Residual channel noise

[kozikowski]

I think that's the limit of the currently available chipset. I can get amazingly low noise floors when the machine is paying attention to the S/PDif input, but the instant I switch to the stereo analog input....

[kozikowski]

Here's one with the UCA-202 idling with nothing analog connected.

[kozikowski]

Here's one with the regular analog input and a dummy plug so it doesn't try to go to S/PDIF. The Audacity input fader is all the way down. It does jump every so often from, I assume, electronic popcorn noise. Koz

[steve]

The point that I find very strange about the original poster's question is (emphasis mine):

I have constant residual noise that fluctuates between -72 db and -84 db when I click pause and then record with 1.3.12 on a Windows 7 PC with the Audacity input all the way down to zero. After relocating cables, trying everything unplugged, plugged in but all gain controls muted or set to zero, same problem. It is on both channels. Used on board sound (HD AC97) a couple of PCI sound cards with the on board disabled in BIOS, same problem.

Installed 1.3.12 on a different Windows 2000 PC, tried all the same sound cards, on board and PCI sound cards, same problem. Doesn’t matter what is plugged in or if nothing is plugged in and all inputs on the PC turned down or muted.

If all gain controls muted or set to zero there should be no signal at all, a signal level of -infinity dB, all sample values 0.000000.

Of course I agree with Gale's comment that muting everything is "not a useful thing to do", but it is still curious why it is happening, and it does have real world implications.

Perhaps audio_guy_77 has simply missed one audio input, and there is an input still active, but I don't think that we can assume this is the case when he has assured us that all gain controls muted or set to zero. If anyone can reproduce his results, then we need to take it seriously. If everyone else gets absolute silence when all gain controls muted or set to zero, then I think we can only draw the less than satisfactory conclusion that it is probably an unresolved problem on his computers.

Why does it matter?
A "residual noise that fluctuates between -72 db and -84 db" is equivalent to a randomization of the sample values of between 1 and 3 bits for each 16 bit sample. As this is of unknown origin, we cannot assume that this noise source will magically disappear as soon as we start recording for real. It would seem more logical that this noise is going to be present on ALL recordings, IN ADDITION to any other sources of noise. Thus the "amazingly low noise floor" that Koz reports when using S/PDIF will not be achievable, and in all cases, the SNR will be around 6 dB worse than it should be.

I think that we need to try to get to the bottom of what is causing this if we possibly can, either by discovering the source of the noise, or by establishing that there is a problem on (some) machines.
To this end:

1. Is everyone else able to fully mute their recording input and "record" total silence in 32 bit float format? (When applied to total silence, the "Amplify" effect will show the "New Peak Amplitude (dB)" to be "-infinity".)
2. What is the correct way to totally mute all input on a Windows 7 machine?

[Gale Andrews]

Just to clarify, I only get the "-infinity" recording if I mute the input using the mute button on the "Recording" tab of Sound. Zeroing the input level on the Audacity input slider (double-click shows "0") or on the Windows slider (value at 0) gives a recording of noise at about -55 dB. If I record for real with the input slider up at 0.8, passages between audio are around -53 dB, hardly any noisier. So the noise I get seems just consistent Windows noise to me.

To mute or adjust input levels in Windows Vista or 7, right-click over the speaker icon > Recording Devices, which brings you to the "Recording" tab of "Sound". Select or right-click the input, choose Properties, click "Levels". You have a slider (which Audacity controls) and a mute button (which Audacity has no access to). The value displayed for level does not change when you press the Mute button.

You can then click OK on the "Levels" tab of "Recording", click "Playback", select or right-click over the playback device, choose "Properties", click "Levels" then adjust or mute playback of the output. On a few machines, you may also have here the ability to adjust or mute playback of your inputs. This would be extremely useful if it worked, as it would be low latency "hardware playthrough", but most machines won't have sliders or mute buttons for inputs (or they won't have any useful effect).

I only see these input playback sliders on my desktop, but they do not give playback of the input (no drivers I can find change that, but it is perfectly possible there are registry hacks or inf file hacks to make input playback work).

Muting playback of the inputs does however shave a couple of dB off the system noise (when the input being recorded from isn't muted).



Gale

[PGA]

Windows 7, RealTek HD Audio, Audacity 1.3.14 recording from "Stereo Mix"
There was no ability through Sound to mute the recording inputs. The only mute options were in the Volume Mixer. So I ran some tests on those. The first test was with both System Sounds and Firefox muted (Speakers and Audacity unmuted). I then recorded a 5-6 second sample of "silence" and ran that through Analyze...Plot Spectrum. I took two screengrabs (the Sound Device status and the Frequency Spectrum plot). I then unmuted System Sounds and repeated the test process and screengrabbing. Finally I unmuted Firefox and re-ran the test process and screengrabbing. I'd love to show you all six screen grabs but it seems there is a forum limit of three attached files per post. So all I can show you is the Frequency plots.

