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Experiments in audio-visual transmissions (tags: radio, PAL, sstv)
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README.md Update 'README.md' 1 week ago

README.md

tvaria

Experiments in audio-visual transmissions (tags: narrowcasting, microfm, microtv, radio, PAL, sstv)

tx

video

Prior Art

Based on Tetsuo Kogawa's simplest tv transmitter https://anarchy.translocal.jp/microtv/howtotvtx.html PDF: https://anarchy.translocal.jp/microtv/simplestTVtx03.pdf

Disclamer

This is probably illegal in your counrty. Don't build this. What can you transmit with license:

  • You have a HAM radio license and call sign (legally usable section of the band (6meter, 50MHz)) > TV signal's bandwidth is too high to be legally transmitted (4.5MHz). Only sstv is allowed on carrier: 50.510 MHz, with Bandwidth of: 1.2000Mhz (Our signal is 4.5Mhz....)

  • 4 ingredients: Frequency, Bandwidth, Transmit Power, Mode

  • Novice callsign:

(ref: https://www.by-rutgers.nl/Amateur%20frequentie%20banden.htm)

Instead of saying 144.500 MHz, these amateur bands are often refered to in meters rather than frequencies. These are the literal wavelenghts of the signal. The 'meter' bands are calculated by speed of light in a vacuum 300 million meters per second divided by frequency in million Hertz/sec (so 300/50Mhz=6 = 'the 6 meter band' )

Diagram of the setup

[TV] ....(air)..... [TRANSMITTER]---(RCA CABLE)---[RPI]---(INTERNET*)

*optional

Making

Tools Needed

  • Side Cutters
  • 5mm drill
  • Soldering Iron
  • Solder
  • Small set of screwdrivers for tuning (plastic prefered)
  • Utility knife
  • Super Glue Handy to have:
  • Vernier Calipers
  • Tweezers (to place components and not burn your fingers)
  • Electrical tape to isolate the variable resistor from the ground plane

Parts Needed

Complete Bill of Material: https://octopart.com/bom-tool/QOJ00mDh 10 parts, total costs: $1.49 excluding the battery and tools needed.

Winding coils

Use a 5mm drill bit to wind your coil around. (coil = 7 tight turns of 0.7mm (measured 0.6mm) around a 5mm drill bit, after winding TIGHT, spread the coil out to be around 6.7mm/7mm in lenght, coild-end to coil end...).

Transmitting

Possible hardware

  • Anything putting out a 4MHz PAL (or NTSC?) signal: game consoles (NES), VCRS, Video Title Generators..or:
  • Raspberry Pi 1 B (fcc id: 2ABCB-RPI21), the first raspbberry pi board (2011-2012) has fullsize composite out (RCA) plug, with correct polarity...
  • Raspberry Pi 3b v2. It has composite out through the minijack trrs connector. NOTE: the ground and + are inverted on this plug... (so centerpin on yellow plug is ground and sleeve is your signal). This also affects the sound output on the trrs...great job rpi!

Tuning the transmitter

Roughly 6 variables:

  • Input Voltage
  • Antenna (Type+ Size)
  • Coil (Material, diameter, length, wire gauge)
  • Variable Capatitor 1 and 2
  • Variable Resistor
  • You position to the transmitter(+ transmitter is microphonic like a contact mic) Preferably you use plastic screwdrivers to adjust the variable caps and resistors..this is not always possible, maybe coating your screwdriver in non conductive paint could work.

The exact transmitting frequency will always be a surprise. Usingt a SDR like HackRF + software like GQRX helps to find your transmitter's frequency in its 'waterfall' display...You can also just 'auto-tune' your tv and wait until you see something, it prolly won't lock-on, so besure to stay ready to stop the auto scanning and do some manual tuning (also see: Building the transmitter below for more details).

Software

In case you are using a Raspberry Pi to generate the analog video signal for your transmitter:

  • this pi image: TODO
  • enable PAL on rpi 1 (sdtv mode 2) in /boot/config.txt
  • $ sudo apt-get install omxplayer, a command line media player that is designed for HW acceleration on the RPI. Its capable of playing video RTMP streams as well.
  • connect to tty1 from ssh: $ sudo chmod 666 /dev/tty1 $ script tty1

Playing / streaming youtube videos NOTE: download / update to the LATEST youtube-dl!!

$ omxplayer 'youtube-dl -g -f mp4 <youtube-url>'

( can also be youtube ID, mind that when your ID has a dash, you must encapsulatethe id with single quotes) (-f mp4: force mp4 (omxplayer cant play webm)) see: https://raspberrypi.stackexchange.com/questions/43245/omxplayer-doesnt-play-audio-when-streaming

Receiving

Analog capable TV with antenna. We used an Amstrad CTV 140 with antenna input.

Tuning your tv

It's handy to fisrt check your RPI composite output on the AV/AUX channel of your TV using a cinch/rca/tulip cable. It should be crisp and colorful. If so, you can go wireless. Else try to enable PAL on rpi 1 (sdtv mode 2?) in /boot/config.txt When all is good, you can manually tune your TV to the beginning of the VHF1 band or just use 'auto tune' to find the transmitted signal.

Notes: this transmitter is designed and tested to work on the VHF1, also called 'Band 1' (47 to 68 MHz for the European Broadcasting Area), specifically on 50 MHz, allocated to amateur radio operators ('hams'). The bandwidth of the PAL signal is around 4.5MHz though, and might not be allowed on this frequeny...Also, this transmitter is rather dirty and can possibly transmit on other frequencies (multiples of the base carrier wave). A filter would be good...Trivia: The '6 Meter band' is called the 'magic band' because it shares the treasured porperties of both High Frequency (HF) and Very High Frequency (VHF) radio wave propagation, namely: skywave, meteorscatter and Multiple-hop sporadic E propagation allowing for intercontinental communications at distances of up to 10,000 kilometres.

Receiving using Software Defined radio

Cheap rtl-sdr sticks dont have enough (4MHz) bandwidth to receive full color pal. Black and white is supposed to be possible using 'TV Sharp': https://www.rtl-sdr.com/analogue-tv-with-rtl-sdr/ (no sound, atleast not withouth 2 stikcs)!

Improvements

Find a way to transmit legally on this band: Either look into SSTV (and use computers to receive using a cheap sdr dongle) or look into amateur televisie (ATV) that uses much higher frequencies (thus allowing for more bandwidth...but good luck hand soldering such a transmitter/receiver): https://vhf-uhf.veron.nl/atv/hoe-start-ik-met-amateur-televisie-atv/

Link Dump

https://www.youtube.com/watch?v=rmGAd_MVaWw (hacking variable inductor to add audio to your fm tv transmitter) https://www.youtube.com/watch?v=3sVIOg-PlKc (video tutorial of kogawa fm audio transmitter)