VK4ZXI

Modifying cavity filters for DATV TX or for repeaters Introduction I am currently doing further work on using notch cavity filters for DATV DVB-T transmitters. My earlier efforts were with what I had at hand and not knowing the solution; I (re)discovered that notch filters clean up DVB-T TX very well. However, it was at low power, 10 W, and high losses, >6 db because of the six cavities in a mobile duplexer. Here, I will report on modifying high power >100 W individual filters. In the next post I will report on using them and determining is just one pair are sufficient. The other goal of this post is to show how easy it is to modify older commercial filters for DATV or repeater use. Modifying cavity filters Old commercial filters are relatively easy to modify as the only thing that changes is the coupling loop, provided they are on frequency (not too hard to change that too!). Notch filters are the simplest as they use a single simple coupling, just a loop of metal. Old co

Decontis dtvtools DVB-T/S measurement, analysis and monitoring software (draft)  Introduction There has been a lack of good DVB-T monitoring software for both TX monitoring and RX measurement, unlike DVB-S that has Tutioune. I came across a commercial grade package from decontis that is relative inexpensive and uses a cheap USB-T dongle. While comprehensive, it is not particularly easy to use, but is network-based. I have managed to get it going and plan to use it for TX power amplifier modification and monitoring. My favorite element is a proper constellation chart. Other software and hardware The available DVB-T measurement,   analysis and monitoring  software is limited. CrazyScan2 for terrestrial/cable DVB-tuners https://sourceforge.net/p/crazyscan/wiki/Info/ uses PCTV USB tuner. The other alternative is to use a TV tuner, which gives MER and BER, but not constellation diagrams. A standalone TV installer instrument can be used, but good ones are expensive. Professional equi

MiniTiouner DVB-S receiver build (draft) Works well, some fiddly soldering. Gives me DVB-S capability Assembled version coming, about $100 from DATV Express group. Links Unique hardware: https://www.batc.org.uk/shop/minitiouner https://wiki.batc.tv/MiniTiouner_hardware_Version_2 https://wiki.batc.tv/MiniTioune_software https://www.videohelp.com/software/LAV-Filters All codecs: https://www.videohelp.com/software/sections/codec-packs https://wiki.batc.tv/File:Notes_on_building_the_BATC_v2_Minitiouner.pdf

Power measurements of DVB-T transmitters- first draft Introduction I have been puzzled for some time about how power of DVB-T amplifiers were measured relative to other modes. This is an important issue in DATV as DVB-T has been criticized as inefficient compared to DVB-S. It would appear that some of the debate comes down to how the power is measured. Most amateurs use simple diode power meters that do not give an accurate reading for DVB-T. In this post, I outline the theory and practice of measuring DVB-T power correctly with either a thermal power sensor meter or envelope power using a spectrum analyser. It is important that the correct measurements be used otherwise it is comparing apples with oranges. Different power measurements are accepted in amateur radio, vis, CW (peak power, key down) compared to SSB (peak envelope power). I wonder if another measurement unit for digital modes may be more appropriate, such as the data rate to the DC power input and spectrum width,

Commercial DVB-T Amplifiers and filters and implications for DATV (first draft) Introduction Commercial solid-state DVB-T amplifiers use about 250 W "pallet" amplifiers; a pair of  amplifiers, each using a pair of LMDOS transistors in a single package. The circuit boards are mounted on a piece of thick copper then on a heat sink. To get higher power, 1 to 50 KW, many pallet amplifiers are used in parallel with a system of splitters and combiners. The amplifier output passes through a series of filters, to stay within a standardized spectrum mask to limit adjacent channel interference. To conclude I note some implications for DATV. Pallet amplifers A typical pallet amplifier is pictured below (bought on eBay). The gold rectangle on the left splits the input to a pair of Doherty amplifiers, while the one on the right combines the amplified signal to the output. The combiner uses a type of circulator to dump RF to a dummy load, the black rectangles on either side of the

DVB-T TX Filter: 23 cm filter using 70 cm duplexer Introduction I came across a commercial UHF bandpass cavity filter that was as long as a VHF cavity. I removed a coupling to see inside. The probe was nearly 500 mm long, rather than the 170 mm in most UHF filters. They were using the probe as a three quarter length, rather than the typical quarter wave length. I checked the cavity at VHF and, sure enough, there was another peak. The obvious question then was whether the 70 cm duplexer filter would have another bandpass at three times the frequency, about 23 cm? It does! With careful tuning it should be possible to use the ubiquitous 70 cm duplexer at 23 cm. Analysis Raw response With the filter set to 7 MHz at 446.5 MHz, I had scanned three times that frequency and a very wide span. Sure enough, there was a passband, although at a different frequency and three times wider. The frequency of these cavity tuners are determined by a fixed length probe (tube) and a capac

Duplexer DVB-T TX filter: Technical details Introduction Analysis Bandpass is addition of two notches  Three cavities RX to ANT Reverseded notch TX to ANT Sum of two notches with six cavities RX to TX Two left hand couplings are conventional, the two right hand couplings, with plastic spacers are unusual, give reverse notch (not sure how). The image shows internal construction of 4 cavity duplexer. Tuning is achieved by slug into the top of the probes, like a capacitive hat on an antenna. Tubes or probes are slightly less than a quarter wave length. Basically antennas in a box with couplings in and out. Ratio of tube diameter to outside diameter determines impedence. One to three gives about 50 Ohm. SWR of Duplexer filter 7 MHz on 70 cm, not great but probably affected by tuning, possibly by coupling design. Conclusion Coupling for reversed notch needs further investigation

