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I'm analyzing an audio recording made on a newish MacBook Pro in Logic, using the internal mic. Looking at the spectrogram, there's nothing above 11025 Hz, which is disturbing— if it were, say, 15kHz I wouldn't be as worried, but this makes me feel like I'm not getting the sample rate I signed up for at some stage in my ADC.

Any idea what's going on here? Have you seen anything like this? File is definitely 44100 and sounds normal.

Edit:

File was converted (original bit depth/sample rate/format, so should be literally the same file) using Cmd-click->Save As New Audio File(s) in Logic's Arrange window. Looking at the originally-recorded files shows normal content up to the expected 22050Hz.

Submitted a bug to Logic on bugreport.apple.com...

Can anyone reproduce this?

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What program do you use for the spectrogram? – leftaroundabout Nov 21 '11 at 21:21
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Ways the problem might not be in the spectrogram: The microphone itself may not any sensitivity at that frequency (Apple doesn't say), or it might be filtered out to eliminate acoustic or electrical noise from inside the computer or to avoid getting close to the Nyquist frequency. If you play a tone above 11025 Hz through your speakers and record that, do you hear anything in the recording? – Kevin Reid Nov 21 '11 at 22:00
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Well, I posted an answer explaining why this happened, but the answer was deleted by a moderator. I have no idea why this site is stuck in beta. – buildsucceeded Nov 29 '11 at 0:16

1 Answer

up vote 2 down vote

Double check all your settings and make sure that "Sample rate" and "Bandwidth" are not being used interchangeably. I'm thinking of this as an issue because you mention 22050 Hz, which just happens to be 2 x 11025.

If you sample something at 22050 Hz, your maximum usable bandwidth is 11025 Hz.

If you have the opportunity to set the bandwidth, your sampling rate will be roughly twice that.

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