Yamaha OPL

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The OPL (FM Operator Type-L) series is a family of sound chips developed by Yamaha. It consists of low-cost sound chips providing FM synthesis for use in computing, music and video game applications. The OPL series of chips enabled the creation of affordable sound cards for IBM PC compatibles in the late 1980s such as the AdLib and Sound Blaster, effectively becoming a de-facto standard until they were supplanted by "wavetable synthesis" cards in the early-to-mid 1990s. The internal operation of the chips is completely digital. Each FM-tone is generated by a digital oscillator using a form of direct digital synthesis. A low-frequency oscillator and an envelope generator drive an FM operator to produce floating-point output for the DAC. Decapsulation of the chips shows two look-up tables, one for calculating exponents and one for log-sine. This allows the FM operator to calculate its output without any multipliers, using the formula exp ⁡ [ log ⁡ sin ⁡ [ φ 2 + exp ⁡ [ log ⁡ sin ⁡ [ φ 1 ] + A 1 ] ] + A 2 ] {\displaystyle \exp[\log \sin[\varphi _{2}+\exp[\log \sin[\varphi _{1}]+A_{1}]]+A_{2}]} and two 256-entry look-up tables. Both tables are stored as pairs of values rounded to the nearest whole number, with the second value represented as the difference between it and the first value. A quarter of the log-transformed sine waveform is stored as a sampled approximation in a 256-word read-only memory (ROM) table, computed by 256 × − log 2 ⁡ ( sin ⁡ ( ( x + 0.5 ) × π 512 ) ) {\displaystyle 256\times -\log _{2}\left(\sin \left({\frac {(x+0.5)\times \pi }{512}}\right)\right)} for values of 0 to 255.