libsidplayfp  1.8.3
Public Member Functions | List of all members
reSIDfp::Filter6581 Class Reference

#include <Filter6581.h>

Inheritance diagram for reSIDfp::Filter6581:
reSIDfp::Filter

Public Member Functions

int clock (int voice1, int voice2, int voice3)
 
void input (int sample)
 
void updatedCenterFrequency ()
 
void updatedResonance ()
 
void updatedMixing ()
 
void setFilterCurve (double curvePosition)
 
- Public Member Functions inherited from reSIDfp::Filter
void enable (bool enable)
 
void reset ()
 
void writeFC_LO (unsigned char fc_lo)
 
void writeFC_HI (unsigned char fc_hi)
 
void writeRES_FILT (unsigned char res_filt)
 
void writeMODE_VOL (unsigned char mode_vol)
 

Additional Inherited Members

- Protected Attributes inherited from reSIDfp::Filter
unsigned int fc
 Filter cutoff frequency.
 
unsigned char res
 Filter resonance.
 
unsigned char vol
 Current volume.
 
bool filt1
 Routing to filter or outside filter.
 
bool filt2
 
bool filt3
 
bool filtE
 
bool voice3off
 Switch voice 3 off.
 
bool hp
 Highpass, bandpass, and lowpass filter modes.
 
bool bp
 
bool lp
 

Detailed Description

The SID filter is modeled with a two-integrator-loop biquadratic filter, which has been confirmed by Bob Yannes to be the actual circuit used in the SID chip.

Measurements show that excellent emulation of the SID filter is achieved, except when high resonance is combined with high sustain levels. In this case the SID op-amps are performing less than ideally and are causing some peculiar behavior of the SID filter. This however seems to have more effect on the overall amplitude than on the color of the sound.

The theory for the filter circuit can be found in "Microelectric Circuits" by Adel S. Sedra and Kenneth C. Smith. The circuit is modeled based on the explanation found there except that an additional inverter is used in the feedback from the bandpass output, allowing the summer op-amp to operate in single-ended mode. This yields filter outputs with levels independent of Q, which corresponds with the results obtained from a real SID.

We have been able to model the summer and the two integrators of the circuit to form components of an IIR filter. Vhp is the output of the summer, Vbp is the output of the first integrator, and Vlp is the output of the second integrator in the filter circuit.

According to Bob Yannes, the active stages of the SID filter are not really op-amps. Rather, simple NMOS inverters are used. By biasing an inverter into its region of quasi-linear operation using a feedback resistor from input to output, a MOS inverter can be made to act like an op-amp for small signals centered around the switching threshold.

In 2008, Michael Huth facilitated closer investigation of the SID 6581 filter circuit by publishing high quality microscope photographs of the die. Tommi Lempinen has done an impressive work on re-vectorizing and annotating the die photographs, substantially simplifying further analysis of the filter circuit.

The filter schematics below are reverse engineered from these re-vectorized and annotated die photographs. While the filter first depicted in reSID 0.9 is a correct model of the basic filter, the schematics are now completed with the audio mixer and output stage, including details on intended relative resistor values. Also included are schematics for the NMOS FET voltage controlled resistors (VCRs) used to control cutoff frequency, the DAC which controls the VCRs, the NMOS op-amps, and the output buffer.

SID filter / mixer / output

* ~~~

| | | –1R1– -- D7 | | —R1– | | | | | | |–2R1– --| D6 | | ---------—<A]--—| | $17 | | | |–4R1– --| D5 1=open | (3.5R1) | | | | | | | –8R1– --| D4 | (7.0R1) | | | | $17 | | (CAP2B) | (CAP1B) | 0=to mixer | –R8– —R8– —C—| —C—| 1=to filter | | | | | | | | ---—R8–|--—[A>–|–Rw--—[A>–|–Rw--—[A>–| ve (EXT IN) | | | | D3 \ ------------—R8–| | | (CAP2A) | (CAP1A) | v3 | | vhp | vbp | vlp D2 | \ --------—R8–| --— | | | | v2 | | | | D1 | | \ ----—R8–| | -------------— | | | | v1 | | | | D0 | | | \ —R8– | | ------------------------— | | | | | | | R6 R6 R6 R6 R6 R6 R6 | | | | $18 | | | $18 | \ | | D7: 1=open \ \ \ D6 - D4: 0=open | | | | | | | ------------------------------— 12V | | D3 –/ –1R2– | | —R8– | | —R2– | | | | D2 |–/ –2R2–| | | ||– ---—[A>------—| |--—[A>--—|| D1 |–/ –4R2–| (4.25R2) ||– $18 | | | 0=open D0 –/ –8R2– (8.75R2) |

                                      vo (AUDIO
                                          OUT)

v1 - voice 1 v2 - voice 2 v3 - voice 3 ve - ext in vhp - highpass output vbp - bandpass output vlp - lowpass output vo - audio out [A> - single ended inverting op-amp (self-biased NMOS inverter) Rn - "resistors", implemented with custom NMOS FETs Rw - cutoff frequency resistor (VCR) C - capacitor

NMOS FET voltage controlled resistor (VCR)

Filter frequency cutoff DAC

"Op-amp" (self-biased NMOS inverter)

vi - input vo - output

Output buffer (NMOS voltage follower)

Member Function Documentation

RESID_INLINE int reSIDfp::Filter6581::clock ( int  v1,
int  v2,
int  v3 
)
virtual

SID clocking - 1 cycle

Parameters
v1voice 1 in
v2voice 2 in
v3voice 3 in
Returns
filtered output

Implements reSIDfp::Filter.

void reSIDfp::Filter6581::setFilterCurve ( double  curvePosition)

Set filter curve type based on single parameter.

Parameters
curvePosition0 .. 1, where 0 sets center frequency high ("light") and 1 sets it low ("dark"), default is 0.5
void reSIDfp::Filter6581::updatedCenterFrequency ( )
virtual

Set filter cutoff frequency.

Implements reSIDfp::Filter.

void reSIDfp::Filter6581::updatedMixing ( )
virtual

Mixing configuration modified (offsets change)

Implements reSIDfp::Filter.

void reSIDfp::Filter6581::updatedResonance ( )
inlinevirtual

Set filter resonance.

In the MOS 6581, 1/Q is controlled linearly by res.

Implements reSIDfp::Filter.


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