Audio Fading, Mixing and Amplification
Figure 1: Audio Mixing and Amplification
The ADPCM signal and the CD audio left and right channels can be faded out at 2 different rates via register 180Fh; 2.5s or 6s. In normal operation i.e. when not fading is applied, TR6 (A144) is constantly saturated by pulling the base to GND via R33 and this allows C12 to charge to 5V via R39. When fading is enabled, the IC output (which I expect to be an open collector output) to pull up the base to 5V and thus stop saturation. This will have the effect of discharging C12 through R40 in fast fading mode and through R40 and R41 in slow fading mode. The fading mode is controlling TR7: when the IC pulls the base up, it is saturated which makes it short R41 giving the fast fading mode. When the IC output is floating, the base no longer receives current and the transistor is open giving the slow fading mode.
Figure 2: C12 Charging and Discharging
The time constant for the charging of C12 is:
Discharging will happen at 2 different rates based on the state of TR7. TR7 close = fast discharge through R40; TR7 opened = slow discharge through R40 and R41. The time constant for the fast discharging of C12 is:
Figure 3: Fast and Slow Discharge Graphs
https://www.redcrab-software.com/en/Calculator/Electrics/C-Discharge-State
Figure 3: Analogue Switch
IC4053 is a 3 channel 2:1 analogue switch. Pin X, Y and Z are the inputs pins for the stereo signals from the CD-ROM and the ADPCM signal. ADPCM already has 3.9V biasing applied, however the CD-ROM signals are coupled with C19 and C20 to remove any biasing that may be present from the CD-ROM but then get biased to 3.9V via R53 and R52. When FADE_SELn connected to pin A, B and C is high, the signals are passed to the rest of the circuitry as is. When FADE_SELn is low, the signals are fed to the volume control IC instead, MM51131L. Note that Inh is tied to GND to always enable it.
This is the register that controls the fading. From looking at the bios, there is only 1 place where this register is written to with value 0Fh. From looking at IC connections and using existing observations from Charles MacDonald, the following conclusion can be made: Main IC P81 is FADE_SELn and is connected to bit1, Main IC P80 is FADE_SPEED and is connected to bit2, Main IC P79 is FADE_EN and is connected to bit3.
Table 1: Register 180Fh
| bit3:FADE_EN | bit2:FADE_SPEED | bit1:FADE_SELn | Audio State |
|---|---|---|---|
| 0 | x | x | No fading |
| 1 | 0 | 0 | 8 second fading |
| 1 | 0 | 1 | No fading (but C12 is discharging slowly) |
| 1 | 1 | 0 | 2.5 second fading |
| 1 | 1 | 1 | No fading (but C12 is discharging fast) |
Figure 5: M51131L Attenuation against Volume Control Voltage
With control pin (P12) connected to REF (5.2V), Volume 1 is applied to both left and right channels called simultaneous volume mode in datasheet and Volume 2 is actually used for balance control. In this case, Volume 2 is set to Vref/2 which makes L and R volumes equal. Attenuation is maximum when C12 voltage reaches 0.5V. Based on C12 discharge rates the maximum attenuation will be reached in 4.22s with
The ADPCM signal and the CD audio channels are fed to dual op-amp IC115 configured as an adder with negative gain. From looking at the resistor values, we can see that that gain will be less than 1 and that ADPCM signal will be amplified a bit less than the CD audio channels.
Figure 6: IC115A&B ADPCM and CD Audio Mixing
At DC level, we will omit the audio signals and only include the biasing voltages.
Figure 7: IC117A Configured as Adder
This opamp mixes the IFU signals (CD and ADPCM) to send then back as mono to the PC-Engine to be present on the RF connector (which is always mono). At DC level, we will omit the audio signals and only include the biasing voltages.
Figure 8: PCE Audio Signals
PC-Engine console left and right audio signals are fed to the IFU PCB in order to be mixed with the CD and ADPCM signals. When the console is not inserted or not powered, TR21 and TR22 are saturated taking these signals to ground probably to stop picking up noise. C56 and C57 are coupling capacitors removing any DC. Each channel has a passive low-pass filter with
Figure 9: IC116A&B Adders
The purpose of these opamps are to mix the PCE audio signals with the IFU audio signals. IC116A&B are configured as an adder with negative gain. From looking at the resistor values, we can see that that gain will be greater than 1 in such way that it will recover the CD audio amplitude. ADCPM amplitude will be slightly less because of the previous stage. At DC level, we will omit the audio signals and only include the biasing voltages.