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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! I'm porting Linux to Apple Silicon Macs at @AsahiLinux. http://patreon.com/marcan  | http://github.com/sponsors/marcan 

Tokyo, Japan
marcan.st
Joined May 2009

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    1. Hector Martin‏ @marcan42 28 Aug 2018
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      Replying to @11rcombs

      The same interface they're being played on.

      1 reply 0 retweets 1 like
    2. Ridley @ a safe distance‏ @11rcombs 28 Aug 2018
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      Replying to @marcan42

      is there an extremely evil resampler or DAC (or its conversion from PCM to delta-sigma) involved? or is there some sort of extremely evil in-band signaling?

      1 reply 0 retweets 1 like
    3. Ridley @ a safe distance‏ @11rcombs 28 Aug 2018
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      Replying to @11rcombs @marcan42

      my first thought was "maybe if on input something upsampled by inserting 0 every other sample, and on output something's downsampling by dropping every other sample, so it works half the time and is completely silent the other half"

      1 reply 1 retweet 9 likes
    4. Ridley @ a safe distance‏ @11rcombs 28 Aug 2018
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      Replying to @11rcombs @marcan42

      which should be too absurd to even contemplate but I dunno, I've seen some extremely bad audio hardware and software

      1 reply 0 retweets 0 likes
    5. Hector Martin‏ @marcan42 28 Aug 2018
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      Replying to @11rcombs

      That is *exactly* what happened. Well done! It was some kind of confusion in the interface (a power cycle fixed it) that caused it to drop every second sample and replace it with a 0. The aliasing wasn't entirely obvious due to the audio I was using.

      2 replies 0 retweets 6 likes
    6. Hector Martin‏ @marcan42 28 Aug 2018
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      Replying to @marcan42 @11rcombs

      I noticed when I turned on spectrogram view for 'B' and saw the mirror image spectrum... which wasn't *completely* implausible given 'B' was chiptune stuff recorded digitally, but bizarre... then I zoomed in to the PCM and saw every other sample was 0.

      1 reply 0 retweets 2 likes
    7. Hector Martin‏ @marcan42 28 Aug 2018
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      Replying to @marcan42 @11rcombs

      It wasn't strictly speaking up/downsampling since everything involved was nominally 48kHz (so I was getting aliased 24kHz audio), but same effect.

      1 reply 0 retweets 1 like
    8. Ridley @ a safe distance‏ @11rcombs 28 Aug 2018
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      Replying to @marcan42

      was I right that the recording had made the same error

      1 reply 0 retweets 0 likes
    9. Hector Martin‏ @marcan42 28 Aug 2018
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      Replying to @11rcombs

      Yeah, that was why 'B' was special like that. 'A' was just some random stuff from another source. And the (digital) level meter of the interface was before the point in the pipeline where the sample dropping happened, so it always showed activity.

      1 reply 0 retweets 1 like
    10. Hector Martin‏ @marcan42 29 Aug 2018
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      Replying to @marcan42 @11rcombs

      This actually got me thinking of the DSP implications. I know how sampling and aliasing work in the frequency axis, but clearly phase matters too. Shifting the sampling phase shifts the phase at which the upper spectrum half folds over? I need to think about this.

      1 reply 0 retweets 1 like
      Hector Martin‏ @marcan42 29 Aug 2018
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      Replying to @marcan42 @11rcombs

      Like, obviously when the downsampling aligns with the nonzero samples you get constructive aliasing interference, and when it aligns with the zeros you get destructive interference, but why?

      12:04 AM - 29 Aug 2018
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        2. Hector Martin‏ @marcan42 29 Aug 2018
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          Replying to @marcan42 @11rcombs

          Paging @xiphmont. Any pointers as to how to explain this time-domain phenomenon in the frequency domain? How does the phase of aliased downsampling affect the phase of the spectral components that get folded together?

          1 reply 0 retweets 1 like
        3. Monty‏ @xiphmont 29 Aug 2018
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          Replying to @marcan42 @11rcombs

          Honestly, never thought about aliasing in detail except how best to avoid it. In the early says of ADC/DAC, the imaging filters purposely allowed some aliasing to get perflectly 'flat' response to nyquist, but that fell out of favor. Possibly due to related question :-)

          1 reply 0 retweets 1 like
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