Sound: MakeEngineSound

       Name: MakeEngineSound                                         [Show more]
       Type: Subroutine
   Category: Sound
    Summary: Make the engine sound, with the choppiness and pitch affected by
             thrust and airspeed
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ToggleEngineSound calls MakeEngineSound


.MakeEngineSound

 LDA engineStatus       \ If engineStatus is zero, then the engine is not
 BEQ engs4              \ running, so jump to engs4 to return from the
                        \ subroutine

 LDA thrustHi           \ Set (K A) = (thrusthi thrustLo)
 STA K                  \           = Thrust
 LDA thrustLo

 LDY #3                 \ Set Y = 3 to act as a shift counter in the following
                        \ loop, where we right shift (K A) four times

.engs1

 LSR K                  \ Set (K A) = (R A) / 2
 ROR A                  \           = Thrust / 2

 DEY                    \ Decrement the shift counter

 BPL engs1              \ Loop back until we have shifted right by 4 places, so
                        \ we now have:
                        \
                        \   (K A) = (K A) / 8
                        \         = Thrust / 8
                        \
                        \ We now ignore the high byte in K, so presumably it is
                        \ zero

 CLC                    \ Set K = A + zVelocityPHi
 ADC zVelocityPHi       \       = (Thrust / 8) + zVelocityPHi
 STA K

 LDA #50                \ Set A = 50 - K
 SEC                    \       = 50 - ((Thrust / 8) + zVelocityPHi)
 SBC K

 BEQ engs2              \ If A = 0, jump to engs2 to set the engine choppiness
                        \ to 1 (a very smooth engine sound)

 BPL engs3              \ If A is positive, jump to engs3 to set the engine
                        \ choppiness to A

.engs2

 LDA #1                 \ If we get here then A is either 0 or negative, so we
                        \ set A = 1 to use as the choppiness value in the engine
                        \ sound, giving a very smooth engine sound

.engs3

                        \ By the time we get here, A is in the range 1 to 50 and
                        \ represents the engine choppiness, with a lower value
                        \ indicating a smoother engine, and a higher value
                        \ making the sound jump up and down in pitch more
                        \
                        \ In other words, the lower the forward airspeed and
                        \ thrust, the choppier the engine sound
                        \
                        \ We can apply this to the sound envelope for the engine
                        \ by altering the amount that the pitch changes in each
                        \ stage of the sound envelope. These values are set in
                        \ the third, fourth and fifth values in the envelope
                        \ definition, with larger values giving us a larger
                        \ amount of pitch change, and hence more choppiness in
                        \ the engine sound

 STA envelopeData+2     \ Set the third and fifth values of sound envelope 1 to
 STA envelopeData+4     \ the value in A, and set the fourth value to the
 EOR #&FF               \ inverse of A, which is -(A + 1) as this is a signed
 STA envelopeData+3     \ value

                        \ We now want to set the pitch of the engine sound so
                        \ that higher thrust and airspeed give us a higher
                        \ pitch, making our Spitfire scream through the air
                        \
                        \ The engine pitch is controlled by byte #5 of sound #7,
                        \ at soundData+60, which contains the low byte of the
                        \ engine pitch, so we now need to calculate a suitable
                        \ pitch value

 LDA K                  \ Set A = K + 80
 CLC                    \       = (Thrust / 8) + zVelocityPHi + 50
 ADC #80                \
                        \ so A is a higher value when we have higher thrust and
                        \ airspeed, which is what we want

 CMP soundData+60       \ If the pitch in byte #5 of sound #7 is already at this
 BEQ engs4              \ value, jump to engs4 to return from the subroutine, as
                        \ we don't need to change the pitch

 STA soundData+60       \ Otherwise set byte #5 of sound #7 to our new pitch in
                        \ A, so the pitch of the engine sound goes up with our
                        \ thrust and airspeed

 LDA #0                 \ Call DefineEnvelope with A = 0 to redefine the first
 JSR DefineEnvelope     \ sound envelope with the new choppiness values

 LDA #7                 \ Make sound #7, the engine pitch, to change the engine
 JSR MakeSound          \ sound's pitch and choppiness

 LDA #1                 \ Make sound #1, the engine amplitude, to ensure that
 JSR MakeSound          \ both engine sounds are being made, as we need to make
                        \ both #1 and #7 for the sound to work

.engs4

 RTS                    \ Return from the subroutine