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Annoying distortion: Sansa Clip+ w/ Rockbox, Audible but not easily measurable
Northpack
post Mar 4 2011, 18:42
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Yes, I didn't mean the external SD specifically. It doesn't make any difference whether the file is on the internal or external memory.
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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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Arnold B. Kruege...
post Mar 5 2011, 13:06
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QUOTE (googlebot @ Jan 22 2011, 08:52) *
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.

n 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.


Has this annoying sound ever been recorded and posted for inspection?
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googlebot
post Mar 5 2011, 15:02
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As I have written, my ALC889's line-in is not sensitive enough to capture the noise. I guess the Clip+ drives it with a larger voltage than the IEMs and then its output is fine.

I have uploaded my test file. It is nothing special, you could also just create your own, a mp3 with a forced high bitrate (so that it doesn't get buffered completely), with nothing but a high pitched tone inside. Copy it several times onto the SD card (not the internal storage) and then listen while you skip between tracks. The noise you hear is the same that annoys me during playback, when the SD card is accessed for re-buffering.

Maybe NwAvGuy can repeat the recording with his much better equipment. Then we would have an example.

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NwAvGuy
post Mar 5 2011, 18:21
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QUOTE (googlebot @ Mar 5 2011, 06:02) *
Maybe NwAvGuy can repeat the recording with his much better equipment. Then we would have an example.

I'll work on that. Right now I'm working on posting recorded clips of 3 different DACs to hopefully shine a little light on the manufacture's response to my NuForce uDAC-2 review. I can try for a recording and measurement of the Clip+ running the last fully released RB version. If anyone wants to be more specific about exactly what they want recorded, and when they want it recorded (i.e. just as the clip starts, etc.), that would be useful?

As for Saratoga, I had him confused with another user (I guess I'm on too many forums to keep everyone straight?). So yeah, there's no need to go to ABI.


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googlebot
post Mar 5 2011, 18:31
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QUOTE (NwAvGuy @ Mar 5 2011, 18:21) *
I can try for a recording and measurement of the Clip+ running the last fully released RB version. If anyone wants to be more specific about exactly what they want recorded, and when they want it recorded (i.e. just as the clip starts, etc.), that would be useful?


That'd be great! Recorded should be the Clip's sound during re-buffering reads from the SD drive. If you play back the track that I have uploaded, it should happen at some point during playback, bitrate and length are sufficient. A shortcut would be putting the uploaded file several times on a SD card and record the noise during track skipping, which is nearly identical to the latter.
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Arnold B. Kruege...
post Mar 7 2011, 17:02
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QUOTE (googlebot @ Mar 5 2011, 09:02) *
As I have written, my ALC889's line-in is not sensitive enough to capture the noise. I guess the Clip+ drives it with a larger voltage than the IEMs and then its output is fine.

I have uploaded my test file. It is nothing special, you could also just create your own, a mp3 with a forced high bitrate (so that it doesn't get buffered completely), with nothing but a high pitched tone inside. Copy it several times onto the SD card (not the internal storage) and then listen while you skip between tracks. The noise you hear is the same that annoys me during playback, when the SD card is accessed for re-buffering.


I don't see any need for sensitive measurements as the transients that are generated by my Clip+ with 01.02.15 firmware when I switch between 2 copies of the test file are pretty gross. The signal I record starts at the baseline, dives negative for about 50 milliseconds to an amplitude of about 20% of the test signal, bounces up for about 20 milliseconds, and then the test tone starts and its DC value starts out about 20% high and then bounces down to a nice symmetrical HF wave.

The most improtant characeristic of the test file is that it has a consdierable signal but is at such a high frequency that it can't be heard very easily.

Looks to me like the Clip's output is disconnected from the headphones when its changing files, and then gets hooked back up and there's a transient something like coupling capacitors charging up. That's odd because the performance of the Clip generally suggests that tehre are no output blocking caps. I guess the servo system that eliminates any need for them still simulates some of the odd things they might do with the power supply remoted and then applied to start playing the file.
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saratoga
post Mar 7 2011, 17:32
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Heres a patched build if someone wants to try it. It tweaks a few settings to the DAC. We tested this ages ago and no one could detect any difference in output at all, but someone suggested to me that they made some difference on his new clip+ so maybe these registers somehow matter on new players:

http://duke.edu/~mgg6/rockbox/rockbox-patched.7z
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googlebot
post Mar 7 2011, 17:34
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QUOTE (Arnold B. Krueger @ Mar 7 2011, 17:02) *
I don't see any need for sensitive measurements as the transients that are generated by my Clip+ with 01.02.15 firmware when I switch between 2 copies of the test file are pretty gross.


