405 line TV with the HackRF

[[fsphil]]

Hi everyone,

I've been working on a program to transmit video with the HackRF, which I unimaginatively called HackTV. It can transmit the usual 625 line modes but it can also transmit 405 line TV (without sound at the moment). I don't have any real 405 line hardware to test this on and I'm curious if it works.

The program lives at: https://github.com/fsphil/hacktv

It was primarily developed for Linux, but I believe it can also be built on a Mac.

[rambo1152]

Hi there and a warm welcome to the Forum.

Hack RF seems to be a customisable SDR transmitter, so does that mean the one box can be a video source and RF modulator?
Is it just for still images or will it accept a movie file input?
Will it output in Band I?
If it’s an SDR I bet it could do vestigial sideband, it sounds interesting.

Building upon a decade old idea called FOTH-TV A couple of us have been using a PC graphics card under Windows to generate 405 (and Baird 240) line material, you just need to add your own modulator to complete the project

https://www.vintage-radio.net/forum/...d.php?t=130716
www.nostalgiatech.co.uk/Baird%20240%20lines.htm
https://www.youtube.com/user/pmscott6135?hl=en-GB&gl=GB

[[fsphil]]

The AM/VSB/FM modulation is done in the software, the HackRF converts it to an analogue waveform and mixes it up to some frequency. In theory you could transmit any kind of signal that fits within its bandwidth limits, I've had DVB-T working in the past. Its output range is 1 MHz to 6 GHz so Band I should be fine.

It's not really designed to be connected directly to a TV, though I have done it. Even at low power it outputs considerably more power than a typical RF modulator would. I always make sure there is a good high value attenuator with a DC block installed to protect both it and the TV.

HackTV works with video files. The only requirement is that the frame rate must match the selected mode.

I do plan to add VSB at some point. For now it's just AM.

[peter_scott]

So for 405 you'd need two of them?

Peter

[[fsphil]]

The audio will be modulated and sent along with the video to the hackrf, so it still only needs one. This is how it works for 625 now, I just need to add an AM audio option for 405. SDR allows any kind of signal or signals to be transmitted / received, as long as they all fit within the bandwidth of the radio. The HackRF is ~20MHz, so there is plenty of room. In theory it could transmit several channels at the same time, but I've no plans to do that with hacktv

[[fsphil]]

Just for fun I've just added the Baird 240 line mode. I tried to figure it out from a scanned page so it might not be too accurate. Does anyone know the details of how audio was transmitted in that system?

[SteveCG]

fsphil. As far as I know audio was transmitted with AM modulation, the sound power being 1/4 of the vision power (ie the same as for the 405 line system). The very early 240/405 dual standard TVs (Televisors??) only had to change the vertical scanning parameters, not the audio.

[dtvmcdonald]

That's great. Close to what I want. I'd love to get a 405 file to play on my Marconi 702.
Even a couple of minutes will do.

What I really want as well is a player that will play static files, pixel exact,
such as test cards, and also something to generate test signals like a sweep.
IF I can get your to work I could get the other things to also. I've tried and failed
to get transmit to work usefully on my HackRF except just plain CW.

[[fsphil]]

An IQ file for transmitting with hackrf_transfer? I can do that for you, any particular video?

[dtvmcdonald]

Originally Posted by [fsphil]

An IQ file for transmitting with hackrf_transfer? I can do that for you, any particular video?

That's what I tried. I got it to sort of work on Linux, but it failed on Windows.

I can take your code for Linux and mutate it directly to do what I need.
I don't want to play a file ... I want it to generate an in-memory array and
repeatedly, forever, play that. That I can do easily ... it getting
it out that failed all my attempts to be reliable.

At least I can try. Any Gnuish code is so horribly opaque that
its hard, but often I succeed. I will look at the stuff you posted,
andhope to work on it over the Christmas break ... the power is going to
be off at the office, I just pray no more broken pipe floods!

[[fsphil]]

Originally Posted by dtvmcdonald

That's what I tried. I got it to sort of work on Linux, but it failed on Windows.

I have only had partial success at compiling hacktv for windows. It's been requested a few times so I may work on this some more. I don't have any PCs running windows here at the moment so I can't do any testing.

Originally Posted by dtvmcdonald

I can take your code for Linux and mutate it directly to do what I need. I don't want to play a file ... I want it to generate an in-memory array and
repeatedly, forever, play that. That I can do easily ... it getting it out that failed all my attempts to be reliable.

The test pattern generator in test.c is almost exactly what you need. At the moment it's just a very simple pattern I put together for testing 625 line PAL, but it could easily be modified and/or extended.

Originally Posted by dtvmcdonald

At least I can try. Any Gnuish code is so horribly opaque that its hard

I try to keep my code clear and simple. Don't always succeed though
Feel free to ask about any part of it.

[beery]

Hi fsphil,

Sounds like an interesting idea. You will need to be aware that 405 line transmissions used positive modulation for the picture, whereas nearly all other TV standards used negative modulation.
I have a very basic standards converter (I also have a couple of Auroras) which does indeed use intercarrier sound at 3.5MHz. It seems to work well on superhet sets (multi channel), but not TRF sets (fixed channel). There were a few TRF sets before WW2, but they really became popular in the late 40s to early 50s. They became less popular as more repeater transmitters came on line (still only one BBC station though) and finally disapeared off the market when ITV came along on VHF band 3 in 1955.

The Baird 240 line system used the same RF standard as the 405 line system. Details of the Baird system can be found here:-
http://www.nostalgiatech.co.uk/new_p...0line%20System

Cheers
Andy

[[fsphil]]

Thanks for that. I have been using data from: http://www.radios-tv.co.uk/Pembers/World-TV-Standards/ which seems to be quite accurate.

I've yet to find a mention of the typical audio bandwidth used, so I'll probably go with 10khz for now.