SOUNDCHIPS

Here i will mention all important audio chipsets/soundcards in demoscene music history.

PAULA [AMIGA]

The Paula is an audio chipset, that was used across all models of Amiga computers. The historical value is priceless. Paula has four DMA-driven 8-bit PCM sample sound channels. Two sound channels are mixed into the left audio output, and the other two are mixed into the right output, producing stereo audio output. The only supported hardware sample format is signed linear 8-bit two's complement. Each sound channel has an independent frequency and a 6-bit volume control (64 levels). Internally, the audio hardware is implemented by four state machines, each having eight different states.

Additionally the hardware allows one channel in a channel pair to modulate the other channel's period or amplitude. It is rarely used on the Amiga due to both frequency and volume being controllable in better ways, but could be used to achieve different kinds of tremolo and vibrato, and even rudimentary FM synthesis effects.

Audio may be output using two methods. Most often, DMA-driven audio is used. As explained in the discussion of Agnus, memory access is prioritized and one DMA slot per scan line is available for each of the four sound channels. On a regular NTSC or PAL display, DMA audio playback is limited to a maximum output rate of 28867 values per channel (PAL: 28837) per second totaling 57674 (PAL: 57734) values per second on each stereo output. This rate can be increased with the ECS and AGA chipsets by using a video mode with higher horizontal scan rate. Alternately, Paula may signal the CPU to load a new sample into any of the four audio output buffers by generating an interrupt when a new sample is needed. This allows for output rates that exceed 57 kHz per channel and increases the number of possible voices (simultaneous sounds) through software mixing.

The Amiga contains an analog low-pass filter (reconstruction filter) which is external to Paula. The filter is a 12 dB/oct Butterworth low-pass filter at approximately 3.3 kHz. The filter can only be applied globally to all four channels. In models after the Amiga 1000 (excluding the very first revision of the Amiga 500), the brightness of the power LED is used to indicate the status of the filter. The filter is active when the LED is at normal brightness, and deactivated when dimmed (on early Amiga 500 models the LED went completely off). Models released before Amiga 1200 also have a static "tone knob" type low-pass filter that is enabled regardless of the optional "LED filter". This filter is a 6 dB/oct low-pass filter with cutoff frequency at 4.5 or 5 kHz.

A software technique was later developed which can play back 14-bit audio by combining two channels set at different volumes. This results in two 14-bit channels instead of four 8-bit channels. This is achieved by playing the high byte of a 16-bit sample at maximum volume, and the low byte at minimum volume (both ranges overlap, so the low byte needs to be shifted right two bits). The bit shift operation requires a small amount of CPU or blitter overhead, whereas conventional 8-bit playback is almost entirely DMA driven. This technique was incorporated into the retargetable audio subsystem AHI, allowing compatible applications to use this mode transparently.


56K [ATARI FAlCON]

The most advanced Atari machine was based around Motorola DSP56000 (aka 56K). The 56k series was quite popular for those times. And was used not only in Atari, but in a number of computers, including the NeXT and SGI Indigo workstations. Upgraded 56k versions are still used today in audio gear, radars, communications devices (like mobile phones) and various other embedded DSP applications. The 56000 was also used as the basis for the updated 96000, which was not commercially successful. The DSP56000 uses fixed-point arithmetic, with 24-bit program words and 24-bit data words. 24 bits were selected as the basic word length because it gave the system a reasonable number range and precision for processing audio (sound), the 56000's main concern. 24 bits correspond to a large 144dB dynamic range, sufficient in the 1980s when analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) rarely exceeded 20 bits.


GRAVIS ULTRASOUND [PC SOUND CARD]

Before sound cards were a standard part of any home computer, games were played in which the sound was nothing more than a series of beeps and blips that came out of the PC speaker. By the early 90's, a digital sound card had become standard equipment on any computer used even casually for gaming. When the Ultrasound was released in 1992, it was meant to compete head-to-head against the ubiquitous "Sound Blaster" series from Creative Labs. Duleepa Wijayawardhana summed it up best on his blog, dups.ca: "In the days of the 386 and 486, when Windows had yet to supplant the game world as the operating system of choice, there was only one sound card choice if you were a gamer, music-lover and a rebel: The Canadian-produced Gravis Ultrasound."

While the Sound Blaster's FM synthesis produced lackluster sound quality, its strength lay in its compatibility with any game that supported the use of a digital sound card. The GUS used sample-based synthesis, which made the music it produced sound light years ahead of anything the Sound Blaster could output, especially if the user upgraded the onboard memory from 256k to a full 1MB. While FM synthesis artificially approximates the sound of an instrument, sample-based synthesis uses real-world recordings. The GUS would load these instrument patches into its onboard memory in order to play a song. Increasing the amount of RAM on the GUS allowed the card to load additional instrument patches which benefitted music requiring a wide variety of instruments. As a result, some MIDI files could not be played if the card had less than a full megabyte of memory. Later versions of the Ultrasound could accept as much as 8MB of onboard RAM, which was a large amount of memory at that time. The Musical abilities of the GUS made it popular on the demoscene during the 1990's.

To a gamer though, perhaps the best reason to buy an Ultrasound was its ability to simultaneously emulate both a Sound Blaster and either a Roland MT-32 or a Roland Sound Canvas. This meant that, assuming the game supported it, the user could enjoy both digitized sound effects and fill MIDI music. Many game companies started adding native GUS support to their games, and the sound quality was noticeably better when using a GUS than with a Sound Blaster. Unfortunately, in the end the lack of widespread support for the card by the game companies was its undoing. The vast majority of games were not compatible with the card, forcing the user to emulate a Sound Blaster using the "SBOS" software included with the GUS. Additionally, even with the Ultrasound's ability to emulate the Sound Blaster, compatibility problems still existed. In an era when it was already hard enough to get a game running in DOS, the hassle of having to deal with these issues were enough to push the casual gamer away. For those with the patience to deal with it however, the rewards outweighed these problems and the Ultrasound still enjoys a small cult following today. 


   There were many revisions of this wonderful sound card. 
If you are interested read on this article.



Correct me and send your suggestions if i'm wrong or missed something.


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