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353 lines
14 KiB
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Atari POKEY Chip Emulator V2.3
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==============================
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by Ron Fries
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19 Jan 98
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The PokeySound Chip Emulator is designed to emulate the functionality of the
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Atari POKEY Chip Hardware through 'C' Sourcecode. The emulator is able to
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produce sounds which are essentially identical to the original POKEY chip,
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including the exact distortions and pitches.
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The emulator is designed to run in a 32-bit environment. Though it can be
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compiled and run in a 16-bit environment, it is slow.
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I would like to give special thanks to Neil Bradley. He provided excellent
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testing support and was also the driving force behind the multiple POKEY
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emulation.
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New Features:
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-------------
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Version 2.3:
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1) Added configurable support for signed/unsigned samples. Default is
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unsigned, define SIGNED_SAMPLES if necessary.
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2) Included SBDRV1.2 which adds DJGPP support and corrects the AUTO-INIT
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detection problem.
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Version 2.2:
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1) Changed to signed 8-bit samples.
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2) Increased gain range by a factor of 16. Divide gain from previous version
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by sixteen to produce the same results.
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3) Removed DOS dependent routines to provide cross-platform support.
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4) Added conditional defines for TRUE and FALSE
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5) Added support for big/little endian machines. Defaults to little endian.
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Define BIG_ENDIAN if necessary.
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Version 2.0:
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1) Support for multiple POKEY chips. The maximum supported is configured
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at compile time.
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2) An adjustable gain. The previous releases had a built-in gain of 64.
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3) A clipping option. Depending on the number of chips emulated and the
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configured gain, it is possible for the output to exceed 8-bits.
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Clipping can be enabled to prevent this, though it does increase the
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processing time.
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Standard Features:
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------------------
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The 'PokeySound' emulator supports the following functions:
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1) All polynomial sound generators:
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a) 4-bit poly - actual bit pattern determined from sampled sound
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b) 5-bit poly - actual bit pattern determined from sampled sound
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c) 17-bit poly - simulated random bit pattern
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d) 9-bit poly - derived from simulated 17-bit poly
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2) Full support of all 'Divide by N' counter clocks:
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a) 1.79 MHz (high limited to playback sample rate)
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b) 64 KHz (high limited to playback sample rate)
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c) 15 KHz
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3) Full support of all 'Divide by N' resolutions:
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a) 8-bit - single channel
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b) 16-bit - double channel
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4) Full support of all distortions
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a) 5-bit poly, then 17-bit poly
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b) 5-bit poly only
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c) 5-bit poly, then 4-bit poly
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d) 17-bit poly only
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e) no poly counters (pure tone)
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f) 5-bit poly only
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5) Full support of volume control
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6) Full support of all pitches - distortions will vary exactly as the
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original Atari based on different pitches
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7) Accurate pitch generation
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8) Support of any playback sample rate (e.g. 22050)
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The 'PokeySound' emulator does not currently support the following functions:
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1) High pass filters
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Though I don't believe adding support for the High-Pass filters is very
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complicated, I decided not to add support right now because I don't
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believe this feature is used much. I'm also not sure how much impact it
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would have on performance. Let me know if you find an application that
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uses it.
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In the 2.0 release, I've removed the non-optimized version. It was only
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left in for reference. If you would still like to see the non-optimized
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version, it's available in the 1.2 release.
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One of the unique features of the emulator is that the processing time varies
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based on the frequency. Since the routine only calculates new output values
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when a change is sensed, the lower frequencies (which change less frequently)
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will require less processing time.
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Differences Between the Emulator and the Actual POKEY Chip:
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-----------------------------------------------------------
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The biggest difference between the emulator and the original hardware is
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that the emulator emulates an 'ideal' POKEY chip. All output from the
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emulator is a based on a precise square wave, whereas the output from the
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original chip has decay. Though the output is slightly different, I
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don't believe this difference is easily discernible.
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Another slight difference is the 17-bit/9-bit poly. Since the polynomial
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is large (2^17 bits), I choose to create the sample using a random number
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generator rather than a table. I don't believe this difference is
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significant.
