An exciting aspect of the trend in single board computers towards ever faster processors has been the clever use of their digital I/O with DSP software to synthesize complex signals in the analogue and RF domains that would previously have required specialist hardware. When we use a Raspberry Pi to poll a sensor or flash an LED it’s easy to forget just how much raw processing power we have at our fingertips.
One of the more recent seemingly impossible feats of signal synthesis on a Raspberry Pi comes from [Evariste Courjaud, F5OEO]. He’s created a DVB-S digital TV transmitter that produces a usable output direct from a GPIO pin, with none of the external modulators that were a feature of previous efforts required. (It is worth pointing out though that for legal transmission a filter would be necessary.) Dvbs Satellite Receiver
DVB is a collection of digital TV standards used in most of the world except China and the Americas. DVB-S is the satellite version of DVB, and differs from its terrestrial counterpart in the modulation scheme it employs. [Evariste] is using it because it has found favor as a digital mode in amateur radio.
This isn’t the first piece of [F5OEO] software creating useful radio modes from a GPIO pin. He’s also generated SSB, AM, and SSTV from his Pi, something which a lot of us in the amateur radio community have found very useful indeed.
We’ve covered digital TV creation quite a few times in the past on these pages, from the first achievement using a PC VGA card almost a decade ago to more recent Raspberry Pi transmitters using a USB dongle and a home-built modulator on the GPIO pins. Clever signal trickery from digital I/O doesn’t stop there though, we recently featured an astoundingly clever wired Ethernet hack on an ESP8266, and we’ve seen several VHF NTSC transmitters on platforms ranging from the ESP to even an ATtiny85.
Thanks [SopaXorzTaker] for the nudge to finally feature this one.
Now all we need is a $5 programmable filter bank.
i wonder if it would be possible to create a variable mechanical LPF using a metal iris.. and if so, what level of attenuation to expect ..
It’s not unusual to see “mechanical” filters in the microwave range, like cavity or interdigital filters. Here’s a nice pic: http://exceedmicrowave.com/uploads/3/3/0/2/3302376/4627451_orig.jpg
But in the VHF/UHF domain the wavelength is a bit large.
Wow, I kept eyeing the 8VSB and thought it was infeasible… But DVB-S does anyone know if it does the same crazy thing with like a thousand subcarriers?
Sorry for blowing it, but it is a single QPSK carrier. It is just a square wave with phase shifts.
Yep QPSK. And the symbolrate is only around 250ksps. So 250kHz 3dB bandwidth, or 320kHz bandwidth.
With 7/8 FEC, gives a 403.186275 KBit/s transport stream bitrate. With 1/2 FEC, gives a 230.392157 KBit/s transport stream bitrate.
So generally, in the 426×240 (240p) or 640×360 (360p) range of video quality. Might scrape in 854×480 (480p), for mainly static video.
I guess if you were using hevc / h.265, vp9 or vp10 you could increase the resolution.
Anyone know if this will also lead to demodulation of DVB-S2 Turbo FEC?
Hahahah I know what you want to do….. :p check china mate!
OK, but can it generate an ATSC signal?
why would you want to support outdated backwards standards?
What is the the reach? And how is the latency? Could so fly FPV?
End-to-end latency: I experienced this myself last fall at a booth at Surplus Party Zofingen (HB9): from jumping in front of the camera until I saw myself on the Rx-Screen nearby _several_ minutes passed by. (can’t tell where in the chain the big’hunk’o’delay takes place: the encoding on the RasPi or maybe also on the (unspecified) Rx…)
Does anyone know of aDVB-T project out there ? Commercial units are in the order of $500. It would be great to populate some extra channels with home brew content.
Agreed – DVB-T would be useful. A colleague’s been looking for something to extend the output from a set-top box in one room to another, without using massively long HDMI cables.
Google for “WS-6990” – if you are lucky you’ll find one for less than 200 US$. Hook up an amplifier and give it a try.
Yeah, there is a company called Hides that makes affordable DVB-T hardware. oe7dbh did a lot of stuff with it, if you search for his callsign and ‘Hides DVB-T’ or something you should find some stuff.
Call me when it’s an FCC certified transmitter
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