Twin Famicom's low-pass filter

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NewRisingSun
Posts: 1510
Joined: Thu May 19, 2005 11:30 am

Twin Famicom's low-pass filter

Post by NewRisingSun »

The Twin Famicom has a low-pass filter that is responsible for its muffled sound. What are its exact characteristics? I have read that the C205 capacitor responsible has a 0.022uF value, but I have not found a value for the respective resistor with which it forms the low-pass filter, and am unable to determine it from inspecting pictures.
lidnariq
Posts: 11430
Joined: Sun Apr 13, 2008 11:12 am

Re: Twin Famicom's low-pass filter

Post by lidnariq »

To answer this question basically involves reverse-engineering the audio path.

But if I were going to take a stab in the dark ... we know that the bare FDS has an extreme lowpass with a corner frequency of 1.75kHz (first order). I would hazard that maybe they just mixed the 2A03 audio in and subjected it to the same lowpass filter.
famiac
Posts: 19
Joined: Mon Dec 26, 2011 5:22 pm

Re: Twin Famicom's low-pass filter

Post by famiac »

@NRS this doesn't address your question directly, but I'm putting the info on here for future reference

Here's a great Japanese resource for twin famicom audio fixes: http://vaot.mydns.jp/fc/tfaudio.htm
The end of this page also provides info: https://etim.net.au/nesrgb/installation-famicomtwin/

Here's a short guide for improving twin famicom audio output.

First off:
Make sure you have the correct values for R103 and R104. There was a mistake in component placement for some consoles.
The correct values are
R104: 1.2 MOhms
R103: 2.2 MOhms


Next off, to increase the volume, the Japanese site mentions two options:
1- Swap R201 and R220
or
2- put a 1kOhm resistor in parallel with R201.
This sets a higher gain for the line audio output

Now, for the LPF involving C205, you have three options:
1- if you did not alter the r201 and r220 configuration, replace C205 with a 270pF cap
2- if you swapped r201 and r220, replace C205 with a 820pF cap
3- if you put a 1kOhm resistor in parallel with R201, replace C205 with a 2700pF cap

Lastly, you can replace C113 on the main board with a 10uF cap for even more relaxed filtering.
The side effect is that some games that play a sound immediately after power-on will be muffled as the capacitor is still charging. This is a minor detail in comparison to the long-run benefits of having better sound during actual gameplay.

I haven't traced the twin famicom audio, but this should be enough information for anyone looking to get good sound out of their twin famicom
NewRisingSun
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Joined: Thu May 19, 2005 11:30 am

Re: Twin Famicom's low-pass filter

Post by NewRisingSun »

Why replace the LPF capacitor with a different one? I just removed it, and it works and sounds nice.
famiac
Posts: 19
Joined: Mon Dec 26, 2011 5:22 pm

Re: Twin Famicom's low-pass filter

Post by famiac »

To raise the corner frequency of the LPF to something more reasonable and still allow for smoothing of undesirable high-frequency noise.

I haven't calculated the new -3dB corner; I took it for granted that it was raised enough. Calculating it should be extremely straightforward if you retrace the circuit, but I haven't yet.

EDIT: In any case, if it sounds nice to you now, then I'd say that's a job well done :D
lidnariq
Posts: 11430
Joined: Sun Apr 13, 2008 11:12 am

Re: Twin Famicom's low-pass filter

Post by lidnariq »

If someone can read the label on C205 I could make an educated guess as to what the original corner frequency is. Maybe. (Maybe additionally a picture of the solder side)

The only possible problem with omitting the lowpass filter altogether is some audio hardware does bad things (e.g. get problematically hot) when fed ultrasonics.
famiac
Posts: 19
Joined: Mon Dec 26, 2011 5:22 pm

Re: Twin Famicom's low-pass filter

Post by famiac »

forum post here claims 0.022uF as the c205 capacitance
lidnariq
Posts: 11430
Joined: Sun Apr 13, 2008 11:12 am

Re: Twin Famicom's low-pass filter

Post by lidnariq »

Well, if the original lowpass was C205 · R201 = 22nF · 8.2kΩ → 880Hz
Switching to 8.2kΩ · 270pF would then be 72kHz

3.3kΩ · 820pF would be 59kHz, and 890Ω(1k//8.2k) · 2.7nF → 66kHz

If these were chosen as the first point at which audio stopped sounding muddy, I'm probably making a bad assumption, because that normally be a corner frequency closer to 10kHz.
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