I’m using v2.0.3 of Audacity, and I’m confused about the “show clipping” results I’m getting.
I have some wav files (and flac and mp3) that are sourced from vinyl. I know that if people aren’t careful when transferring their vinyl it can result in clipping. So I’ve been scanning my files in Audacity, looking for clipping, and some of my files are lit up all over the place in red, indicating clipping. Yet, when I run the same wav files through the software “wave repair”, and select “find clipping in selection”, it either says “no clipping” or the clipping is drastically less. So what do I make of this? Do my files have clipping or not? The same if I convert some of my mp3 or flac files (just for testing purposes because wave repair will only read wave files). There are some that are really bad and even wave repair shows clipping, but most files that are showing clipping in Audacity, even lots of clipping, seem to have zero clipping in Wave Repair. Am I misunderstanding something here?
Does it still do that in Audacity 2.1.0? It might not.
Going into and out of MP3 can cause real clipping. MP3 changes the electrical characteristics of the sound and sometimes, if the wind isn’t at your back, the sound gets louder. So nobody’s shocked about that.
You can’t measure clipping. You can infer clipping. Clipping is overload distortion where the blue waves go higher than the system can handle. The digital system “runs out of numbers.” So a close inspection of the overload point seems to be in order, except a lot of head-banger music is produced where the blue waves are designed to live at the overload point…and there is no distortion. Now what?
So now you do things like count the sequential overload points and if the number goes over a certain limit, you might possibly assume that the system may be in overload/clipping. Note all the slippery words in there.
So that’s the problem. This may work a lot better in Audacity 2.1.0.
Thanks for the reply.
So I downloaded the zip version of 2.1.0, and I get the same results. Files that are showing lots of red lines in Audacity say “no clipping” in wave Repair. The file I’m testing now is a wav file, with no mp3 conversion involved - just a straight wav file of a cleaned vinyl record.
So I don’t know what to make of this - how would I know if I have clipping if the software doesn’t agree. In wave repair’s options for clipping, clipping level is set to 0.0 dB (“soundcard is able to deliver 0dB”). There are 2 other options - “soundcard with analogue clipping” and “overcompressed CDs”. “soundcard with analogue clipping” sets it to -4.0 and “overcompressed CDs” sets it to -0.5. With this file, wave repair shows no clipping if set to “soundcard is able to deliver 0dB” or “overcompressed CDs” but shows hundreds of instances of clipping if set to “soundcard with analogue clipping”. What is the proper setting for what I’m trying to do? And does Audacity have adjustable settings for clipping detection? If so, what should it be set to?
It sounds like it may not be clipped just because it is in the red? So then what is the best way to tell if clipping is an issue with these files?
I don’t know there is a good way. Flirting with clipping/overload is dangerous and complicated. If, for example, you have lots of crisp drums in the work, the effects of clipping may never be heard. The damage overtones and harmonics will be up where only dogs can hear.
We recommend live recording peaks around -6dB or anywhere in the new Audacity sound meter yellow region. Delivery to ACX AudioBooks specifies sound peaks no higher than -3dB. Running any work up against the clip region is not desirable.
Audacity’s clipping indicator is simple. If the peaks hit 0dB or go over 0dB, clipping (red) is indicated. I’d consider it an indicator of potential clipping.
Your analog-to-digital converter (and your digital-to-analog converter) is an integer device and it’s hard-limited to 0dB. If you digitize an LP and clipping is indicated (before you do any processing) it’s very-likely truly-clipped because it’s very unlikely that your analog peaks hit exactly 0dB without “trying” to go over.
If there is clipping on the LP itself it’s virtually impossible to see or detect. The process of cutting and playing back a record means that squared-off flat-topped waves don’t remain flat-topped (although the resulting distortion obviously still remains).
But if you normalize the volume for 0dB peaks, Audacity will indicate clipping wherever the peaks hit 0dB even if there is no true clipping. Most modern CDs are compressed, limited, and normalized, so it’s common for Audacity to show some clipping from a ripped CD.
As Koz says, the process of MP3 compression can boost the peaks. MP3 is lossy compression… The wave shape changes and some peaks get higher and some peaks get lower. If you compress a normalized WAV to MP3, usually some peaks will go over 0dB. MP3 can go over 0dB without clipping, so Audacity may indicate clipping even though the peaks are not actually clipped. Most commercial MP3s will show clipping in Audacity. If you export that MP3 file to WAV, it will truly be clipped at 0dB.
If you have a truly-clipped file and you reduce the volume so that the peaks/clips no longer hit 0dB, Audacity will not indicate clipping although the waveform will obviously still be clipped.
