Turn a click into a tone

Since unit 10, a hit has made a sound, but you could miss it: twelve quick pushes of the speaker, over in about a thousandth of a second. We will turn that tick into a note you can hear, 523 Hz for a fifth of a second, which is the C an octave above middle C.

The Spectrum has no sound chip. Its speaker moves only when a program tells it to, one bit at a time, so the program makes every vibration itself. That makes sound a timing problem, and timing is something we can calculate exactly.

Hear the impact

Tick Sound beside the buttons under the editor, then press Assemble and run. Sound starts unticked, because a browser plays sound only after you ask it to. Once you tick it, the other lessons remember the choice.

Launch with Space and leave the ship where it is. The first meteor reaches it about two seconds later, you hear the tone, and the run ends. R retries; Q returns to the title.

To hear it outside this page, press Download tape and load meteor-storm.tap into the site’s Spectrum player with its Sound ticked, into Emu198x on the desktop, or into any other 48K Spectrum emulator with sound turned on.

One bit moves the speaker

In Meet Assembly’s first program we turned the border red with out ($FE),a. The same port drives the speaker. Bits 0 to 2 of the value written are the border colour, bit 3 drives the MIC socket used for saving to tape, and bit 4 drives the speaker. Writing a byte with bit 4 set pushes the speaker one way; writing one with it clear lets it back.

One push and one release make a cycle. Repeat it steadily and the air carries a square wave: two levels, switched at regular intervals. The time one cycle takes is its period, and the number of cycles in a second is its frequency, measured in hertz (Hz). Shorter periods mean higher notes. The number of cycles decides how long the sound lasts.

So a tone needs two numbers: how long to wait between writes, and how many cycles to play. The new impact_sound gives each one a name at the top of the source:

IMPACT_HALF equ 255
IMPACT_CYCLES equ 105

impact_sound:
 ; Port $FE bit 4 drives the speaker; bits 0-2 are the border colour.
 ; Every write ORs in the border, so the tone leaves it unchanged.
 ld a,(border)
 and 7
 ld d,a
 ld e,IMPACT_CYCLES
impact_sound_cycle:
 ld a,d
 or 16
 out ($FE),a
 ld b,IMPACT_HALF
impact_sound_high: djnz impact_sound_high
 ld a,d
 out ($FE),a
 ld b,IMPACT_HALF
impact_sound_low: djnz impact_sound_low
 dec e
 jr nz,impact_sound_cycle
 ret

Each cycle sets bit 4 and writes, counts B down from IMPACT_HALF with djnz, clears bit 4 and writes, counts down again, then decrements E. D holds the border bits throughout; the next section explains why.

Count the period in T-states

The Z80 in a 48K Spectrum runs from a 3.5 MHz clock: 3,500,000 T-states each second. Every instruction takes a fixed number of them, listed in Zilog’s manual. ld b,n takes 7, out ($FE),a takes 11 and ld a,d takes 4. djnz takes 13 when it jumps back and 8 when B reaches zero and it falls through, so a djnz that starts with n in B costs 13(n-1)+8, which is 13n-5.

Add up the time between one speaker write and the next:

With n=255 a period is 26×255+57 = 6687 T-states. Divide the clock by the period: 3,500,000 ÷ 6687 = 523.4 cycles a second, so 523 Hz. The length is 105 × 6687 = 702,135 T-states, and 702,135 ÷ 3,500,000 is 0.2006 seconds: about 200 ms. A frame of the 50 Hz picture is 69,888 T-states, so the tone lasts ten frames.

The old routine used the same idea with much shorter waits: 313 T-states a cycle, which is 3,500,000 ÷ 313 = 11,182 Hz, for only 12 cycles. That is 3756 T-states in all, 1.07 ms. A sound that short reaches the ear as a click, not a note: it ends before there is time to hear a pitch. That is why the old impact barely registered.

An Emu198x audio capture of this program measured 522 Hz for 105 cycles, 199 ms, and the site’s browser player produced the same pitch and length.

The two halves differ by 23 T-states, so the speaker spends slightly longer released than pushed. That changes the tone’s colour a little, not its pitch: pitch follows the whole period. The code runs at 32768, above the 16K at $4000 to $7FFF that the ULA, the chip that draws the picture, shares with the processor. Its instruction fetches are never held up. Only each OUT to the ULA’s port may wait a few T-states while the picture is drawn, a fraction of a percent here.

Try it: with Sound ticked, edit the value, press Assemble and run and let a meteor hit the ship. Compare what you hear with your prediction, then restore 255 and run again to hear the two notes one after the other.

Keep the border bits

The old routine cleared its speaker bit with xor a and out ($FE),a. That writes zero to every bit, including the three border bits, so each impact also set the border to black. The border was already black, so nobody noticed. Any program that chose another colour would lose it the moment the ship was hit.

This program keeps the border colour in a byte of its own, border, placed outside the run’s state so retry does not clear it. Start-up now loads that byte with ld a,(border) before its first out ($FE),a, where it used xor a before. impact_sound loads the same byte, keeps only the colour bits with and 7, and holds the result in D. Every speaker write starts from D: ld a,d then or 16 to push, ld a,d alone to release. The speaker moves; the colour bits are written with their existing values each time, so the border stays as it was.

Assemble with border: defb 2 near the end of the source. The border is red from launch and stays red through the impact, where the old routine would have turned it black. Restore 0 afterwards.

Wait while it plays

The whole tone runs inside impact_sound, which returns only when the last cycle ends. For 200 ms the game does nothing else. The frame interrupt still counts frames, but the storm does not move and the result screen waits. Here that costs nothing: hull is already zero, so the run is over and the result follows as soon as the tone ends.

A sound during play would be different. Unit 25 gives the other events sounds and measures what they take from an update, and unit 26 plays them without delaying the game. Unit 27 uses that for the crash and unit 28 colours the storm. Units 29 to 35 turn the game into a voyage of five storms, and Keep and share the game packages it, sounds included, as a tape you can load on a fresh machine.