Clipping: How to Recognize It

[Sounder]

I know that clipping results from overdriving a speaker (not enough power when it's needed leading to the amp putting out a major burst of energy that can destroy the speaker). How does one recognize it AND how can one tell if the clipping is in the recording itself or if it is occurring "live" during the listening?

A couple days ago, I got some pipe organ recordings with really deep bass that sometimes cause a scratchy/rattling sound. This occurs even when played at relatively low sound levels.

I don't want to destroy some newly built speakers!

Thanks, very much, everyone.

John

[theophile]

Clipping is a phenomenon that occurs to those who don't use Altecs.(Humour intended)

[Sounder]

Bfish,

Thank you for the very informative description of clipping. Honestly, I had heard about it but never (knowingly, at least) encountered it so I had no idea what to listen for.

I wonder if I could impose on your generosity even more? Could I send you the link to the organ selection for you to evaluate? Obviously, you would recognize clipping instantly and put the question to rest.

Theo,

How right you are. I am using new GPA 604's and older (AlNiCo) University 6303's and fear for the latter.

Thank you, thank you.

John

[Todd W. White]

EXCELLENT reply, bfish!

Exactly what I was thinking (only better!)...

[bfish]

Originally posted by Sounder:
...How does one recognize it AND how can one tell if the clipping is in the recording itself or if it is occurring "live" during the listening?...

<font size="2" face="Verdana, Helvetica, sans-serif">With a dual trace oscilloscope, by comparing the amplifier's input with the output.

Or...turn it down and see if the effect goes away. If not, it's either on the recording, or some other system artifact (which is what your description sounds like).

Amps and preamps clip, speakers don't (they exceed Xmax).

Clipping is what happens when an amp (or preamp) is overdriven, caused by an input signal level that exceeds the amp's maximum sensitivity rating. In the picture below, the blue line represents a clean sine wave. The red line shows what an overdriven amp stage does to it. It amplifies what it can, but it can't do no more. The signal peaks are then "clipped" off.



The flattened peaks of a clipped waveform cause a couple of problems when sent to a speaker (besides being less-than-perfect representations of the source). The flat portions are essentially DC, and since they're at the amp's peak output are strong levels of it...meaning potentially damaging heat for voice coils. Clipped waveforms also generate high levels of harmonics both in the amplifier and the speaker. By carefully setting a system's gain structure, the potential for clipping can be effectively eliminated.

On the other hand, musicians are free to create any tonal qualities they care to, and clipping is often used intentionally for that purpose in instrument amps. Used in moderation in a tube amp, all you hear is a richer harmonic structure. In extremes (or with SS), you get a "fuzzbox" effect.

So....you play a Hendrix album recorded with huge levels of instrumental clipping and harmonic distortion. Your system however, if properly setup and not overdriven, is merely reproducing all that distortion, not adding to it. This is where I feel the term distortion is somewhat inadequate. We go to great lengths to avoid it like the plague in our hifi endeavors, yet much of what we're playing is lousy with it...and we love it. Whether good or bad depends on which side of the recording console you're standing.

The artifacts you're noticing on organ music is more likely the result of low frequencies that your speakers can't properly deal with. A (big) organ's low pedal is 16Hz. At frequencies below the box tuning of a bass reflex system, excursion increases rapidly, quickly becoming non-linear, followed by damage.

A good reference on clipping;

http://sound.westhost.com/clipping.htm

<font color="#FFFFFF" size="1">[ January 08, 2008 03:03 AM: Message edited by: bfish ]</font>

[bowtie427ss]

Here's another oldie but goodie from the JBL site:

www.jblpro.com/pub/technote/lowpower.pdf

[bfish]

Sure John, just post or send the link, and I'll be glad to check it out. If it's an online audio file, I can even load it and look at the waveform directly.

Could you give some detail of what you're hearing in particular? Like is it an intermittent or continuous thing, only hear it in certain passages, etc.

[donaldpatten]

John
What BFISH said is right on.
If you want to Hear clipping , try this:
Connect a tuner, a walkman, cassette, CD player... any audio source with a volume control and a headphone or line out to a microphone input on a R2R, cassettet recorder, computer...
Adjust the volume controls to a very low level. As you increase the source volume you wil reach a point when the mic preamp clipps. As the source volume is further increased the sound will be totally distorted , but not get louder.

Power amps clip when over driven. Most cases we are not aware of the clipping. I have done live concert sound and have experienced preamps , condenser mics and most other electronics clipping. What I did observe on many occasions was power amps in constant clipping. The signal on the speaker line was a series of square waves, that were really clipped sine waves, and yet the sound from the speakers was very good. Go figure.
Don

[Sounder]

I've been investigating this clipping-like phenomenon in my system and found that the MP3 track does NOT distort unless it is played through iTunes.

Here is a summary:
MP3 + iTunes + Airport = distortion in loud, low Hz passages
MP3 + iTunes (on computer) = distortion
MP3 + QuickTime Player + Airport = no problem
MP3 + QuickTime Player (on computer) = no problem
MP3 + QuickTime Player + Airport = no problem

I play a LOT of such tunes on my Altecs via Airport and iTunes, and this is the first problem I have encountered. Perhaps iTunes compresses the files and information is being lost in the low frequencies.

I learned a lot, guys (esp Bfish), so thank you for your help.

John

[bfish]

Hmmm...iClipping...

Sounds like you're iOn the right iTrail.

Could be a software glitch, but more likely your iSettings.

Think of a media player as a preamp, essentially it is, because it controls/modifies the signal before it gets to your amp. If it sends your amp too hot a signal, it'll cause clipping. All those groovy little features like DFX, bass boost, etc are easier to overlook/misuse than if you just had knobs to twiddle on a pre. Analog preamps didn't usually provide features that could get you in near as much trouble as software can. Analog gear doesn't have hidden little controls you forget about or never found, it's all literally up front.

This is something I alluded to in the evaluation/tuning thread. For measurement apps, you've gotta have full control and all the garbage turned off.

A quick look at the iTunes support page shows you're not alone;

http://discussions.apple.com/search....o&q=distortion