Testing a cheap Chinese duplexer as a 70cm DATV DVB-T 7 MHz TX filter- Wow! Introduction In my last post I described a cheap Chinese duplexer re-tuned as 70 cm DATV DVB-T 7 MHz TX filter. The duplexer uses notch cavity filters, six in all. The notch filters have a much sharper edge, compared to a band-pass filter. The sharp notch seems suited to the vertical edges of a DVB-T signal. I initially check the signal source, a HiDes camera with direct DVB-T output at 1080P. I was a little surprised at the spread, but the filter cleaned it up well. This would indicate the need for a filter before the main power amplifier. I pressed on with just one filter and tried it at the output of the amplifier, a 10 W device, from Darko OE7DBH, using a RA60H4047M1 60 W module. Even with the indifferent input, the filter was able to reduce the spread to -60 dB and give a clean 10 W output. The notch duplexer/filter seems to overcome some of the major hurdles with DVB-T amplifiers and warrants

A 70cm DATV DVB-T 7 MHz band-pass filter using a cheap Chinese duplexer Introduction DATV transmitters for DVB-T are notorious for "spread" outside the channel, to the point that keeping it 30 dB or more below the signal becomes a limit for power output, typically 10 W out of a 70 W module amplifier. Even with -30 dB spread, it is desirable to have a band-pass filter before further amplification or transmission. Usually an interdigital filter is used, but they are either expensive to buy or a bit difficult to build. VK4JVC suggested using a cavity filter duplexer instead. I tried a four cavity notch duplexer, but the pass-band losses were too high, more than 20 dB. I had bought a cheap, ~A$100, Chinese Jiesai  duplexer, but had put it aside as the response looked bad. After try other duplexers (notch and pass-reject types), I tried the Chinese one again, this time successfully. The result is that the Chinese filter seems to provide a good pass-band for the 7

Digital Video Processing with HD CCTV: Noise reduction In this post I examine some of the sophisticated technology used in digital high definition CCTV cameras, including a the powerful underlying SOC electronics, and the similarities with audio noise reduction in audio, as used in amateur radio. Noise reduction in HD CCTV I read very widely and like to see or create connections between apparently disparate ideas and technologies. From an earlier post on working with a Pentax lens I have, I read about the image processing abilities of some Pentax lenses.   Pentax/Richo have an award-winning image processing system, PENTAX Atmospheric Interference Reduction (PAIR). http://www.ricoh.com/fa_security/security/pair/  The results are amazing:       From a first reading, the processing seems to be done in the lens itself, something that had me mystified as to how such spectacular results are possible with just optics. However, on closer reading, the output of the camera i

BlackMagic Design ATEM TV Studio problems with 1080p; a hardware limitation? In this post, I think I have discovered the problem of the ATEM TV Studio not accepting 1080p input, a hardware limitation. Alternatives are then considered. BlackMagic Design ATEM TV Studio As noted in earlier posts, I use BlackMagic Design ATEM TV Studio as the main component of my DATV studio to connect multiple cameras and other video sources. Overall it performs brilliantly well allowing for most features needed in a professional TV switcher, and with nothing else even remotely in the price range; $1000. http://vk4zxi.blogspot.com.au/2014/06/datv-dvb-t-blackmagic-designs-atem-tv.html     No 1080p input, despite being common format for cameras However, it has one very annoying problem, it will only accept 1080i input, not 1080p, which to a point is reasonable as 1080i is the highest resolution for broadcast TV. The big problem is that almost all cameras output 1080p in li

Achieving 4K UHD DATV- very draft Perhaps a little early, but 4K DATV may be more achievable than I first thought. It would be a bit of a technological coup if amateur radio can do 4K before regular free-to-air broadcast TV. 4K video cameras and monitors are already relatively inexpensive. 4K TV capture/switchers are available and not too expensive. The missing link are modulators, transmitters and receivers, but may be possible using inexpensive SDR TRX; they can already do DVB-T/S. As far as I am aware, broadcast TV is still struggling with Full HD digital TV in some countries, notably the USA with a very large number of small TV stations and the not insignificant cost of having to replace virtually everything, other than their antenna. I suspect the same across some of Europe and Asia. For both terrestrial and satellite, while they may have digital TV, most of it is SD (standard definition) or HD (high definition 720p), rather than wide-screen Full HD (1080i; wish it was 1080p

My journey in DATV and the future: 4K UHD or internet-linked DATV repeaters; Not that crazy? Draft In this post I want to briefly outline my DATV journey and a quest for Full HD DVB-T. I have achieved this in a relatively short time and out of some of my difficulties, have wondered if first, 4K UHD DATV and, second, network-linked DATV repeaters, are possibly not that distant. In this post I will outline my journey, as the future is path dependent, history matters! In the following two posts I will consider ways to achieve 4K UHD and internet-linked DATV repeaters. My DATV journey: Live Full HD DVB-T Personally, coming late to DATV at the beginning of 2013, with a 40 year break in my amateur radio activities (see my first post), I have not had to put in the extreme effort and expense of either analogue TV or digital TV, particularly over the last decade, such as by the DATV Express team, among many others. Before returning to amateur radio I had spent considerable time and