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This channel can be used to output digital sound, the length of the sample buffer (Wave RAM) can be either 32 or 64 digits (4bit samples). This sound channel can be also used to output normal tones w hen initializing the Wave RAM by a square wave. This channel doesn't have a volume envelope register. 4000070h - SOUND3CNT_L (NR30) - Channel 3 Stop/Wave RAM select (R/W) Bit Expl. 0-4 - Not used 5 R/W Wave RAM Dimension (0=One bank/32 digits, 1=Two banks/64 digits) 6 R/W Wave RAM Bank Number (0-1, see below) 7 R/W Sound Channel 3 Off (0=Stop, 1=Playback) 8-15 - Not used The currently selected Bank Number (Bit 6) will be played back, while reading/writing to/from wave RAM will address the other (not selected) bank. When dimension is set to two banks, output will star t by replaying the currently selected bank. 4000072h - SOUND3CNT_H (NR31, NR32) - Channel 3 Length/Volume (R/W) Bit Expl. 0-7 W Sound length; units of (256-n)/256s (0-255) 8-12 - Not used. 13-14 R/W Sound Volume (0=Mute/Zero, 1=100%, 2=50%, 3=25%) 15 R/W Force Volume (0=Use above, 1=Force 75% regardless of above) The Length value is used only if Bit 6 in NR34 is set. 4000074h - SOUND3CNT_X (NR33, NR34) - Channel 3 Frequency/Control (R/W) Bit Expl. 0-10 W Sample Rate; 2097152/(2048-n) Hz (0-2047) 11-13 - Not used 14 R/W Length Flag (1=Stop output when length in NR31 expires) 15 W Initial (1=Restart Sound) 16-31 - Not used The above sample rate specifies the number of wave RAM digits per second, the actual tone frequency depends on the wave RAM content, for example: Wave RAM, single bank 32 digits Tone Frequency FFFFFFFFFFFFFFFF0000000000000000 65536/(2048-n) Hz FFFFFFFF00000000FFFFFFFF00000000 131072/(2048-n) Hz FFFF0000FFFF0000FFFF0000FFFF0000 262144/(2048-n) Hz FF00FF00FF00FF00FF00FF00FF00FF00 524288/(2048-n) Hz F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0 1048576/(2048-n) Hz 4000090h - WAVE_RAM0_L - Channel 3 Wave Pattern RAM (W/R) 4000092h - WAVE_RAM0_H - Channel 3 Wave Pattern RAM (W/R) 4000094h - WAVE_RAM1_L - Channel 3 Wave Pattern RAM (W/R) 4000096h - WAVE_RAM1_H - Channel 3 Wave Pattern RAM (W/R) 4000098h - WAVE_RAM2_L - Channel 3 Wave Pattern RAM (W/R) 400009Ah - WAVE_RAM2_H - Channel 3 Wave Pattern RAM (W/R) 400009Ch - WAVE_RAM3_L - Channel 3 Wave Pattern RAM (W/R) 400009Eh - WAVE_RAM3_H - Channel 3 Wave Pattern RAM (W/R) This area contains 16 bytes (32 x 4bits) Wave Pattern data which is output by channel 3. Data is played back ordered as follows: MSBs of 1st byte, followed by LSBs of 1st byte, followed by MSBs of 2n d byte, and so on - this results in a confusing ordering when filling Wave RAM in units of 16bit data - ie. samples would be then located in Bits 4-7, 0-3, 12-15, 8-11. In the GBA, two Wave Patterns exists (each 32 x 4bits), either one may be played (as selected in NR30 register), the other bank may be accessed by the users. After all 32 samples have been played, ou tput of the same bank (or other bank, as specified in NR30) will be automatically restarted. Internally, Wave RAM is a giant shift-register, there is no pointer which is addressing the currently played digit. Instead, the entire 128 bits are shifted, and the 4 least significant bits are outp ut. Thus, when reading from Wave RAM, data might have changed its position. And, when writing to Wave RAM all data should be updated (it'd be no good idea to assume that old data is still located at the same position where it has been written to previously).