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There are also a few differences which are introduced by aliasing. This is
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a direct result of using an output sampling rate which is not identical to
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the original sound rate. It is most evident with high frequencies.
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A final difference is the lack of support for the High-Pass Filter
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functionality. I plan to add this in a future release if necessary.
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Sample/Test Application:
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------------------------
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The test program I've distributed is a 16-bit DOS application created with
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the Borland 'C' compiler. The only reason I used 16-bit was because I
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already had a set of working SB drivers in 16-bit. Since the test system
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is dedicated to generating sounds, the performance in 16-bit is more than
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adequate.
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POKEY.C
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=======
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The POKEY.C file is the heart of the PokeySound Emulation program.
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Although the routines in the file must work together, no other files are
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modules are required for operation. A header file, 'POKEY.H', has
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been included for use in other modules, and provides the necessary
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function prototypes. I've attempted to make the routines as portable as
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possible, so the file should compile on almost any compiler with little
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or no modification.
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I have made some attempts at optimizing the routines, though I am sure
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more optimization can be done. They are currently only available in 'C'.
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I'll be happy to convert them to assembly language if desired. Please feel
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free to send me e-mail at rfries@tcmail.frco.com.
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The routines are easy to use. Detailed descriptions on the function calls
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are listed below.
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The POKEY.C module can be compiled in a 32-bit or 16-bit environment.
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Since these routines are optimized for 32-bit use, the code will default
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to 32-bit. To compile in 16-bits, use a command line option to define
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the variable COMP16.
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GENERAL OVERVIEW
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----------------
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On start-up of the system, a single call should be made to Pokey_sound_init.
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This routine will prepare the structures for sound output. This routine
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can be called again if necessary during warm-start or other reset.
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Once in the main loop, there are two other functions that will be used.
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Whenever the system needs to write to either the AUDC or AUDF values,
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a call should be made to the Update_pokey_sound routine. This routine will
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take care of updating the internal registers. It will pre-calculate several
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values to help with optimization.
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The only other routine that is called is the Pokey_process function. This
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function will fill a audio buffer with a specified number of bytes. This
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function should be called whenever a new audio buffer is required.
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For best results, I recommend using at least two output buffers. Using this
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scheme, the sound card can be playing one buffer while the system is filling
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the other.
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DETAILED FUNCTION DESCRIPTIONS
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------------------------------
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Pokey_sound_init(uint32 freq17, uint16 playback_freq, uint8 num_pokeys)
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-----------------------------------------------------------------------
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This function initializes the structures used by the PokeySound routines.
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This function takes three parameters: the main clock frequency, the
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playback frequency and the number of POKEY chips to emulate.
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The maximum number of POKEY chips emulated is configured at compile time.
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Though the maximum number of chips can be configured as one, the PokeySound
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1.2 routines are recommended if only a single chip is to be emulated since
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they have will provide better performance.
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NOTE: The following information on the 1.79MHz clock applies only to the
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Atari 800 line of home computers. Many Arcade games also use the
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POKEY chip, but most use a different frequency for the POKEY clock.
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Refer to the schematics for more details.
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The main clock frequency is the frequency of the 1.79MHz source clock.
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To provide exact results, freq17 should be set equal to 1789790 Hz. As an
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alternative, freq17 can be set to an approximate frequency of 1787520 Hz.
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Using this approximate frequency will reduce aliasing and thus produce a
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clearer output signal.
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A constant has been defined for both of these values for your convenience.
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The names are FREQ_17_EXACT and FREQ_17_APPROX.
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The playback frequency is the frequency of the sound playback (the frequency
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used by the sound card). For best results, the playback frequency should
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be an even division of the main clock frequency. Since most of the sounds
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will be generated using the 64kHz clock, I also recommend making the
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playback frequency an even division of the 64kHz clock.
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The 64kHz clock is exactly equal to the main clock divided by 28. For
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the playback frequency, I recommend one of the following values:
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1) FREQ_17_APPROX / (28*1), which is equal to 63840. Of course, most sound
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cards can't reproduce this frequency.
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2) FREQ_17_APPROX / (28*2), which is equal to 31920. All of the newer cards
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will support this frequency.
