Screens

Cochise · Post by **Cochise** » Tue May 13, 2008 2:51 pm

I still have a 15'' LCD as my main monitor, not that bad being used to it, but since I spend time with other computers having larger screens it's like becoming a bit too tight...
Then I'm looking for a larger LCD screen.

Beyond the price/quality issue, which is a point I would like to discuss too, it looks like there's another thing I've to face with:
Some time ago, working at an audio/video project, I temporarily used a 17'' LCD Philips monitor and it was bringing humm in the audio.
The humm was present whatever the resolution; disappearing only with black screens.
Of course I use unbalanced audio connections, but there wasn't any change moving the cables around, and there's no humm with the 15''...

ADDED:

Using a Luna Box Z-link connected, and having analog audio cables far off from the screen and it's cable, I don't think it's due to unbalanced connections, however

Post by **garyb** » Tue May 13, 2008 3:00 pm

what kind of power supply does it have and where is it plugged in? is it on the same circuit as the rest of the audio AC supplies?

Cochise · Post by **Cochise** » Tue May 13, 2008 3:19 pm

The 17'' has built in PSU; the 15'' has external one. All the devices are connected to two wall socket (one beside the other; the same AC line).
I also considered the PSU, being the humm anyway similar to AC noise (although, if I well remember there was also high frequencies component), but it disappeared having the monitor connected and turned on, but displaying black screens (I noticed it during boot and XP startup. I know speakers should be turned off at startup but... it happens).
It also disappeared unplugging the video cable at the computer end or at the monitor end.

I haven't tried yet with the new machine. There's more space now between audio and graphic boards..

Cochise · Post by **Cochise** » Tue May 13, 2008 3:31 pm

Can the PSU bring humm in connection with the absorption?

Post by **garyb** » Tue May 13, 2008 3:42 pm

maybe. it depends on where the sheilding problem is.

well, i guess that particular monitor won't work for you....

Cochise · Post by **Cochise** » Thu May 15, 2008 6:34 am

Yes, it can be reverse connected.

I will try, although I'm not sure it's AC related.
I could manage to record the noise, then post it...

Cochise · Post by **Cochise** » Thu May 15, 2008 6:10 pm

I actually don't know if AC here is sine or square. I think it's sine, then the high freq components couldn't be harmonics of the 50 Hz from the mains...

Couldn't the noise anyway be related to the screen refresh rate?

Post by **garyb** » Thu May 15, 2008 6:58 pm

AC is sine. DC is square. the wall is AC although the power suppy inside the monitor certainly outputs some DC voltages..there are many sources for noise, however, poor shielding being the most likely...

husker · Post by **husker** » Thu May 15, 2008 8:11 pm

DC is square? wow!?! there's a lot of harmonics in that straight line

Post by **garyb** » Thu May 15, 2008 9:46 pm

Cochise · Post by **Cochise** » Fri May 16, 2008 7:44 am

AC is sine. Now I remember time ago I was reading about problems with cheap inverters (from 12V DC to mains voltage AC) due to square wave output...

The monitor video cable is quite thin in comparison with the others I've seen...

Cochise · Post by **Cochise** » Fri May 16, 2008 8:06 am

Cochise wrote: I could manage to record the noise, then post it...

I could even try to analyze it by oscilloscope/spectrum analyzer tools..

astroman · Post by **astroman** » Fri May 16, 2008 2:01 pm

the internal PSU of the Monitor is a switching power supply, which is capable to produce a lot of noise, which may leak into the PC/Audio by the video cable.
The noise level increases with load of the PSU - if the screen goes black it also goes into energy saving mode, drawing almost no power, so the noise disappears.

Another source might be the high voltage supply for the TFT's backlight, aka 'inverter'. Dunno which frequencies are used, tho.

using a different videocable may help to identfy the source

cheers, Tom

Cochise · Post by **Cochise** » Sat May 17, 2008 8:43 am

astroman wrote: Another source might be the high voltage supply for the TFT's backlight, aka 'inverter'. Dunno which frequencies are used, tho.

using a different videocable may help to identfy the source

cheers, Tom

Of course, the neon tubes... AFAIK even freq higher far from the top of the audio range are used with little transformers... but who can guess how it works for that monitor?

The cable has male connectors at both the ends; not the most easy to find.
Moreover I'm not going to use that specific monitor. Just trying to identify the characteristic to avoid, buying a new one.

Cochise · Post by **Cochise** » Sun May 18, 2008 11:04 am

Ok, I just had the Philips TFT here to make some try.

1) the noise is going on whatever orientation the AC plug has
2) no noise on the DAW using the monitor with the other PC, connected to the same AC socket
3) the pitch of the noise don't change using different refresh rate
4) the noise is present at Luna box output even if SFP ain't running
5) the noise do is present when the screen is black but the backlight is on, it disappears when backlight is off

these are waveform and spectrum of the noise I picked up at Lunabox output:

Cochise · Post by **Cochise** » Sun May 18, 2008 11:16 am

At now I don't think changing the cable will solve.
The other cables here are a bit thicker, but they also are older...
The Philips' cable has small 'nuts' (sorry, don't know the name of those things) at both the ends and it is marked: AWM E101344 STYLE 20276 VW-1 80°C 30V SPACE SHUTTLE-D.

chriskorff · Post by **chriskorff** » Tue May 20, 2008 8:29 am

I get something called coil whine from my LCD monitors - it's a right nuisance! But maybe that's what you're experiencing? It's fairly high-pitched, and it apparently has something to do with cheap power supplies (my monitors are both cheap, and have those in-line power supplies, a bit like laptops).

In my case it definitely isn't DC hum (balanced cables throughout, no 50Hz hum, active monitors and LCDs all sharing the same AC power source). Like you, it stops when the monitors go black (either screensaver, or, like you mentioned, when the screens blink off and on again during startup, and also just when you turn the monitors off). It doesn't have any effect on recorded audio, it's just interference between the monitors and the LCD screens' power supplies.

It's also a real bitch.

Does that sound like what might be happening to you?

astroman · Post by **astroman** » Tue May 20, 2008 3:05 pm

oh yeah, I hear you - it's a real pity...

a MacBook Pro (the old 15" model) would just drive me nuts - I was totally surprised about the beeeep level when I had to service one from the office and took it home.
My TFT starts to emit hearable sounds when I set the brightness too low, i.e changing brightness and contrast might drive it out of the hearing range.

Fortunately I don't have any of the noise in my guitar gear, except when the TFT would return from powersaving I hear the 'whining', too

cheers, Tom

Cochise · Post by **Cochise** » Wed May 21, 2008 11:13 am

@ chriskorff

Something very similar is happening here with the Philips 170S. Just the noise fundamental freq is lower, as you can see.

I just checked the screen in a ordinary PC configuration (onboard audio + Creative 2.1) and there's no noise...

astroman · Post by **astroman** » Wed May 21, 2008 4:01 pm

is the chassis of the PC properly grounded, the card sits firm in the slot and has a tight connection to the PC chassis ?

cheers, Tom