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Amiga as test pattern generator.

 
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Amiga as test pattern generator.

Post by a4000bear » Sat Mar 14, 2015 2:50 am

About 15 years ago I decided to make myself a high quality test pattern generator, using an old Commodore Amiga A1200.

I built it into a rack case, along with an old PC power supply, a modified Amiga A520 PAL modulator and a surplus video camera viewfinder CRT, which was driven by a PCB from an old B&W monitor.

Front.jpg
Amiga Test Pattern Generator.


A momentary pushbutton was wired into the former left mouse button. This allows the user to cycle through the patterns stored in the memory. The selected pattern is displayed on the camera viewfinder CRT, and output is provided on the rear panel in the following formats:
Modulated RF - provided by the existing A1200 RF modulator.
CVBS video - provided by the existing PAL encoder in the A1200.
S-Video - provided by the added and modified A520 PAL encoder.
RGB - provided by the existing video out socket that was extended to the back panel
Component - provided by an added circuit, which converts the original RGB signal.

Rear.jpg
Rear panel showing outputs.


Originally I had an old hard drive attached to the inbuilt IDE interface that automatically booted the Amiga straight into an image viewer program called 'ppshow' which displayed the images stored on the hard drive. Later I replaced the hard drive with a compact flash card from an old camera. I'm able to fit quite a few patterns in the tiny 8MB card.

Insides.jpg
Insides


Patterns included the following:

PM5544
PM5644 (widescreen PM5544)
PM5540
Colour bars
Multiburst
Comb filter test
Luma/Chroma delay test
Zoneplate patterns

It's not necessary to do all these modifications to have an Amiga work as a test pattern generator. As a minimum all you need is a standard A1200 or A4000, preferably with Kickstart 3.1 ROMS (3.0 causes slight cropping on the left of the picture) It's also possible to use a bootable floppy to store the patterns on, however you are limited to a maximum of 5 or 6 patterns.

These patterns are not suitable for earlier Amiga models such as the A500 or A2000, as these cannot display enough colours or have the required 'super high res' screen modes (1440 x 582) that these patterns require.

If anyone is interested, I can provide the files for you to create a bootable Amiga hard drive or floppy.

 
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Re: Amiga as test pattern generator.

Post by johntheboffin » Sat Mar 14, 2015 3:02 am

That is most impressive.

I had an Amiga many years ago, but the kids took it over. I still wonder what happened to it.

Well done for a clever and well implemented project!

John

 
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Re: Amiga as test pattern generator.

Post by a4000bear » Sat Mar 14, 2015 3:13 am

johntheboffin wrote:That is most impressive.

I had an Amiga many years ago, but the kids took it over. I still wonder what happened to it.

Well done for a clever and well implemented project!

John


Thanks, John.

It's amazing what can be built up from those spare bits and pieces hanging around.

I just had a thought too.

For those people into 405 line TV, I know that these Amigas have programmable line and frame rates. It could be possible to force them to output a 405 line video signal. Unfortunately I don't have a 405 line TV to try this out on.

 
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Re: Amiga as test pattern generator.

Post by crustytv » Sat Mar 14, 2015 9:21 am

What an excellent idea, great attention to detail and a very professional looking unit. Thanks for sharing it with us. :thumb

p.s.
viewfinder on the front and the selectable option was a neat touch.

 
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Re: Amiga as test pattern generator.

Post by johntheboffin » Sat Mar 14, 2015 11:16 am

a4000bear wrote:For those people into 405 line TV, I know that these Amigas have programmable line and frame rates. It could be possible to force them to output a 405 line video signal. Unfortunately I don't have a 405 line TV to try this out on.


You could program the timebases on the Commodore PET as well. This was done by using the POKE command to change the values in specific memory locations.

As the PET had a built in CRT and associated circuitry, this meant that you were directly modifying its line and frame scan rates. Not sure how much truth there is in it, but I once heard that if you set the line scan rate to a setting where the LOPT was particularly inefficient you could cause it to overheat and sometimes catch fire. There are times when I wish Windows had such a feature!

