by Steve Anderson » Sun Mar 06, 2011 5:04 pm
This all started perhaps over a year ago when someone (I forget who) asked is there a replacement for the CA3306 6-bit A-D chip used in Peter Smiths' 625-Line to 30-Line Converter. These chips are now obsolete although the Club Shop has a limited supply as do one or two retailers in the UK. BEWARE of those "Obsolete Component Suppliers', they're mostly a bunch of sharks!
The problem is that there is a very limited choice of replacement when you don't want to spend a fortune and you want it home-construction friendly, i.e. in a DIP package, not SMD.
The nearest I've come across is the Analogue Devices AD7822 which is a 20-pin DIP and under ten quid in the UK, Farnells' price is 7.71 plus VAT etc.. The AD7822 has the advantage(?) of being 8-bit but can only sample up to 2Mhz as opposed to 10MHz for the CA3306. For converting 625 to NBTV it is fast enough however, Peters design samples at around 1MHz.
The manner in which the AD7822 operates is slightly different to the CA3306 and requires some changes to Peters design. In pondering over this I thought, "Why not start afresh?" So here we are.
Peters converter design is now over 20 years old and quite a number have been built and it works as advertised. But it does use 24 chips not including any there may be in the power supply.
Thus far I have been able to reduce the chip-count down to six (again not including the power supply). Three 8-pin DIPs, one 16-pin and one 28-pin (0.3") PIC which replaces all the 'glue logic' and memory. At the moment I'm still using the 18-pin CA3306 but when the AD7822's turn up it will be replaced.
Where this isn't as good as Peters is in the sampling, each NBTV line has only 72 active (video) pixels, whereas Peters has around 150. The reason is simply the limited amount of RAM in the PIC. I'm using a 18F2620 which has the largest RAM available in the 18F series at just under 4kB. The actual RAM used for video storage is 2304 bytes (32x72), a few more bytes are used for counters etc.. The syncs are generated, not stored, and in line with club standards it outputs 32-line NBTV, I haven't planned for 30-lines but it should be possible.
So at this stage the hardware is almost defined...it may grow a little in chip-count but I am trying to keep it to an absolute minimum, certainly under ten.
I'm gearing this as a 625 to 32 converter, nothing else, no bells, no whistles. But perhaps in a MkII version additional features could be added. External RAM will help immensely but adds to the chip-count. An SD Card to store the converted video could be useful, the micro has the required SPI interface. Suggestions?
There may be a small item regarding this in the next (March) newsletter, I'm hoping, but not promising to get this ready for the June newsletter.
A PCB is not beyond the realms of possibility as it's quite simple.
Now...the hard part...the software! Wish me luck!
Steve A.
P.S. The copy I have of Peters converter is from the BATC's CQTV magazine, No. 189, published in 2000. I assume it also appeared in the NBTV newsletter, but I don't know which issue(s). Does anyone know?