Audio Level Meter vs Audio Output Levels

[radiodungog]

Hello Dmitry,

At Radio Dungog we have added a radio studio mixer to our system.  RadioBOSS feeds into the mixer and so does the presenter and guest mics. and we now have the luxury of headphones and VU meters.  Yay !  We have done this to help make live broadcasts a little easier for the presenters.

I noticed something weird as we did the audio lineups.

We used a 400 Hz test tone at 0 VU on RadioBOSS

We set the mixer fader to a preset to show 0 VU on the studio mixer.

We then reduced the audio fader on the screen of RadioBOSS 1dB at a time to observe the linearity.  This was the slider labelled "Gain".  We wound the level down from 0 VU by 1dB at a time until we got to about -6dB.

The RadioBOSS output bargraphs reduced as expected by 1dB at at time BUT the VU meter on our mixer remained the same until we had reduced the RadioBOSS level to below about -6dB.

Then with the RadioBOSS "Gain" control at -6dB and the bargraphs at -6dB the studio mixer VU meter finally dropped from 0 VU.

Any ideas on what might cause this ?


Thanks, Jamie C.

 

[djsoft]

Maybe the signal is being amplified somewhere between RadioBOSS and Mixer? Some audio cards can output at +6dB for example... AFAIK level meter in RadioBOSS is pretty accurate.

 

[radiodungog]

Hello Dmitry,

It's not the output level that I was concerned with - it was the linearity between 0 VU and -6dB ... but I will do some checks again and see what I can find.

Thanks, Jamie C.

 

[DJSTU]

Hello Jamie C

Have just been reading this post....
That is never going to work Jamie. You cannot match the output of RB vu versus a mixing desk vu. You are testing oranges against lemons. The two were never designed to match.
The sound must be absolutely clean and untainted coming from the software. For instance did you have the AGC turned off ? Are there any other influences that could have affected RB output ? Any software sound processing ?
Are there gain settings in say Win7 mixing desk that could affect it ?
What input levels is the desk set at.
My commercial soundcards output at -10db. The mixing desk faders are set to - 10db mark when on-air. The output on the vu meters on the desk when playing a tone should peak at 0db.
In a commercial operation you work at -10db so as if someone pushes the fader up too far it doesn't over modulate.

You will need to measure the output at the sound card itself before it enters the mixing desk inputs.

 

[djsoft]

It's not the output level that I was concerned with - it was the linearity between 0 VU and -6dB ... but I will do some checks again and see what I can find.

You can try creating test tones (in any audio editor) playing at 0dB, -1dB and so on and check how level is displayed in RadioBOSS and Mixer.

 

[radiodungog]

DJSTU, Dmitry,

This is what I did:
1. Created a 400 Hz test tone with Audacity with an output of 1.0 - so I assumed that was 0 VU
2. Put that test tone on RadioBOSS and played it
3. The VU meter on RadioBOSS came up with a full scale reading at 0 VU
4. I then adjusted the "Gain" slider on RadioBOSS in 1 dB steps downwards and observed the bar graph respond accordingly ... down to -6 dB
5. I then wound up the "Gain" slider to 0 level again
6. I then fed the output of the PC to our studio mixer and adjusted it's fader so that 0 VU was displayed ... hence 0 VU on RadioBOSS corresponded to 0 VU on the studio mixer  ...  YES !
7. I then adjusted the RadioBOSS "Gain" slider down in 1dB steps and observed the VU meter on the studio mixer which indicated as follows:
RadioBOSS setting in dB ... Studio Mixer VU Meter reading in dB
0 ... 0
-1 ... 0
-2 ... 0
-3 ... 0
-4 ... 0
-5 ... 0
-6 ...  -1

8. Compression, Auto-Gain and all those things were switched off on RadioBOSS.

So what I am saying is that I observed a non-linear response between RadioBOSS and the Stidio Mixer VU meter when it really should have been more consistent.

Does this make sense Stu ?

Jamie C.

 

[radiodungog]

DJSTU, Dmitry,

I called up the manufacturer of the radio mixer panel (Elan Audio, in Perth, Western Australia) and they were VERY helpful.  They stated that they go to a lot of trouble to ensure that the calibration on the analog VU meter is accurate and that they calibrate them in the factory.

I described the symptoms to them and they think that the problem is some built in compression in the PC sound card.  They said that many sounds cards do this to prevent them being driven too hard into distortion.  They said that using their mixer VU meter as a mrasurement device for calibration was quite acceptable.

So now I will check the output of the PC sound card with a CRO to see if the output temains the same at -5dB and up on RadioBOSS and if so go for another sound card with slightly better performance .... like this one:

http://www.pccasegear.com/index.php?main_page=product_info&products_id=19169


Making progress ...