John

 
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Re: Amiga as test pattern generator.

Post by Cathovisor » Sat Mar 14, 2015 11:35 am

Anno Domini is probably playing tricks with me now, but ISTR that the A4000 could be coupled to a Questech Charisma to create custom versions of the CLEO* patterns...

(*CLEO = Curvi-Linear Effects Option)

 
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Re: Amiga as test pattern generator.

Post by a4000bear » Sat Mar 14, 2015 12:48 pm

johntheboffin wrote:You could program the timebases on the Commodore PET as well. This was done by using the POKE command to change the values in specific memory locations.

As the PET had a built in CRT and associated circuitry, this meant that you were directly modifying its line and frame scan rates. Not sure how much truth there is in it, but I once heard that if you set the line scan rate to a setting where the LOPT was particularly inefficient you could cause it to overheat and sometimes catch fire. There are times when I wish Windows had such a feature!

John


Indeed there was a 'killer poke' that reputedly could damage later models of the Commodore PET.

The early versions of the PET used a number of TTL logic chips to generate the sweep signals for the inbuilt monitor. Also these early models had slow video RAM access. It was found that typing 'POKE 59458,62' sped up this memory, though 'sparkles' appeared on the screen.

Later versions of the PET has faster video RAM access as well as a CRTC chip which generated the sync and blanking signals for the monitor. The POKE mentioned above now had no effect on speed, but there was now an unwelcome effect on the retrace signal that was fed to the monitor electronics.

I think Commodore fixed the problem fairly quickly as I have a late model PET, and typing the killer poke does nothing to the display.


Incidentally, I did have first-hand experience of a self-destructing Windows PC. I had just assembled a new PC, and on powering it up for the first time, I noticed that it was dead. As I reached over to switch it off, there was a loud bang, something whizzed past my head, and the PC beeped and started to boot! I found that an electrolytic capacitor had blown itself off the motherboard. Presumably it was a short circuit, and after it had removed itself from the motherboard, the short was also gone....

 
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Re: Amiga as test pattern generator.

Post by BluePilot » Sat Mar 14, 2015 7:51 pm

Ah yes, the IBM instruction set. I remember others such as PED (punch and eject disk) and the classic for anyone who ever tried to use tapes with an IBM system, WWLR (write wrong length record). If only there had been a RWLR.

Here's a more complete list:
http://ruthless.zathras.de/fun/top-secret/NewOpCodes.txt

 
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Re: Amiga as test pattern generator.

Post by freya » Mon Mar 16, 2015 9:10 am

I spent many years playing with amigas, started with a 500 then the tragic CD32, then a 1200 with blizzard 12060, then a 4000.
Very happy times, even built the 1200 into a mini case years before they were widely converted. They were used for rendering some of the first CGI graphics shown on film and television I believe.
The A1200 is still in the wardrobe, I wonder if it still works.

 
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Re: Amiga as test pattern generator.

Post by a4000bear » Tue Mar 17, 2015 12:18 am

freya wrote:I spent many years playing with amigas, started with a 500 then the tragic CD32, then a 1200 with blizzard 12060, then a 4000.
Very happy times, even built the 1200 into a mini case years before they were widely converted. They were used for rendering some of the first CGI graphics shown on film and television I believe.
The A1200 is still in the wardrobe, I wonder if it still works.


It might be a good idea to check it out. Quite a few of the later model Amigas like the A4000 and A1200 had surface mount electrolytic capacitors that are prone to leaking electrolyte, causing corrosion on the motherboard.

 
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Re: Amiga as test pattern generator.

Post by Doz » Tue Mar 17, 2015 9:35 am

Video Toaster anyone?

 
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Re: Amiga as test pattern generator.

Post by freya » Tue Mar 17, 2015 10:52 am

The capacitors all seem to be ok, it powers up but wont read anything from the floppy, and the hard disk isn't spinning either. Been asleep too long.

 
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Re: Amiga as test pattern generator.

