HydrogenAudio

CD-R and Audio Hardware => Audio Hardware => Topic started by: skamp on 2013-05-01 10:51:30

Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 10:51:30
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 (http://www.hydrogenaudio.org/forums/index.php?s=&showtopic=94673&view=findpost&p=825534) that problem, but it's bugging me, and I guess it doesn't hurt to start a dedicated thread.
Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-01 11:03:24
RMAA always add dither in its test signal so you are not really testing digital silence.
Expand the
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 11:23:06
You must have done something wrong, your sample lasts 26s while the original lasts 71s. Also, don't compress with 7zip, just use FLAC.
Title: RMAA fails to pick-up audible hiss
Post by: Brazil2 on 2013-05-01 11:23:10
Maybe give Visual Analyser a try:
http://www.sillanumsoft.org/intoduction.htm (http://www.sillanumsoft.org/intoduction.htm)
http://www.sillanumsoft.org/prod01.htm (http://www.sillanumsoft.org/prod01.htm)
Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-01 11:28:27
You must have done something wrong. Your sample is too short and doesn't look like the original signal at all.

Audacity screenshot of the original: http://outpost.fr/stuff/20130501122056.png (http://outpost.fr/stuff/20130501122056.png)
Audacity screenshot of your sample: http://outpost.fr/stuff/20130501122019.png (http://outpost.fr/stuff/20130501122019.png)


My test signal did not contain some test so it is shorter, including:
Frequency response (swept sine)
THD (set of tones)
Impulse/Phase response

Other than these there is nothing wrong, except I modified the "Noise level" test to remove the dither. You can safely use my signal and generate a nice looking html report with it.
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 11:42:27
I don't think that's working properly. RMAA reports a noise level of -18.6dB with both my Fuze+ and my O2/ODAC (which is not possible, it's dead silent).
Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-01 11:47:35
Try to uncheck the following in RMAA before testing. Those tests won't show in the html report anyway.

Frequency response (swept sine)
THD (set of tones)
Impulse/Phase response

If it still doesn't work I will upload a full length test file again.
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 11:50:12
I did that. RMAA wouldn't accept the file if I didn't.
Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-01 11:57:28
Full test file
http://s000.tinyupload.com/index.php?file_...990245098663944 (http://s000.tinyupload.com/index.php?file_id=04429990245098663944)
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 14:22:18
Thanks. Dynamic Range is still the same (95dB), while the noise level is even lower: -98.6dB.
Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-01 14:46:37
Do you mean you can still hear the hiss but the measurement is -98.6dB? How did you perform the load test? Is it similar to this?

http://www.xbitlabs.com/articles/multimedi...-kamabay_5.html (http://www.xbitlabs.com/articles/multimedia/display/scythe-kamabay_5.html)
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 15:01:56
Do you mean you can still hear the hiss but the measurement is -98.6dB? How did you perform the load test?


As always, with the Shures attached to a stereo splitter, which is plugged into the headphone out of the Fuze+ and connected to the line-in of my ADC.
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 15:05:46
Also note that RMAA does pick up noise, when it's there. I used a fairly bad ADC when I first started doing RMAA tests, and the noise level was significantly high.
Title: RMAA fails to pick-up audible hiss
Post by: 2Bdecided on 2013-05-01 15:13:26
Could the maximum signal output vs the background hiss be a greater range than 16-bits (96dB)? Is your ADC capable of capturing this range?

FWIW there's a feint but audible hiss on the Sansa Clip+ with factory software using sensitive earphones. I couldn't cope with listening to a loud recording at full volume through the same equipment. Never measured the dynamic range though - don't think I have the equipment to do it.

Cheers,
David.


Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 15:18:08
Could the maximum signal output vs the background hiss be a greater range than 16-bits (96dB)? Is your ADC capable of capturing this range?


I'm not sure what you mean. Surely if the difference was greater than 96dB, I wouldn't be able to hear it. FYI, my ADC (EMU 0204 USB (http://outpost.fr/rmaa/EMU_0204_USB.htm)) only records in 24 bit, and the maximum DR that I ever measured with it, is its own: 112.7dB.
Title: RMAA fails to pick-up audible hiss
Post by: 2Bdecided on 2013-05-01 15:27:06
Surely if the difference was greater than 96dB
That depends on how loud "full scale" at "full volume" is with your IEMs.

Cheers,
David.

Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-01 15:44:02
In RMAA's test signal, 2.5s to 6.5s is the noise level test. Save the recorded file and listen to it with your 0202. If you can hear nothing then it simply means your test method cannot reveal the hiss.

I have no confidence to test headphone amps with RMAA as well, I only trust line level tests.
Title: RMAA fails to pick-up audible hiss
Post by: saratoga on 2013-05-01 17:59:20
My guess is that you don't have enough dynamic range on your ADC to detect it.  The full scale voltage on your ADC could easily correspond to an extremely high level on your headphones. If the noise is independent of the volume level, it may be below the noise floor of the ADC.

For example if the noise is -100 dbfs and your headphones are 125 db per volt, and your ADC had 100 db SNR, you will not detect it with the ADC but will easily hear it with a silence file. This is what happened with some of the clip players for example.
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 18:05:21
My guess is that you don't have enough dynamic range on your ADC to detect it. […] if your ADC had 100 db SNR […]


112.7dB (http://outpost.fr/rmaa/EMU_0204_USB.htm).
Title: RMAA fails to pick-up audible hiss
Post by: saratoga on 2013-05-01 18:29:52
My guess is that you don't have enough dynamic range on your ADC to detect it. […] if your ADC had 100 db SNR […]


112.7dB (http://outpost.fr/rmaa/EMU_0204_USB.htm).


Is that enough to record the noise?
Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-01 18:45:31
Is that enough to record the noise?


Yes, compared to a 24 bit silent file, with the volume cranked up ridiculously high.
Title: RMAA fails to pick-up audible hiss
Post by: 2Bdecided on 2013-05-03 10:05:39
So if you just record using your equipment the noise is there in the resulting wav file, but RMAA doesn't see it?

Title: RMAA fails to pick-up audible hiss
Post by: skamp on 2013-05-03 11:06:46
I find this all very confusing, so instead of drawing the wrong conclusions, I uploaded two recordings (http://www.hydrogenaudio.org/forums/index.php?showtopic=100682) of my Fuze+: one with nothing playing (just the Fuze+ powered on), and one with digital silence playing.

Note that previously, I had generated my 16 bit silent file with Audacity. As I suspected, it does something more than just generate silence (it adds some very audible noise). This time I generated my 16 bit silent file with shngen from shntool. With the new silent file, I don't hear a difference between playing it, and playing nothing.

EDIT: I forgot to mention that both recordings were made loaded with my Shure IEMs.
Title: RMAA fails to pick-up audible hiss
Post by: bennetng on 2013-05-03 11:49:14
Note that previously, I had generated my 16 bit silent file with Audacity. As I suspected, it does something more than just generate silence (it adds some very audible noise).

The noise is probably dither. You should be able to disable that in Audacity's preferences. 16-bit dither without noise shaping is around -96dB. It should be inaudible unless you turn up the volume to an uncomfortable level. Also, digital silence means nothing, 8-bit digital silence is identical to 16-bit and 24-bit digital silence.
Title: RMAA fails to pick-up audible hiss
Post by: Arnold B. Krueger on 2013-05-03 13:47:27
My guess is that you don't have enough dynamic range on your ADC to detect it. […] if your ADC had 100 db SNR […]


112.7dB (http://outpost.fr/rmaa/EMU_0204_USB.htm).


Is that enough to record the noise?



One relevant parameter is what 0 dB means in terms of real world voltages.

An audio interface designed for pro audio generally puts out 4-6 volts with digital FS.

Digital Fs on a portable digital player would probably be more like 1 volt. 

There is 12 dB difference between 1 volt and 4 volts.

For the purpose of measuring portable digital players, the Emu 0204 may have only about 100 dB dynamic range, and that puts it well within 10 dB of the UUT, which is starting to be too close for good accuracy.

Title: RMAA fails to pick-up audible hiss
Post by: 2Bdecided on 2013-05-03 15:08:32
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  .

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.

Cheers,
David.

P.S. a test for another day would be a -90dB and then -100dB sinewave with noise shaped dither.
Title: RMAA fails to pick-up audible hiss
Post by: stephan_g on 2013-05-05 23:56:53
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 µVrms (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...)
Title: RMAA fails to pick-up audible hiss
Post by: stv014 on 2013-05-06 10:04:37
Tests in this (http://www.hydrogenaudio.org/forums/index.php?showtopic=100682&view=getnewpost) 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).