Sound Blaster 16 Programming Document

Written by Ethan Brodsky

Version 3.5 - 6/26/97


Note: 97/6/11
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Disclaimer

This information is distributed AS IS. The author specifically disclaims responsibility for any loss of profit or any consequential, incidental, or other damages resulting from the use or misuse of this information. This information may be freely distributed in any form as long as this disclaimer remains intact.

Introduction

The Sound Blaster 16 is capable of both FM and digitized sounds. Digitized sound capibilities range from 8-bit mono 5000 HZ sound all the way up to 16-bit stereo sound at 44khz. This FAQ documents programming the SB16 DSP CT1341 chip for recording and playback of digitized audio. Prior knowledge on programming earlier Sound Blaster sound cards is necessary.

The Sound Blaster 16 DSP I/O Ports

The SB16's DSP chip is programming using several I/O ports at a base I/O address determined by jumper settings. On the SB16, there are 16 I/O ports which are used for FM synthesized music, mixer settings, DSP programming and CD-ROM access. Five of these ports are used in programming the DSP. They are listed below.

Resetting the DSP

You have to reset the DSP before you can program it. The DSP can be reset using the following procedure:
  1. Write a 1 to the reset port (2x6)
  2. Wait for 3 microseconds
  3. Write a 0 to the reset port (2x6)
  4. Poll the read-buffer status port (2xE) until bit 7 is set
  5. Poll the read data port (2xA) until you receive an AA
The DSP usually takes about 100 microseconds to initialized itself. After this period of time, if the return value is not AA or there is no data at all, then the SB card may not be installed or an incorrect I/O address is being used.

Writing to the DSP

To write a byte to the SB16, the following procedure should be used:
  1. Read the write-buffer status port (2xC) until bit 7 is cleared
  2. Write the value to the write port (2xC)

Reading from the DSP

To read a byte from the SB16, the following procedure should be used:
  1. Read the read-buffer status port (2xE) until bit 7 is set
  2. Read the value from the read port (2xA)

Programming the DMA Controller

The DMA (Direct Memory Access) controller controls data transfers between I/O devices and memory without using the CPU. An Intel 8237 DMAC integrated circut is used to control this. An IBM compatible computer has two DMA controllers, one for 8-bit transfers and one for 16-bit transfers. The DMA controller, coupled with an external page register, is capable of transfering blocks of up 64k to the SB16. Here is information on I/O ports and register settings necessary for sound card I/O:

I/O port addresses for the DMA Address and Count Registers

I/O port addresses for the control registers

I/O port addresses for lower page registers

Mode register bit assignments

Write single mask bit assignments

DMAC2 is used for 16-bit I/O and DMAC1 is used for 8-bit I/O. The procedure for starting a transfer is complicated, so I'll list the steps for starting the type of DMA transfers used for sound I/O:

  1. Calculate the absolute linear address of your buffer
      LinearAddr := Seg(Ptr^)*16 + Ofs(Ptr^)); 
  2. Disable the sound card DMA channel by setting the appropriate mask bit
      Port[MaskPort] := 4 + (Channel mod 4); 
  3. Clear the byte pointer flip-flop
      Port[ClrBytePtr] := AnyValue; 
  4. Write the DMA mode for the transfer
    The mode selection bits should be set to 01 for single-mode. The address inc/dec bit should be set to 0 for address increment. The auto-initialization bit should be set appropriately. I will discuss auto-initialized DMA later. The transfer bits should be set to 10 for playback and 01 for recording. The channel select should be set to the sound card DMA channel. Be aware that "read" means a read from memory (Write to sound card) and that "write" means a write to system memory.
      Port[ModePort] := Mode + (Channel mod 4); 
    Some often used modes are:

  5. Write the offset of the buffer, low byte followed by high byte. For sixteen bit data, the offset should be in words from the start of a 128kbyte page. The easiest method for computing 16-bit parameters is to divide the linear address by two before calculating offset.
    if SixteenBit
      then
        begin
          BufOffset := (LinearAddr div 2) mod 65536;
          Port[BaseAddrPort] := Lo(BufOffset);
          Port[BaseAddrPort] := Hi(BufOffset);
        end
      else
        begin
          BufOffset := LinearAddr mod 65536;
          Port[BaseAddrPort] := Lo(BufOffset);
          Port[BaseAddrPort] := Hi(BufOffset);
        end;
    
  6. Write the transfer length, low byte followed by high byte. For an 8-bit transfer, write the number of bytes-1. For a 16-bit transfer, write the number of words-1.
      Port[CountPort] := Lo(TransferLength-1);
             Port[CountPort] := Hi(TransferLength-1); 
  7. Write the buffer page to the DMA page register.
      Port[PagePort] := LinearAddr div 65536; 
  8. Enable the sound card DMA channel by clearing the appropriate mask bit
      Port[MaskPort] := DMAChannel mod 4; 

Setting the sampling rate

Unlike earlier Sound Blasters, the SB16 is programmed with actual sampling rates instead of time constants. On the SB16, the sampling rate is set using DSP commands 41h and 42h. Command 41h is used for output and 42h is used for input. I have heard that on the SB16, both these command currently do the same thing, but I would recommend using the individual commands to guarantee compatibility with future sound cards. The procedure for setting the sampling rate is:
  1. Write the command (41h for output rate, 42h for input rate)
  2. Write the high byte of the sampling rate (56h for 22050 hz)
  3. Write the low byte of the sampling rate (22h for 22050 hz)

