It's for DC bias. They put out a square wave on the 3.5mm sleeve pin (which isn't connected to USB ground) and then the same 50% square wave on the line channel is silence, lower duty cycle goes negative, higher duty cycle goes positive. No AC coupling caps.
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OH, one of those! Yeah, it's trying to be a cheap-ass class D, and those are generally designed to assume they're driving a coil. The 51nb people use them in their audio daughtercards for the X62 and T70.
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People get into trouble because the X62 has a single mono speaker, but the amp is being driven in stereo, and so most builders connect the speaker to left + and right -. Except the amp is not always driving the same squarewave to both channels.....
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"why is my computer speaker squealing all the time?"
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Ha. The datasheet for this one actually lists using L- as common for both L&R in a 3.5mm application, so I assume it's the same 48 kHz square wave on both. Looks like ~393MHz = 13-bit PWM, which is better than I expected TBH.pic.twitter.com/NkbLmUMJkE
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It's kind of amazing how much you can abuse audio and it's still passable.
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Remember that time my FireWire interface went stupid and was zeroing out every other sample? I didn't notice until I tried to play back something I'd recorded on it and 50% of the time it was silence (but other audio mixed in SW worked, as did the level meters). Drove me nuts.
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That definitely would have stumped me for a good long time!
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I once tried to download and use this audio: https://www.youtube.com/watch?v=HlqrwOoLW4E … The catch: The left channel is a perfect inverse of the right one. After downmixing the file was completly silent. That one took some time to figure out
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Audible at least (if you listen to the original)!
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Yeah, really obvious on headphones, and suspicious enough on speakers :-)
12:13 AM - 20 Sep 2018
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