Post by Refugee » Tue Mar 17, 2015 11:25 am

I have had old hard disks that will not spin.
The fix I found is to remove the unit from the case and power it up. They usually spin without a data cable connected.
Hold it up in one hand and tap on the corners with your knuckle so that the case rotates sharply in alternate directions around the platter axle. This often frees them up.

 
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Re: Amiga as test pattern generator.

Post by freya » Tue Mar 17, 2015 11:36 am

I must have been thinking on the same line, its sitting on a hot water bottle presently I was then going to give it a tap when its nice and warm.

 
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Re: Amiga as test pattern generator.

Post by Refugee » Tue Mar 17, 2015 11:40 am

Warming them up also helps but a little tap and a few minutes of spinning is best so that it warms itself up.

 
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Re: Amiga as test pattern generator.

Post by Cathovisor » Tue Mar 17, 2015 2:36 pm

You wouldn't like what I've seen done to older Mac hard drives, where the things are slammed flat onto the bench to free them off... :aae

Once had a disk-based video keyer come in for repair. The heads had jammed at one point on the disk, and stayed there... until they cut their way through the aluminium platter :ccf

 
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Re: Amiga as test pattern generator.

Post by freya » Tue Mar 17, 2015 2:41 pm

I used to do similar to some huge full height ( physically) scsi drives I used to maintain. Always getting frozen by the heads in the park position.
My hard drive in the Amiga is now working, the floppy however is still refusing to read disks, I suspect it has dirty heads. Now where do I get a floppy cleaner disk these days :bbd

 
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Re: Amiga as test pattern generator.

Post by Refugee » Tue Mar 17, 2015 2:54 pm

I would guess that FDD cleaning kits are NLA but there is always a strip down and manual clean to fix the problem.

 
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Re: Amiga as test pattern generator.

Post by Katie Bush » Wed Mar 18, 2015 11:46 pm

If you can strip down far enough to get access to the heads, you can get away with a piece of tissue, or blotting paper, soaked in methylated spirit and very carefully work it back and forth in the gap between the heads.. I've had to do this on odd occasions, and has always been very successful.

Marion

 
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Re: Amiga as test pattern generator.

Post by freya » Thu Mar 19, 2015 10:25 am

All done and working, thanks for the Tip Marion.

 
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Re: Amiga as test pattern generator.

Post by Katie Bush » Thu Mar 19, 2015 9:21 pm

freya wrote:All done and working, thanks for the Tip Marion.


Glad to be of service... :aad

Marion

 
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Re: Amiga as test pattern generator.

Post by a4000bear » Sat Mar 21, 2015 12:33 am

Refugee wrote:I have had old hard disks that will not spin.
The fix I found is to remove the unit from the case and power it up. They usually spin without a data cable connected.
Hold it up in one hand and tap on the corners with your knuckle so that the case rotates sharply in alternate directions around the platter axle. This often frees them up.


Another fix that can tried for a HD that won't spin up:

Remove the HD from the computer. Holding the HD horizontally in your hand, swivel your wrist back and forth so that the HD rotates back and forth. It's a bit hard to explain easily, but I hope you get the idea.

The idea is to jerk the platter a little, hopefully overcoming any stiction that may have happened in the bearings. In the case that worked for me, the HD had not been used for a while. but after it was freed up, it worked OK.

 
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Re: Amiga as test pattern generator.

Post by Alistair D » Sat Mar 21, 2015 1:05 am

Like trying to waggle an old fashioned watch or alarm clock to get it going again after it has been overwound?

Al

 
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Re: Amiga as test pattern generator.

Post by Terrykc » Sat Mar 21, 2015 11:45 am

A description more appropriate to this forum would surely be rotating an Avo 8 back and forth to demonstrate the movement damping when the meter is correctly left in the AC-DC position ...

 
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Re: Amiga as test pattern generator.

Post by johntheboffin » Sat Mar 21, 2015 11:58 am

I seem to recall that, in my days of servicing and calibrating Avometers, we would leave the AC switch set to 10 A and the DC switch set to 50 uA, as this provided better damping than Avo's recommended settings.

John

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