Getting the Picture Right (HDMI & VGA)¶

When you first run AExp — the Amiga 500 core for the MEGA65 — you may notice that the Amiga's picture is not perfectly placed on your screen. It might sit a little too far to one side, an edge might be cut off, or a demo might show strange-looking leftovers in the border.

This is a well-known quirk of how the real Amiga produced its video signal. The good news: AExp gives you an easy, safe way to fix it, usually just by copying one file onto your SD card.

This guide explains, in plain terms, why the problem exists and how to make the picture look just right on your particular screen.

You are not alone: MiSTer has the same problem¶

If you have used MiSTer, you may know that it, too, offers a "screen centering" adjustment for its Amiga core. That is not a coincidence. Any faithful Amiga recreation runs into the same wall, because the wall is the Amiga itself, not the recreation. AExp solves it in the same spirit as MiSTer, so if you have centered a picture on MiSTer before, this will feel familiar.

Why the Amiga is tricky to place¶

Two things work against a perfectly placed picture:

The Amiga was generous and a little sloppy with its borders. It drew a wide picture surrounded by a border, and different programs placed their image at slightly different spots inside that border. There was never one single "correct" position — a 1985 game and a demo from 1992 might not agree. Old TVs hid this because they overscanned (they cropped the edges anyway). A modern, pixel-exact display shows everything, including the wonkiness.
HDMI and VGA need different tools. AExp can send its picture two ways at once: as a modern HDMI signal to a TV or monitor, and as an old-school analog VGA signal for retro monitors. These two paths work completely differently, so each gets its own adjustment. One knob cannot fix both.

Remember: The core is not outputting a VGA signal at all. VGA is used here as a shortcut for "analog signal". We use this shortcut because the hardware connector looks like a VGA connector.

The three controls¶

AExp gives you three independent controls, each stored per Amiga screen mode:

HDMI crop — picks a rectangle of the Amiga picture and scales it to fill your HDMI screen. Moving an edge re-frames (and slightly zooms) the HDMI picture. HDMI only.
Analog position (pan_x, pan_y) — moves the complete analog picture, on-screen menu included, left/right/up/down. Works in all three VGA modes (Standard, 15 kHz HS/VS, 15 kHz CSYNC).
Analog overscan (os_l, os_r, os_t, os_b) — hides or reveals the Amiga's border edges on the analog output. Handy when a demo leaves odd-looking material in the overscan area. It changes what is visible; it does not move anything.

One thing none of these controls do: change the true size of the picture. Position moves it, overscan trims it, HDMI crop re-frames it — but if the analog picture as a whole is too wide or too narrow for your monitor, that is what the monitor's own H-size/V-size controls are for.

The easy way: try one of the two ready-made files¶

AExp ships with two ready-made settings files, each tuned for a different kind of screen:

aexp_screen.cfg_16_9 — for a plain 16:9 display that just fills the screen and has no "4:3" aspect mode of its own. It leaves the picture almost exactly as the Amiga draws it (only a tiny nudge on the hires modes). Tested to work well on a cheap 16:9 monitor with no 4:3 emulation.
aexp_screen.cfg_4_3 — a gentle "underscan" that pulls all four edges in a little. Tested to work very well on a 4:3 Checkmate display, and — perhaps surprisingly — just as well on a Samsung 55" 4K TV.

Because every TV and monitor overscans and stretches a little differently, there is no single file that is perfect everywhere. So the plan is simple: try both and keep whichever looks best on your screen.

Copy both files into the /amiga folder on your SD card — the same folder that holds kick.rom and your disk images.
Pick one to try and rename it to aexp_screen.cfg — that is, remove the trailing _16_9 or _4_3 so the name ends in just .cfg. Only a file named exactly aexp_screen.cfg is read by the core.
Start the core, or — if it is already running — open the on-screen menu (press the Help key) and choose "Reload screen cfg".
Look at the picture. Happy? You are done. If not, rename the other file to aexp_screen.cfg (replacing the first one) and use "Reload screen cfg" again to compare. Each reload takes about a second — no reboot needed.

The core adjusts automatically as programs change the picture shape: the Amiga can show a few different "screen modes" (see below), and both files carry the right settings for each mode, so AExp always applies the matching one for you. You do not have to do anything.

