The A2320 Display Enhancer

Karl Jeacle

In the beginning...

When I first got my Amiga - I was pretty happy. I had a nice setup. A2000, bridgeboard, partitioned HD, ah yes, this was the machine for me - what on earth had I been doing with a PC for that long?

But as time passed, I started to get frustrated at the limitations of my setup - there was more I could do with this machine - if only I had a larger hard disk, some more memory, a faster modem....and wow, wouldn't a flicker fixer be nice...

Unfortunately, flicker fixers were about STG#325, and a multisync at least that much again. Bit out of my price range. However, at the start of this summer, Microway reduced the price of their flicker fixer to STG#125! They claimed this was due to the boards selling so well, however, a more likely explanation, was due to competition from Commodore's A2320.

The A2320 Display Enhancer

The A2320 is essentially the same video circuitry that's inside the Amiga 3000 which eliminates the irritating "flicker" you get when in interlaced modes, but placed on a board which can be plugged into the video slot of an A1500/2000.

The board comes in a compact (26x17x5cm) box (small enough to dodge customs :-) which includes a "Test/Demo" disk, 26-page manual, and fine-tune adjustment tool. The demo disk contains an impressive collection of both interlaced and non-interlaced pictures to test your board.

Setting it up

Before installing the A2320 you must check whether your Amiga has a 2 or 4-layer mother board. This is determined by looking at the front left hand-hand corner of the motherboard. You should find a label that says "AMIGA B2000 - CR" which indicates that your motherboard does indeed have a 2-layer motherboard. If you are unfortunate enough to have an old Amiga with a 4-layer, you are instructed to get a replacement motherboard from your local authorized Commodore service center.

However, provided you've got the right motherboard, and you've got a suitable monitor, installation should be a breeze. Just plug into the video slot, and away you go. On the back is your DB15 VGA style monitor connector, an enable/disable switch and a fine tuning adjustment screw.

The enable/disable switch controls what frequency output comes from the DB15 connector. This does not affect output from the existing video port on the Amiga.

First time you use your new toy, you'll probably notice some slight jittering on the display, a quick twiddle of the fine-tune adjustment tool soon puts this to rest. A couple of "testcard" pictures on the demo disk are helpful here.

Inside the A2320

When enabled, two things that the A2320 does are

  1. De-interlacing:

    While in interlaced mode, this removes flicker.

    The reason an Amiga flickers on a high res screen is because it first outputs all the even display lines followed by all the odd display lines. It does this so fast, we can see a full image on screen, but it appears to flicker. The display enhancer output frequency is twice that of a stock Amiga, so with the help of 384Kb of on-board memory, it can output both the odd and even display lines at the same time, thus eliminating flicker.

  2. Scan-doubling:

    While in non-interlaced mode, this removes visible scan lines.

    This is achieved by drawing/refreshing the screen twices as many times as usual giving thus giving a rock steady display without any scan lines. While de-interlacing, the display enhancer uses its high frequency output to display both odd and even display lines; here, it just outputs the same display lines twice. This is known as "double-scanning".

The display enhancer can work in both NTSC and PAL modes - handy if you have an ECS Agnus, and want to work in NTSC. Since we're in Europe, the board will be delivering PAL frequencies: Vertical PAL frequency is fixed at 50 Hz. Horizontal PAL frequencies are 15.625 KHz (A2320 disabled), and 31.25 KHz (A2320 enabled).

15.625 Khz is the frequency that a stock Amiga outputs, driving your standard 1084/8833 monitor. It flickers in interlaced mode. The A2320 outputs this frequency when you flick the switch to disable mode.

31.25 KHz is the frequency that the display enhancer outputs when enabled. It does *not* flicker in interlaced mode, but is too high a frequency for a 1084, requiring a multisync quality monitor.

Choosing the right monitor

There are basically three types of monitors you'll come across - standard 1084/8833, SuperVGA/low-end multisyncs, and high-end multisyncs. Sample specifications for these types of monitors, along with the A2320 output is given below:

----------------------------------------------------------------------------
   Monitor Name           Description     Vertical Freq      Horizontal Freq
----------------------------------------------------------------------------

CBM1950 / NEC3D        High-end Msync        45->120 Hz         15->36.5 KHz

NEC2A / My SVGA        Low-end / SVGA             50 Hz           30->40 KHz

    1084 / 8833      Standard monitor             50 Hz           15.625 KHz

----------------------------------------------------------------------------
          A2320              Enabled              50 Hz            31.25 KHz
          A2320             Disabled              50 Hz           15.625 Khz
----------------------------------------------------------------------------

As you can see, a 1084 simply won't cut it. It can only handle a disabled A2320 output - pretty useless.

A high-end multisync on the other hand has no problems. It will happily take a flicker free 31.25 KHz signal from the display enhancer. It will even "scan down" to the original 15.625 KHz Amiga output if you disable the A2320. This is useful if you're considering buying something like HAM-E. Go for a monitor like this if you can afford it.

If however, you're a little short of cash, you can get away with a low-end multisync which will only recognize an enabled A2320. For most applications (except HAM-E etc) this is very acceptable. I managed to get my hands on a SuperVGA monitor which has a variable Horizontal frequency of 30->40 Khz which works just fine.

However, I must stress that many VGA/SVGA may *NOT* work fully (+) with the A2320. Many VGA/SVGA monitors can only handle a 31.5 KHz input, and this is not what the display enhancer outputs. So, if you are thinking of an SVGA, make sure you try it out with your system before you buy.

[(+) FOOTNOTE: I say fully, since they may work in NTSC mode, but not PAL.]

Conclusions

The only real criticism of the display enhancer can be the top half-line of flicker which remains on screen even when the board is enabled. This occurs on the 3000 too. Many people have given out about it in the past, but they're really making a big deal over nothing. Just adjust the vertical stretch knob on your monitor, and it'll disappear off the top of the screen...

The display enhancer is a great product and I recommend it highly. Once you get a taste of flicker-free Amiga, you won't ever want to go back. It gives the Amiga the professional feel it's been lacking - and if you've ever used Workbench 2.0 on a 3000, you'll know just what I mean - an interlaced Workbench screen with custom fonts and colors looks just great.

What more can I say? Buy one.