' ============================================================
' BBC LEVEL / DISTORTION TEST  (works on b6 or newer, no V7 needed)
' Purpose: find out whether the distortion heard in the bass
' section comes from the analog output stage (PWM, amp, speaker)
' or from the digital mix.
'
' Plays one held A-minor sound: noise bass (ch0, tuned by silent
' ch1) + melody (ch2) + harmony (ch3), with the same base levels
' as the game: bass -7, melody -5, harmony -6.
'
' Keys:
'   1 = bass on/off      2 = melody on/off      3 = harmony on/off
'   + = all louder (1 step)    - = all quieter (1 step)
'   v = PLAY VOLUME down 10    V = PLAY VOLUME up 10
'   b = bass tuner pitch: 141 -> 161 -> 181 -> 41 -> 141 ...
'       (game uses ~141..190 = about 120-250 Hz; 41 = 29 Hz is far
'       too low, only for comparison: it sounds like rattling clicks)
'   ESC = quit
' ============================================================
OPTION EXPLICIT
OPTION BASE 0

CONST BASS_AMP = -7
CONST MEL_AMP  = -5
CONST HARM_AMP = -6
CONST MEL_PITCH = 117   ' E5
CONST HARM_PITCH = 101  ' C5

DIM INTEGER on_bass, on_mel, on_harm
DIM INTEGER shift, vol, bass_tune
DIM STRING k$

on_bass = 1 : on_mel = 1 : on_harm = 1
shift = 0 : vol = 100
bass_tune = 161   ' game range is about 141..190 (note pitch + 140)

PLAY VOLUME vol, vol
Apply

DO
   k$ = INKEY$
   IF k$ <> "" THEN
       SELECT CASE k$
           CASE "1" : on_bass = 1 - on_bass : Apply
           CASE "2" : on_mel  = 1 - on_mel  : Apply
           CASE "3" : on_harm = 1 - on_harm : Apply
           CASE "+" : shift = shift - 1 : Apply     ' more negative = louder
           CASE "-" : shift = shift + 1 : Apply
           CASE "v" : vol = vol - 10 : IF vol < 0 THEN vol = 0
                      PLAY VOLUME vol, vol : Report
           CASE "b" : bass_tune = bass_tune + 20
                      IF bass_tune = 201 THEN bass_tune = 41
                      IF bass_tune = 61 THEN bass_tune = 141
                      Apply
           CASE "V" : vol = vol + 10 : IF vol > 100 THEN vol = 100
                      PLAY VOLUME vol, vol : Report
           CASE CHR$(27) : EXIT DO
       END SELECT
   ENDIF
   PAUSE 20
LOOP

PLAY BBC SOUND &H10, 0, 0, 0
PLAY BBC SOUND &H11, 0, 0, 0
PLAY BBC SOUND &H12, 0, 0, 0
PLAY BBC SOUND &H13, 0, 0, 0
PRINT "Done."
END

' Clamp an amplitude to -15..-1 (louder = more negative)
FUNCTION Lv(base AS INTEGER) AS INTEGER
   LOCAL INTEGER a
   a = base + shift
   IF a < -15 THEN a = -15
   IF a > -1 THEN a = -1
   Lv = a
END FUNCTION

SUB Apply
   ' flush all four channels, then restart the ones that are on
   PLAY BBC SOUND &H10, 0, 0, 0
   PLAY BBC SOUND &H11, 0, 0, 0
   PLAY BBC SOUND &H12, 0, 0, 0
   PLAY BBC SOUND &H13, 0, 0, 0
   IF on_bass THEN
       PLAY BBC SOUND 1, 0, bass_tune, 255      ' silent tuner for the noise
       PLAY BBC SOUND 0, Lv(BASS_AMP), 3, 255   ' noise rate taken from ch1
   ENDIF
   IF on_mel  THEN PLAY BBC SOUND 2, Lv(MEL_AMP), MEL_PITCH, 255
   IF on_harm THEN PLAY BBC SOUND 3, Lv(HARM_AMP), HARM_PITCH, 255
   Report
END SUB

SUB Report
   PRINT "bass=";on_bass;" mel=";on_mel;" harm=";on_harm;
   PRINT "  levels: ";Lv(BASS_AMP);" ";Lv(MEL_AMP);" ";Lv(HARM_AMP);
   PRINT "  PLAY VOLUME=";vol;"  bass tuner=";bass_tune
END SUB