Frequency spectrum of "silence" #1

Audacity7B.JPG (70.82 KiB) Viewed 1681 times

Frequency spectrum of "silence" #2

Audacity7D.JPG (67.94 KiB) Viewed 1681 times

Frequency spectrum of "silence" #3

Audacity7F.JPG (69.77 KiB) Viewed 1681 times

Having noticed that there was a difference; and having also noticed that the hard-drive was doing some background "chattering", I re-ran the test process. Without changing the mute status of the Sound Devices I discovered I could get different but very similar frequncy curves.

Conclusions:
It would seem that some components within the desktop PC are capable of generating electronic "noise". The soundcard (which may well be the culprit) is capable of picking up this "noise" and placing it onto its outputs. Audacity faithfully records whatever it is given.

regards,
Peter
P.S. The Upload Attachment tab states "one or more files", implying that it is possible to select all the files in one go. I could only select one file at a time. Painfully slow!

[waxcylinder]

And here is my Edirol UA-1EX idling with nothing but my passive switches' empty channel selected, I get a small mains spike.

Edirol UA-1EX idling.PNG (20.26 KiB) Viewed 1679 times

Curiously with my ART DJ-Pre11 preamp connected and my TT on and rotating (no record) it looks even quieter.

Edirol UA-1EX idling with pre-amp.PNG (18.5 KiB) Viewed 1679 times

With this I'm assuming that the mains spike disappears as the TT is grounded to the preamp, which is grounded to the house mains (a common single mains plug for all my reocording kit - fed to a surge-protected multi-way drop)

WC

[PGA]

Further to my post above, I've done a little more investigation...

I used the Windows Control panel to disable all the jack socket inputs on the front and rear panels of my desktop PC (i.e. they are all now greyed out on the Recording tab of the Sound window; the only enabled device is "Stereo Mix"). I then re-ran the tests - same sort of result, a low level of low frequency noise; and the disk drive occasionally "chattering". But I think I now know what the disk drive activity is: I think it's Audacity"! Drive C: is where the Temporary files are stored; presumably these are being written to as the recording is being made? And to correct one point of detail in my previous post: when I said Firefox I should have said Thunderbird (the Mozilla e-mail software). That sound control will, I presume, be for the "New mail" alert sound.

One positive to come out of this: I now realise that I need to mute the System Sounds and Thunderbird sources before I start a Stereo Mix recording. I never did it before and, thus far, have never been hit with an unwanted sound in the middle of a recording. I guess I've just been lucky!

[steve]

On Windows you should be able to switch off system sounds. Exactly how to do that varies depending on which version of Windows, but the settings can be found in the Windows Control Panel. If you record from Stereo Mix and you want to avoid recording system sounds such as alerts and incoming mail, it is a good idea to disable all system sounds.

Just to clarify, I only get the "-infinity" recording if I mute the input using the mute button on the "Recording" tab of Sound.

Windows 7, RealTek HD Audio, Audacity 1.3.14 recording from "Stereo Mix"
There was no ability through Sound to mute the recording inputs.

@PGA
Are you using "DirectSound" in Audacity? ("Host" in the Device Toolbar)
Do you have your soundcard set to use "Exclusive" mode?

I guess it's not that surprising that there may be some residual noise if the inputs are just "turned down". In fact, the more I look at it, the less surprising it seems to be.

When I record from my Behringer UCA 202 I can either connect directly to it through ALSA, or via PulseAudio (on Linux)

When connected directly, there are no recording level controls and no mute. In effect, the input signal to Audacity comes straight from the A/D converters in the UCA 202. With no input to the UCA 202 I see an RMS noise level of -90 dB, around -76 dB peak, which is about as good as can be expected from a 16 bit sound card.

When connecting via PulseAudio, recording level control is available as is "Mute".
PulseAudio is able to adjust the signal level by controlling the digital signal as it passes between the UCA 202 and Audacity. In effect it can scale the digital signal.
When the recording level is turned up to maximum, the noise level is the same as when connecting directly with ALSA. Lowering the recording level reduces the noise floor so that at zero recording level (or mute) the recorded signal is absolute silence.

I see now that it makes sense that there will be some residual noise level if Audacity is connecting directly to the hardware rather than being scaled. That noise will just be the noise floor of the sound card's A/D converter, and a peak level of around -76 dB is to be expected.

[PGA]

@Steve,
No, I wasn't using DirectSound, I was using MME. I'm rushing to get ready to take some AV sequences to show to a local organisation so don't have time to report more fully. I'll be online again tomorrow am (UK time) so will post a full update ready for you tomorrow.