CD frequency sweep - can hear "birdies". - diyAudio
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Old 19th May 2004, 12:36 PM   #1
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Default CD frequency sweep - can hear "birdies".

First off, I didn't make up the term "birdies". It's ones that radio people use to describe spurious squeaks and tweets in the sound because of RF intermodulation or something...

Anyway, I while ago I was listening to a sinewave sweeping through the range (geek-city or what?) on the car CD player and every now and then I could hear a low level heterodyne whistle that seemed to come and go at regular intervals, i.e every X amount of swept Hertz. Does this indicate some corners cut in the D-A design or filtering? It would be interesting for others to try this too and tell of your results.
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Old 19th May 2004, 12:53 PM   #2
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Birdies within digital signal processing are called tones. One arrangement that is most susceptible to them is the delta-sigma modulator (either in AD or DA converters). They have to be properly dithered in order to avoid these tones.
Your problem might also be some aliasing problem or intermodulation problem (i.e. linearity problem in the anti-aliasing filter).
There might be other possibilities, but this is what came to my mind.

Regards

Charles
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Old 19th May 2004, 01:46 PM   #3
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Sounds like it might be good as a simple test to separate the good, the bad and the ugly at the HiFi shop.
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Old 19th May 2004, 02:15 PM   #4
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Do you have a radar nearby?
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