openMSX
Public Member Functions | Static Public Attributes | Protected Member Functions | List of all members
openmsx::YM2413Core Class Referenceabstract

Abstract interface for the YM2413 core. More...

#include <YM2413Core.hh>

Inheritance diagram for openmsx::YM2413Core:
Inheritance graph
[legend]

Public Member Functions

virtual ~YM2413Core ()=default
 
virtual void reset ()=0
 Reset this YM2413 core. More...
 
virtual void writePort (bool port, uint8_t value, int offset)=0
 Write to the YM2413 register/data port. More...
 
virtual void pokeReg (uint8_t reg, uint8_t value)=0
 Write to a YM2413 register (for debug). More...
 
virtual uint8_t peekReg (uint8_t reg) const =0
 Read from a YM2413 register (for debug). More...
 
virtual void generateChannels (float *bufs[11], unsigned num)=0
 Generate the sound output. More...
 
virtual float getAmplificationFactor () const =0
 Returns normalization factor. More...
 

Static Public Attributes

static constexpr int CLOCK_FREQ = 3579545
 Input clock frequency. More...
 

Protected Member Functions

 YM2413Core ()=default
 

Detailed Description

Abstract interface for the YM2413 core.

We currently have two concrete implementations:

This interface separates the actual YM2413 emulation from the rest of the (sound) emulation details. This allows to more easily share this implementation between different emulators. It also allows to more easily test the YM2413 emulation in isolation.

There are two main functions in this interface: write to registers and get output samples. All timing information is implicit in the order of the calls to these functions (e.g. write some register, generate 10 output samples, write another register, ...).

Definition at line 26 of file YM2413Core.hh.

Constructor & Destructor Documentation

◆ ~YM2413Core()

virtual openmsx::YM2413Core::~YM2413Core ( )
virtualdefault

◆ YM2413Core()

openmsx::YM2413Core::YM2413Core ( )
protecteddefault

Member Function Documentation

◆ generateChannels()

virtual void openmsx::YM2413Core::generateChannels ( float *  bufs[11],
unsigned  num 
)
pure virtual

Generate the sound output.

Parameters
bufsPointers to output buffers.
numThe number of required output samples.

The requested number of samples must be strictly bigger than zero.

The output of the different channels is put in separate output buffers. This makes it possible to e.g. record individual channels or to pan, mute or adjust volume per channel. The YM2413 can operate in two modes: 9 channels or 6 channels + 5 drum channels. The latter mode requires a total of 11 output buffers. In the first mode, the last two output buffers are filled with silence (this is very efficient, see below). Each output buffer should be big enough to hold at least 'num' number of ints.

The output is not simply stored in the buffer, but added to the existing data in the buffer. So you'll have to zero the content of the buffer before passing it to this function. OTOH if you want to combine (some of) the channels, you can pass the same buffer for all those channels. This approach may have a little overhead when you're interested in all channels, but it is very efficient if you're only interested in the combined channels (the most common case). You can even directly combine this output with other chips with the same native frequency (like Y8950 or YMF262).

When the core detects that some channel is silent, it will assign nullptr to the buffer pointer (so the content of the buffer is left unchanged, but the pointer to that buffer is set to zero). When all the channels you're interested in are silent you can even skip all subsequent audio processing for this channel (e.g. skip resampling to 44kHz). It is very common that some or even all of the channels are silent, so it's definitely worth it to implement this optimization. Also the cores internally try to detect silent channels very early, so an idle YM2413 core generally requires very little emulation time.

◆ getAmplificationFactor()

virtual float openmsx::YM2413Core::getAmplificationFactor ( ) const
pure virtual

Returns normalization factor.

The output of the generateChannels() method should still be amplified (=multiplied) with this factor to get a consistent volume level across the different implementations of the YM2413 core. This allows to internally calculate with native volume levels, and possibly results in slightly simpler and/or faster code. It's very likely that subsequent audio processing steps (like resampler, filters or volume adjustments) must anyway still multiply the output sample values, so this factor can be folded-in for free.

Implemented in openmsx::YM2413OriginalNukeYKT::YM2413, openmsx::YM2413Okazaki::YM2413, openmsx::YM2413NukeYKT::YM2413, and openmsx::YM2413Burczynski::YM2413.

◆ peekReg()

virtual uint8_t openmsx::YM2413Core::peekReg ( uint8_t  reg) const
pure virtual

Read from a YM2413 register (for debug).

Note that the real YM2413 chip doesn't allow to read the registers. This returns the last written value or the default value if this register hasn't been written to since the last reset() call. Reading registers has no influence on the generated sound.

Implemented in openmsx::YM2413OriginalNukeYKT::YM2413, openmsx::YM2413Okazaki::YM2413, openmsx::YM2413NukeYKT::YM2413, and openmsx::YM2413Burczynski::YM2413.

◆ pokeReg()

virtual void openmsx::YM2413Core::pokeReg ( uint8_t  reg,
uint8_t  value 
)
pure virtual

Write to a YM2413 register (for debug).

Because of the timing constraint on writing registers via writePort(), writing registers via that way is not very suited for use in a debugger. Use this method as an alternative.

Implemented in openmsx::YM2413OriginalNukeYKT::YM2413, openmsx::YM2413NukeYKT::YM2413, openmsx::YM2413Burczynski::YM2413, and openmsx::YM2413Okazaki::YM2413.

◆ reset()

virtual void openmsx::YM2413Core::reset ( )
pure virtual

◆ writePort()

virtual void openmsx::YM2413Core::writePort ( bool  port,
uint8_t  value,
int  offset 
)
pure virtual

Write to the YM2413 register/data port.

Writing to a register is a 2-step process: First write the register number to the register port (0), then write the value for that register to the data port (1). There are various timing constraits that must be taken into account (not described here).

We emulate the YM2413 as-if it generates a stream at 49.7kHz and at each step it produces one sample for each of the 9+5 sound channels. Though in reality the YM2413 spreads out the generation of the channels over time. So it's like a stream at 18 x 49.7kHz where the 9+5 channels have a fixed position in the 18 possible slots (so not all 18 slots produce a (non-zero) output).

In other words: we emulate the YM2413 18 steps at a time. Though for accurate emulation we still need to take the exact timing of register writes into account. That's what the 'offset' parameter is for. It must have a value [0..17] and indicates the sub-sample timing of the port-write.

Implemented in openmsx::YM2413Okazaki::YM2413, openmsx::YM2413Burczynski::YM2413, openmsx::YM2413OriginalNukeYKT::YM2413, and openmsx::YM2413NukeYKT::YM2413.

Member Data Documentation

◆ CLOCK_FREQ

constexpr int openmsx::YM2413Core::CLOCK_FREQ = 3579545
staticconstexpr

Input clock frequency.

An output sample is generated every 72 cycles. So the output sample frequency is effectively 49716Hz. If you need the output at e.g. 44100Hz you need to resample the data. Generating the output at the native YM2413 frequency allows for quite some simplifications in the implementation of the cores. It also generally results in better sound quality. At least if the resampler step itself doesn't degrade the quality again (Certainly don't simply skip some of the samples to get to 44kHz, at the very least do linear interpolation. But preferably use a better resample algorithm).

Definition at line 40 of file YM2413Core.hh.

Referenced by openmsx::YM2413Burczynski::YM2413::generateChannels().


The documentation for this class was generated from the following file: