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I have an mp3 128 Kbps music file with particular parts where a series of single note only on a cello is played initiated with a bell click. Less than a second after a bell click and cello note playing starts a very faint Morse code like series of bursts in 2 - 5 kHz range for a couple of seconds then fades away.

I was able to localize their frequency range and duration with iZotope RX on a time-frequency spectrum.

They happen only in the parts of a cello note playing after a bell click in some parts even fainter than in the others.

Could they be attributed to errors in mp3 compression or to other factors?

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    Can you post a sample of the audio? – AJ Henderson Feb 8 '14 at 18:40
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    It would help if you attached the file. – Sergey Stadnik Feb 11 '14 at 0:46
  • Could it be just slight distortion from overdriving the channel? The bell could "trigger" the clipping. – Tobias Schmidt Feb 12 '14 at 22:57
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Yes - most definitely this is to do with mp3 compression. Strings in particular require a much higher bitrate to be reproduced through mp3 without artifacts. You're not going to get much mileage out of a strings recording under 192kb/s. Adding a bell is just asking for trouble. The way that the mp3 encoding works will mean that the available bit rate is taken up encoding the bell sounds which are of a particular set of harmonics while the remaining bits in the stream probably aren't enough to handle the cello - which has an immensely complex harmonic structure. This sounds very much like a test case recording. Is there a copy of the recording available?

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"Morse code like series of bursts in 2-5kHz range for a couple of seconds" very much sounds like RF interference from a mobile phone (those tend to occur before a call/SMS or when doing a call, and in sparse patterns in between). Now you associate this sound not with just a single passage but rather a recurring combination of elements. That could be

a) because a sound sample is being used repeatedly b) a mobile phone (ring tone?) is involved in the sound production. This would be rather hard to time so it's unlikely unless we are talking about a combination of a) and b): a ring tone sampling being used.

Reuse of a sample disturbed by RF interference sounds to me more plausible than compression artifacts of similar (rather than identical) music resulting in an audibly unique pattern like the one you describe.

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