#include <ExternalFilter.h>

Public Member Functions
int	clock (int input)

	ExternalFilter ()

void	setClockFrequency (double frequency)

void	reset ()

Detailed Description

The audio output stage in a Commodore 64 consists of two STC networks, a low-pass RC filter with 3 dB frequency 16kHz followed by a DC-blocker which acts as a high-pass filter with a cutoff dependent on the attached audio equipment impedance. Here we suppose an impedance of 10kOhm resulting in a 3 dB attenuation at 1.6Hz.

                                9/12V
-----+
audio|       10k                  |
     +---o----R---o--------o-----(K)          +-----
 out |   |        |        |      |           |audio
-----+   R 1k     C 1000   |      |    10 uF  |
         |        |  pF    +-C----o-----C-----+ 10k
                            470   |           |
        GND      GND         pF   R 1K        | amp
         *                   **   |           +-----
 
                                 GND

The STC networks are connected with a BJT based common collector used as a voltage follower (featuring a 2SC1815 NPN transistor).

To operate properly the 6581 audio output needs a pull-down resistor (1KOhm recommended, not needed on 8580) ** The C64c board additionally includes a bootstrap condenser to increase the input impedance of the common collector.

Constructor & Destructor Documentation

◆ ExternalFilter()

reSIDfp::ExternalFilter::ExternalFilter ( )

Constructor.

Member Function Documentation

◆ clock()

RESID_INLINE int reSIDfp::ExternalFilter::clock ( int input )

SID clocking.

Parameters

input input sample, signed 16 bit

Returns: filtered sample, signed 16 bit

◆ reset()

void reSIDfp::ExternalFilter::reset ( )

SID reset.

◆ setClockFrequency()

void reSIDfp::ExternalFilter::setClockFrequency ( double frequency )

Setup of the external filter sampling parameters.