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Annoying distortion: Sansa Clip+ w/ Rockbox, Audible but not easily measurable
googlebot
post Jan 22 2011, 14:52
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After reading several positive posts around here, I replaced my venerable iPod G5 Video with a little Sansa Clip+ and a 16GB flash card. Missing AAC support was a deal breaker, but Rockbox was reported to run fine.

I must say, compared to the absolutely flawless experience with my last iPod, I'm quite dissatisfied. It started with a bug of Rockbox 3.7.1 that large SD cards are sometimes not initialized properly (and thus don't appear in the menu), which affected my Transcend model. An upgrade to the latest daily build fixed this.

An issue, that I can't fix with whatever build, is annoying CPU noise. At the beginning of tracks and during rebuffering of longer tracks there is a short humming sound (like a bumblebee) mixed with a fast sequence of random high frequency tones. The sound differs as a function of data format and content. It is very homogeneous with WAV tracks (only humming) and different with lossy and lossless codecs. The same file encoded with FLAC and AAC leads to different short distortions. It is only audible for tracks with initial silence or very dynamic content, for example classical music with quiet passages or audio books. The original firmware does not show this behavior. It is probably a throttling issue. Rockbox just reads and decodes the file in a regular loop as fast as the CPU can deliver it. Due to cheap hardware design this leads to audible inference on the output. The original firmware probably works around this by some form of flow control and doesn't read the data any faster than required. But, as I've said, due to missing AAC support the original firmware is not an option.

The phenomenon is only audible with my headphones. The Creative EP-630 have a rather large sensitivity of 106 dB. I tried to record the distortion over a line-input and it is non-existent there. That made me think about the common practice of shutting people up with RMAA results or telling them that what they hear must be placebo. High-impedance, line-level inputs do not necessarily capture the same as what a sensitive headphone outputs.

Can anyone, who also owns sensitive earphones, recommend a comparable player (no video, real buttons) with flawless output? It would be nice if I could re-use the SD-card.

This post has been edited by googlebot: Jan 22 2011, 14:53
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NwAvGuy
post Mar 5 2011, 02:30
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As I think the OP may already know, I did a full test of the Clip+ on my blog here:

Sansa Clip+ Measured

I noticed slightly greater subjective hiss, at times, with the Rockbox firmware compared with the Sansa firmware. I also noticed a different slight noise when the display is on. I've encountered display noises with other devices as well. Could that be what at least some of you are hearing?

In terms of measurements, the Clip+ measures fairly quiet, but I only ran that test (I think) with the factory firmware. I might re-run it, if I get the chance, with the RB firmware with both the display on and off. I use a dScope audio analyzer with a noise floor below -140 dB so it should pick up *any* audible noise regardless of the type or source.

What others are calling the "CPU Noise" in this thread is likely related largely to the hardware and it's just showing up in different ways with different firmware. Put another way, I strongly doubt there's anything in the RB firmware that's actually creating a noise and sending it to the DAC. Instead, it's what's known as "EMI noise" which tiny digital audio devices are often prone to. The CPU draws spikes of current when it executes, those spikes create an electromagnetic field, and the audio circuits pick up a bit of that field. It's really hard to get rid of when you can't physically get the noisy digital stuff away from the sensitive analog stuff. I bet the tiny Apple Shuffle may have some EMI noise as well because of its size.

Given that Rockbox does a lot more than the original Sansa firmware, it's likely to load the CPU in different ways. I've written commercial embedded firmware and there are lots of different approaches to scheduling tasks, etc. And it's entirely possible the Rockbox firmware is simply hitting the CPU in ways that create a bit more audible noise.

I know one of the developers that worked on the Rockbox port to the Clip+ hangs out on AnythingButiPod so it might be worth posting this question in the appropriate forum there. He seems to know the player inside and out.

And, finally, I don't think it's entirely fair to expect a $29 player (the basic Clip+ hardware in 2 GB form) to be the equal in every way of a multi hundred dollar iPod. My iPod Touch 3G compared to the Clip+ in my review was quieter, had less distortion, etc. But, with balanced armature IEMs (which have wild impedance swings) the Clip+ actually sounds better because the iPod has a much higher output impedance that interacts in ugly ways with many headphones. I also can't clip a Touch 3G to my shirtsleeve at the gym. So they both have their strengths.


