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Il MOS Technology SID (Sound Interface Device) 6581/8580 era il chip sonoro dei computer della Commodore: CBM-II, Commodore 64 e Commodore 128. Era uno dei primi chip sonori del suo genere ad essere incluso nei computer, prima dell'epoca della rivoluzione del suono digitale.

Assieme al VIC-II, il SID fu responsabile del successo del C64.

Il SID ha il brevetto americano numero , archiviato nel 27 febbraio del 1983 e approvato nel 7 luglio 1987. Il brevetto è scaduto il 7 luglio 2004.

Progettazione

The SID was devised by engineer Robert "Bob" Yannes, who later founded the Ensoniq digital synthesizer company. Yannes headed a team that included Yannes, two technicians and a CAD operator running Applicon (nowadays a part of the UGS Corp.), who designed and completed the chip in five months' time in the latter half of 1981. Yannes was inspired by previous work in the synthesizer industry and was not impressed by the current state of computer sound chips. Instead, he wanted a high-quality instrument chip, which is the reason to why the SID has features like the envelope generator, previously not found in home computer sound chips.

Emphasis during chip design was on high-precision frequency control, and the SID was originally designed to have 32 independent voices, sharing a common oscillator. However these features could not be finished in time, so instead the mask work for a certain working oscillator was simply replicated three times across the chip surface, creating three voices with a unique oscillator for each voice. Another feature that was not incorporated in the final design was a frequency look-up table for the most common musical notes, a feature that was dropped because of space limitations. The support for an audio input pin was a feature Yannes added without asking, even though this has no practical use in a computer. The masks were produced in 7-micrometer technology in order to gain a high yield: the current state-of-the-art at the time was 6-micrometer technologies.

The chip, like the first product using it, the Commodore 64 was finished in time for the Consumer Electronics Show in the first weekend of January 1982. Even though Yannes was partly displeased with the result, his colleague Charles Winterble said: "This thing is already 10 times better than anything out there and 20 times better than it needs to be".

The specifications for the chip were not used as a blueprint. Rather, they were written as the development work progressed, and not all planned features made it into the final product. Yannes claims he had a feature-list of which three fourths made it into the final design. This is the reason why some of the specifications for the first version (6581) were accidentally incorrect. The later revision (8580) was revised to match the specification. For example, the 8580 can make a logical AND between two waveforms, something that the 6581 could never handle. Another feature that differs between the two revisions is the filter: the 6581 version is far away from the specification.

Caratteristiche

  • tre canali audio indipendenti e programmabili, con un suono ad 8 ottave e un limite da 16 a 4000 Hz;
  • quattro diverse onde per gli oscillatori audio (sega, triangolo, quadrato e rumore)
  • tre filtri sonori (low-pass, high-pass e band-pass)
  • sincronizzazione degli oscillatori
  • due convertitori a 8 bit dall'analogo al digitale (utilizzato per i controller da gioco)
  • input audio esterno
  • generatore di numeri casuali

Technical details

The SID is a mixed-mode integrated circuit, featuring both digital and analog circuitry. All control ports are digital, but the output ports are analog. The SID features three-voice synthesis, where each voice may use one of four different waveforms: square wave (with variable pulse width), triangle wave, sawtooth wave and a pseudo-random (but not white noise) wave. Each voice may be ring modulated with one of the other waves, i.e. the frequency spectrum is multiplied and output. The ring modulation, filter, and programming techniques for switching between different waveforms at high speed make up the characteristic sound of the SID.

Each voice may be passed through a common, digitally controlled analog filter with variable cut-off frequency and resonance, which is constructed with the aid of capacitors external to the circuit. An external audio in port enables external audio to be passed through the filter.

The 6581 had a quirk that caused changes in volume level on a channel to result in a slight "pop". Eventually this bug was found to be useful for producing a fourth voice of percussive sounds, digitized speech, and even short excerpts of digital recordings (the Commodore 64 lacked the memory capacity for full-length digital recordings of songs). Unfortunately, this "defect" was partially corrected in the 8580 used in the Commodore 64C and the Commodore 128. This made digitized sound (samples) very quiet. Fortunately, a hardware modification could be made to the 8580 to restore the volume to 6581 levels.