As I have repeatedly written, I'm totally fine with the sound quality of the stock firmware, which sadly leaves a lot to be desired with regard to supported codecs (not even AAC) and suitability for large audio book playlists. The short click between track transitions on the original firmware does not bother me, it also does not occur during rebuffering. It is unrelated to the reported issue.

QUOTE (saratoga @ Mar 7 2011, 17:32) *
Heres a patched build if someone wants to try it. It tweaks a few settings to the DAC. We tested this ages ago and no one could detect any difference in output at all, but someone suggested to me that they made some difference on his new clip+ so maybe these registers somehow matter on new players:

http://duke.edu/~mgg6/rockbox/rockbox-patched.7z


Great! I'll give it a try. There is also another debug build attached to my bug report, that is still waiting to be tested. I haven't found the time, yet.

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Arnold B. Kruege...
post Mar 7 2011, 18:06
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QUOTE (googlebot @ Mar 7 2011, 11:34) *
QUOTE (Arnold B. Krueger @ Mar 7 2011, 17:02) *
I don't see any need for sensitive measurements as the transients that are generated by my Clip+ with 01.02.15 firmware when I switch between 2 copies of the test file are pretty gross.


As I have repeatedly written, I'm totally fine with the sound quality of the stock firmware, which sadly leaves a lot to be desired with regard to supported codecs (not even AAC) and suitability for large audio book playlists. The short click between track transitions on the original firmware does not bother me, it also does not occur during rebuffering. It is unrelated to the reported issue.



What then am I supposed to be looking for?

I'm sitting here with some really great recordings of track switching - with a -107 dB noise floor.

And that is pretty interesting all by itself - between tracks the headphone jack of the Clip+ is *that* quiet.
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googlebot
post Mar 7 2011, 19:12
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Yes, that's impressive. If you find the time, I'd be interested in the same measurement with the Rockbox firmware.
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Arnold B. Kruege...
post Mar 7 2011, 20:12
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QUOTE (googlebot @ Mar 7 2011, 13:12) *
Yes, that's impressive. If you find the time, I'd be interested in the same measurement with the Rockbox firmware.


I currently have no interest in loading my Clip+ with Rockbox. Not that I have anything against it, just that I'm really pleased with the OEM firmware.

If its not broken...
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greynol
post Mar 7 2011, 20:15
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To me a lack of gapless playback = broken.

This post has been edited by greynol: Mar 7 2011, 20:32
Reason for edit: the all important article "a"


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googlebot
post Mar 7 2011, 20:47
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The only match, that I have found in my favorite consumer portal for the criteria "no touchscreen", "AAC support", and "user extensible storage", is this this epic design failure, the Colorfly Pocket. I almost spilled my dinner over my monitor.




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Arnold B. Kruege...
post Mar 7 2011, 22:21
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QUOTE (greynol @ Mar 7 2011, 14:15) *
To me a lack of gapless playback = broken.


I understand but do not share the opinion
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grums
post Mar 8 2011, 20:30
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On the Clip+/Rockbox there's a possibility to tweak the amplification between +/- 12dB:
Settings->Playback Settings->Replaygain->Pre-amp.

Maybe tweaking this can hide the noise a bit more?

Ole
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Lear
post Mar 8 2011, 22:04
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QUOTE (grums @ Mar 8 2011, 20:30) *
On the Clip+/Rockbox there's a possibility to tweak the amplification between +/- 12dB:
Settings->Playback Settings->Replaygain->Pre-amp.

Maybe tweaking this can hide the noise a bit more?


That's just a digital volume knob, so I doubt it helps much. And it is only applied when ReplayGain is used on a file.
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grums
post Mar 9 2011, 13:26
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QUOTE (Lear @ Mar 8 2011, 22:04) *
QUOTE (grums @ Mar 8 2011, 20:30) *
On the Clip+/Rockbox there's a possibility to tweak the amplification between +/- 12dB:
Settings->Playback Settings->Replaygain->Pre-amp.

Maybe tweaking this can hide the noise a bit more?


That's just a digital volume knob, so I doubt it helps much. And it is only applied when ReplayGain is used on a file.


Right - but then maybe he could use ReplayGain...
(I agree that the noise and the music might have been mixed before that, in which case it doesn't help - I don't know the inner workings of the player+software - but it's quite easy to test)
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googlebot
post Mar 9 2011, 13:29
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ReplayGain does DAC wise nothing different than the volume knob. So it won't help.

