Annoying distortion: Sansa Clip+ w/ Rockbox, Audible but not easily measurable Options

[googlebot]

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

[Soap]

I believe the charge against the use of RMAA numbers is an improper one based on faulty practices:

A properly done RMAA test is performed by recording the loaded output of the player. You did not describe loading the output during your recording attempt therefore I assume you didn't.

Your recording attempt was the same as an unloaded RMAA test, a test which would be dismissed by most objectivists as being suggestive but not telling.

Use a splitter to plug in headphones to the player while at the same time feeding your sound card's line-in.

Bet you dollars to donuts you successfully capture the noise (not distortion) then.

This post has been edited by Soap: Jan 22 2011, 16:18

[googlebot]

Bet you dollars to donuts you successfully capture the noise (not distortion) then.

While loading reveals LF roll-offs and the like, which is related to output impedance, I'm not so certain that loading must reveal sensitivity related issues.

I have all necessary equipment but a spare 3,5 mm socket at hand right now. So I cannot setup a loaded test without cutting my headphone cable. A splitter might accomplish the same without soldering. But are they always just connected in parallel without additional circuitry?

PS Since the signal is highly correlated to the encoded signal, it's not necessarily false to speak of distortion instead of noise.

This post has been edited by googlebot: Jan 22 2011, 16:34