smeezekitty wrote:The bandwidth of NTSC is quite high so you need specialized hardware.
To a degree that's true, the hardest part is finding an A-D which is fast enough, easily available and at a sane price. The rest of it is reasonably straightforward.
The closest I've seen that meets those criteria is the AD7822, it can sample at 2MHz (just fast enough) and is eight bit. There is the old CA3306 which can sample at 10+MHz, but is only six bit and almost impossible to find these days, though it 'would do'. There's also the eight bit version, the AD3308, but at ridiculous prices...don't even bother trying to source them!
Similar to the SSTV-625 up-converter but in reverse, it would use two processors, one to write to RAM, the other to read at the slower rate.
PDF for the AD7822 attached, they're around 10 quid in the UK, say US$14.00...I would guess similar to a pint of beer these days in the UK...
Steve A.
It is possible to use two CA3306s in 'tandem' to generate a seven bit output. Also using oversampling and decimation you possibly could get eight bits, but I doubt its worth the effort. Anyway, it would be more complex and costly than using a single AD7822, so why bother?
I must admit I haven't looked within the past few years for a suitable A-D, maybe there are newer devices that I'm unaware of...time for some research...
Later...well, I did have a look around for similar spec'ed parts, few and far between, they're either
too fast (56GS/S...what!), too expensive, or in ridiculously small SMD packages, i.e. not hobbyist friendly...so it looks like the AD7822 it has to be...it may be worthwhile stocking up for the future before they are also deemed obsolete...at least those in DIL/DIP packaging...
Farnell/Element14 UK..1079304
RS UK......................709-6777
Digikey US................AD7822BNZ-ND
I did try finding the Australian websites for the above suppliers, but kept getting redirected back to the Thai sites, I guess due to my location...though it doesn't happen when I visit the UK or US websites...even though all three suppliers listed have Thai websites...