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Aviator on the BBC Micro

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