After looking at the code, I rather suspect it is SD controller related anyway. It is reverse engineered and full of assumptions and hacks.

Ofc that's not the developers' fault. They're doing their best to make it work often completely without any proper documentation.

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grums
post Mar 9 2011, 14:29
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You're right - just did a quick test, on my (new) Clip+ the noise level is constant (no matter what volume level is chosen) - and mostly related to 'disk' access.
I use Sennheiser CX400.
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kritip
post Aug 5 2011, 17:20
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QUOTE (saratoga @ Mar 7 2011, 17:32) *
Heres a patched build if someone wants to try it. It tweaks a few settings to the DAC. We tested this ages ago and no one could detect any difference in output at all, but someone suggested to me that they made some difference on his new clip+ so maybe these registers somehow matter on new players:

http://duke.edu/~mgg6/rockbox/rockbox-patched.7z


Hi Saratoga.

I finally got RB on my Clip+ thanks to the modified bootloader (my post on www.rockbox.org)

I have now started playing with it, and instantly noticed this noise issue. I have no SD card installed. On track load, and what I assume is re-buffering (playing FLAC at the moment) I get random easily audible hiss/noise.

I tried the latest stable, development and also this version, and all have the same issue. Just for your information as I couldn't see any feedback on this patched version.

Shame, as it makes listening a pain.


I should note that the original firmware ALSO has a noise issue playing. It's a bit quieter, but it's constant, almost like a helicopter in the background, making quiet passages very dirty and noisy.

I'd suggest that BOTH firmwares have an issue with noise, but the OEM firmware is probably making small reads almost constantly whilst the RB firmware is reading in much larger chunks., making no noise in the mean time, but more noise when it makes a large read.

I'd imagine a lot of people are simply missing the OEM firmware noise as it's less noticable.

It's very obvious when I turn the volume off on both. The OEM noise still exists, the RB doesn't until a rebuffer.

//EDIT -- Noise also exists in both firmwares for MP3, though on the OEM it sounds different to FLAC (faster helicopter)

BTW, I'm using Jays Q-Jays headphones.


Cheers,

Kristian

PS. This isn't a TOS8 issue I don't feel. The noise is totally noticable and repeatable.

I can try and make recording of both firmwares output if needed, though I'm not sure how good they will sound on my laptop soundcard line in.

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saratoga
post Aug 5 2011, 19:42
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QUOTE (kritip @ Aug 5 2011, 12:20) *
QUOTE (saratoga @ Mar 7 2011, 17:32) *
Heres a patched build if someone wants to try it. It tweaks a few settings to the DAC. We tested this ages ago and no one could detect any difference in output at all, but someone suggested to me that they made some difference on his new clip+ so maybe these registers somehow matter on new players:

http://duke.edu/~mgg6/rockbox/rockbox-patched.7z


Hi Saratoga.

I finally got RB on my Clip+ thanks to the modified bootloader (my post on www.rockbox.org)

I have now started playing with it, and instantly noticed this noise issue. I have no SD card installed. On track load, and what I assume is re-buffering (playing FLAC at the moment) I get random easily audible hiss/noise.

I tried the latest stable, development and also this version, and all have the same issue. Just for your information as I couldn't see any feedback on this patched version.



You can also try the patch from googlebot's bug report:

http://www.rockbox.org/tracker/task/11907

Since no one tested it on a player with SD noise, I have no idea if it'll help. But worth a shot. I compiled a build with it here:

http://duke.edu/~mgg6/rockbox/rockbox-fs11907.7z
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kritip
post Aug 6 2011, 07:41
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Thanks. I have tried that and the issue is still there.

It may not be possible to remove the noise at all, just handle it in different ways?

In 3 or 4 days time, I will pick up a cable and try to display the noise patterns visually between the firmwares. I think that will help explain a lot smile.gif
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saratoga
post Aug 7 2011, 18:40
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If it happens in then Sandisk firmware too, then yeah theres probably no easy fix. That said, it might be possible to work around the issue like we did with the pitch error in the sandisk firmware. I'd be interested to see what recordings of the noise look like.

FWIW on my Clip+ there is a bit of background noise, but I can only really see it on a scope, since at +6dB volume, its only a few mV (and it does scale with volume). It sounds like theres a batch of players out there that are much more susceptible to this background noise. Interestingly when the DAC is powered off it doesn't entirely go away (but gets much, much weaker), so I tend to think its power supply noise being amplified by headphone amp.
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