VHDL Teletext: Overview of the project and the FPGA Teletext hardware

Project objective

The aim of this project was to build a fully-featured Teletext decoder using an FPGA. The open-source firmware was written in VHDL. The completed device is the world's first standalone Teletext decoder with an HDMI output, but initially,I implemented a VGA output to learn the basics of generating video from an FPGA.

A brief introduction to Teletext

Teletext was introduced in Europe in 1970s as a system to display text and simple graphics on television sets. A signal contained within the Vertical Blanking Interval (VBI) (of the composite video signal) is decoded by (typically) a custom ASIC in the TV set, which displays the page requested by the user when it is received. World System Teletext (WST) became very popular in Europe and remains in use today (2021) in countries including Germany, Italy, and Ireland. Teletext was also launched in the United States, where it was much less successful, and didn't last very long.

Teletext most commonly carried information for human use such as news, but in the past it also carried programs for computers to use - called Telesoftware.

In the 1990s, Level 2.5 teletext was introduced. It allows Teletext artists to use more colours and more complex graphics. Level 2.5 is not widely supported by television sets, but VHDL Teletext will include some Level 2.5 decoding ability.

Teletext survives despite the switchover from analogue to digital television; the data originally carried in the Vertical Blanking Interval of the analogue picture is now carried as part of the DVB-S signal, and is encoded into the composite video output of most satellite receivers when tuned to an appropriate channel.

Outside of broadcast, Teletext is used as an art form. The Teletext signals can be generated by a Raspberry Pi using vbit2. TEEFAX, started by Peter Kwan in 2016, is one such service that can be run from a Raspberry Pi.

Development process

I've documented key parts of the project development process in detail in my blog:

1. Driving a VGA monitor from an FPGA - I gained an understanding of the basics of FPGA video generation by displaying simple test patterns on a VGA monitor.
2. Page display including Text and Graphics - A basic implementation of the Teletext display generator, incorporating most Level 1 features and using RAM pre-loaded with a fixed page for testing purposes.
3. Data recovery, processing and page extraction - Created the logic for recovering the data from the composite video signal, identifying packets belonging to the requested page, and writing them to dual-port RAM.
4. A sidestep post - programming the FPGA using a MAX 10 FPGA - The FPGA Teletext board (details of the board are below in this blog entry) can be programmed using a Raspberry Pi, offering an alternative to those who do not have or wish to procure a USB Blaster. The Raspberry Pi is a popular generator of Teletext signals, so it is incredibly useful to be able to reprogram the FPGA using it.
5. Generating a high resolution HDMI/DVI output from an FPGA using Double Data Rate I/O - Added the logic to produce HDMI signals at resolutions up to 1280x720.

VHDL-Teletext firmware features

• Open-source firmware GitHub
• Level 1.0 teletext decoder with single-page memory
• Selected Level 2.5 and Level 3.5 features implemented
• Resolution selection via DIP switches: 720x576 or 800x600 at 50Hz or 60Hz (1280x720 available via firmware branch, and 640x480 also possible with minor code modifications)
• Alpha Black enable switch for selecting between adherence to the original specification or compatibility with certain modern teletext services
• Power-on page number selection via DIP switches
• Mix function enables/disables the display of background colours (note that this board does not display the video portion of the composite video signal over HDMI)
• Subpage selection
• Reveal button displays and hides "concealed" page content
• Page selection via number keys, up/down buttons, and Fastext keys
• Level 2.5/3.5 store function for applying enhancements to Level 1.0 pages
• Parameterised design enables design tweaks without any programming knowledge

The hardware: the FPGA Teletext PCB

An FPGA development board designed for the application of decoding WST (World System Teletext) and displaying it on a HDMI monitor/TV. The board is programmed with my open-source VHDL-Teletext firmware and all that's needed to complete the setup is a micro USB power supply, HDMI or DVI monitor, and a source of teletext data with composite video out such as a Raspberry Pi running VBIT2 or a satellite receiver. 50/60Hz output and choice of 576p and 800x600 resolutions for compatibility with international TVs.

Development board hardware features

• 10M08SCE144C8 FPGA
• HDMI output with arbitrary resolution support* with proper current-mode logic level shifter IC
• Composite video input with high-speed comparator for WST sampling
• Micro USB power supply connection - typical current when running teletext decoder firmware is 100mA
• 3.3V 2A switching regulator (plenty of power for external electronics) and 2.5V LDO regulator
• 3-channel programmable oscillator
• 16-way DIP switch
• 20 buttons (including detachable keypad)
• Three I/O headers with 23 I/O pins
• Footprint for optional 512K SRAM for development purposes (footprint can alternatively be used as extra I/O)
• Footprint for optional video processor IC (SAA7113) (this could be used to implement full composite video to HDMI conversion)

* Resolution support tested up to 1280x720. The master branch of the firmware includes support for 720x576 and 800x600 resolutions, and the 720p mode uses the feature/720p branch.

The board is suitable for all manner of FPGA projects. The Altera MAX 10 FPGA has 8k logic elements which is enough for complex hobby FPGA projects. In addition to HDMI connectivity and the built-in keypad and buttons, 23 I/O are connected to headers to allow connection to other devices, providing the potential for diverse applications such as a Telesoftware decoder or a replica of an old microcomputer.

Why did you make it?

I made it because I wanted to create a Teletext decoder which does not depend on obsolete components and has a high-quality, modern, video output connection.

The unique combination of hardware features needed made existing FPGA development boards unsuitable. Limited stocks of old obsolete decoder chips are still available but these only have low-quality composite video outputs and usually only work with PAL TVs.

What makes it special?

This is the world's only standalone teletext decoder with HDMI output.

Hardware for sale

I have assembled a small number of boards by hand and they are up for sale below. If there is serious demand for this PCB, then I will consider getting a proper production run done, and I'll offer a self-assembly kit too. Please comment below on my blog if these are of interest.

Click here to buy the VHDL Teletext PCB on Tindie (fully assembled) - ships from the UK (though all pricing is in USD on Tindie) - please remember that this is currently a very low volume product so I cannot achieve the low pricing of high-volume consumer gear.

Also available on eBay for those who would encounter high fees from their bank/card provider for paying in USD on Tindie or who wish to use the eBay Global Shipping Programme - eBay link.