Yamaha YM3812
Encyclopedia
Sound chip
A sound chip is an integrated circuit designed to produce sound . It might be doing this through digital, analog or mixed-mode electronics...
(i.e. integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
) created by Yamaha Corporation in 1985 and famous for its wide use in IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
-based sound card
Sound card
A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...
s such as the AdLib
AdLib
Ad Lib, Inc. was a manufacturer of sound cards and other computer equipment founded by Martin Prevel, a former professor of music and vice-dean of the music department at the Université Laval...
and Sound Blaster
Sound Blaster
The Sound Blaster family of sound cards was the de facto standard for consumer audio on the IBM PC compatible system platform, until the widespread transition to Microsoft Windows 95, which standardized the programming interface at application level , and the evolution in PC design led to onboard...
.
It is backwards compatible with the OPL aka YM3526
Yamaha YM3526
The YM3526, aka OPL , is a sound chip developed by Yamaha as a low-cost nine channel, two operator FM synthesis chip. It was notably used in a Commodore 64 expansion, the Sound Expander, as well as several Arcade games, such as Bubble Bobble.A very closely related chip is the Y8950, or MSX-AUDIO,...
, to which it is very similar – in fact, it only adds 3 new waveforms. An upgraded version of the OPL2, the OPL3 aka YMF262
Yamaha YMF262
The Yamaha YMF262, also known as the OPL3 , is an FM synthesis sound chip. It is an improved version of the Yamaha YM3812 , adding the following features:*twice as many channels...
, was also popular in later sound cards such as the Soundblaster 16. Another related chip is the YM2413
Yamaha YM2413
The YM2413, aka OPLL, is a cost-reduced FM synthesis sound chip manufactured by Yamaha Corporation and based on their YM3812 . To make the chip cheaper to manufacture, many of the internal registers were removed...
(OPLL), which is a cut down version.
The circuit has 244 different write-only registers. It can produce 9 channels of sound, each made of two oscillators or 6 channels with 5 percussion instruments available. Each oscillator can produce sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
s which may also be modified into 3 other waveforms – the negative part of the sine can be muted or inverted, and pseudo sawtooth waves (1/4 sine waves upward only with silent sections in between) can also be produced. This odd way of producing waveforms give the YM3812 a characteristic sound. Each wave generator has its own ADSR envelope generator. Its main method of synthesis is Frequency modulation synthesis
Frequency modulation synthesis
A 220 Hz carrier tone modulated by a 440 Hz modulating tone with various choices of modulation index, β. The time domain signals are illustrated above, and the corresponding spectra are shown below ....
, done by phase modulation of the phase of one of the channel's oscillators by the output of the other.
For the whole channel:
- Main frequency (10 bits)
- Octave (3 bits)
- Note on/off
- Synthesis mode (FM or just additive)
- Feedback (0–7, the modulator modulating itself)
For each one of the 2 oscillators:
- Frequency multiply (can be set to 1/2, 1 to 10, 12 or 15)
- Waveform (Sine, half-sine, absolute-sine, quarter-sine)
- Volume (0–63, logarithmic)
- Attack, Decay, Sustain, Release (4 bits each, logarithmic)
- Tremolo (On or off)
- Vibrato (On or off)
- Sustain (On or off)
- Envelope scaling per key (On or off)
- Volume scaling per key (0–3)
There are also a few parameters that can be set for the whole chip:
- Vibrato depth
- Tremolo depth
- Percussion mode (uses 3 channels to provide 5 percussion sounds)
- Composite sine mode (very rarely used; doesn't work on the OPL3)
Internal operation
The internal operation of the chip is completely digital. Each FM-tone is generated by a digital oscillator using a form of direct digital synthesisDirect digital synthesis
Direct Digital Synthesizer is a type of frequency synthesizer used for creating arbitrary waveforms from a single, fixed-frequency reference clock...
. A quarter of the log-transformed sine waveform is stored as a sampled approximation in a 256-word read-only memory
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
(ROM) table. The rest of the sine-waveform is extrapolated via its property of symmetry. Scaling the output of an oscillator to a wanted volume would normally be done by multiplication, but the YM3812 avoids multiplications by operating on log-transformed
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
signals, which reduces multiplications into computationally cheaper additions. Another 256-word ROM stores the exponential function
Exponential function
In mathematics, the exponential function is the function ex, where e is the number such that the function ex is its own derivative. The exponential function is used to model a relationship in which a constant change in the independent variable gives the same proportional change In mathematics,...
as a lookup table, used to convert the logarithmic scale signal back to linear scale when required, as the final stage where the oscillator-outputs are summed together (just prior to the DAC-output bus.) The YM3812's output, a sequence of floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
numbers clocked at a sampling frequency of approximately 49720 Hz, is sent to a separate digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
(DAC) chip, the YM3014B
Yamaha YM3014B
thumb|YM3014BThe YM3014B, aka DAC-SS, is a floating point digital-to-analog converter chip developed by Yamaha. It generates one analog output from a single serial input, the sound data containing a 10-bit mantissa and 3-bit exponent. It was featured alongside many Yamaha sound synthesis chips,...
.