Give every event a sound

The impact now sounds like something, but it is the only event that does. Collecting a star, pressing boost and reaching clear space all happen in silence. We will give each of them a sound without writing four more copies of unit 24’s loop.

The idea is to treat a sound as data. A short list of numbers describes each sound, and one routine plays whichever list it is given. Changing a sound then means changing numbers, not code.

Space launches; O/P steer; hold Space for 2X. To hear each sound, collect a star, press Space during flight, finish the course, or leave the ship still until a meteor hits it. R retries a result; Q returns to the title.

Tick Sound beside the buttons under the editor to hear them on this page. If you ticked it in unit 24, it is already on.

A sound is a list of notes

Each note is two bytes: a half-period count, the n that unit 24 loaded into B, and a cycle count. A sound is any number of notes followed by a single 0. A half-period of 0 would make djnz count 256 times, a note none of our sounds needs, so 0 is free to mean “end of sound”.

; Period 26*half+51 T-states at 3.5 MHz. In play, keep sounds short:
; the game loop waits while they play.
star_sound: defb 40,8, 30,10, 0
boost_sound: defb 100,4, 70,6, 0
arrival_sound: defb 127,63, 100,79, 84,94, 62,250, 0
impact_sound: defb 150,30, 200,30, 255,60, 0

star_sound plays 8 cycles with n=40, then 10 cycles with n=30: a short rising chirp. impact_sound falls through three notes and ends on n=255, unit 24’s pitch.

One routine plays any list

play_sound takes HL pointing at a sound. It loads the border bits into D once, as unit 24 did. Then, for each note, it reads the half-period into C and the cycle count into E, advances HL past both bytes and calls tone. A zero half-period returns.

; HL = sound: notes of (half-period count, cycle count), ended by 0.
; Changes AF/BC/DE/HL. Blocks until every note has played.
play_sound:
 ld a,(border)
 and 7
 ld d,a
play_sound_note:
 ld a,(hl)
 or a
 ret z
 ld c,a
 inc hl
 ld e,(hl)
 inc hl
 call tone
 jr play_sound_note

; D = border, C = half-period count, E = cycle count (1-255).
; Period 26*C+51 T-states. Changes AF/B/E.
tone:
 ld a,d
 or 16
 out ($FE),a
 ld b,c
tone_high: djnz tone_high
 ld a,d
 out ($FE),a
 ld b,c
tone_low: djnz tone_low
 dec e
 jr nz,tone
 ret

tone is unit 24’s cycle loop with one change. It cannot use ld b,IMPACT_HALF any more, because the half-period now arrives in C. ld b,c copies it instead, and takes 4 T-states where ld b,n took 7. It appears once in each half, so a period is now 26n+51 T-states: 6 fewer than before. The register comments above each routine record what it needs and what it changes, so a caller knows which values to save.

The cycle count lives in E, a single byte, so a note can have 1 to 255 cycles. arrival_sound needs 250 for its last note; a longer note would have to be split into two.

Check the table’s arithmetic

Work each note out the same way as unit 24: period 26n+51, frequency 3,500,000 ÷ period, length cycles × period.

Take the star’s first note: 26×40+51 = 1091 T-states, and 3,500,000 ÷ 1091 is 3208 Hz. Eight cycles are 8,728 T-states; the second note adds 10 × 831 = 8,310, for 17,038 in all. Reading the table adds under 100 T-states per note.

The arrival notes climb C, E, G and the C above: a C major arpeggio, as a fanfare. The impact falls from about A through E to C. Emu198x audio captures measured every pitch within 2% of the table. The highest pitches read slightly low, because a capture at 44.1 kHz has only about ten samples in each of their cycles.

Four events, one call

Each event loads HL with its sound and calls play_sound:

• Star. In the pool loop, after the score is added. B is counting the pool’s slots, and play_sound changes BC, so the call is wrapped in push bc and pop bc.
• Impact. In the pool loop too, where impact_sound was called before, with the same push bc and pop bc.
• Arrival. In won, after the finish bonus and records are settled and phase is set to 3. The run’s time and score are fixed before the sound starts, so 299 ms of fanfare cannot change them.
• Boost. In boost, which needs more thought.

Sound on the press, not the hold

boost runs at every update and stores 1 in boost_time while Space is held. Calling play_sound whenever it saw 1 would chirp 25 times a second for as long as you hold boost. The chirp should mark the moment boost starts: the update where Space is held now but was not held at the previous update. That change is called an edge.

boost:
 ld bc,$7FFE
 in a,(c)
 and 1
 xor 1
 ld (boost_time),a
 ; Sound only on the press: held now, not held at the previous update.
 ld c,a
 ld a,(boost_last)
 ld b,a
 ld a,c
 ld (boost_last),a
 cp b
 ret z
 or a
 ret z
 ld hl,boost_sound
 jp play_sound

boost_last remembers the previous update’s value. The routine keeps the new value in C, loads the old one into B, stores the new one for next time and compares them. If they are equal, nothing changed and it returns. If they differ and the new value is 0, Space has just been released, and it returns too. Only a change from 0 to 1 reaches the sound.

The last instruction is jp play_sound, not call. play_sound ends with ret, which returns straight to whoever called boost. A call followed by ret would do the same work with an extra return.

boost_last sits inside the run’s state, so a new run starts with it clear. The title waits for Space to be released before a run begins, so the Space that launches never counts as a boost press.

What a sound costs

play_sound does not return until its last cycle ends, so the game waits while a sound plays. Unit 24’s impact could afford that because play had ended. A star and a boost press happen in the middle of an update.

An update has two frames: 2 × 69,888 = 139,776 T-states. A star takes 17,038 of them, about 12%. A boost press takes 21,830, about 16%. The routes the course was checked with still keep their two-frame cadence, with the same times, scores and records as before. That tells us these sounds fit in the time those routes had to spare. It is not a guarantee: a dense stretch of the storm, with boost doubling the work, has less to spare.

Try it with Space held while collecting stars and watch frame_delta. With Sound ticked you also hear each star hold its note for about 25 ms instead of 5. The inspector samples values, so it may miss a single 4; a few stars make one more likely to show. Then restore the original line.

Arrival and impact block for longer, 299 ms and 193 ms, but play has already ended when they start. Unit 26 measures the cost of in-play sound properly and spreads a sound across frames so the storm keeps moving while it plays.