CVBS and Y/C signals can be converted to an MPEG video stream using the SR-Systems and AGAF DATV systems. Professional broadcast systems can sometimes accommodate CVBS if an appropriate option is fitted to the MPEG ecnoder.
Analogue Video Mixer/Desk: Both CVBS and Y/C signals can be converted to an MPEG video stream using the SR-Systems and AGAF DATV systems.
Interdigital/Bandpass Filters: Interdigital/BandPass filters can be reused on DATV systems and help to clean up DATV TX RF as well as off-air received signals
Low Noise Block Converters (LNBs): LNBs can be used for DATV and are especially useful for downconverting 13 cm, 9 cm and 3 cm DATV signals to an L-Band IF where low cost domestic DVB receivers can be used to decode and display the MPEG-2 video stream.

The following analogue ATV components must be used carefully or replaced in a DATV station:

Power Amplifiers: Most existing linear amplifier designs are not sufficiently linear to cope with the demands of QPSK modulation. The extreme dynamic range required to cope with QPSK calls for existing power amplifiers to be under-run (by at least 80%) or to be replaced with modern, extremely linear amplifiers. The problem with amplifying QPSK signals is the high crest-factor, which is the difference between average and peak power. If standard linear amplifiers are driven too hard the digital signal will suffer spectral regrowth resulting in severe signal distortion. For further information on the effects of driving power amplifiers into saturation with digital modes follow this link to the Dutch ATV website. GMSK modulation is easier to amplify using existing power amplifier blocks but care still needs to be taken to avoid distortion and corruption of the digital video transmission.

How can I scan the band for DATV signals?

Whilst it's not as easy as spinning the tuning dial on a rotary tune analogue satellite receiver, there are technical solutions to the basic need to tune the band looking for video signals. Many of the cheaper DVB receivers, such as those from Fortec Star, include a facility to rapidly band scan for new DVB satellite channels, automatically identifying symbol rates and other parameters. If you are looking for a more interactive approach to finding DX signals you may wish to consider modifying a satellite receiver to make a spectrum analyser, with spectrum surveillance capability, via a host PC.

Reproduced with kind permission. Copyright (c) Fred Bruenjes - moonglow.net

Screen Shot from BLSA Project. Timed scan: 500MHz of spectrum scanned repeatedly for six hours shows DVB newsfeeds come and go. Frequency is top to bottom and the passage of time is left to right.

How do I grade a DATV Contact?

Many aspects of the well established "P rating" system for ATV contacts are no longer relevant to grading a digital ATV contact. The following values have been proposed for a "D rating" system to be used when grading DATV reception:

P Rating	Analogue Meaning	S/N	Signal Strength	D Rating	Digital Meaning	BER
P0	Trace of a picture barely visible, if at all.	<3 db	<5 uV	D0	Station ident received but no picture resolved
P1	Picture visible but only large objects.	3-8 dB	5-15 uV	D1	Severely corrupted picture freezes for long periods
P2	Picture is visible but very noisy.	8-20 dB	15-50 uV	D2	More Mosaics than clear picture,
P3	More picture detail than noise.	20-35 dB	50-200 uV	D3	Picture regularly turns into mosaics and/or audio drops
P4	Good picture detail, slight noise.	35-45 dB	200-1000 uV	D4	Infrequent audio and/or picture corruption
P5	Perfect picture, no noise.	>45 dB	> 1000 uV	D5	Perfect picture and sound

Additional notes to explain the significance of Bit Error Rates etc will go here. Sample screen shots to be captured and incorporated into primer.