The 6581 and 8580 differ from each other in several ways. The original 6581 was manufactured using the older NMOS process, which used 12V DC to operate. The 8580 was made using the HMOS-II process, which required less power (9V DC), and therefore made the IC run cooler. The 8580 was thus far more durable than the 6581. Additionally, a better separation between the analog and the digital circuits made the 8580 chip's output less noisy and distorted.

A HMOS-II version of the 6581 was produced, the 6582. It was never shipped in new Commodore 64s.

The original manual for the SID mentions that if several waveforms are enabled at the same time, the result will be a logical AND between them, but only the 8580 actually has this functionality: on the 6581 some waveform combinations will only yield silence or be close to inaudible, depending on the chip revision. The filter is also different between the two models, with the 8580 being closer to the actual specification.

Despite its documented shortcomings, many SID musicians prefer the flawed 6581 chip over the corrected 8580 chip. The main reason for this is that the filter produces strong distortion that is sometimes used to produce simulation of instruments such as a distorted electric guitar. Also, the highpass component of the filter was mixed in 3 dB attenuated compared to the other outputs, making the sound more bassy. In addition to nonlinearities in filter, the D/A circuitry used in the waveform generators produced yet more additional distortion that made its sound softer and smoother.

Revisioni

Il chip 6581 R1 non fu mai venduto nel mercato. Le revisioni del chip furono:

  • 6581 R2 - detto anche come "MOS" "6581"
  • 6581 R3 - "6581 R3" o "6581 CBM"
  • 6581 R4 AR - "6581 R4 AR"
  • 6582 A - prodotto nel 1992
  • 8580 R5

Suono nei giochi

Il Commodore 64 aveva un protocollo di trasferimento dati dalle cassette e dai dischi lento, impiegava minuti per caricare 64K nella memoria. Di conseguenza, durante il caricamento molti giochi erano soliti produrre musica attraverso un programma, detto "loader".

I più noti compositori di queste musiche erano Martin Galway, noto per molti giochi, come Wizball. Rob Hubbard ha prodotto le musiche per ACE 2, Delta, International Karate, IK+ e Monty on the Run. Altri musicisti noti erano Jeroen Tel (Cybernoid e Myth) e Chris Hülsbeck.

Sviluppo moderno

  • Nel 1997 fu rilasciato uno strumento musicale elettrico che utilizzava il chip SID. Era chiamato SidStation e fu prodotto dall'azienda svedese Elektron. Nel 2004, la Elektron rilasciò il sequenziatore musicale Monomachine con tastiera opzionale. Il Monomachine aveva diversi sintentizzatori, compreso un oscillatore basato sul 6581, che veniva emulato con un chip DSP.
  • Negli anni '90 fu venduta una scheda sonora per PC, detta HardSID. La scheda utilizzava da 1 a 4 chip SID.
  • Nel giugno del 1998 fu rilasciato un emulatore SID: reSID. L'emulatore, interamente software, è disponibile sotto forma di codice sorgente C++ e rilasciato con licenza GPL dall'autore, Dag Lem.
  • Il MIDIbox SID è un sintentizzatore MIDI che poteva contenere fino a 4 chip SID. Il controllo del sintetizzatore era eseguito via software o via panello di controllo.
  • Il progetto PhoenixSID 65X81 (2006) ha come scopo il tentativo di creare un chip SID utilizzando hardware moderno utilizzando chip FPGA, progetti originari e confronti con i veri chip SID.

Note

Un file con estensione .SID è un file sonoro che contiene non solo le tracce sonore, ma anche codice macchina per il 6502 necessario ad eseguire la musica sul SID. La collezione The High Voltage SID Collection contiene più di 30 mila musiche. I file SID hanno il tipo MIME: audio/prs.sid

Riferimenti

  • Appendice O, "6581 Sound Interface Device (SID) Chip Specifications", tratto da Commodore 64 Programmer's Reference Guide.
  • Bagnall, Brian. On The Edge: The Spectacular Rise and Fall of Commodore, pp.231–238. ISBN 0973864907.

Vedere anche

Collegamenti esterni

Sound Interface Device | MOS Technologies SID | MOS Technology SID | MOS Technology SID | MOS 6581 | SID (microprocesseur) | MOS Technology SID | SID | MOS Technology SID | SID

 

This article is licensed under the GNU Free Documentation License. It uses material from the "MOS Technology SID".

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