Casio MG-880 Digit Invader Remake

I believe everybody (ok, most, not every) would have come across this Casio pocket calculator (model: MG-880) game called digit invader in childhood, if one was born before 1989. This project is an attempt to remake it. I know there are already a number of remakes out there but I have to say I feel they are not good enough because they loosely stick to the original game specifications, which I think is actually very important. I have tried my best to make it as close to the original game as possible, such as the speed, game flow, scoring and even the sound was analyzed using the Fourier Analysis function in Audacity in order to reproduce the sound with the buzzer. Please find the game instruction below the demo videos.

Please see the demo video here for the short edition:

And here for the long edition:

Game Instructions:

The game is actually called "Digit invader". It is about there are invaders from space represented by digits attacking our missile base. It was implemented on a pocket calculator that just has an 8 digit LCD panel and each digit is just a 7-segment-LCD with 2 buttons.

The left button is aim and the right button is fire. The aim button is for aiming the missile, pressing it would make the left most digit on the screen to cycle through 0 to 9 and then n and then 0 again. This left most digit represents the type of invader you can kill if you press the fire button. Invaders appears from the right side of the screen flying leftwards gradually. If any invaders successfully land on your base before they are killed you lose a life. The game is over if you lose 3 lives in the same mission. The digit second from the left on the screen is the number of lives remaining in that mission.

Every mission has 16 invaders coming and you got 30 missiles to kill them. If all 30 missiles are fired but there are still remaining invaders the game is over immediately too.

If the invaders killed in the mission accumulates to be multiple of 10, a special saucer will appear (represented by an n). Shooting an n has extra high score.

Missions get faster and faster but after finishing 9 missions the game speed would slow down again. The first 9 missions are using 6 spaces for the invader, but the 9 missions after the first 9 missions would have 1 space less (i.e. having 5 spaces only). Finishing this set of 9 missions would get back to 6-space-missions.