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RMAA fails to pick-up audible hiss, reports nearly perfect noise level
post May 1 2013, 10:51
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We've been scratching our heads about this on Anything but iPod and in #rockbox on Freenode. With sensitive IEMs (like my Shure SE425s), there's a faint, but audible, hiss with the Rockboxed Fuze+, when no music is playing. The hiss is a lot more pronounced when playing a digitally silent file. I don't hear hiss with the iPod Classic. At least 2 other people have made the same observation.

The problem is, RMAA can't seem to pick it up. The Fuze+ gets exactly the same noise and dynamic range scores as the Classic, unloaded as well as loaded with the Shures and the Denons: 95dB, a nearly perfect score for a 16 bit device. More to the point, while the Fuze+ and the Classic are audibly different (with regards to hiss), why do they get identical scores in all situations?

You could slap me with the TOS #8 hammer, but I don't know how we can prove what we hear. The difference is easily heard when turning the Fuze+ on and off. And since we can't determine the nature of the noise, or its cause, the Rockbox developer (pamaury) doesn't really know what to do about it, or if it's even fixable at all.

I'm no electrical engineer. Is there something special about what we perceive as "hiss", that would be audible, but that wouldn't show up in RMAA-type tests / recordings? There's no way I could hear noise if it actually was at -95dB.

I have no idea if this is relevant, but FWIW, the Fuze+ has an output impedance of 0.9Ω, and the iPod Classic, 5.5Ω. The Shures are 22Ω / 109 dB/mW, and the Denons are 25Ω / 100-106 dB/mW (depending on whom you ask).

Edit: I forgot that I already mentionned that problem, but it's bugging me, and I guess it doesn't hurt to start a dedicated thread.

This post has been edited by skamp: May 1 2013, 10:55

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post May 3 2013, 15:08
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Your files show identical behaviour in both conditions.

I've tried the same thing with my clip+, stock firmware (V01.02.15A), and I get a clear difference (using my own silent files). I tried 16-bit digital silence, 16-bit dithered silence, and 16-bit noise-shaped dithered silence, all generated in Cool Edit Pro from 32-bit silence wink.gif .

I don't have a Y splitter, so could only listen via earphones or connect directly to the PC - hence the direct connection to the PC is not a proper test of the Clip+'s performance when feeding earphones.

Through earphones at full volume (an impossibly loud volume setting for listening to full-scale waveforms without destroying your ears) I can hear that the noise level from a silent (all zeros, no dither) file drops dramatically when I press pause. With pause pressed, the output is essentially silent, whereas when playing silence it is not. Putting a fixed DC offset in the file vs all zeros, or a very slow ramp - no difference - same level of hiss playing all three. I could not hear any difference between dithered and undithered. I can hear the attenuator in the Clip+ acting rapidly when pause is pressed, especially when pausing the DC offset - it's not a hard click - it's a buzz caused by the attenuator stepping rapidly through a few discrete settings (about 12), which gives a stepped wave on the output.

Recording into my PC, the 0dB FS signal replayed at full volume on the Clip+ registered -10.3dB via my audiophile 2496 hence I am not using its full resolution.

Recording at 24-bits, you can see (and hear, if you crank the volume or digitally boost the signal) the change in noise level when pause is pressed. The recording of the paused state is not quite so silent as through earphones, but it is equally as silent as recording with the device switched off and/or disconnected, so I think this is the limit of my 24-bit sound card. You cannot see or hear any difference between dithered and undithered source but a third "silent" file with just noise-shaped dithering has the noise-shaped high frequency part easily visible on the recording (though within the audible range it looks and sounds the same, so all benefit of noise shaping is lost).

Recording at 16-bits (actually at 24-bits and converting, so I could control the conversion), the paused parts are still audibly and measurably less noisy than the playing-silence parts, but the difference is less pronounced than with the 24-bit recording. With dithered conversion, the difference is detectable but small.

Occasionally, especially when activated near the end of a file, the pause function is not silent at all, but is just as noisy as when "paying" - this seems to be a little (usually inaudible) "bug" which is worth watching out for.

It seems my sound card, even with more than 1-bit of headroom lost due to too low a level, is more than sufficient to capture the noise performance of the Sansa clip+ when its playing, but 16-bits is insufficient to capture quite how silent it is when paused. 24-bits on this sound card may be sufficient, but it's impossible to make that call without doing the test properly (Y splitter, correct levels somehow?).

Hope this helps.


P.S. a test for another day would be a -90dB and then -100dB sinewave with noise shaped dither.

This post has been edited by 2Bdecided: May 3 2013, 15:10
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Posts in this topic
- skamp   RMAA fails to pick-up audible hiss   May 1 2013, 10:51
- - bennetng   RMAA always add dither in its test signal so you a...   May 1 2013, 11:03
- - skamp   You must have done something wrong, your sample la...   May 1 2013, 11:23
|- - bennetng   QUOTE (skamp @ May 1 2013, 18:23) You mus...   May 1 2013, 11:28
- - Brazil2   Maybe give Visual Analyser a try: http://www.silla...   May 1 2013, 11:23
- - skamp   I don't think that's working properly. RMA...   May 1 2013, 11:42
|- - bennetng   Try to uncheck the following in RMAA before testin...   May 1 2013, 11:47
- - skamp   I did that. RMAA wouldn't accept the file if I...   May 1 2013, 11:50
|- - bennetng   Full test file http://s000.tinyupload.com/index.ph...   May 1 2013, 11:57
- - skamp   Thanks. Dynamic Range is still the same (95dB), wh...   May 1 2013, 14:22
|- - bennetng   Do you mean you can still hear the hiss but the me...   May 1 2013, 14:46
|- - skamp   QUOTE (bennetng @ May 1 2013, 15:46) Do y...   May 1 2013, 15:01
- - skamp   Also note that RMAA does pick up noise, when it...   May 1 2013, 15:05
- - 2Bdecided   Could the maximum signal output vs the background ...   May 1 2013, 15:13
|- - skamp   QUOTE (2Bdecided @ May 1 2013, 16:13) Cou...   May 1 2013, 15:18
|- - 2Bdecided   QUOTE (skamp @ May 1 2013, 15:18) Surely ...   May 1 2013, 15:27
|- - bennetng   In RMAA's test signal, 2.5s to 6.5s is the noi...   May 1 2013, 15:44
- - saratoga   My guess is that you don't have enough dynamic...   May 1 2013, 17:59
|- - skamp   QUOTE (saratoga @ May 1 2013, 18:59) My g...   May 1 2013, 18:05
|- - saratoga   QUOTE (skamp @ May 1 2013, 13:05) QUOTE (...   May 1 2013, 18:29
|- - skamp   QUOTE (saratoga @ May 1 2013, 19:29) Is t...   May 1 2013, 18:45
|- - Arnold B. Krueger   QUOTE (saratoga @ May 1 2013, 13:29) QUOT...   May 3 2013, 13:47
- - 2Bdecided   So if you just record using your equipment the noi...   May 3 2013, 10:05
- - skamp   I find this all very confusing, so instead of draw...   May 3 2013, 11:06
|- - bennetng   QUOTE (skamp @ May 3 2013, 18:06) Note th...   May 3 2013, 11:49
- - 2Bdecided   Your files show identical behaviour in both condit...   May 3 2013, 15:08
- - stephan_g   The amount of dynamic range required in measuremen...   May 5 2013, 23:56
- - stv014   Tests in this other thread suggest that the noise ...   May 6 2013, 10:04

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