STE documentation

4. DMA-Sound - the simple way to make music

Registers:

  DMA-Sound Control Register:

    $FFFF8900  0 0 0 0 0 0 0 0 0 0 0 0 0 0 X X

Writing a "00" to the last 2 bits terminate DMA sound replay.

Bit 0 controls Replay off/on, Bit 1 controls Loop off/on (0=off, 1=on).

DMA-Sound Start Address Register:

    $FFFF8902  0 0 X X X X X X X X X X X X X X   Hibyte

    $FFFF8904  X X X X X X X X X X X X X X X X   Midbyte

    $FFFF8906  X X X X X X X X X X X X X X X 0   Lowbyte

These three registers contain the 24-bit address of the sample to play. Even though the samples are built on a byte-base, the DMA chip also only allows even addresses

  DMA-Sound Count Register:

    $FFFF8908  0 0 X X X X X X X X X X X X X X   Hibyte  (ro)

    $FFFF890A  X X X X X X X X X X X X X X X X   Midbyte (ro)

    $FFFF890C  X X X X X X X X X X X X X X X 0   Lowbyte (ro)

Used internally for the DMA-soundchip to count from start- to end-address. No write access.

  DMA-Sound End Register:

    $FFFF890E  0 0 X X X X X X X X X X X X X X   Hibyte

    $FFFF8910  X X X X X X X X X X X X X X X X   Midbyte

    $FFFF8912  X X X X X X X X X X X X X X X 0   Lowbyte

The address that the sample ends at. When the count registers have reached this address, the DMA-sound system will either stop or loop.

  DMA-Soundmode Register:

    $FFFF8920  0 0 0 0 0 0 0 0 X 0 0 0 X X X X

Allows to toggle between several replay methods. Bit 7 switches Mono/Stereo (1 = Mono, 0 = Stereo), Bit 0 and 1 encode the replay rate:

      0 0  -  6258 Hz

      0 1  - 12517 Hz

      1 0  - 25033 Hz

      1 1  - 50066 Hz

Sounds fairly easy, right ? Unfortunately, it's not.

? I set all the registers, but there is no sound at all.

! The DMA-Soundsystem expects you to write the high-byte of the Start- and Endaddress first. Even though this serves no purpose at all, writing the highbyte clears the others. Hence it must be written first.

? I can't hear anything on my Falcon when trying to replay a 6 KHz sample.

! The Falcon DMA-soundsystem does not support 6 KHz. The value "00" in the Soundmode-register means "OFF" on the Falcon.

? The sound is awful. This does not sound like my sample.

! On the STE, the DMA-soundsystem only works with signed sample files, featuring values from -128 over 0 to +127. Some sample programs use unsigned formats, ranging from 0 to 255 with 128 representing zero-line of the sample. Those samples need to be converted first.

? I want to replay stereo samples. How can i know which sample will be played on which channel ?

! Stereo Samples have to be organized wordwise like Lowbyte -> right channel

  Hibyte  -> left channel

? My STE program to replay samples does not work very well on the Falcon.

! No, the Falcon's soundsystem is way more complex and can without any major programming interfere very well with the parts of the STE-soundsystem, especially since identical addresses are used for some purposes. If you want Falcon-compatibility of your STE-code, do not use "move"-instructions to set/unset bits of $FFFF8900 and $FFFF8920 as this might override Falcon-specific registers. Best, leave them as they are and use AND to unset and OR to set certain bits.

? Will my STE code work on the TT ?

! Yes, it will. The TT's DMA-sound subsystem is identical to the one of the STE.

? I am trying to change the addresses of the sample while the DMA-sound plays, but it does not work.

! No, the DMA-Soundsystem latches the Start- and End-Register internally, so writing to these values only takes influence when the values are re-read, which happens when the sample has been played, even if the DMA-soundsystem is switched to loop.

? Argh, i have now implemented DMA-sound to my program and now my whole screen-management goes wild

! This easily happens. The DMA-sound subsystem of the STE houses the "shifting logic" in the STE-Shifter. When starting the DMA sound to play a sample, you should not try to access the Shifter's registers directly afterwards but "wait" a few cycles. The simplest solution is to wait a single VBL after starting the DMA-sound before proceeding with your program. Once the DMA-sound plays, you can change DMA-sound registers without risking to screw up screen management.

? Can i get a "notice" from the DMA-soundsystem when it finished playing a sample ?

