TV out audio RC filter design

[jameswilddev]

Hello!

I've been experimenting with https://github.com/rossumur/Arduinocade and one of the things I'd like to improve upon is the audio.  I've got a few devices and it works on all of them, but is very quiet.

I've put a pocket DSO on the audio output and it seems to be a little low amplitude (600mV peak to peak) which roughly halves when a cable is connected (a breadboard was attached in both cases).

Was the design here (100K resistor between PWM output and audio RCA jack) intended to form a RC filter with the cable acting as the capacitor? 

If so, could I replace this with something similar to the Pi's RC filter? https://hackaday.com/2018/07/13/behind-the-pin-how-the-raspberry-pi-gets-its-audio/  There might need to be component substitutions as I'm running 5V here rather than the Pi's 3.3V, and the protection diodes can probably be dropped too (though if I do need it, I might want one on the video too).  My system's mono so I'd only need one half of this.

No real EE experience by the way, just poking components mostly.  I would have posted in the beginners' forum but it appears to be having problems right now (502).

Thanks!

[JohnnyMalaria]

My guess is that the Arduinocade is relying on the inherent finite capacitance of the PWM output circuit.

In the Pi case, the RC network is 270R and 33nF. To get the same RC constant with a 100K resistor, the capacitance would need to be 33 x 270/1000000nF = 89pF. A typical TTL pin has a capacitance of about 5pF. So, it's certainly within the realm of possibility that the use of the 100K resistor and the inherent pin capacitance will give a filter with a cut in the high 10s of kHz.

I'd try the Pi version.