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Northpack
post Mar 5 2011, 11:33
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QUOTE (NwAvGuy @ Mar 5 2011, 02:30) *
In terms of measurements, the Clip+ measures fairly quiet, but I only ran that test (I think) with the factory firmware. I might re-run it, if I get the chance, with the RB firmware with both the display on and off. I use a dScope audio analyzer with a noise floor below -140 dB so it should pick up *any* audible noise regardless of the type or source.

AFAIK the Clip+'s output was extensively tested during rockbox development and it didn't show any issues concerning distortion or noise floor. However, if only more recent hardware revisions are affected it's quite proable that noone tested an affected device.

QUOTE
I know one of the developers that worked on the Rockbox port to the Clip+ hangs out on AnythingButiPod so it might be worth posting this question in the appropriate forum there. He seems to know the player inside and out.

You don't have to go to the ABI forums though, saratoga hangs around here too wink.gif

QUOTE
And, finally, I don't think it's entirely fair to expect a $29 player (the basic Clip+ hardware in 2 GB form) to be the equal in every way of a multi hundred dollar iPod.

Of course not. When it comes to Apple, I expect design and sturdy build quality. Concerning the sound quality I wouldn't expect much of a difference, as even the ICs avaiable today will always provide good enough quality if the curcuit design isn't messed up.

