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Video to NBTV converter

PostPosted: Sun Mar 06, 2011 5:04 pm
by Steve Anderson
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 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?

PostPosted: Mon Mar 07, 2011 5:24 pm
by gary
Volume 25 Number 3

PostPosted: Mon Mar 07, 2011 5:39 pm
by Steve Anderson
gary wrote:Volume 25 Number 3

Thanks Gary....well before my time, my copies don't start until Vol. 28.

On that note, has anyone ordered the newsletter back-issue DVDs? I'm hoping they've been compiled by now. There was no mention of them on the web-site the last time I visited and I don't recall seeing any reference to them on the flyer that comes with the newsletter.

Pointers anyone? (I don't want to bother Jeremy with something like this).

Steve A.

PostPosted: Mon Mar 07, 2011 6:32 pm
by gary
Steve, it's a CD, it is listed on the loose leaf "CLUB SALES" page that comes with your newsletter under "CDs" - Newsletter Archive CD £15 - well worth it IMHO.

Edit: and, of course, contact Vic Brown for a copy.

PostPosted: Mon Mar 07, 2011 9:14 pm
by Steve Anderson
Hmm...must have missed them on the separate Club Sales sheet. It's something I don't bother scanning, just the newsletter itself. It just ends up in the trash ten minutes before the I wish we could have pdf copies...

I'll get in contact with Vic, thanks again.

Steve A.

PostPosted: Mon Mar 14, 2011 6:02 pm
by Steve Anderson
Isn't technology amazing....the posting above was a week ago, now I have a pristine copy of said CD! At long last Pay-Pal works here!

I have only had a quick skim through the CD but I know that it will afford much more interesting evening entertainment than the 'box'.

Fifteen Quid...including postage here...worth every one of those 731 Thai Baht it cost me.

Steve A.

P.S. Newsletter arrived in same postal delivery, (having been franked in the UK on the 9th) so hopefully everyone should have theirs already or will receive it in the next few days.

PostPosted: Mon Mar 14, 2011 7:08 pm
by gary
Which newsletter is that Steve?

PostPosted: Mon Mar 14, 2011 8:08 pm
by Steve Anderson's Vol. 36 No.3 which if things are getting back on schedule is due right about now, mid-March in preparation for the April convention. The "No. 3's" historically appear in March.

Steve A.

PostPosted: Mon Mar 14, 2011 9:26 pm
by Dave Moll
My copy arrived at the end of last week, so yours should be winging its way to you at this very moment.

PostPosted: Mon Mar 14, 2011 9:40 pm
by gary
Excellent! It seems I have only just received the last one...


PostPosted: Fri Dec 06, 2013 9:21 pm
by Steve Anderson
Whatever has happened to the newsletter I don't know...

But to resurrect this thread work has slowly continued on this converter and basically comes down to six chips...

1) An LM1881 FSTV sync separator (8-pin DIP)
2) An LF356 op-amp for post-filter gain (8-pin DIP)
3) An 74HC4052 for black-level clamping (16-pin DIP)...this could be changed to a 74HC4066. (14-pin DIP)
4) An AD7822 8-bit A-D (20-pin DIP)
5) A pre-programed PIC uC which I'll make available at cost and/or via the club shop at around a fiver (28-pin DIP)
6) An CA3240 dual op-amp for PWM filtering (8-pin DIP)

That's it. No other exotic components, no SMD devices...the rest from the usual suppliers...

It requires +5V and +12V only, both regulated at less than 100mA...

Steve A.

Added 9/12/2013...I've just realized that the 74HC4066 requires an opposite polarity switch control pulse. This could be fixed with an inverter, but that would require another forget that idea...perhaps it was the reason I chose the '4052 in the first was some time ago.

PostPosted: Thu Jan 16, 2014 8:53 pm
by Steve Anderson
...another interruption to the progress of this project called 'real work'. But that's done with so maybe I can actually be able to spend some time on this!

Photo attached which shows the entire hardware extent of the 625-32 converter, including the power supply, though less the transformer.

More to come,

Steve A.

The reason for two small boards rather than a single larger one is that perhaps in the future I may upgrade the 'Logic Board' (on the right) to use external RAM, the analogue board (left) would need no changes.

Two of the pre-sets on the left-hand board will eventually vanish and become front-panel controls.

PostPosted: Thu Jan 16, 2014 9:22 pm
by AncientBrit
Looks good Steve,



PostPosted: Fri Jan 17, 2014 4:36 pm
by Steve Anderson
As a note, should I decide to build an external RAM version this is mechanically how I plan to do it.

It is possible to add external RAM to the existing 28-pin version posted yesterday, there is just enough free space on the board to accommodate it. But it would mean losing a few niceties which are probably very useful.

This means simply more pins on the uC, so it has to be a 40-pin device, currently every pin on the 28-pin version is used. I would need four more pins, (the external RAM is a SPI device), I2C is simply too slow.

Steve A.

From here on in once the inter-board wiring is done and hardware certified (mostly done already), it's just a software exercise (ho-ho)...

PostPosted: Fri Jan 17, 2014 8:13 pm
by AncientBrit
Hi Steve,

If you migrate to a 40pin device you might consider using one of the extra pins to switch to a test pattern on the NBTV ouput.

I found that quite useful for setup purposes in the converter I built many moons ago.