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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 / 109ádB/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 5 2013, 23:56
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The amount of dynamic range required in measurements can be surprisingly high. As mentioned, there's the issue of differing reference levels, and then there still is the difference of a device's instantaneous dynamic range vs. its total dynamic range. The DAC inside a Clip+ may only give about 90 dB of instantaneous DR, but it's followed by a 40-odd dB PGA. Ultimately minimum noise level at low volume settings (or muted) will be the PGA's at minimum gain, plus whatever comes in via the virtual ground buffer (see AS3543 datasheet).

Even assuming pretty horrid voltage noise performance from these CMOS amps, minimum noise floor should be around or under 10 ÁVrms (20 kHz BW), so with the typical maximum output of 800 mVrms, total DR should be around 100 dB or thereabouts. With low-noise amps and higher supply voltages, total DR can be 130+ dB, 140+ dB if you're really good. (Actually these are the very same factors limiting SNR in converters these days, and consumer DACs and ADCs usually are CMOS parts operating from +5V only. So even if you do get noise down, you won't be able to match a super low noise bipolar amp on +/- 15 V. You would have to run 8 channels in parallel - with externally combined output - at the very least, as in fact some parts can do.)

Having total DR > instantaneous DR is important since it allows shifting DR up and down as required. You won't need more than about 70 dB of instantaneous DR for audio playback, but headphones alone may show a variation in sensitivity of 40 dB and more (see e.g. super.fi 5 Pro vs. K240M). Total DR requirements are often misjudged. You'd be surprised by the amount potentially required even in a nominally "lo-fi" application such as shortwave reception. (There's nothing quite as annoying as broadband hiss when your actual signal stops at like 3 kHz...)

This post has been edited by stephan_g: May 6 2013, 00:00
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post May 6 2013, 10:04
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Tests in this other thread suggest that the noise voltage of the Fuze+ only decreases by a few dB at low volume vs. full volume. This explains why there is audible hiss when using very sensitive IEMs. Although the EMU 0204 does add some noise to the recording, even at a low volume setting of the Fuze+ it only seems to make the overall noise level worse by 1-2 dB (considering also that the noise measurement of the EMU also included its DAC).
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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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