Creating Video From A ROM

We’re used to computers with display screens, yet how many of us have created the circuitry to drive one directly? Sure, we’ve coded up an SPI display driver on a microcontroller, but create the hardware to generate a usable video signal? That’s a little more difficult. [Jdh] has given it a go though, with a TTL video card.

In this case it’s not a card so much as a collection of breadboards, but all the logic is there to generate the complex array of video timings necessary for synchronisation, and to output the bits sequentially at the right voltage levels for the analogue monitor. It’s worth pointing out though that it’s not a composite video signal that’s being created sinceit’s monochrome only with no subcarrier.

In the end he encounters the problem that his ROM isn’t fast enough for the pixel rate and thus the image has artefacts, but it does at least produce a recognisable and readable something on the screen. Old hands in the video business might point out that analogue TVs were a bit forgiving when it came to exact timings and line counts so the circuit could quite possibly be simplified, and also that trading away some of the resolution might fix the ROM speed issue. But it’s an impressive piece of work, and should be of particular interest for anyone interested in how video works.

Fans of video cards on breadboards should also check out [Ben Eater’s] 7400-series video card.

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History Of Closed Captions: The Analog Era

Closed captioning on television and subtitles on DVD, Blu-ray, and streaming media are taken for granted today. But it wasn’t always so. In fact, it was quite a struggle for captioning to become commonplace. Back in the early 2000s, I unexpectedly found myself involved in a variety of closed captioning projects, both designing hardware and consulting with engineering teams at various consumer electronics manufacturers. I may have been the last engineer working with analog captioning as everyone else moved on to digital.

But before digging in, there is a lot of confusing and imprecise language floating around on this topic. Let’s establish some definitions. I often use the word captioning which encompasses both closed captions and subtitles:

Closed Captions: Transmitted in a non-visible manner as textual data. Usually they can be enabled or disabled by the user. In the NTSC system, it’s often referred to as Line 21, since it was transmitted on video line number 21 in the Vertical Blanking Interval (VBI).
Subtitles: Rendered in a graphical format and overlaid onto the video / film. Usually they cannot be turned off. Also called open or hard captions.

The text contained in captions generally falls into one of three categories. Pure dialogue (nothing more) is often the style of captioning you see in subtitles on a DVD or Blu-ray. Ordinary captioning includes the dialogue, but with the addition of occasional cues for music or a non-visible event (a doorbell ringing, for example). Finally, “Subtitles for the Deaf or Hard-of-hearing” (SDH) is a more verbose style that adds even more descriptive information about the program, including the speaker’s name, off-camera events, etc.

Roughly speaking, closed captions are targeting the deaf and hard of hearing audience. Subtitles are targeting an audience who can hear the program but want to view the dialogue for some reason, like understanding a foreign movie or learning a new language.

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Scanimate Analog Video Synths Produced Oceans Of Motion Graphics

Why doesn’t this kind of stuff ever happen to us? One lucky day back in high school, [Dave Sieg] stumbled upon a room full of new equipment and a guy standing there scratching his head. [Dave]’s curiosity about this fledgling television studio was rewarded when that guy asked [Dave] if he wanted to help set it up. From that point on, [Dave] had the video bug. The rest is analog television history.

Today, [Dave] is the proud owner and maintainer of two Scanimate machines — the first R&D prototype, and the last one of only eight ever produced. The Scanimate is essentially an analog synthesizer for video signals, and they made it possible to move words and pictures around on a screen much more easily than ever before. Any animated logo or graphics seen on TV from the mid-1970s to the mid-80s was likely done with one of these huge machines, and we would jump quite high at the chance to fiddle with one of them.

Analog television signals were continuously variable, and much like an analog music synthesizer, the changes imposed on the signal are immediately discernible. In the first video below, [Dave] introduces the Scanimate and plays around with the Viceland logo a bit.

Stick around for the second and third videos where he superimposes the Scanimate’s output on to the video he’s making, all the while twiddling knobs to add oscillators and thoroughly explaining what’s going on. If you’ve ever played around with Lissajous patterns on an oscilloscope, you’ll really have a feel for what’s happening here. In the fourth video, [Dave] dives deeper and dissects the analog circuits that make up this fantastic piece of equipment.

Here’s another way to play with scan lines: delay the output to some of them and you have a simple scrambler.

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Full-Colour, Full-Motion Video – On An Audio Cassette!

A lot of projects we feature use video in some form or other, but that video is invariably digital, it exists as a stream of numbers in a computer memory or storage, and is often compressed. For some of us who grew up working with composite video there is a slight regret that we rarely get up-close and personal with an analogue stream, so [Kris Slyka]’s project putting video on a conventional audio cassette is a rare opportunity.

It's fair to say this isn't the highest quality video.
It’s fair to say this isn’t the highest quality video.

Readers with long memories may recall the Fisher-Price PixelVision toy from the late 1980s which recorded black-and-white video on a conventional cassette running at many times normal speed. This system does not take that tack, instead it decreases resolution and frame rate to a point at which it can be recorded at conventional cassette speeds. The result is not particularly high quality, but with luminance on one side of a stereo recording and chrominance on the other it does work.

The video below the break is a run through the system, with an explanation of how video signals work. Meanwhile the code for both encoder and decoder are available through the magic of GitHub. If you’re interested further, take a look at our examination of a video waveform.

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AV Synth Is Psychedelic Analog Mayhem

Digital video is cool and all, but it can’t compete with analog in terms of smooth, creamy glitches and distortion. [gieskes] has developed an analog audio-visual synthesizer that is a great example of the old-school retro visuals you can create with a handful of simple components.

Known as the 3TrinsRGB+1c, it’s available both assembled and in kit form. It’s probably best to start with the manual. Synthesis is achieved through the use of a HEF40106 hex inverting buffer – a cheap and readily available part that nonetheless provides for excellent results. Video can be switched between RGB oscillators and a series of inputs, and there are various controls to create those classic scrolling effects and other visual oddities.

Additionally, a series of connections to the underlying circuitry are broken out on a header connector. This allows for extra modules to be plugged in, and several designs are available to expand the unit’s capabilities.

Analog video isn’t used so much on a day-to-day basis anymore, but it’s a great technology to tinker and experiment with. We’ve seen some of [gieskes] experiments in this arena before, too – like this Arduino video sampler. Video after the break.

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