[sdiy] Video Synth...

[Pete Swarbrick]

Hey Nils
I only just got back online after the Christmas/New year thing...are you
still needing a PAL sync pulse generator? I have NOS  Ferranti ZNA134
and crystals which would do what you want. In fact the ZNA234 version
(pattern generator) may be more suitable for video synth experiments- I
have used both of these for this over the years. Somewhere I have
probably got one of these as well. Another alternative would be Philips
SAA1043 which I was even more flexible and had a companion subcarrier
generator the SAA1044. There was a further version(SAA1101) that
integrated both of these into one. Maybe I even have one of these in a
parts drawer somewhere.
Although these are long out of production I still use them on occasion
since a) I've done so for years b) I've got some and c) I am not much of
a PIC programmer!

www.datasheet4u.com/html/S/A/A/SAA1043_PhilipsSemiconductors.pdf.html
http://electrickery.xs4all.nl/comp/dai/doc/ZNA134.pdf
http://www.datasheetarchive.com/pdf/zna234-datasheet/zna234-datasheet.ht
ml

 Let me know if any of this is of interest.
(and don't be fooled by the job title below, I'm an analogue guy)

Pete

Peter Swarbrick
Head of Digital Imaging
Panavision Europe
Greenford UB6 8GD
UK
+44 208 839 7338
+44 7958 597777
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-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Magnus
Danielson
Sent: 22 December 2007 15:07
To: np at planetarc.de
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Video Synth...

From: np <np at planetarc.de>
Subject: [sdiy] Video Synth...
Date: Fri, 21 Dec 2007 22:21:41 +0100
Message-ID: <476C2E65.1060002 at planetarc.de>

Hej Nils!

> I'm playing with the idea to build a little video synth related
circuits over the next days and I have a couple of questions. Hope you
fine guys can help me even if it's a bit off-topic:
> 
> First problem: Signal generation for a PAL BAS signal.
> 
> For my video signal generator I need two sync-signals at relative
stable frequencies. VSync is at 50Hz (well - that's easy). HSync at
15625 Hz.
> 
> I did some math and found out that I can get both frequencies by
dividing down a 27Mhz crystal oscillator. That would be a cheap way to
do this, but I haven't ever done anything that runs at these high
frequencies. Will prototype board (the one with the dots, not the
strips) work at that high frequencies or am I asking for trouble? For
the frequency division job I plan to use the 74HCT4059.

Be sure to have the standard 100 nF ceramic caps decoupling the power of
the
chips and you will be fine. Most importantly you want to ensure that the
caps
sits very near the chip and connects with smallest possible traces over
to the
power pins. The key is to minimize loop area between the chip and the
capacitor. For 27 MHz you will be fine if you just put it on the
short-end of
the chip and make traces to go directly to the power pins.

As for the frequency itself, 27 MHz is a standard video frequency, so
you can
get the classic CCIR 601 (now ITU-R BT.601) sample rates, which matches
both
PAL and NTSC thanks to math magic. For the frequencies you required, you
can
do with a significantly lesser frequency if you only require PAL
compatibility.

> Is there a simpler way to get my frequencies? Any idea that gets me
out of the need to do HF stuff is welcome. Maybe I've overlooked another
crystal that runs at a lower frequency, or I could do things with PLLs,
or there is a even simpler solution. I don't want to use a
microcontroller btw.

Your frequencies are

f      = 25 = 5^2
 frame

f      = 50 = 2*5^2
 field

f      = 15625 = 5^6
 line

Common term is 5^2 so you end up having the multiples of 1, 2 and 5^4
and a
common source for these is 2*5^4 thus, the lowest common frequency
becomes

f    = 2*5^4*5^2=2*5^6 = 31250 Hz
 com

f      = f    / n      => n      = f    / f      = 2*5^6/5^2 = 2*5^4 =
1250
 frame    com    frame     frame    com    frame

f      = f    / n      => n      = f    / f      = 2*5^6/(2*5^2) = 5^4 =
625
 field    com    field     field    com    field

f      = f    / n      => n      = f    / f      = 2*5^6/5^6 = 2
 line     com    line      line     com    line

Recall that both PAL and NTSC is interlaced, so the two fields only hold
half
of the lines in each frame.

> Next problem: for the image synthesis I need a bunch of two quadrant
multipliers. I've looked up the datasheet of the SSM2024 as I have lots
of them lying around here but I found nothing about their slew rate in
the spec. _That_ makes me wonder how they perform at higher frequencies
at all.

It only tells you that it was not a prioritized area since their main
target
was the audio products. You should be fine.

> The worst thing they will do frequency wise is a VCA job for a 15khz
saw-wave. I do not want much ringing and smearing at the sawtooth reset
though.

If you use a high frequency like 27 MHz, your sawtooth can be the
counter
output through a DAC. 13,5 MHz is a particularly nice frequency to use
as a
step in that equals the length on the screen matching the heigth of a
line.
You might want that. That divisor would be at 864 or 1728 if you go from
27 MHz.

>  From experience, can you tell me how high the SSM2024 can practically
go?

Don't have a SSM2024 to play with. I'd expect several MHz of BW, but
then I
would expect that to change with control current. I also expect it to
depend
on the hookup and especially the capacitive output loading.

In general, I recommend you to learn the more HF stuff as they will
become a
limiting aspect for you.

Cheers,
Magnus
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