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== Technical details ==
The Ricoh 2A03<ref name="visual6502">{{cite web|url=http://visual6502.org/images/pages/Nintendo_RP2A_die_shots.html|title=Nintendo RP2A03 and RP2A07 - an high quality picture of the die|last=Sattler|first=Christian|website=visual6502.org|access-date=2020-08-12}}.</ref> contained a [[second source]]d [[MOS Technology 6502]] core, modified to disable the 6502's [[binary-coded decimal]] mode (possibly to avoid a MOS Technology patent<ref>[https://patents.google.com/patent/US3991307 Patent US3991307]</ref>). It also integrated a [[programmable sound generator]] (also known as APU, featuring twenty two [[memory-mapped I/O]] registers),<ref>{{Cite web|url=http://www.atkinsoft.com/soundgenerators.html|title=Sound generators of the 1980s home computers|website=www.atkinsoft.com|access-date=2019-12-02}}</ref> rudimentary [[Direct memory access|DMA]], and game controller polling.<ref name="2A03techref">{{cite web|url=https://www.nesdev.com/2A03%20technical%20reference.txt|title=2A03 technical reference|last=Taylor|first=Brad|date=April 23, 2004|website=NesDev.com|archive-url=https://web.archive.org/web/20181017163239/https://www.nesdev.com/2A03%20technical%20reference.txt|archive-date=October 17, 2018|url-status=live|access-date=June 6, 2008}}</ref>
== Sound Hardware ==
The Ricoh 2A03's sound hardware has 5 channels, separated into two APUs (Audio Processing Unit). The first APU contains two general purpose pulse channels with 4 duty cycles, and the second APU contains a Triangle wave generator, a [[Linear-feedback shift register|LFSR]]-based Noise generator, and one Delta-Modulated PCM (DPCM) channel. While a majority of the NES library used only 4 channels, more games later into the NES's life span used the 5th DPCM channel, as memory mappers became more common and prices of ROM chips became cheaper. For example, [[Super Mario Bros. 3]] uses it for simple drum sounds, while other games like [[Journey to Silius]] use it for sampled basslines. An interesting quirk of the DPCM channel is that the bit order is reversed compared to what is normally expected for input, as a result, many games often ignore this quirk, which results in distorted sounding samples.<ref>{{cite web |url=https://tcrf.net/Ufouria:_The_Saga#Sound_Design_Quirks |website=The Cutting Room Floor |access-date=9 October 2021}}</ref>
The sound output by the Ricoh 2A03 is present on two pins, which are then mixed using a non-linear mixing circuit. On Famicom and Dendy systems, this is then output to the cartridge slot, where an expansion sound chip may add its own mixed sound into the resulting output, which is then fed back into the console. International NES systems instead output the 2A03's sound directly to the TV, but expansion sound functionality can be restored by a simple pin mod on the expansion port, allowing the internal mixer to add the cartridge's expansion sound output from the expansion port.<ref>{{cite web |last1=Burke |first1=Kevin |title=NES Expanded Audio: 100k Pot Mod |url=https://curriculumcrasher.com/2017/01/24/nes-expanded-audio-100k-pot-mod/ |website=The Curriculum Crasher |access-date=9 October 2021}}</ref>
== Regional variations ==
PAL versions of the NES (sold in Europe, Asia, and [[Video gaming in Australia|Australia]]) used the '''Ricoh 2A07''' or '''RP2A07''' processor, which was identical to the 2A03 except for a different [[Frequency divider|clock divider]] and the 50 [[Hertz|Hz]] [[Refresh rate|vertical refresh rate]] used in the PAL television standard, with the exception of the 16 hard-coded sampling rates for the [[Delta modulation]]-encoded sample playback unit. The changes were made in order for sampled sounds to maintain the original pitch of the RP2A03 with the new clock divider. Most developers however, usually lacked the resources to properly convert their games to the 2A07, leading to most games often sounding out of tune or off-tempo.
==See also==
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