Hack the Nemesis ROM to convert it to Gradius (Japanese Bubble System version)

[michael2448]

Hack the Nemesis ROM to convert it to Gradius (Japanese Bubble System version)

Using @cmonkey's Gradius-style graphic modification, I want to recreate the Bubble System version of Gradius on the Nemesis board.
The Nemesis board has many differences from the Bubble System version.

Changes

・Recreate Bubble System startup (audio countdown during bubble memory warm-up, numerical countdown)

・Adjusting game speed (timing of volcanic eruptions, Zabs, tentacle rushes, reducing frame drops, etc. - high-speed specification)
to match the Bubble System version

(Dip Switch Function: The game generally starts at default difficulty (1loop), but settings allow starting from the (2loop or 3loop playthrough (increased revenge bullets). If you rank 1st in name input, unused BGM1 plays)

I want to modify it with these specs... but I'm struggling with where to start in the program (Should I disassemble it?)
This year marks Gradius's 40th anniversary (though the 41st is also approaching)
Gradius maniacs of the world. Assemble!

 

[michael2448]

For reference on bubble system behavior
Upon power-up, it speaks “Presented by KONAMI” using synthesized voice.
The GX400's synthesized voice (VLM5030) performs a bubble memory warm-up countdown before the numerical countdown. (The visuals displayed here vary per video, but this is to transfer and initialize game data to the main board after warm-up. There is a theory that the visuals were caused by uninitialized VRAM.)
Warm-up Countdown
Countdown begins from 50.
Get it ready. Fifty.
Get it ready. Forty nine.
Get it ready. Forty eight.
:
Countdown completion time varies depending on the environment (temperature) where the bubble memory is located (the memory warms up to an optimal temperature of approximately 35°C). In other words, until warming is complete, startup is faster in summer (countdown 35–29: heating complete) and slower in winter (countdown 23–19: heating complete). (We also want to make the warming duration adjustable via dip switch settings.)
Once the warming count ends, it transitions to the numerical countdown.
(If the bubble memory does not reach the optimal temperature...)
Get it ready. Two.
Get it ready. One.
Get it ready. Presented by KONAMI.
Presented by KONAMI.
Get it ready. Fifty.
Get it ready. Forty nine.
Get it ready. Forty eight.
:
Repeat? (Unclear)

 

[michael2448]

(After warm-up countdown ends)
A crackling white noise sound occurs,
the warm-up countdown voice interrupts, the VRAM display initializes, and switches to a numerical countdown.
Morning music plays while the countdown continues from 99 to 00. (The music extends for about 1 second at the 13th count). (https://www.nicovideo.jp/watch/sm119684 )
When it reaches 00, the ROM check screen appears. After a crosshatch pattern scrolls, the game launches.
This is the entire sequence.

 

[michael2448]

The main change is this bubble system sequence. (I want to focus on this)
Since it doesn't involve significantly altering the program, I don't think it's particularly difficult.
The concern is reproducing the VLM5030's synthesized speech.
Since it's not on the Nemesis board... PCM? Recording the audio?

 

[michael2448]

Reference videos

グラディウス　バブルシステムカウントダウン（高画質版）

グラディウス　バブルシステムカウントダウン（高画質版） ほぼ２年ぶりにバブル基板に火を入れてみたところ無事に起動。記念に高画質版のバブルシステム起動...

www.nicovideo.jp

グラディウス【アーケードアーカイブス】バブルシステム起動

グラディウス【アーケードアーカイブス】バブルシステム起動 [ゲーム] コナミコマンド→起動→デモシリーズ一覧【 https://www.nicovideo.jp/user/15021151/series 】マイリ...

www.nicovideo.jp

www.youtube.com/live/ize-W4yfvyU?t=7m55s

uninitialized VRAM.