405 lines from Raspberry Pi 5

[Mike Watterson]

Almost 20 years ago I wrote an article for the BVWS Bulletin saying that you couldn't us an ordinary PC for real-time standards conversion.

Some have suggested using computers or digital signal processors (DSP). Conceptually there is no problem. I’m a pretty lousy programmer and I could write you the conversion algorithms in a few lines of a high level language such as BASIC. These would take a 625 line image, already in the computer, and convert it to a 405 line image, also within the computer. And there’s the snag. You still need to get the picture in and out and you have to ensure that the computer can keep up with the data. Video comes at you continuously so real time means exactly that – you cannot put your hand up, take time out and catch up later. So when you say you have built a converter with a cast off 486 PC I won’t believe you.

In 2006 Kat Manton proved me wrong by showing 405 out from a PC. This used MythTV Linux. Others worked on similar methods, both in Windows and Linux. These all used the capabilities of modern graphics cards to do just about any line standard.

Yes, and in 2007 I needed to demo Digital TV in a 2 MHz channel. I found claims of realtime H.264 encoding using a top of the range GFX card GPU misleading. It could decode in realtime, but they never released the software for encoding. I had a PCI card to generate DVB that could do 1MHz to 8MHz bandwidth, but I had to pre-encode all my content to an Mpeg2 transport stream saved to a file that the PCI card could use. That ran in a loop and the card set to output on VHF as an IF for a 10.45 GHz transmitter. A 10.45 GHz receiver down converted to UHF which was then fed to a Motorola setbox.

Now at least one channel can be encoded in H.264 cheaply in realtime. I have an HDMI to UHF DVB-T box that does 6, 7 or 8 MHz and various encoding settings from Technomate. Probably intended for guest houses and hotels. At less than a tenth of the price I have a USB HDMI encoder that takes up to 4K and provides 1080p. An Analogue USB capture gadget works with NTSC or PAL and provides up to 1080p H.264 or 720 x 576 MPEG. Both work on the Raspbery Pi4B in real time to output at 720p60 (tested) to 1080p50 (tested) to 4Kp30 (tested) or 625 PAL composite. The Analogue one seems to have a comb filter as composite PAL has no patterning at all on the real time HDMI output. OBS supports the audio, because mysteriously sound doesn't work on VLC with many capture devices. More astoundingly, both work in real time on my Android tablet either live on screen, capture to file, screen cast via WiFi to 4K TV in Full HD, or stream on the LAN.

PC Standards:
I never did more than 625 line RGB on a Trident ISA VGA card with adaptor to SCART that added the H &V syncs to SCART composite pin. A small DOS program reprogrammed the VGA card registers. An external PAL encoder with RGB SCART in converted to composite. A DOS video player then could output files pre-encoded at 768 x 576 25i for recording on to VHS. This worked on a 286. Not much later I got an ISA monster card with HW video overlay, PAL capture and playback (Y/C or composite), SVGA/XVGA loop through and MJPEG recording / playback with Adobe Premiere Video editing. An 800 Mbyte HDD for the video. Next we created SVCDs better than the commercial VCD using a CD writer (no DVD writer yet) and used the DVD player to convert to PAL for VHS or TV viewing. Then we progressed to Firewire for video capture (digital recordings or analogue bridge) as no affordable PC video capture was as good and DVD writing for Video out.

So finally after 30 years messing with analogue video on PCs, I have cheap gadgets and a suitcase of analogue 8mm tapes, some Digital8 (they are no issue) and a shelf of VHS tapes (maybe one is S-VHS) to capture. I won't bother using DVDs for distribution (many I know can now only play those on their PS4 etc), but files on micro SD cards. A 256 G card (about 36 hours of H.264 HD?) is only £18. A decent 32G card is about £5 for about 8 hours. My first HDD was 5 Mbytes. First HDD for video was 800 MByte. My server now has 6 T byte for user files. I may get the old analogue 8mm camera going as it's likely more compatible to play its recordings than the Digital8.

Big changes since I joined BBC in 1975. Sound in Syncs was the only obvious digital. Maybe 20 years ago I visited RTE Lyric FM in Limerick City. Everything was digital except the mixing desks (to be replaced soon) and most files were on a server in Dublin.
In 1971 my project presentation at the Young Scientist Exhibition in Dublin was the idea of Digital TV (BBC had published some research) and communication of it on laser. One of the judges was the RTE Chairman: "Fascinating, but of course we'll never have digital TV or laser communication." The next year saw the demo of laser comms via fibre optics. Free space Laser comms is rare, but all carrier fibre is laser based and you can buy home 10 Gbps laser-fibre kit. Irish Analogue TV was turned off in 2012 and test transmissions of H.264 SD were at least in 2007 or maybe 2006, I forget.
My broadband was Cable protocol (DOCIS 2.0, then DOCSIS 3.0) over 13 km outdoor 10GHz duplex link from November 2005 to November 2023 (8Mbps/1Mps). Same outdoor radio that duration (About 45 MHz up. 485 MHz down) to a cable modem swapped once. Finally on fibre. My first computer comms
was with CP/M and 1200/75 BBS in 1981, then 300 baud dialup to an X.25 pad to access a BT server in London in 1986 for bitnet and email, and a fake Telex address.
Hotel in 1987: We can only take advance bookings by post or Telex, not phone. So I ran my PCW8256 dialled a Belfast PAD from Co. Clare, navigated to and logged into Telecom Gold in London and sent them a telex.

Big changes in computers, comms, audio and video in the last 50 years.