DOC ROM Studio — edit Derby Owners Club's game data

DOC ROM Studio is a free, offline fan tool that turns the arcade program ROM into friendly editable tables — no hex editor needed. It reads the same ROM-verified data the Breeder Studio uses, and writes your changes back byte-for-byte so the file stays a valid 4 MB ROM. It works with all four versions, and it understands the Japanese versions natively (Japanese text is read and written as the game stores it). This is an advanced tool: you supply your own program ROM, and you're responsible for what you do with it — see the legal note.

What you need

Your game's ROM zip — the easiest path. In your game pack folder it's the <set>.zip for the version you want (see the table). Load that and the tool hands you back a finished, ready-to-play zip — no unzipping, no hunting for files.
A modern browser (Chrome / Edge / Firefox). The tool runs entirely on your computer; nothing is uploaded.

Advanced: you can also load a bare .ic22 on its own and use Export .ic22 only to get just the edited ROM back.

Version	Load this zip from your game folder	(program ROM inside)
World Edition (Rev C)	`ROMs\drbyocwc\drbyocwc.zip`	`epr-22336c.ic22`
World Edition EX (Rev D)	`ROMs\derbyocw\derbyocw.zip`	`epr-22336d.ic22`
DOC 2000 (Japanese)	`ROMs\derbyo2k\derbyo2k.zip`	`epr-22284a.ic22`
DOC Original / '99 (Japanese)	`ROMs\derbyoc\derbyoc.zip`	`epr-22099b.ic22`

Step by step

Open the tool and load your game's ROM zip. Click Launch DOC ROM Studio, then drag the <set>.zip from your game's ROMs\… folder onto the page (or click Load .ic22 or .zip). The tool reaches inside the zip, pulls out the program ROM, auto-detects the version (a badge shows which), and is ready to edit.

Edit, using the tabs. Every change is written into the ROM in memory the moment you type or pick it; edited fields get an orange outline. The tabs depend on the version (below).

Build your ready-to-play zip. Click Build ready-to-play .zip. The tool rebuilds the complete <set>.zip right in your browser — your edited program ROM swapped in, and all the other ROM files (the “mask” ROMs and headers) kept exactly as the game expects. It downloads with the same name. The ROM stays exactly 4 MB, so the result is always valid.

Drop it in and play. Move the new <set>.zip into your game's ROMs\<set>\ folder, replacing the old one (same name). Double-click the matching Play file. That's it — your edits are live, with nothing else to set up. (Tip: keep a copy of the original zip so you always have a clean starting point.)

No zip, just a bare .ic22? You can still load it and use Export .ic22 only to get the edited ROM back — but the one-click ready-to-play zip needs you to load the .zip so the tool has the mask ROMs to pack alongside.

English versions (Rev C / Rev R / Rev D)

These show the full set of tables. Text is plain English, so it's the simplest to edit.

Racing Horses (244) — the CPU opponents: name plus their running stats (start, corner, out-of-bounds, competing, tenacious, spurt), internal stats (stamina, speed, sharp), coat colour, dirt aptitude and grade.
Sires / Dams — the breeding stock: name, the six externals, and the composite value.
G1 Races and Tracks — race and course names (each shows its byte limit; shorter is fine).
Game Text — on-screen messages and dialogue. Rev C is organised into labelled sections; Rev D shows a single searchable list (its text was re-laid-out, so search is the quickest way to find a line).

Names on the English versions are ASCII; each field shows how many characters fit. Keep the %s (a name) and %d (a number) placeholders in any message you edit.

Japanese versions (DOC 2000 & DOC '99) — read this

The Japanese versions work differently, and they're worth a careful read. Two things make them more involved than the English versions:

The text is Japanese, stored as EUC-JP. Each Japanese character is more than one byte, so editing isn't just "type ASCII." DOC ROM Studio reads and writes EUC-JP for you — you type real Japanese (or English) and it handles the bytes — but space is measured in bytes, not letters.
Horse identity lives in the cabinet, not always on the ROM. On the Japanese versions a player horse's name/pedigree is tied to its card and the machine, so the editable text here is the game's built-in strings — menus, messages, race commentary, and the breeding-stock names — rather than your saved horses.

When you load a Japanese ROM you get two tabs:

① Racing Stats (244)

The same 244-horse roster as the World Edition, with each horse's verified numbers (start / corner / OOB / competing / tenacious / spurt, plus stamina / speed / sharp and dirt aptitude). These are just numbers, so there's no Japanese to deal with here — type a value and it's written. The horse names are Japanese text; you edit those in the next tab by searching for the name.

② JP Text & Translate — the main event

This tab finds every Japanese string in the ROM and lets you revise it in place. Each row looks like this:

offsetJapanese (editable)romajiEnglish (your notes)bytes 0x105714このカードは競走馬です。konokādo wa kyōsōba desu.This card is a racehorse.24/24

Scan. Choose a region (the default Text region covers the dialogue and name area; Full ROM finds everything but with more noise; Custom range lets you enter your own bounds), then click Scan. The tool lists every Japanese string it found.
Read it. Each row shows the offset (where it lives), the Japanese text (the editable field), an automatic romaji reading to help you sound it out, an English column for your own translation/notes, and a byte budget.
Edit the Japanese field to change what the game displays. You can type Japanese or English (English shows fine — it's valid in this encoding). As you type, the byte counter updates.
Mind the byte budget. The counter shows used / max. The game packs lines tightly (often separated by line-break codes), so you can match or shorten a line, but the field stops you from overrunning into the next one. If you make a line shorter, the tool pads the gap safely and preserves the line-break that follows. In short: equal-or-shorter is always safe; longer is capped.
Search to jump to anything — it matches the Japanese, the romaji, your English, or the offset. This is how you find a specific horse name or a particular message fast.

Why "bytes," not "letters"? A Japanese character is two bytes in this ROM, a normal English letter is one. So a slot that fits 12 Japanese characters (24 bytes) also fits 24 English letters. The counter always speaks in bytes so you know exactly how much room you have.

Translating a section into English

The romaji and English columns are reading aids for your use — they are never written into the ROM. Two ways to fill the English column:

Offline (no account, recommended): click Copy bilingual table (paste into Excel/Google Sheets) or Download bilingual CSV. You get offset · Japanese · romaji · English to translate in whatever tool you like, at your own pace.
Optional auto-translate: pick a provider (Claude, Google Translate or DeepL), paste your own API key, and click Translate loaded rows to auto-fill English for the rows currently shown (tip: search to a section first, then translate just that).

About the optional API key: it's entirely optional. If you use it, the key stays in your browser and is sent only to the provider you choose — never to this website, and it is not saved anywhere. Leave it blank and everything except auto-translate still works. The whole tool is offline otherwise.

A worked example

Load derbyo2k.zip (DOC 2000) from your game folder → it opens on JP Text & Translate after a scan.
Type a horse or phrase in Search (e.g. paste a katakana name, or search the romaji like oguri) to find the line.
Click Translate loaded rows (or use the offline export) to see the English.
Edit the Japanese field to your new text, watching the byte counter stay at or under the max.
Click Build ready-to-play .zip and drop the new derbyo2k.zip into your game's ROMs\derbyo2k\ folder, then double-click Play (Steps 3–4 above).

Safety & good habits

The file size never changes. All edits are in place, so the ROM stays 4 MB and valid.
Edit a copy. Keep the original .ic22 so you always have a clean base to go back to.
Respect the byte budgets. A field that turns red is over its limit and won't be written until it fits.
Your data stays yours. Nothing you load, edit, or translate is uploaded to this site — it all happens in your browser.