Digitized sound I/O

To record or play back sound, you should use the following sequence:
  1. Allocate a buffer that does not cross a 64k physical page boundary
  2. Install an interrupt service routine
  3. Program the DMA controller for background transfer
  4. Set the sampling rate
  5. Write the I/O command to the DSP
  6. Write the I/O transfer mode to the DSP
  7. Write the block size to the DSP (Low byte/high byte)

Upon interrupt when using single-cycle DMA:

  1. Program DMA controller for next block
  2. Program DSP for next block
  3. Copy next block if double-buffering
  4. Acknowledge the interrupt with the SB by reading from port 2xE for 8-bit sound or port 2xF for 16-bit sound.
  5. Acknowledge the end of interrupt with the PIC by writing 20h to port 20h. If the sound card is on IRQ8-15, you must also write 20h to A0h.

DSP commands

D0
Pause 8-bit DMA mode digitized sound I/O initiated by command Cxh.
Applicable to both single-cycle and auto-initialized DMA I/O.
D4
Continue 8-bit DMA mode digitized sound I/O paused using command D0.
Applicable to both single-cycle and auto-initialzied DMA I/O.
D5
Pause 16-bit DMA mode digitized sound I/O initiated by command Bxh.
Applicable to both single-cycle and auto-initialized DMA I/O.
D6
Continue 16-bit DMA mode digitized sound I/O paused using command D5.
Applicable to both single-cycle and auto-initialized DMA I/O.
D9
Exit 16-bit auto-initialized DMA mode digitized sound I/O after the end of the current block.
DA
Exit 8-bit auto-initialized DMA mode digitized sound I/O after the end of the current block.
E1
Get DSP version number. After sending this command, read back two bytes form the DSP. The first byte is the major version number and the second byte is the minor version number. A SB16 should have a DSP version of 4.00 or greater. Check this before using an SB16 specific commands.
Bx
Program 16-bit DMA mode digitized sound I/O
Command sequence: Command, Mode, Lo(Length-1), Hi(Length-1)

Command:

Common commands:

Mode:

Cx
Program 8-bit DMA mode digitized sound I/O
Same procedure as 16-bit sound I/O using command Bx
Common commands:

The FIFO is used to eliminate inconsistencies in the sample period when the sound card is not able to get DMA when it needs it. With FIFO disabled, the card attempts DMA at precisely the instant that it needs a sample. If another device with a higher priority is accessing DMA, the sound card waits for the sample and the sampling rate may be decreased. The FIFO allows the DMA sample period to be more erratic without affecting the audio quality. The FIFO is cleared whenever a command is sent to the DSP. In Single-cycle mode, the DSP is constantly being reprogrammed. The FIFO may still contain data which has not been output when it cleared. To avoid this problem, the FIFO should be turned off for single-cycle mode. When in auto-initialized mode, the DSP is never reprogrammed. The FIFO can be left on and sound quality will be improved.

Auto-initialized DMA

When single-cycle DMA is used, sound output stops at the end of each block. The interrupt handler can start another transfer, but there will be a break in output. This causes a click between each block, reducing sound quality. When auto-initialized DMA is used, sound output loops around at the end of the buffer. The DMA controller keeps transfering the same block of memory that the DMA transfer was initiated with. When the end of the buffer is reached, it will start sending the buffer again by auto-initializing the current offset and count registers with the values stored in the base offset and count registers. The usual method for achieving click-less output is to allocate a buffer and divide it into two blocks. Program the DMA controller with the length of the whole buffer, but program the SB16 with the length of a block. (Half of the buffer) An interrupt occurs for each sound card block, so two interrupts will occur each time the buffer is played, once at the midpoint (Start of the second block) and once at the end (In effect, the start of the first block) The interrupt handler should copy data into the block that was just finished so that the data is ready when it is needed for output. The programming procedure for an auto-initialized DMA transfer is identical to the procedure for a single-cycle DMA transfer, except that bit 4 of the DMA mode register and bit 3 of the DSP command are set.

Upon interrupt when using auto-initialized DMA:

  1. Copy next chunk into output buffer block that just finished
  2. Acknowledge the interrupt with the SB by reading from port 2xE for 8-bit sound or port 2xF for 16-bit sound.
  3. Acknowledge the end of interrupt with the PIC by writing 20h to port 20h. If the sound card is on IRQ8-15, you must also write 20h to A0h.

To stop sound immediately:

Both commands stop sound immediately, without an interrupt.

To stop the sound at the end of the currently block:

These two commands will stop the sound at the end of the current block. If your program is not prepared for an interrupt after output is finished, it may cause problems.

You can also end auto-initialized mode by reprogramming the DSP for single-cycle mode. The card then switches from A/I mode to S/C mode after the next interrupt. It will then contiue to play or record for the length specified, generate an interrupt and stop. This will allow you to stop output exactly at the end of the data, without requiring the remainder of the DMA buffer to be filled with silence. This technique may or may not be useful to you. I would recommend using the pause commands documented in in the immediate stop section unless another method is more suited to your purpose.

References

Thanks to Douglas Kaden at Creative Labs for information on 16-bit DMA, FIFO mode, and other topics.


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Ethan Brodsky <brodskye@cae.wisc.edu>