Loud pops between songs

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analogwalt
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Loud pops between songs

Post by analogwalt » Sun Jan 03, 2021 6:39 am

My apologies if this isn't the best place to ask this. I welcome any assistance in finding a better forum. My question involves low-level technical details, so that's why I decided to ask here.

I'm running Fedora 33 with the Fedora versions of Audacity, Sound Juicer, and RhythmBox. I have ripped several CDs to FLAC using Sound Juicer. I have also used Audacity to record several vinyl LPs to FLAC. When I play these various files back in RhythmBox, there is a nasty pop when transitioning to/from files that were created by Audacity. If I jump around between different FLAC files created from different CDs using Sound Juicer, there is no noise between songs. If I play all the Audacity-created songs (in order) from a given LP, it sounds fine; no loud pops between the Audacity songs. But as soon as there is a transition between a file created by Sound Juicer and a file created by Audacity (or vice versa), there is a horribly loud pop.

I *did* use Audacity's "Normalize" feature to remove the DC level from my LP recordings.

My audio interface hardware is a Focusrite Scarlett 2i2. My PC has an Intel Core i7-3770, 16 GB of RAM, and a mostly-empty 1TB SATA hard drive (spinning magnetic). CPU utilization is typically very low.

It seems to me that there is something different between the files created by Audacity and the files created by Sound Juicer. My guess is that it involves DC levels or something.

I have a degree in ECE, and I write embedded software for a living. I have broad general knowledge of A/D conversion, audio sampling, FLAC, etc., but I lack detailed knowledge of how Audacity, Sound Juicer, RhythmBox, Linux sound drivers, etc. all work down in the nitty-gritty details.

Another datapoint: Before recording with Audacity, I shut down PulseAudio, since it drops samples. After I'm done recording, I exit Audacity. I restart PulseAudio and use RhythmBox to listen to all my FLAC files.

I welcome any thoughts and advice.

Thanks!!!
Walt

kozikowski
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Re: Loud pops between songs

Post by kozikowski » Sun Jan 03, 2021 11:25 am

I *did* use Audacity's "Normalize" feature to remove the DC level from my LP recordings.
That would have been my first guess. DC Pops are a relationship between two different sound recordings. Did you Normalize the other sounds? You may have corrected the recording that wasn't broken.

If it's bad enough, you can see DC offset. When the performance goes quiet, the blue waveforms don't settle to zero. You can cheat a little and Effect > Amplify a fixed amount and see what turns up.

Image

Koz

analogwalt
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Re: Loud pops between songs

Post by analogwalt » Sun Jan 03, 2021 10:40 pm

Thank you for the DC level picture. The way I've been using Audacity is to record one project per vinyl album, pressing the pause button after the first side of the LP, then releasing the pause mode after flipping the LP. I then delete any unwanted portions of the recording (stylus up/down, long lead-in and lead-out, etc.). I then select the entire project (Ctrl-A) and use the Click Removal tool. I then listen to the audio and manually fix any remaining clicks/pops/noise that I find sufficiently objectionable. I use a combination of different techniques depending on the size and severity of the noise: sometimes the Repair tool, sometimes Amplify with about -20 dB, and sometimes deleting a single AC cycle of music (matching up corresponding zero-crossings of the music waveform). Once I'm finally satisfied, I select the entire project (Ctrl-A) and run the Normalize tool. (I leave the "process stereo channels separately" box *unchecked*, since I didn't like what happened to the left/right stereo balance with the checkbox turned on. Could that be my problem??? I will have to experiment...) I then apply labels to the individual songs within the project, and Export Multiple to FLAC.

<shrug>

kozikowski
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Re: Loud pops between songs

Post by kozikowski » Mon Jan 04, 2021 2:58 am

Effect > Normalize (Remove DC) has to be used before you mix and edit. The songs have to be DC-Free before you combine them.

You can get pops of you edit sound wrong.

This is one silly example. Top one is very bad.

Image

There's two distortions. Get the mating waves going the same direction and always cut on zero crossings.

I think there's a setting where you can force Audacity to look for zero crossings, always.

You can also cut by not cutting. you can do a really fast cross-fade. That's how video editors do it. They generally cut on video frame boundaries. None of this fine-tuning down to the waveform business. People who need surgical editing have been known to split the sound track off, edit it in Audacity, and apply it back to the video show. Audacity will not do the remixing.

Koz

kozikowski
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Re: Loud pops between songs

Post by kozikowski » Mon Jan 04, 2021 3:02 am


analogwalt
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Re: Loud pops between songs

Post by analogwalt » Sun Jan 10, 2021 6:08 am

kozikowski wrote:
Mon Jan 04, 2021 2:58 am
Effect > Normalize (Remove DC) has to be used before you mix and edit. The songs have to be DC-Free before you combine them.

Koz
I'm not certain I see how your point is relevant to my situation. I'm only recording one LP per Audacity project. I'm not mixing tracks from separate recordings. I got the idea from the Audacity manual to use the Normalize tool near the end of the overall process. Thus the normalization would be *after* removing egregious clicks and noise that might otherwise have high amplitudes (digital clipping) and would throw off the normalization.

Note that I don't get any loud clicks/pops when playing the entire project in Audacity. After exporting the individual songs to FLAC, I can play the FLAC files in RhythmBox with no clicks or pops between FLAC files from the same LP. I get the loud clicks/pops in *RhythmBox* when there is a transition from a file (MP3 or FLAC) that was created in Audacity, to a file ripped from CD by SoundJuicer (or vice versa). But no clicks/pops between FLAC files from *different* CDs, all ripped by SoundJuicer.

So, it seems to me that SoundJuicer produces FLAC files that are somehow different from the FLAC files produced by Audacity. And somehow when I let RhythmBox play all these files in shuffle mode (or even entire albums sequentially), there are loud clicks/pops when changing between files generated from Audacity vs SoundJuicer.

???
<shrug>

jademan
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Re: Loud pops between songs

Post by jademan » Mon Jan 11, 2021 3:38 pm

analogwalt wrote:
Sun Jan 10, 2021 6:08 am
So, it seems to me that SoundJuicer produces FLAC files that are somehow different from the FLAC files produced by Audacity. And somehow when I let RhythmBox play all these files in shuffle mode (or even entire albums sequentially), there are loud clicks/pops when changing between files generated from Audacity vs SoundJuicer.
Would you like to post sample FLAC files so that we can look at them ourselves? If they are too big for this forum put them on dropbox or some such and post a link.

DVDdoug
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Re: Loud pops between songs

Post by DVDdoug » Mon Jan 11, 2021 5:21 pm

I'm still assuming it's an offset... I don't think there is a "perfect" DC offset algorithm and some algorithms can make it worse if you have an asymmetrical waveform. DC offset is normally the result of a hardware defect so you're more like to see it with digitized LP files than with ripped CDs.

Try a high-pass filter (at 15 or 20Hz if you want to preserve all of the deep bass). That's one of the "Classic Filter" effects in Audacity. DC is zero Hz so any high pass filter will kill it.

A high-pass filter can leave a "glitch" at the beginning of the file so if that happens you'll have to cut it out, or add a short (of long) fade-in, etc.

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