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3) FREQ_17_APPROX / (28*3), which is equal to 21280. All of the SB
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compatibles should support this frequency.
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4) FREQ_17_APPROX / (28*4), which is equal to 15960. This may be the
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best choice, as it offers good sound reproduction with good performance.
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Of course, these options also assume you are using the approximate
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frequency for the main clock as well. Any of these choices will offer the
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best results when the main 64kHz clock is used, reasonable results when the
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15kHz clock is selected, and marginal results when the 1.79MHz clock is
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selected (the only way to produce good results in all cases is to set the
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playback frequency to 1.79MHz!)
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Feel free to experiment to find other alternatives as well.
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This function has no return value (void).
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Update_pokey_sound (uint16 addr, uint8 val, uint8 chip, uint8 gain)
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-------------------------------------------------------------------
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This function should be called each time an AUDC, AUDF or AUDCTL value
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changes. This function takes four parameters: the address to change,
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the new value, the chip to be updated, and the gain to be used.
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The lower four bits of the address should be one of the following values:
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Addr Description
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------ -----------
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0x00 AUDF1
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0x01 AUDC1
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0x02 AUDF2
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0x03 AUDC2
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0x04 AUDF3
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0x05 AUDC3
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0x06 AUDF4
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0x07 AUDC4
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0x08 AUDCTL
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In order to support multiple POKEY chips, only the lower four bits of
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the address are used. Note that this routine can no longer be called with
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any address as it will affect the operation of the specified chip.
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The routine pre-calculates several values that are needed by the
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processing function. This is done to optimize performance.
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The output will be amplified (multiplied) by the gain (note that POKEY 2.0
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used gain/16). If the output exceeds the maximum value after then gain and
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clipping is enabled, the output will be limited to reduce distortion.
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The best value for the gain depends on the number of POKEYs emulated and
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the maximum volume used. The maximum possible output for each channel is 15,
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making the maximum possible output for a single chip to be 60. Assuming all
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four channels on the chip are used at full volume, a gain of 4 can be used
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without distortion. If 4 POKEY chips are emulated and all 16 channels are
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used at full volume, the gain must be no more than one to prevent distortion.
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Of course, if only a few of the 16 channels are used or not all channels are
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used at full volume, a larger gain can be used.
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To enable clipping, define the logical CLIP before compiling. The default
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mode of operation is no clipping as this provides the best performance. If
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necessary (only required if the gain is too large), remove the comments from
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around the CLIP definition in the POKEY.H file.
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To enable signed output, define the logical SIGNED_SAMPLES before compiling.
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The default is unsigned 8-bit centered around 128. If unsigned samples are
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necessary, remove the comments from around the SIGNED_SAMPLES definition in
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POKEY.H.
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This function has no return value (void).
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Pokey_process (uint8 *buffer, uint16 n)
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---------------------------------------
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This function calculates and fills a buffer with signed or unsigned 8-bit
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mono audio. This function takes two parameters: a pointer to the buffer to
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fill and the size of the buffer (limited to 65535). This function fills the
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buffer based on the requested size and returns. It automatically
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updates the pointers for the next call, so subsequent calls to this function
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will provide a continuous stream of data.
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The size of the buffer that is needed depends on the playback frequency.
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It is best to keep the buffer as small as possible to maximize response time
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to changes in the sound. Of course, the minimum size is dependent on
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system and emulator performance.
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Selecting the correct buffer size is a careful balance. Selecting a buffer
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size that is too small will produce noticeable clicks in the output, though
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selecting a size that is too large will cause a poor response time and
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possible delays in the system when the new buffer is filled.
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This function has no return value (void).
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License Information and Copyright Notice
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========================================
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PokeySound is Copyright(c) 1996-1998 by Ron Fries
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This library is free software; you can redistribute it and/or modify it under
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the terms of version 2 of the GNU Library General Public License as published
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by the Free Software Foundation.
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This library is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more
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details.
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To obtain a copy of the GNU Library General Public License, write to the Free
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Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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Any permitted reproduction of these routines, in whole or in part, must bear
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this legend.
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