Btw. your measurements look well done and quite comprehensive.
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Posts in this topic
- googlebot   Annoying distortion: Sansa Clip+ w/ Rockbox   Jan 22 2011, 14:52
- - odigg   I think people experienced with RMAA measurements ...   Jan 22 2011, 15:52
- - Soap   I believe the charge against the use of RMAA numbe...   Jan 22 2011, 16:03
|- - googlebot   QUOTE (Soap @ Jan 22 2011, 16:03) Bet you...   Jan 22 2011, 16:28
|- - odigg   QUOTE (googlebot @ Jan 22 2011, 10:28) I ...   Jan 22 2011, 19:00
- - Peter   Just for reference: I believe I'm having a sim...   Jan 22 2011, 16:44
- - bryant   There is a mention on this Rockbox page about anno...   Jan 22 2011, 20:25
- - Arnold B. Krueger   QUOTE (googlebot @ Jan 22 2011, 08:52) Af...   Jan 22 2011, 23:25
|- - Northpack   QUOTE (Arnold B. Krueger @ Jan 22 2011, 23...   Jan 25 2011, 11:14
- - probedb   You could post on the rockbox forums, plus also ra...   Jan 23 2011, 10:43
|- - odigg   I installed the latest version of Rockbox on my Cl...   Jan 24 2011, 21:30
- - greynol   I hope this gets fixed or Sansa implements gapless...   Jan 24 2011, 21:40
|- - 2Bdecided   QUOTE (greynol @ Jan 24 2011, 20:40) Sans...   Jan 25 2011, 10:25
|- - odigg   QUOTE (greynol @ Jan 24 2011, 15:40) I ho...   Jan 25 2011, 14:42
- - botface   @googlebot I have a Fuze v1 and have no problems ...   Jan 25 2011, 13:37
- - googlebot   I'm pretty sure that the noise cannot be heard...   Jan 25 2011, 20:21
- - Northpack   I can hear that noise too but as long as I am not ...   Jan 25 2011, 22:20
- - eevan   I can confirm that there is high noise level on Fu...   Jan 26 2011, 14:33
- - saratoga   googlebot just pointed me to this thread. I am ...   Mar 4 2011, 00:45
|- - googlebot   Which version of Rockbox are you using?   Mar 4 2011, 00:56
|- - odigg   QUOTE (saratoga @ Mar 3 2011, 18:45) I...   Mar 4 2011, 16:56
- - saratoga   r29342. But the Clip+ has been my preferred playe...   Mar 4 2011, 01:02
- - googlebot   Very interesting! Do you have a Rockbox build ...   Mar 4 2011, 02:11
- - Northpack   For me the noise is related to SD access. It's...   Mar 4 2011, 17:45
|- - odigg   QUOTE (Northpack @ Mar 4 2011, 11:45) For...   Mar 4 2011, 18:38
- - Northpack   Yes, I didn't mean the external SD specificall...   Mar 4 2011, 18:42
- - NwAvGuy   As I think the OP may already know, I did a full t...   Mar 5 2011, 02:30
|- - Northpack   QUOTE (NwAvGuy @ Mar 5 2011, 02:30) In te...   Mar 5 2011, 11:33
- - Arnold B. Krueger   QUOTE (googlebot @ Jan 22 2011, 08:52) Af...   Mar 5 2011, 13:06
- - googlebot   As I have written, my ALC889's line-in is not ...   Mar 5 2011, 15:02
|- - NwAvGuy   QUOTE (googlebot @ Mar 5 2011, 06:02) May...   Mar 5 2011, 18:21
||- - googlebot   QUOTE (NwAvGuy @ Mar 5 2011, 18:21) I can...   Mar 5 2011, 18:31
|- - Arnold B. Krueger   QUOTE (googlebot @ Mar 5 2011, 09:02) As ...   Mar 7 2011, 17:02
|- - googlebot   QUOTE (Arnold B. Krueger @ Mar 7 2011, 17...   Mar 7 2011, 17:34
|- - Arnold B. Krueger   QUOTE (googlebot @ Mar 7 2011, 11:34) QUO...   Mar 7 2011, 18:06
|- - googlebot   Yes, that's impressive. If you find the time, ...   Mar 7 2011, 19:12
|- - Arnold B. Krueger   QUOTE (googlebot @ Mar 7 2011, 13:12) Yes...   Mar 7 2011, 20:12
- - saratoga   Heres a patched build if someone wants to try it. ...   Mar 7 2011, 17:32
|- - kritip   QUOTE (saratoga @ Mar 7 2011, 17:32) Here...   Aug 5 2011, 17:20
|- - saratoga   QUOTE (kritip @ Aug 5 2011, 12:20) QUOTE ...   Aug 5 2011, 19:42
- - greynol   To me a lack of gapless playback = broken.   Mar 7 2011, 20:15
|- - Arnold B. Krueger   QUOTE (greynol @ Mar 7 2011, 14:15) To me...   Mar 7 2011, 22:21
|- - grums   On the Clip+/Rockbox there's a possibility to ...   Mar 8 2011, 20:30
|- - Lear   QUOTE (grums @ Mar 8 2011, 20:30) On the ...   Mar 8 2011, 22:04
|- - grums   QUOTE (Lear @ Mar 8 2011, 22:04) QUOTE (g...   Mar 9 2011, 13:26
|- - googlebot   ReplayGain does DAC wise nothing different than th...   Mar 9 2011, 13:29
- - googlebot   The only match, that I have found in my favorite c...   Mar 7 2011, 20:47
- - grums   You're right - just did a quick test, on my (n...   Mar 9 2011, 14:29
- - kritip   Thanks. I have tried that and the issue is still t...   Aug 6 2011, 07:41
- - saratoga   If it happens in then Sandisk firmware too, then y...   Aug 7 2011, 18:40
|- - kritip   Indeed, this noise doesn't really scale with v...   Aug 7 2011, 20:21
- - saratoga   Which rockbox version did you test?   Aug 7 2011, 20:48
- - kritip   the latest development and stable build, and the o...   Aug 7 2011, 21:19
- - saratoga   Yes, but which version did you test when you recor...   Aug 7 2011, 21:21
- - kritip   sorry: r30259-110806   Aug 7 2011, 21:24
- - Rezal   The Clip+ I have bought some weeks ago is also aff...   Aug 20 2011, 12:33
- - Rezal   Some additional findings (why can't I edit my ...   Aug 20 2011, 16:47
|- - db1989   QUOTE (Rezal @ Aug 20 2011, 16:47) why ca...   Aug 20 2011, 17:01
- - Sik_Lescinovid   I'm sorry to be bumping an old thread, but I c...   Dec 29 2012, 16:52
|- - saratoga   QUOTE (Sik_Lescinovid @ Dec 29 2012, 10:5...   Dec 29 2012, 20:33
|- - Sik_Lescinovid   QUOTE (saratoga @ Dec 29 2012, 21:33) I d...   Dec 29 2012, 21:39
- - saratoga   I'm not sure what the best way is. When I...   Dec 29 2012, 22:11
- - kritip   Well, after the clip broke (literally the clip), I...   Apr 6 2013, 22:46
- - kritip   Just to update, the new player doesn't exhibit...   Apr 15 2013, 17:53


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