One thing to know: these two presets adjust the HDMI picture and leave the analog output untouched. If you use the analog output and it sits off-center, tune it by hand as described below.

The four Amiga screen modes¶

An OCS PAL Amiga displays one of these screen modes:

Lores
Hires
Lores interlaced
Hires interlaced

Each mode can want slightly different values, so the file holds one row of settings per mode, and AExp picks the right row automatically.

Fine-tuning it yourself (for the tinkerers)¶

If your specific TV or monitor still trims an edge or sits slightly off, you can adjust the numbers to taste. AExp ships with a small helper program, aexp_screen_cfg.py, that writes the settings file for you. You only need Python 3 installed on your computer (Windows, macOS or Linux) — nothing else.

Tip: the tool edits an existing aexp_screen.cfg in place if one is present, so the easiest starting point is to rename whichever ready-made preset looked best (aexp_screen.cfg_16_9 or aexp_screen.cfg_4_3) to aexp_screen.cfg first, and then fine-tune it from there.

Open a terminal in the folder that contains the tool and run it with no extra options to enter its friendly, interactive mode:

python3 aexp_screen_cfg.py

It shows the current settings as a table and asks which mode row you want to edit. Pick a row, and it walks you through the three groups — HDMI crop, analog position, analog overscan. Press Enter at a group prompt to skip that whole group, and Enter on any value to keep it as-is. When you are done, press Enter at the row prompt to save the file.

Prefer one-liners? Everything works from the command line too:

python3 aexp_screen_cfg.py --mode lores --pan-x 8            # analog picture right
python3 aexp_screen_cfg.py --mode all --pan-y -3             # analog picture up, all modes
python3 aexp_screen_cfg.py --mode lores --himin 32           # trim HDMI left edge
python3 aexp_screen_cfg.py --mode all --reset pan            # undo all analog panning
python3 aexp_screen_cfg.py --mode lores-i --copy-from lores  # reuse tuned values
python3 aexp_screen_cfg.py --list                            # show the current table

The tool prints, after every change, which way the picture will move — so you never have to memorize sign conventions.

HDMI crop: pick a rectangle, then blow it up to fill the screen¶

The HDMI side runs through a digital scaler. It takes a rectangle out of the Amiga's picture and stretches that rectangle to fill your entire HDMI screen. Each of the four numbers pulls one edge of that rectangle inward:

himin — the LEFT edge. Only 0 or a positive value does anything; a bigger number cuts more off the left.
himax — the RIGHT edge. Only 0 or a negative value does anything; a more-negative number cuts more off the right.
vimin — the TOP edge (0 or positive).
vimax — the BOTTOM edge (0 or negative).

Why the mixed signs? Because each number is measured from its own edge and can only move toward the middle. Giving himin a negative value to reveal more on the left does nothing — there is nothing to the left of the Amiga's own picture, so it is ignored (it clamps to "full"). A positive himax is ignored for the same reason. On HDMI you always trim inward, never expand outward.

One consequence: HDMI adjustment is a re-frame, not a pure slide. Whatever rectangle you choose is always blown up to fill the same screen, so every change moves the content and zooms it a little. To move the picture right you trim the right edge (himax negative); the remaining part is stretched to fill the screen, so the content shifts right, but it also becomes slightly bigger. In practice this is exactly what you want for centering.

Quick recipe:

Move the picture left -> himin a little positive.
Move it right -> himax a little negative.
Move it up -> vimin a little positive.
Move it down -> vimax a little negative.
Whole picture cut off on all sides by an overscanning TV -> trim all four a touch (himin +, himax -, vimin +, vimax -). This is the digital equivalent of "underscan."

Because this all happens on the source side, one HDMI setting centers every HDMI resolution (16:9, 4:3, 5:4) at the same time.

Analog position: move the whole picture¶

pan_x and pan_y move the complete analog picture — Workbench, demo, border and the on-screen menu together — without touching its size or the HDMI output. Positive values move right and down, negative values left and up:

pan_x — one step is one hires pixel (half a lores pixel). The step size is the same in Standard and in the 15 kHz modes.
pan_y — one step is one Amiga picture line.

