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FS#8106 - Clipping/clicking with Crossfeed+Dithering

Attached to Project: Rockbox
Opened by Philip Barton (audiophyl) - Wednesday, 07 November 2007, 07:36 GMT
Last edited by Thom Johansen (preglow) - Tuesday, 04 December 2007, 17:00 GMT
Task Type Bugs
Category Music playback
Status Closed
Assigned To No-one
Operating System iAudio X5
Severity Low
Priority Normal
Reported Version Daily build (which?)
Due in Version Undecided
Due Date Undecided
Percent Complete 100%
Votes 0
Private No

Details

On my X5, the newest builds (post-metadata-on-buffer) will click/clip the audio if Crossfeed and Dithering are both enabled. This is only audible if the audio is mastered *loud* (metal: yes, NPR.org podcasts: no).

If Crossfeed or Dithering are disabled/enabled exclusively, the problem does not exist.

For testing purposes (build 15510):
1) all settings were reset to default
2) Crossfeed was enabled
3) Dithering was enabled
This task depends upon

Closed by  Thom Johansen (preglow)
Tuesday, 04 December 2007, 17:00 GMT
Reason for closing:  Fixed
Additional comments about closing:  Should be fixed now, please open a new bug report if it isn't.
Comment by Philip Barton (audiophyl) - Wednesday, 07 November 2007, 07:37 GMT
It appears that the clipping/clicking is only happening with lower frequencies: the beat of a kick drum, the thump of a deep bass note, etc.
Comment by Thom Johansen (preglow) - Wednesday, 07 November 2007, 13:07 GMT
Are you sure the dithering has any role here? Dithering just does minute changes to the audio, and while it might cause a small, small clip, this should never be noticable. The crossfeed however, is quite capable of making audio clip in a manner much more apparent, especially with audio that has much bass. Try disabling dither to see if the clipping is still there. If it is, try adjusting the parameters of the crossfeed to get rid of it.
Comment by Daniel Siebrecht (DimensionSix) - Wednesday, 07 November 2007, 16:58 GMT
I had a similar problem. Some days ago I disabled dithering because of strange noises (don't know if it's clipping because I don't know what that means). Without dithering it's no problem, but when I enable it, quality of the music is very bad. I don't know if this also happens in current builds because I didn't enable it since the problems.
Platform: H320
Comment by Steve Bavin (pondlife) - Wednesday, 07 November 2007, 17:41 GMT
Daniel - r15369 (on 29th October) fixed a problem with dithering/clipping. Please ensure you're running at least this version.
Comment by Daniel Siebrecht (DimensionSix) - Wednesday, 07 November 2007, 18:41 GMT
OK, thank you. Seems that I didn't notice this change. With a build from yesterday I have no problems.
Comment by Thom Johansen (preglow) - Wednesday, 07 November 2007, 19:17 GMT
Out of curiosity, which codec did you use when you noticed this? That bug should only affect WMA files.
Comment by Philip Barton (audiophyl) - Thursday, 08 November 2007, 01:46 GMT
My original message does note that this behavior is audible in 15510, and I have now tested it in 15522 with the same results. The file I am listening to is an MP3. I may have an OGG or WMA here or there, but nothing loud enough to cause this behavior.

Once again, the problem only surfaces when Crossfeed and Dithering are both enabled. If I am just using Crossfeed or just using Dithering, there is no clicking/clipping.

What is the worst-case scenario for output volume with Crossfeed + Dithering? 0dbFS? Even -0.1 dbFS can clip...

-Philip
Comment by Philip Barton (audiophyl) - Thursday, 15 November 2007, 03:43 GMT
This problem is still present in build 15627. Will it help if I send someone an MP3 in which this is audible?

-Philip
Comment by Thom Johansen (preglow) - Sunday, 18 November 2007, 23:42 GMT
Well, sure it would. I can't really hear the problem myself so a sample would be nice.
Comment by Philip Barton (audiophyl) - Tuesday, 20 November 2007, 08:15 GMT
Still exists in 15716.

I'm too lazy to make a shorter clip (sorry!), so I've just uploaded a song:

http://people.cs.und.edu/~pbarton/problem_file.mp3

I only have the X5 to test this on. Once again, this occurs even after a full settings reset and only with Crossfeed and Dithering *both* enabled. The clipping/clicking is audible at any volume level.

I hope this helps in tracking down the cause!

-Philip
Comment by Linus Nielsen Feltzing (linusnielsen) - Tuesday, 20 November 2007, 08:28 GMT
I opened the file in Cool Edit Pro, and it's easy to see that the track is clipping even without applying Crossfeed or Dithering.
Comment by Philip Barton (audiophyl) - Tuesday, 20 November 2007, 08:53 GMT
Linus, thanks for the quick response.

Zooming in on the wave-form should reveal that 0dBFS is neither hit nor sustained with enough frequency to cause the amount of clipping/clicking audible during playback. Still, any clipping visible in the file is not audible *until* Crossfeed & Dithering are enabled simultaneously. That tells me the audio is being altered in manner which is cause for (some, possibly small) concern.

I'm presently listening to the second newest Dream Theater album, and am hearing more of the same. The audio is normalized to 98% and the clipping is still audible. I'd have uploaded one of these songs instead, but they tend to be very long.

-Philip
Comment by Philip Barton (audiophyl) - Tuesday, 20 November 2007, 09:10 GMT
An additional note: This was not audible until recent builds. My taste in music hasn't suddenly changed; these files have been in steady rotation on my playlist for far too long, and were when I used the OF. This clipping/clicking is a recently introduced phenomenon.

I can live without Dithering, but shrugging off the potential problems doesn't seem like a long-term solution. I really wish I understood digital audio enough to sort this out on my own, but alas I am a lowly CS undergrad with not much useful experience in programming. :)
Comment by Dave Greenidge (Dave G) - Tuesday, 20 November 2007, 09:17 GMT
I have the same problem on my Sansa e280. Distortion is minimised by running bass at -2db or turning dithering off. I like the crossfeed feature too much to disable that. Replay gain is on with clipping prevention also turned on and the distorion happens at all volume levels with both mp3 & ogg files. The problem only seemed to appear some time around build 15447.
Comment by Philip Barton (audiophyl) - Tuesday, 20 November 2007, 09:38 GMT
I unintentionally lied, the Dream Theater was not normalized. The audio data is straight from the CD with no modification other than having been MP3-encoded, unless Windows Media Player did something I was unaware of.

Also, the problem does go away if Crossfeed's direct gain is lowered to the lowest possible setting of -6.0dB (-4.0dB for Dream Theater). It still sounds like there is something odd happening with the audio at peaks which doesn't happen without Crossfeed + Dithering, though.

Wish I could be of more help.

-Philip
Comment by Thom Johansen (preglow) - Tuesday, 20 November 2007, 13:19 GMT
Ok, I've had a listen and I can assure you that this file clips like mad when decoded even with no crossfeed and dithering. You are aware that files that don't clip before MP3 encoding can very well clip _after_ being encoded, right?
This is all quite besides the point, though, as when I enable dithering I get nasty overflow clicks, which I assume are what you're talking about. I'll check this out.

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