Has anybody seen this type of behavior when looking at an audio spectrogram? There are missing chunks of frequency information (black parts = no data).

I understand that compression will remove information, but I've never seen anything like this. In example #1, 5kHz of spectrum (5-10kHz) disappears for 1s.

Recordings are 48kHz, stereo, AAC. I believe they were compressed with faac AAC, 128kbps VBR nominal.

It's subtly noticeable when listening, and occurs on both channels. example

  • 3
    "Has anybody seen this…" No. I don't know what kind of answer you want from us. Compare it to the original recording, see if it was a processing error or existed at source.
    – Tetsujin
    Commented Jul 11, 2023 at 7:42
  • It's probably normal behaviour, I don't work with AAC or any lossy compressed format really, so I don't see it a lot, But as some info is removed on purpose using a psychoacoustic model, I think it's completely "normal". You need to find out what settings were used for the encoding and what affect those settings have.
    – n00dles
    Commented Jul 13, 2023 at 11:23
  • Update: Original recording is clean. When bypassing AAC encode and saving to WAV, no blanking is present in spectrogram. Turns out there was something wrong with the libfaac binary (1.29). Downgraded to 1.28 and the issue disappeared. Being lossy, of course some information is lost -- but no more large chunks of missing data.
    – 77RD
    Commented Jul 13, 2023 at 22:39
  • @77RD You can add that as an answer if you want
    – n00dles
    Commented Jul 16, 2023 at 23:24


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