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Old 25th August 2004, 03:33 PM   #1
hacknet is offline hacknet  Singapore
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Default awful output waveforms

i scoped my philips cd634`s outputs and to my disgust. this is what i got.

i got square waves from NCH tone gen. burn them into a cd and ran the in my cdp.

i`ve decoupled all the 5v supplies to the decoder and the servo chips, changed and biased the opamps.

what can i do to improve the situation?
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Old 25th August 2004, 03:39 PM   #2
hacknet is offline hacknet  Singapore
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100hz
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Old 25th August 2004, 03:42 PM   #3
hacknet is offline hacknet  Singapore
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1khz
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Old 25th August 2004, 03:46 PM   #4
hacknet is offline hacknet  Singapore
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i can`t seem to post the 15khz trace.

it basially looks more like a sin wave than a square. whats going wrong?
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Old 25th August 2004, 04:10 PM   #5
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Why do you think something is wrong?

Cheers, John
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Old 25th August 2004, 04:37 PM   #6
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Its caused by the filters on the output of the DAC. Almost every cd player will behave that way. Nothing to worry about.
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Old 25th August 2004, 04:59 PM   #7
MikeB is offline MikeB  Germany
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Yes the result is completely correct !
The DAC needs a lowpass that cuts every frequency above Samplerate/3. (eliminate aliasingdistortion)
This means all frequencies above ~15khz are cut off. This results
in a near sinewave with your 15khz squarewave.
The fact that the squarewaves look so "disgusting", shows that the
filter does a perfect job in cutting off the high freqs, and not only
smooth the signal.

Mike
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Old 25th August 2004, 10:49 PM   #8
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Quote:
what can i do to improve the situation?
Stop listening to CDs. This Gibbs Phenomenon ringing is an artifact of the excessively narrow transition band from non-minimum phase brick wall filtering which is widely acknowledged to add coloration to the high frequencies.
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Old 25th August 2004, 11:17 PM   #9
Gyula is offline Gyula  Hungary
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The ringins are from an interpolation filter and not from a DAC. The interpolation filter reconstructs the original waveform and it has only sinuses and cosinuses () from a few Hz to 22.05 kHz to do this. So the edges containing upper frequencies will be distorted.
Guys what you talking about is a kind of distortion like round off the heel of trailing edges (like a passive integrator with square-wave). Just see the attached picture! This is the output waveform of a TAS5012 drived output filter. The TAS working with 8x ovesampling (interpolating, and resampling with 8x Fs(in)), the sampled signal is an 1 kHz square-wave. The TAS5012 doesn't contain a DAC.

Good Luck!

Gyula
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Old 26th August 2004, 02:12 AM   #10
Wodgy is offline Wodgy  United States
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As a couple others have said, the output you're seeing is perfect. CDs are a band-limited medium, and all band-limited media are subject to the Gibbs phenomenon that you're seeing.

Any introductory signal processing textbook will explain this to you. Alternatively, any university-level mathematics text with a decent explanation of Fourier analysis will also explain this.

You can design a DAC that doesn't have the Gibbs "handles", but such a DAC will actually be less accurate with respect to what is actually recorded on the CD. Filterless DACs have this property. They look intuitively nice in the time-domain on an oscilloscope to people who don't understand signal processing, but they look bad in the frequency domain.

Signal processing is an interesting field. It's worth learning more about it.
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