! Yes, you can use TIMER A as event counter which will be notified when a sample has been played completely.  

? I want to replay a sample backwards.

! Does not work on the STE. The sample-counter can only be increased, not decreased.

5. The National LMC 1992 and the Microwire

Preface:

  There's a little, but common mistake of minor importance when it comes to this combo that allows manipulation of the DMA sound to enhance trebble and bass as well as left, right and main volume. It is not the Microwire interface that manipulates the sound, it is a chip named National LMC 1992. This chip however has not been integrated into the Atari STE hardware directly but can only be communicated with using a 3-bit serial interface, the so-called Microwire. It is a bit hard to handle for a beginner, but luckily, it is also hard to crash the STE using this register. And since the Microwire can basically connect more than just one device, it needs a 2 bit address to which device to transfer data to. The LMC1992 is at "address" 2 (binary 10). Each address-data-pack written to the LMC1992 therefore consists of an 11 bit package. The communication is a bit similar to communicating with the YM2149 since the Microwire also requires to encode data in a certain way.

National LMC 1992

  Adress and Data register

    $FFFF8922  x x x x  x x x x  x x x x  x x x x

This address is being used to feed the National LMC both address and data bits for a certain setting. The choice which bits are being read are being described in the mask register at $FFFF8924. As described above, the first two bits of the 11 bit package need to be a "10" to address the LMC1992. Then there are 3 more "address" and 6 more data bits. The address bits are 3 in total and are being used as follows:

      0 1 1  -  Master Volume (followed by 6 bits of data)

      1 0 1  -  Left channel volume (followed by 6 bits of data)

      1 0 0  -  Right channel volume (followed by 6 bits of data)

      0 1 0  -  Trebble control (followed by 6 bits of data)

      0 0 1  -  Bass control (followed by 6 bits of data)

      0 0 0  -  Mixer (followed by 6 bits of data).

However, not all bits of the 6 general data bits are being used. It is necessary to have a multiple of 6 though since the Microwire is a 3-bit serial interface. The explanation of the 6 data bits are (d means necessary data bit, x means bit is ignored)

          Master Volume: d d d  d d d  (all 6 bits used)

                     0 0 0  0 0 0  = -80 db volume

                     0 1 0  1 0 0  = -40 db volume

                     1 0 1  x x x  =   0 db volume (max)

Each increment represents 2 db. If the 3 left bit encode "101", the last 3 bits are being ignored.

      Left channel:  x d d  d d d  (left bit ignored)

                       0 0  0 0 0  = -40 db volume

                       0 1  0 1 0  = -20 db volume

                       1 0  1 x x  =   0 db volume (max)

Each increment represents 2 db. If the 3 left bit carry "101", the last 2 bits are being ignored.

            Right channel: x d d  d d d  (left bit ignored)

                       0 0  0 0 0  = -40 db volume

                       0 1  0 1 0  = -20 db volume

                       1 0  1 x x  =   0 db volume (max)

Each increment represents 2 db. If the left 3 bit are "101", the last 2 bits are being ignored.

            Trebble:       x x d  d d d  (left 2 bits are ignored)

                         0  0 0 0  = -12 db (min)

                         0  1 1 0  =   0 db (linear)

                         1  1 0 0  =  12 db (max)

Each increment represents 2 db, normalized at 15 KHz.

            Bass:          x x d  d d d  (left 2 bits are ignored)

                         0  0 0 0  = -12 db (min)

                         0  1 1 0  =   0 db (linear)

                         1  1 0 0  =  12 db (max)

Each increment represents 2 db, normalized at 50 Hz.

            Mixer control: x x x  x d d  (left 4 bits are ignored)

                              0 0  = DMA + (YM2149 - 12 db)

                              0 1  = DMA + YM2149

                              1 0  = DMA only

                              1 1  = reserved

Setting "00" mixes the output of the YM2149 and the output of the DMA-sound, but the YM2149 sound is being downsized by 12 db. "01" mixes DMA and YM2149 linearly, "00" means DMA sound output only.

       Mask Register

    $FFFF8924  x x x x  x x x x  x x x x  x x x x

This contains in a bitfield which bits of the Address+Data Register are explicetely used. Since the Microwire, as it is being used in the STE, requires 11 bits of data (in general, the Microwire can transport 14 bits), it is essential to let the Microwire know WHICH of the 16 bits of this register are to be taken into account.As being used in the STE, this register will always feature 11 "1"s and 5 "0"s.