Note that Audacity itself (like most audio editors) is NOT hard-limited to 0dB. If you do something that pushes the peaks over 0dB, such as boosting the volume, or boosting the bass, Audacity will indicate clipping but the waveform won’t actually be clipped… yet. If you export to a regular (integer) WAV file, burn a CD, or send that data to your DAC at full-volume it will be clipped at 0dB.
Some great information here. Thank you very much for all the knowledgeable replies. It sounds like for vinyl rips I find on the 'net, I can’t really assume the files are truly clipped, even though Audacity highlights it as red, since I don’t know what’s been done to the files. With my own vinyl rips, it sounds like the key is to watch the levels when making the recording to be sure not to clip the album to begin with.
One thing I’m still a bit confused over - since both Audacity and Wave Repair are set by default to indicate clipping at 0dB, why then is Audacity showing a wav file (not converted) as clipped in many different spots and yet Wave Repair is showing no clipping. Since they’re both set for 0dB, shouldn’t they be on the same page? Is there a different threshold before Wave Repair indicated clipping?
Perhaps Wave Repair is using an algorithm similar to the “Find Clipping” effect (Audacity Manual) which looks not for single sample values, but for a run of samples.
Since you can’t actually see over 0, a sound value of 0 may or may not be damage. But three successive values at 0 might be damage. The wave would have gone higher had it been allowed to. You descend into fancy English again. Music is unlikely to have three or more successive values at 0.
The behavior of digital clip indicators is a subject for lively discussion.
Even in analog where you would think it’s relatively straightforward. Do you want the indicator at the point the system crashes or as a warning slightly before it crashes?
Interesting. I took that file as an example - the one that Audacity displays lots of red lines in view>show clipping, but wav repair says “no clipping”, and tried analyze>find clipping in Audacity ( thank you - I didn’t even know about that option). Sure enough, it shows no clipping when run through analyze>find clipping.
It must be that there are several single samples that hit 0, but not runs. There’s about 2 minutes that show red lines each time a cymbal is heard in the piece.
You might just want to zoom-in and look at the “waveform”. A clipped analog sine wave looks like [u]this[/u]. But, A digital audio file isn’t a true waveform. It’s a series of samples (or “dots”). Your digital-to-analog converter connects the dots and smooths/filters the waveform, but you can’t see the analog waveform without an oscilloscope.
And of course, real-world audio is not perfect sine waves. But, it would be unusual for a musical instrument to create squared-off sound waves and it would also be unusual for a squared-off sound wave to get recorded and reproduced perfectly squared-off. If you see a squared-off waveform you can be fairly confident that there’s clipping or limiting or something unnatural.
If you want to see what clipping looks like, Amplify for a New Peak Amplitude of +3 or +6dB and click Allow Clipping. Export as 16-bit WAV, open the clipped WAV file and zoom-in on the clips to where you can see the individual samples. If you Amplify again to bring the peaks down by 1dB or so, you’ll still see the squared-off waves, but Audacity won’t show red/clipping.
One thing I’m still a bit confused over - since both Audacity and Wave Repair are set by default to indicate clipping at 0dB, why then is Audacity showing a wav file (not converted) as clipped in many different spots and yet Wave Repair is showing no clipping.
I have Wave Repair, but I’ve never played around with it’s clipping indicator. It’s easy enough to prevent clipping so I don’t worry too much about how clipping indicators work…
When I digitize an LP, I simply scan the recording to find the peak. If it hits 0dB I assume it’s clipped and I start over. (In Audacity the Amplify effect scans the file to find the peak.)
When I’m editing a recording I either scan the file or I normalize to bring the peaks down to 0dB before exporting/saving. That ensures that none of my editing/processing caused clipping (or it ensures that I didn’t make the existing clipping worse).
If I didn’t create the file, there’s nothing I can do about pre-existing clipping and if I don’t hear distortion I don’t worry about it. If I can hear the clipping, there’s still nothing I can do about it. (I’ve tried Audacity’s Clip Fix effect once on a clipped/distorted file and although it made the waveform look better it didn’t make it sound any better.)
If I edit an MP3 or other lossy file* I’ll normalize before exporting/saving to prevent clipping from the boosted peaks caused by MP3 compression. (This may lower the overall average volume by 1 or 2 dB.)
- I try to avoid editing lossy files (when possible) because there is another generation of quality loss when you re-compress to a lossy format. Or, for simple editing tasks I’ll use a lossless MP3 editor.
Clip Fix is another fancy English tool. It uses careful analysis of the waves before and after the damage to try and reconstruct what the wave would have been had there been no damage. It’s makin’ it up.
I do something similar. I have a very high quality turntable, stylus, arm and preamp. I will do a quick analysis of the recording and if any parts of it (other than pops) are too high, I’ll let the vinyl rest a bit and transfer it again at lower volume. After a while I can stop adjusting. There’s only so much volume variation that can be pressed into vinyl and my system has very low noise.