DigiLite Update and Soapbox

Again, this is going to be a shorter version of my blogger ‘blog at http://w0fms.blogspot.com where I intend on having more random thoughts and technical detail than I post here! 🙂

I’ve purchased parts (in the US– all from Mouser for lowest overall cost except for the Analog Devices AD8346ARUZ Modulator IC– which they don’t carry) for the British Amateur Radio Club’s DigiLite “poor man’s” DVB-S SCPC modulator unit.

Almost complete BATC DigiLite DVB-S modulator board. All that is still needed is a MMIC amplifier be added after the QPSK modulator is properly adjusted with the external PLL Oscillator I am still awaiting from Israel.

The unit, as you may remember from my last post, uses an older analog Hauppage PVR-150, 250, 350, 500 and/or probably PVR-USB2 MPEG2 hardware encoder to set fixed constant bit rate (CBR) video and MP2 type CBR audio in a program stream format. This is translated to Transport Stream (TS) format for over the air transmission, Channel, EPG, and other needed DVB-S specific information is also multiplexed by the BATC software which is then sent to a DSPic33 that optionally adds the FEC (particularly at rate 1/2) and randomizes (to take out DC components because RF is inherently AC coupled!) the data– and then forms I/Q data with some pre-distortion with some tricks.. it’s filtered in a atypical way through an ANALOG LC Nyquist filter.. otherwise unfiltered PSK has infinite bandwidth! In commercial products a DSP typically would do this with a digital filter and a DAC. Finally the AD8346 does the I/Q phase shifting and that is the directly at frequency modulated DVB-S.

The LO is generated by a good e-bay find– a PLL L-band Oscillator that you can get from Ultram Tech in Israel custom programmed to one frequency. (I intend on reprogramming the unit with an Arduino [AVR ATMEGA part] board in the future– the chip’s dividers are programmed with SPI.) I have not received the board yet and didn’t want to disassemble and hack the 1152 MHz oscillator I previously bought about two years ago from Israel.

What the software and serializer firmware doesn’t do, thankfully, is care about the type of video that is in the MPEG-2 stream. I verified that tonight. The designers in the UK thought that NTSC would be fine.. and it seems to be.

Here is a video of the DigiLite “doing it’s thing” with rate 2222K video and 128K audio at a bandwidth of 3 MHz- 3MSym/S at 1/2 FEC QPSK. The video was set at NTSC D1 720×480 4:3 and I toyed with the settings until we were absolutely maxed out and were at close to zero null packets. Over the air, about 100K slower video might be a better choice. At 2112K, the null packets averaged 4.5%.. so somewhere in between these settings is about right for NTSC at 3MSym/s QPSK. This proves the digital part of the system out! We are close!

YouTube video of the BATC DigiLite running NTSC!

This is cool.. 1/2 the bandwidth and better quality than most Amateur TV. You’d think it was a no brainer for 70cm to replace 6-8 MHz AM TV? Well it is.. but in my not so humble opinion it’s not legal to run.

—SOAPBOX—
Part 97 defines that a digital phone or image (important definition) signal is legal if it uses the same or less bandwidth than the equivalent analog signal.

DVB-S and other digital standards definitely do. Unfortunately, from commercial license grants researched, I note that ATSC is C7W emission type and DVB-S/S2/C/T is G7W. Image is defined to EXCLUDE “7W” emissions. C7W and G7W are specifically defined as “multiplexed data” in another part of the rules– actually in reference to RTTY in the typical early 1980’s wording of those rules. So because DVB-S is “data” by part 97 definition, the 56K baud/100KHz BW rule applies at 70cm. In my not-so-humble-opinion it is not legal to run DVB-S nor ATSC (?!?) at 70cm because of these antiquated and wrong rules. But it is legal at 33cm and above as wideband data.

There are several who disagree with me on this. But for their definition to be valid technically by the Part 97 rules as they stand ATSC and or DVB-S would have to be G2F (DVB-S) or C2F (ATSC). By the Emissions Designator system that would also imply no audio channel. F2F is what SSTV is and I believe the “image” definition for FM/PM/PSK was specifically written with HF SSTV in mind! NO COMMERCIAL license grant (I found) has it defined that way! I can’t run 70cm DVB-S because I interpret the rules as above. Anyone who disagrees with me and wants to run it is fine with me. Wish you the best and I’m all for you! Likely the FCC nor the ARRL would do a thing to you as they shouldn’t!

What this really means is that the digital rules in US ham radio need fixing in a big way! We need to get rid of all the protectionist crap and realize that for many modes, the handwriting is on the wall for analog. The way the current rules are written in Part 97 makes most new digital modes second-class citizens on the bands. Bogus and Sad IMHO in almost-2012.

— /SOAPBOX —

Fred Spinner, WØFMS, is a regular contributor to AmateurRadio.com and writes from Iowa, USA. Contact him at [email protected].

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