BATC Forum

As mentioned in CQ-TV 270, several members of the ATV community have been working to produce a narrow band DVB-T system - this project is now at beta phase and testing has started on some prototype equipment.

What is and why do we need DVB-T / OFDM?

DVB-T is a multi-carrier modulation system whereas DVB-S is a single carrier system. DVB-T has been available for over 20 years but only in fixed bandwidths for broadcast use (8,7,6 and 1.7MHz) making it unsuitable for DATV use. However, receiver silicon with programmable bandwidths has recently been released and Charles G4GUO has written software enabling it to be used for amateur applications.

For most amateur applications we don't need it!

OFDM does not give any benefits when used over a normal simplex path between stations or into the local repeater. It also does not give any benefit when operating over Oscar 100. In fact, it requires even more linear amplifiers than DVB-S and typical back off will need to be almost 90% (a 100 watt PA will only produce 10 watts of clean OFDM) - this increases the cost and power consumption of the PA you need to use.

Where OFDM is really valuable is where there is a lot of multi-path or phase distortion on the signals making it ideal for mobile video - all the wireless cameras at sports events use a form of OFDM. The application where we think OFDM will be very useful is on the lower VHF bands and HF where we hope it will enable contacts via Sporadic E and other long-distance DX modes!

So what is the Knucker project?
Take a look at the wiki for more details https://wiki.batc.org.uk/Knucker

Receiver:
Mike G0MJW has designed new USB tuner hardware to host the new Serit 4762 DVB-T NIM - this will be available from the BATC shop.
Dave G8GKQ is developing software to run on the Portsdown (beta version was released today) and it is intended to develop STB software in the future which will be similar to but not known as Ryde.

Transmitter:
Charles G4GUO has produced a prototype version of the DATVexpress software - this works well with the DATVexpress card and has support for the Pluto and Lime but this is experimental at this time.

Project timescales
Whilst the Portsdown has been released with a beta version of the receive software, the limiting factor is the availability of the new NIM from Serit with the new programmable silicon.

BATC has been able to purchase a small number of NIMs and these are being used to test the system. We have just ordered 400 more units and these will be available for sale in the shop in March.

Once the NIMs and PCB are available we will do a general release of the project.

We do have a very small quantity of NIMs available if you wish to be involved in beta testing please contact us by email - NIMs are very limited so we are looking for people to commit to a program of testing in the intended applications and not just use it on QO100!

Our thanks yet again to Dave G8GKQ, Charles G4GUO and Mike G0MJW for providing the ATV community with another set of toys to play with!

73

Noel - G8GTZ

Looks a bit like this
It fits into the same case the the Minitiouner V2 but with a different panel as the NIM is not the same.

Mike

My completed board. Noel scribbled out the S after DVB to reiterate" DVB-S is NOT (repeat NOT) supported in any BATC applications"

DVB-T works very well on 29MHz

Dave

G4FRE

Very nice Dave.

Looks very similar to mine apart from the colour of the headers and LED.

Hopefully the new silk screen as above has expunged the S. The reason for not supporting DVB-S is although tuner does support it, it doesn't seem to work very well at low bandwidth compared to the other tuner and it does not support the DVB-S2 16/32 QAM mode. This is a project for terrestrial use where multipath is an issue.

Please be careful using the prototype board images as a guide as the production boards have some changes. It should be obvious and it is largely capacitor orientation now being all the same, very clearly marked on the silk screen, but we know how we can be fully aware the red wire is positive, the black negative, yet diligently connect them with the polarity reversed.

Mike

Sorry but we have sold out of the protype Knucker NIM and PCBs.

More will be available when the project is released in late March.

73

Noel - G8GTZ

I'm really looking forward to this becoming available. This may be the project that helps me recover ATV/DATV locally. As the rest of the US uses DVB-T that has been some resistance to try anything else.

Dear all,

I friend of mine has sent me a link to this very interesting project.

Would you please tell me how the decoding of DVB-T signals with bandwidths outside of the specifications of the DVB-T standard (e.g. 2 MHz) is done in the Knucker project? I suppose that it is done by the tuner (NIM) but normally they are limited to the standard bandwidths, or am I wrong?

I am asking this question because we are currently working on a hardware DVB-T transceiver for the 70cm band that is based on RX and TX modules from HiDes in Taiwan. It would be nice to have a solution that does not depend on these parts.

We have also tried to decode DVB-T signals of up to 2 MHz bandwidth using an RTL-SDR stick and GNU radio. Although others say that this is impossible, in principle it is possible, but there is a software problem in GNU radio we cannot solve. The OFDM symbol recognition block in GNU radio only works as long as the clock frequencies used on the transmitter and receiver sides are 100% identical. If this is not the case, sooner or later (seconds to minutes) the symbols are no longer recognised by the receiver software and reception is lost. The concerned block(s) would have to be modified to take into account differences of the clock frequencies. That's why we have switched back to hardware. I am currently working on the software to control the RX and TX part via the serial port using a Raspberry Pi and a touch screen.

Thank you very much.

73 de Ralf, DL5EU

Hello Ralf,

The BATC Knucker project is based on the Serit FTM 4762 NIM
http://www.serit.hk/product/product70.html
The narrow bandwidth is achieved by reprogramming the demodulator clock in the Avalink AVL6862
demodulator chip. The code we are currently using is under an NDA with Avalink but there
is a Linux driver for that chip and we will eventually transition to something derived from that
so that the S/W can be open sourced.
The Avalink demodulator is essentially a software defined demodulator, internally it consists of a
processor and various hw accelerators, this is the reason it can be re-purposed.

- Charles G4GUO

Ralf,

FYI Serit will only build a minimum quantity of 4762 NIM and BATC has 400 on order which will be in the BATC shop, hopefully at the beginning of March.

Initial testing has gone well and we hope to release the Portsdown transmit and receive software when the tuners arrive in early March. In case you have not seen it, more details on the BATC project here https://wiki.batc.org.uk/Knucker

73
Noel - G8GTZ

Hello Noel,

so in order to get one, I would need to become a member of BATC and order one as soon as possible? Or are the 400 already sold out?

73 Ralf, DL5EU