Example:

Let's say we want to feed the LMC the data "011101000", we would need to write a "10 011101000" to the address+data register. We can use whatever bits we like of the 16 bits of this register, so we use the mask register to mask out the unused bits, which might look like:

      $FFFF8924  0 0 0 0  0 1 1 1  1 1 1 1  1 1 1 1

      $FFFF8922  0 0 0 0  0 1 0 0  1 1 1 0  1 0 0 0

or

      $FFFF8924  0 1 1 0  0 1 1 1  1 1 1 0  1 1 0 1

      $FFFF8922  0 1 0 0  0 0 1 1  1 0 1 0  0 0 0 0

both have the same effect.

Sounds complicated enough but can boost the DMA-sound output of the STE quite a lot. When programming it first time however you might easily see that it did not work as planned. Why ?

? Can't hear any changes on my Falcon ...

! Unfortunately, the Falcon does neither have a Microwire interface nor the National Semiconductors LMC1992. The Falcon cannot manipulate bass, treble, main, left and right volume as easily as the STE can. The Falcon will not report an error either though. The TT does have the Microwire interface as well as the LMC1992.

? I write both address+data and the mask register correctly, still it doesn't have the expected effect.

! You need to write the mask before you write address and data. As soon as address+data register has been written to, the Microwire starts to operate (which means shifting to the left). Writing the mask register after writing the address+data register is therefore useless.

? I write data into the mask-register, then address and data but it still doesn't do what i wanted to.

! Always make sure you have a total of "11" bits, and always make sure, the leading bits on the left side are a "10". Otherwise, the Microwire will try to access other peripherals that the STE does not have - which will not lead to an error, but result in no changes at all.

? I tried to achieve sound manipulation effects by writing a lot of values to the LMC1992 to change DMA sound output. It does seem to ignore a lot of my values.

! The LMC1992 is connected to the Microwire and is being fed data in a serial way. The Microwire is more or less a parallel to serial converter and it does that by shifting the 16 bit value (along with the mask) to the left 16 times and passing each bit for that the mask-bit is "1" to the LMC. That takes some time and during its operational state, the Microwire cannot be written to.

? How can i find out wether the Microwire interface is done ?

! Simply check the value in the address+data register after you wrote your value into it. If the value at $FFFF8922 is identical with the value you wrote into it, the Microwire is done shifting and can once again be written to. In all other cases, the Microwire is still shifting and cannot be written to.

? I successfully wrote to the LMC1992, but now YM2149 sound output is pure torture. What happened ?

! Well, the LMC1992 is not a chip that controls the DMA-sound in its digital form but manipulates the analogue sound that comes out of the DMA chip. If you now put the mixer to mix YM2149 and DMA sound, the LMC1992 will also manipulate the YM sound output. However, the YM2149 as a soundchip is not really meant to have Bass and Trebble enhanced. This might result in a very ugly sound.

? My program works fine and also exits cleanly, but then any subsequent sound output is awful. How come ?

! You should save the contents of the LMC1992 right at the start of your program and when exiting, you should restore the original value - and the easiest way is to save both mask and address+data register. Restoring can be done by just writing mask and address+data registers. The Microwire does not need any further software support once you wrote the values, so it does not harm if your program terminates in the meantime. Programs you launch when your program is terminated that do use DMA or YM2149 sound might be affected by your LMC1992 settings otherwise.

? How come the Falcon does not have this feature ?

! When the Falcon was initially planned (and named Sparrow), it had a chip that was supposed to bear similar features named RASCAL. It is probable that this chip was supposed to "simulate" a Microwire + LMC 1992 duo as well as give enhanced possibilites towards the 16-bit stereo sound of the Falcon (Sparrow) as well as the DSP. Either Atari did not finish this chip in time, it was too expensive or too complex, we don't know. It appeared in the first Sparrow prototypes as well as the first few Falcon (exhibitors) boards. However, it is so far unknown wether this chip is compatible to the Microwire+LMC1992 duo or not and why it has been canned.

? Why is the handling of the LMC1992 so complicated ?Wouldn't there have been an easier way to give the STE these features ?

! Yes, of course, but the LMC1992 was very cheap. The LMC1992 was never meant to serve in a computer but was commonly used in TV sets, Radios etc. and any other Audio-device that had the option to control volumes, bass and trebble electronically, and those preferred a "1-bit serial" implementation. The Microwire is just the connection of the STE's architecture to the LMC1992.

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