Difference between revisions of "AGI/Specifications/Sound"

According to Donald B. Trivette, author of "The Official Book of King's Quest", a year before the IBM PCjr was announced IBM asked Sierra to create a game that would show off the new computers color graphics capabilities. IBM supplied the company with a prototype Junior, and Roberta set to work designing a new type of adventure game. The game produced was called King's Quest. This is important because the IBM PCjr had a different method of sound generation than the IBM compatibles of today. The sound data was stored to make it easy to send to the Juniors sound generators. This format appears to have remained right through the AGI games up until 1989--90 when SCI took over even though the PCjr had long since been surpassed by the 286, and 386.

     .  R0 R1 R2 .  .  .  .      Register number in T1 chip (0, 2, 4).

 

Each voice in the T1 sound chip has an independent sound-level control, which is calculated in terms of decibels of attenuation, or softening. There are four bits uses to control the volume. These bits, labeled A0 through A3, can be set independently or added together to produce sixteen volume levels as shown below.

The attenuation is set by sending a byte of the following format to the T1 sound chip:

     .  R0 R1 R2 .  .  .  .      Register number in T1 chip (1, 3, 5, or 7).

The noise frequency is set by sending a byte of the following format to the T1 sound chip:

     .  1  1  0  .  .  .  .      Register number in T1 chip (6)

The 5503 Ensoniq Digital Oscillator Chip (DOC) contains 32 fundamental sound-generator units, known as "oscillators". Each oscillator is capable of either making an independent tone by itself, or of being paired up cooperatively with it's neighbor in a pairing known as a "generator". The generator arrangement is used by most programs, for it allows more flexibility and a thicker, lusher sound.

The DOC plays 8-bit waveforms, with the centerline at 0x80 (128 decimal). This format is known as "8-bit unsigned". 0x00 (0 decimal too) is reserved for `stop'. If a sample value of 0 is encountered by a DOC oscillator, the oscillator will immediately halt and not produce any more sound. The DOC additionally has an 8-bit volume register for each oscillator, with a linear slope. The dynamic range of the DOC (the `space' between the softest and loudest sounds it can produce) is approximately 42 dB, or about on par with an average cassette tape.

Each oscillator has it's own 16 bit frequency register, ranging from 0 to 65535. In a normal DOC configuration, each step of the frequency register increases the play rate by 51 Hz, and computing the maximum theoretical play rate is left as an exercise for the student.

* Free-run: the oscillator simply plays the waveform and stops. No interaction with it's 'twin' occurs.

The output of an oscillator can be directed to any one of 16 possible channels. Apple only makes 8 channels avalible via the 3 bits on the sound expansion connector, and all current stereo cards limit this to 1 bit, or two channels. However, the "Bernie II The Rescue" IIgs emulator for the Power Mac expands this support to 4 discrete output channels, two of which are encoded to the rear channel for Dolby Pro-Logic compatible output. No IIgs software that I'm aware of supports more than 2 channels however.

 

We now know enough about the PCjr's T1 sound chip to discuss the AGI sound format. The sound is stored as four separate units of data, one for each voice. Each sound file stored in the VOL files has an 8-bit header which contains offsets into file. The format is as follows:

Note that the last three bytes were around the other way in version 1 of the AGI interpreter. The above order is opposite from the order that would be output to the T1 sound chip.

Each voice's data section in the SOUND resource file is usually terminated by two consecutive 0xFF codes. Another way of checking for the end is to see if it has reached the start of the next voice section, or in the case of the noise voise, the end of the SOUND data.

             .  R0 R1 R2 .  .  .  .      Register number in T1 chip (0, 2, 4).

             .  1  1  0  .  .  .  .      Register number in T1 chip (6)

         .  R0 R1 R2 .  .  .  .      Register number in T1 chip (1, 3, 5, or 7).

The sound format used in AGI version 1.12 (and also in the DOS version of the pre-AGI game Winnie the Pooh in the Hundred Acre Wood) is less structured from the format described above for AGIv2 and AGIv3.

In this format the note data is separated to "rows" of fixed duration of 2. Each row contains the data for all the four generators and ends with the byte 0. The data consists simply of register values that the player dumps to the sound chip. Because of this they can be in arbitrary order. An exception is the frequency updates that need two bytes and the chip expects them to come in the correct order. Often a row contains only one 0 which means the chip state is not updated on this row.

MIDI songs have a stram of MIDI data following the resource type. The following dump shows the MIDI data in the King's Quest I opening theme resource:

     0a 80 43 40    Resease note 0x43 in channel 0 with velocity 0x40

Now, that's a bit scary looking, but all the important information is there :-)

The envelope is first: for each segment there's a breakpoint (target volume, which is on a logarithmic scale in 6 decibel units) and an Increment (a 8.8 fixed point number telling how much to adjust the volume on each tick). For instance, if the first segment had BP = 1 and Inc = 0x0001, it would take 256 ticks for the volume to reach 1. The ''rel seg'' tells which segment of the envelope is the final one. Ticks in the Note Synth default to 100 Hz, although the AGI interpreter may well have used a different value -- I'll have to check.

''A wave coun''t and ''B wave count'' tell how many wavelists there are for each 5503 voice. If there's more than 1 wavelist you compare the note being started with the ''top'' value in each wavelist and if ''top'' is greater than or equal to the note you're starting you use that wavelist. In this case there's only 1 possible wavelist so top is naturally 0x7f, the highest possible numbered MIDI note.

 

Case the wave starts 0x5000 bytes into the image: For size you mask off all but the lowest 3 bits and it gives you the basic wave size as follows:

Note that if a zero is encountered in the wave before that size you still stop at the zero.

''Mode'' is the 5503 oscillator mode for the voice in the bits 1 and 2. The lowest bit (bit 0) is a "halt" flag. 0 is looping, 1 is oneshot (play once), 2 is sync/AM (which nobody uses, but I'll try to explain it if they actually are), and 3 is swap (oscillator 0 plays once, generates an IRQ, and oscillator 1 starts automatically. If oscillator 1 is also in swap mode, it will play once, generate and IRQ, and auto-start oscillator 0 again. If oscillator 1 is loop mode it will just loop continuously - this setup is often used to have a sampled "attack" on an instrument followed by a loop.

The full structures look like this: (there is no structure padding on the IIgs - all bytes are jammed right together!)

In Police Quest these structures start at offset 0x8469 in PQ.SYS16. ''(The same offset is used for KQ1 --CM)'' I can't find any of my other old disks with AGI games to locate their offsets. PQ only defines 28 instruments.

Writing a program to play the tunes will require four pointers which keep track of where in each voice segment the program currently is since all four voices are played simultaneously. The first voice is the melody and is the voice that is played on the PC speaker in today's modern PC compatibles, the other two voices being ignored. I'd imagine that other platforms such as the Amiga and Macintosh would probably play all three voices.

A program would start by reading each of the four offsets in the header. It would then go through a loop which begins by reading the first note of each voice section. The duration's are then monitored and when each note finishes, another note is read. Note that the notes for each voice will usually finish at different times. The program finishes when all of the voice sections have been entirely played. This will usually occur for each voice at the same time but not necessarily I don't think.

Then of course you could always convert the AGI SOUND to a MIDI file and play that which will sound a hundred times better :)

 

My program reads in the duration as a 16 bit word. It then loads the two following bytes and calculates the frequency as follows:

Remember also that the SOUND format includes volume information for each voice. The exact conversion from the decible values to the volume control on todays sound cards is uncertain at this stage.