Under the hood, AExp shifts the timing of the sync pulses relative to the picture content — exactly what the H/V-position knobs of a monitor do, just from the core's side. Two honest consequences follow from that:

CRTs and sync-locked monitors follow the pan exactly. This includes SCART/RGB setups on the 15 kHz CSYNC mode — the composite sync is generated from the already-moved timing, so everything stays consistent.
Some analog-input flat panels re-center themselves. An LCD with auto-position logic measures the incoming signal and may quietly undo part or all of your pan, immediately or on its next "Auto" run. That is the monitor being helpful, not a core bug. On such a display, use overscan trimming (below) and the HDMI output for exact placement, or simply use the monitor's own position controls.

Also good to know: the same physical monitor may place two different cores differently. The C64 core and AExp produce different analog timings, and a monitor may even store separate settings per timing. Do not expect one core's numbers to mean anything for another core.

Safety and range. The core never lets the pan push a sync pulse into the visible picture: requests are automatically limited to roughly an eighth of a line horizontally, and to whatever black border room your current settings leave around the sync. If the picture stops moving before it is where you want it, hide a little border on that side first (overscan, below) — that frees up more room and the pan limit grows with it. Vertical pan is limited to ±64 lines. Because of these built-in limits, a pan value cannot produce a broken or lost signal; the worst case is that the picture moves less than you asked for.

Analog overscan: hide or reveal the border edges¶

The four overscan values trim the visible area of the analog picture, edge by edge. This is the tool for demos or games that leave garbage-looking material in the border, and for displays that show more border than you would like:

os_l — left edge: positive hides border on the left, negative reveals more.
os_r — right edge: negative hides border on the right, positive reveals more.
os_t — top edge: positive hides lines at the top, negative reveals more.
os_b — bottom edge: negative hides lines at the bottom, 0 leaves it untouched. (A positive value is a legacy "absolute line" mode you almost never want.)

Horizontal steps are quarter lores pixels; vertical steps are lines. Hidden areas simply turn black — overscan does not move the remaining picture (that is what position is for), and it does not resize anything. The on-screen menu keeps itself inside the remaining visible area, so it stays fully usable while you trim.

If you used the "VGA offsets" of an earlier AExp release: these are the same four values, now with honest names. They always cropped the picture — they never panned it, despite what older documentation said. Use pan_x/pan_y for panning.

What about picture size?¶

If, after positioning and trimming, the analog picture is still too wide, too narrow, too tall or too short as a whole, none of these controls will fix that — stretching the picture needs either your monitor's H/V-size controls or a scaler in the signal path. AExp deliberately keeps the native analog output scaler-free, because that is what makes the 15 kHz modes authentic for CRTs.

At a glance¶

Aspect	HDMI crop	Analog position	Analog overscan
What it does	picks a source rectangle and zooms it to fill the HDMI screen	moves the complete analog picture (menu included)	hides/reveals analog border edges
Moves the picture?	re-frames it (plus slight zoom)	yes — a true slide	no
Resizes anything?	slight zoom is inherent	no	no (trimmed areas turn black)
Fields	`himin` `himax` `vimin` `vimax`	`pan_x` `pan_y`	`os_l` `os_r` `os_t` `os_b`
Units	Amiga source pixels	hires pixels / lines	quarter lores pixels / lines
Works on	HDMI only	all three VGA modes	all three VGA modes
Display caveats	none	auto-positioning LCDs may cancel it; CRTs follow exactly	none

Whichever control you are tuning, the method is the same: change one number by a small amount, save, reload, and look. Repeat until it is right.

The experiment loop: try, look, repeat¶

Here is the whole cycle for dialing in your own perfect picture. It does not require rebooting or re-flashing the core:

Adjust and save the file with the tool (it creates aexp_screen.cfg).
Copy aexp_screen.cfg into the /amiga folder on your SD card.
On the MEGA65, open the core's on-screen menu (press the Help key) and choose "Reload screen cfg".
Look at the picture. Not right yet? Go back to step 1 and nudge the numbers a bit more.

Each reload takes about a second, and your changes appear immediately — no reboot needed. Position changes settle within a blink; you never get a torn or broken picture from a reload.

If the analog picture ever disappears (goes black): an overscan value is hiding everything. Don't worry — nothing is broken. Set the overscan values smaller (or delete aexp_screen.cfg entirely) and reload again. The HDMI output is completely independent, so the menu stays visible on HDMI the whole time, which makes it easy to recover. (Pan values cannot cause this — they are limited in hardware.)