The video modes were chosen to match the Quarter-VGA standard (320x240 pixels). QVGA LCD displays are a good match to the resolution of a non-interlaced TV image, and are also necessary for portable applications.
The data format often differs from the simple RGB sent to the DACs, which in turn requires re-formatting inside the FPGA. QVGA displays are far better than the abysmal displays used in games machines like the Gameboy and Gameboy Advance. It doesn't look too difficult to generate the LCD drive signals.
So I shopped around and got this rough price perspective:
£ 65 Cheap 15" CRT monitor for PC £ 80 5" Colour QVGA LCD, STN (quantities of 1 to 10) £100 El-cheapo portable 14" TV £140 5" Colour QVGA LCD, TFT (quantities of 10 to 100) £160 Good quality 15" Flat-screen TV with SCART input and teletext (Toshiba 15N11) £160 Good quality 21" curved-screen TV with SCART input and teletext (Toshiba) £175 Bargain 15" SVGA (1024 x 768) TFT LCD monitor, analogue RGB input from PC video port £190 5" Colour QVGA LCD, TFT (quantities of 1 to 10)
As you see, LCD price is not simply proportional to the number of pixels. The demand for PC displays means keen pricing on TFT screens of SVGA or greater resolution. The demand for TFT QVGA is much smaller. STN QVGA is reasonably priced but suffers smearing of fast motion such as cursors or game action. It is almost as cheap as a PC CRT monitor and uses less space and power, so okay for use in applications like vending machines or information kiosks.
TV output is good value: you can use your living room TV for a big display, and if you have to buy a small portable TV you can enjoy using it as a spare TV.
QVGA TFT LCD isn't good value right now, though the market for PDA screens is increasing.
It was hoped that driving pixel data directly to an LCD would remove the cost, space and power taken by the three video DACs. Since the TFT QVGA panels are so expensive, the only other way to avoid the DACs would be to drive a display with a DVI interface. The downside is that turning pixel data into serial DVI signals requires specialised high-speed chips that probably require just as much cost space and power as the video DACs.
Organic LED displays hold promise of fast response and not needing backlights. Right now Optrex Plastic LED displays are available in monochrome up to 128 x 64 pixels from CPC but are still costly (c. £30). I look forward to seeing them in QVGA size!
A better choice for now might be to add the ability to drive desktop PC panels. These are just as expensive but can be useful as a desktop display when not being used by this project.