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Forum Index : Microcontroller and PC projects : Low Pass Filter again, a question.
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| Bleep Guru  Joined: 09/01/2022 Location: United KingdomPosts: 509 |
Hi Harm, Ref. your LPF design below. I'm building a portable MMBasic computer and want to have a jack output with sound, I intend to join the L & R channels to give mono and feed to both the L & R connections of the jack, so L & R will be the same mono signal. However due to space and other considerations, I don't want to use 2 LPF, is it reasonable to simply take the 2 PWM outputs, both through 220 Ohm resistors, feeding into one of your filter design and give problem free results? beat frequencies or anything else? I have tried it and it seems to be fine, to my ears, but is there a better way? different resistor values? whatever you and your Spice model suggests? :-) Thanks for any help. Regards, Kevin. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4226 |
Hi Kevin, If anything than use L and R through 470 ohm resistors (not 220 ohm) to the one single filter stage. That will work since the 2 PWM channels work synchronously (they switch simultaneous on one of the edges of the PWM). But I cannot predict what will happen if you feed this mono system with so called "spatial audio". It is used in some 80's music, where L and R are in counter phase. In real stereo this sounds a bit like audio being ultra wide (no mono component, only extreme left and right). In real mono, you would get almost silence. But in this digital mixing, you will still get a mono sound. And maybe that is what you actually want. If you are interested in music playing on this device. For MMbasic sound, this is not straightforward to do (anymore) since Peter took out that functionality. Regards, Volhout PicomiteVGA PETSCII ROBOTS |
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| Bleep Guru Joined: 09/01/2022 Location: United KingdomPosts: 509 |
HI Harm, Which functionality is that, I'm currently able to play compressed WAV music files ok? with Play wav filename, this is on a LCD version, not VGA. Ok great, I'll use 2x 470 Ohm resistors driving one filter, I'm not worried about "spatial stereo" ;-) Regards Kevin. |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6783 |
The function of playing in mono. Using mixing resistors should be fine - the PWM outputs are always a stereo pair. I suppose that ideally you should extract the audio via filters before mixing it, but if you are trying to keep board space down that won't be an option. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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