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DAB Transmitter
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airmedia
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Joined: 29 Jun 2005
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Post DAB Transmitter Reply with quote
How would a DAB transmitter be made ? Since this is the future of radio
i was woundering if this would be possible..

Pirate DAB Stations....????

Alistair
Air Media

RADIO BEDS - RADIO FX - SWEEPERS - VOICE OVERS
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Last edited by airmedia on Wed Feb 24, 2010 11:22 pm; edited 1 time in total
Thu Jun 30, 2005 8:32 pm View user's profile Send private message Visit poster's website
pcs
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Post Reply with quote
Probably not easily and cheaply

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Fri Jul 01, 2005 10:10 am View user's profile Send private message Visit poster's website
Medus
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Post Reply with quote
Well, it isn't so difficult from what I've seen although the skillsets involved are perhaps a little different from those analogue radio engineers may be used to.

1./ Modulation method. DAB uses a QPSK and OFDM which has more in common in WAN networks than conventional radio broadcasts. Certainly it is not such a familiar topic for the FM radio enthusiast. Essentially the digital symbols are DQPSK modulated (by neither frequency nor amplitude but by phase adjustment of the subcarriers in 90 degree increments in order to send the symbols 00, 01, 10, 11) - Then the signals are repeated over many subcarriers (to avoid the effects of multipath fade) before being amplified and output.

2./ Familiarity. Whilst most FM engineers are using digital techniques such as microcontroller management (PICs and such) these simply don't have the speed for encoding DAB which is based on the MP2 codec. This means that you have to ditch microcontrollers and use much faster FPGA technology (or an off-the-shelf MPEG encoder IC - but this has drawbacks)

3./ Coding skills. FPGA's are coded in Verilog/VHDL programming languages which are a radical departure from the ASM code using in microcontrollers (Which are conventional linear code as opposed to concurrent logic and thus more widely understood)

4./ Soldering. This may sound like a silly problem but most FM designs are soldered using conventional means... but DSP's and FPGA's require high-density SMT techniques which again leaves some small-scale developers cold - also, it effectively denies the buyer home-assembled kits... the best you can hope for is part-assemble kits and a higher pricetag.


Other issues...

Now, I mentioned off-the-shelf MP2 encoders... you could use one of these as a design block component to speed your design but you immediately hit a problem. Even if you use a prebuilt MP2 encoder you still need logic to control additional processing after the MP2 encoder block to make up the COFDM modulation. This involves:

- Convolution (viterbi) encoding
- Forward Error Correction
- Frequency division Multiplexing
- Temporal/Frequency interleave
- Fast Fourier Transformations

Again, way more than a microcontroller can handle at these bitrates. So, since FPGA's are going to be needed to handle all of this anyway we may as well ditch stock MP2 encoder IC's and write the MP2 codecs to the FPGA as a first stage of a complete DAB encoder chain. In other words, we cannot really get away from the Verilog/VHDL code requirement... at least, not until some benevolent chip manufacturer releases end-to-end DAB encoder IC's to the public at decent prices.

So, having neither the prebuilt parts or Verilog/VHDL design experience, you are not likely to see many DAB transmitter designs from the current FM kit distributors.



BTW. If anyone is interested I have an FPGA reference design for the Xilinx VirtexII (XC2VP70) FPGA part which implements several small-footprint low-latency MPEG2 audio encoders without any unneccessary features (Based on a stripped down version of DUMA's commercial 'Video DV-X realtime IP encoder' core) ... Theres no reason you cannot add additional FPGA logic to form a complete DAB streamcard. Then all you're looking at is the subchannel reintegrator and a way of adding the resulting broadband subchannels to some form of exciter. Essentially, this should provide all you need for a DAB transmitter.

RF design really isn't my strong point so I wouldn't be able to help with any specifics in the RF output stages. The electronics behind that side of things is out of my depth - but I can at least show how to build the subchannels you need to transmit.

I don't know if the DAB MPEG2 encoder requires any premphasis as a hangover from analogue FM but I very much doubt it. It wouldn't make a great deal of sense at all... but then I've been surprised enough by emergent technology designs to know not make assumptions, even sensible ones : )



I think though, that you'd want to see how far you can push an X-Band STL because you want to keep as much of the processing away from the rooftop as possible. A DAB processor is not something you'd want the FCC/RA taking from you regularly. The problem is that the processors output is wideband... So is an X-Band link so hopefully you wouldn't need to site a DAB plexer on a rooftop ; )

Also, being a multiplex signal I think you'd see pirate stations cooperating in placing these setups and then each applying their streams. Theres enough channel space available in a DAB TX Mode1 unit for a number of pirates and sharing costs makes sense. Theres also the SFN angle which makes the final transmitters harder to locate due to intentional multipathing.

I doubt we'll see this happening until analogue dies though so we've got a long while yet - and by then who knows what we can fit on a single FPGA, perhaps the entire DAB stream processor. It certainly is going to be interesting.


-Medus
Sat Jul 29, 2006 3:29 pm View user's profile Send private message
feel 105fm
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Post Reply with quote
I'm very interested in this so if anyone has any ideas how to build a proper digital rig or has a design that needs building and wants to share let me know.
Tue May 29, 2007 3:25 am View user's profile Send private message
jphastings
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Post Reply with quote
This is an age old thread, but I'll ask anyway: what is there to prevent all the MPEG 2 encoding being done in software on a computer? If WiFi is based on ODFM technology, would it be feasible with some (serious) driver hacking to turn a WiFi card into a modulator for a DSB broadcasting system?

Great to read such a well explained post, thanks for taking the time to write it!
Wed Oct 06, 2010 12:59 pm View user's profile Send private message
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