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10 Sound

The sound chip built into the TI-99/4A has three sound channels and a single noise channel. All channels can be in use simultaneously.

The stack signature for the SOUND word is as follows:

    SOUND ( pitch volume channel -- )

Where:

• pitch is a value between 0 and 1023
• volume is a value between 0 and 15 (0 being loudest)
• channel is a value between 0 and 3, with 3 being the noise channel

Note: When addressing the noise channel, valid values for pitch are 0 to 7 inclusive.

If you are used to TI BASIC you are probably wondering where the duration value is. There isn't one. It's entirely dumb. SOUND simply writes to the sound chip and it's done. It's up to you to end notes/sounds at the appropriate time in your code. To end a note, simply set the volume of that channel to 15.

10.1 Supplementary Sound Words

The only sound-related word in the TurboForth ROM is SOUND, as described above. However, the following supplementary words are very useful and offer a superior level of control over the sound chip:

: VOLUME ( ch# vol -- ) \ set volume of channel ch# to vol $F AND SWAP 2* 1+ 8 + 4 << OR $8400 C! ; : TONE ( ch# freq -- ) \ set freq of channel ch# to freq SWAP 2* 8 + 4 << SWAP DUP $F AND ROT OR SWAP 4 >> SWAP $8400 C! $8400 C! ; : NOISE ( n -- ) \ set noise generator to noise type n 7 AND $E0 OR $8400 C! ;

10.1.1 Example Usage

To use:

   0 1017 TONE

Will play a low A on channel 0. Note: the codes for the notes are given below.

   0 0 VOLUME

...will set channel 0 to maximum volume (15 is off).

Here is an A chord:

   0 1017 TONE
   1 807 TONE
   2 679 TONE

Then set the volume:

   0 0 VOLUME
   1 0 VOLUME
   2 0 VOLUME

Note: You only have to set the volume for each channel once. Unless you want to change the volume, of course.

To stop a tone from playing, set its volume to 15.

Here is a test program:

   1 VALUE STEP
   : TEST 0 0 VOLUME 1017 20 DO 0 I TONE STEP +LOOP 0 15 VOLUME ;

Now, you can play around like this:

   1 TO STEP TEST
   2 TO STEP TEST
   10 TO STEP TEST

etc. etc.

The noise channel takes a value from 0 to 7 for the different types of noise. Use a value of 3 for the channel number number when you want to change the volume of the noise channel.

10.2 Code Equivalents of the Supplementary Sound Words

The words given above are very useful and quite fast. They are also useful to see how to address the sound chip in high-level code. However, in normal use, one would want sound-related words to process as quickly as possible in order to give more time to other things, especially when developing games etc. To that end, machine code versions of the above words are given below. Usage is identical to the words given above. They occupy 114 bytes of memory.

base @ hex \ save current base CODE: volume ( ch# vol -- ) \ set the volume on sound channel ch# to vol C034 0240 000F C074 0A11 0221 0009 0A41 E040 06C1 D801 8400 ;CODE CODE: noise ( n -- ) \ set noise generator to noise type n C034 0240 0007 0260 00E0 06C0 D800 8400 ;CODE CODE: tone ( ch# freq -- ) \ set tone channel ch# to frequency freq C074 C0B4 0A12 0222 0008 0A42 C001 0240 000F E080 0941 06C2 06C1 D802 8400 D801 8400 ;CODE base ! \ restore base

10.3 Frequency Constants

The values corresponding to musical notes are given here. They are taken from the Editor/Assembler Owner's Manual by Texas Instruments (see pages 318 to 320) © 1982 . If anyone has a more accurate table then please get in touch (see contact information on the home page).

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