bgm_devlog.md

Background Music Development Log

calculate how long each note should last

• In general, we can mainly categorize the notes in this sheet music into quarter notes and eighth notes.
• This music's BPM is 180, which means it plays 180 quarter notes per minute, so every quarter notes lasts for 60s/180 = 0.33s. For convenience, we let it be 0.3s. Therefore the eighth note should last for 0.3s/2 = 0.15s.
• We must insert a break between the two notes, otherwise two consecutive eighth notes of the same pitch will sound like a quarter note. Thus we add a additional 0.05s break to distinguish two consecutive notes of the same pitch.
• To sum up, every quarter note would last for 0.3s, followed by 0.05s break. Every eighth note would last for 0.15s, followed by 0.05s break.

calculate every pitch's correspond counter number

• Every pitch come from a frequency divider whose input clock signal is 100MHz, so we must calculate the counter number to get the accurate frequency.
• Formula: 100M / pitch's frequency / 2
  • C4(261Hz): 190839
  • D4(293Hz): 170068
  • E4(329Hz): 151515
  • F4(349Hz): 143266
  • G4(391Hz): 127551
  • G4#(415Hz): 120481
  • A4(440Hz): 113636
  • A4#(466Hz): 107296
  • B4(493Hz): 101215
  • C5(523Hz): 95602
  • D5(587Hz): 85178
  • D5#(622Hz): 80385
  • E5(659Hz): 75873
  • F5(698Hz): 71633
  • F5#(739Hz): 67658
  • G5(783Hz): 63776
  • A5(880Hz): 56818
  • B5(987Hz): 50658

pitch generator

• There're three important inputs
  • counter: Decide which pitch we want to play
  • rest: 1 for rest notes, 0 for regular notes
  • eight: 1 for eighth notes, 0 for quarter notes
• Two outputs
  • next_note: When a note is played to its full duration, next_note would toggle to 1, telling the top module proceed to the next note
  • note: the pitch we want to play

module note (input clk_100MHz,
              input [17:0] counter,
              input rest,
              input eight,
              output next_note,
              output note
);

• we set up a beat_counter which will increment by 1 whenever recieve a posedge clk_1000Hz signal. So, if the eight is 1, the beat_counter will count to 200 and return to 0 (0.15s sound output and 0.05s break). On the other hand, if the eight is 0, it will count to 350 and return to 0 (0.3s sound output and 0.05s break).
• sound output logic

assign note = clk_out & ~rest & ((eight & (beat_counter <= 150)) | (~eight & (beat_counter <= 300)));

Top module: A Finite State Machine

• A note is a state. In every state, we input different counter, rest and eight to generate the specific pitch.
  • Totally 188 states...

always@(*)
  begin
    case(state)
      //first section
      10'd0: begin //eighth E5
               counter = 18'd75873;
               rest = 0;
               eight = 1;             
             end
      10'd1: begin  //eighth E5
               counter = 18'd75873;
               rest = 0;
               eight = 1;     
             end
            .
            .
            .
            .
      10'd187: begin  //eighth G4
                 counter = 18'd127551;
                 rest = 0;
                 eight = 1;
               end
      10'd188: begin  //quarter rest
                 counter = 18'd0;
                 rest = 1;
                 eight = 0;
               end
    endcase
  end

• Next State Logic

  always@(posedge next_note)
  begin
    if(state == 188)
      state <= 0;
    else
      state <= state + 1;
  end