Thanks guys,  Jamie C.

 

[DJSTU]

Tell me how much your sound card cost and I will be able to tell you the answer instantly that you want to know. Is it a USB one at all ?

Digital environments do not handle anything above 0 db well. This is why you should be aiming to peak at 0db in a digital environment.

Commercial soundcards will handle sound better, at a higher level,  because of the extra cost of engineering them. Quality costs, but the cards are engineered for 24/7 operation and cost huge dollars, in the thousands.

Older analogue environments used to be at +4db. This was to allow the sound to drown out the hiss. It improved the Signal to Noise Ratio.

 

[DJSTU]

I have just had a look at that sound card at $169 you had the URL for.

That would never qualify as a good sound card.
But it will do and be adequate just so long as you don't overdrive it by going over 0 db.
In fact any sound cards I have similar to that (I have a few Behringer ones) I make sure the software output peaks at absolute maximum -3db.

Better still, with one sound card I peak it at -6db and then use a hardware compressor to lift the level at output which is in turn followed by a hardware peak limiter.

Also make sure on RadioBoss you have the Low Pass Filter turned on to 15,000Hz.

 

[DJSTU]

As a suggestion Jamie, try getting yourself a software peak vu meter. There are quite a few out there at no cost.
This will enable you to measure your output before going to the desk. It will measure the output directly from RadioBoss before entering the sound chain. That might help you to see if the sound card is doing anything.

I had the feeling that RB had an output peak limiter built in, but cannot find it.
So therefore that is a matter I will raise one day with Dmitry about building one in (based on a db level).

 

[djsoft]

radiodungog
You can create test tones with different levels in Audacity:
- click Generate->Tone, set Amplitude to 1 (which is 0dB)
- click Effect->Amplify and change the level, eg. -3dB.
I've tried -1, -3, -6 and -10dB and RadioBOSS shows levels correctly.

Well, RB and Mixer have correct level meters, which means the problem is in the sound card. I'd started with checking sound card settings, it looks like its output level is set to +5dB, that's why you don't see level decrease until you drop Gain down to -6dB.

 

[djsoft]

I had the feeling that RB had an output peak limiter built in, but cannot find it.
So therefore that is a matter I will raise one day with Dmitry about building one in (based on a db level).

There's no peak VU meter There's level meter on the FX tab, but it shows peaks only for a short time (a black line above the colored level).

And by the way - the level meter shows the level after FX, Autoamp, crossfades and everything was applied. But it doesn't take into account the output volume control. If volume control is at maximum and sound card itself doesn't change the level - RB VU meter and Mixer VU should show the same level.

 

[radiodungog]

Stu, Dmitry,

Thanks for the answers.

We don't need a +4dBm balanced output from the PC as the mixer input it goes into is an RCA input and can be altered and then adjusted on the mixer fader.

The ONLY place we use 600 ohm balanced line and levels is at the mixer output and the equipment that follows the mixer.  Here is a list of the gear that operates 600 ohms balanced at a nominal 0 VU level of +4dBm and an overload dynamic range of 20dB over nominal .... so 24dBm peak before clipping or distortion.

Elan Audio - Merlin 8 Radio Mixer (various input levels and configs, 600 ohms balanced output at +4dBm)
APHEX Compellor 320A (audio processor compressor, inputs and outputs 600 ohm balanced at +4dBm)
BARIX 500 (audio to IP streaming encoder, inputs 600 ohm balanced at +4dBm)
WiFi Radio Link
BARIX 500 (IP streaming to audio decoder, outputs 600 ohm balanced at +4dBm)
APHEX Dominator II 722 (audio brick wall limiter, inputs and outputs 600 ohm balanced at +4dBm)
FM Stereo Transmitter (digital modulation, inputs 600 ohm balanced at +4dBm)

So once we get the PC soundcard sorted out we should have the system under control.

What is it Stu about that external soundcard that you don't like ?

Also, I visited a few community radio stations two weeks ago that normalised their MP3 files by running them through MP3trim and selected the normalised level of .-3dB ... that was their way of preparing the files for broadcast.


Thanks, Jamie C.

 

[djsoft]

Also, I visited a few community radio stations two weeks ago that normalised their MP3 files by running them through MP3trim and selected the normalised level of .-3dB ... that was their way of preparing the files for broadcast.

You don't' need to do it in RadioBOSS. Internally it can process sound levels over 0dB without quality degradation. To control output level it's easier to use the "Gain" slider (or control level via a plugin). No need to change sound files levels.

 

[radiodungog]

Dmitry,

It was their way of normalising all the MP3 music tracks ... run all MP3 files through MP3trim so they are the same level.  Then their replay software didn't need to do any more work.

With ReplayGain on RadioBOSS does it just auto-adjust by itself OR do we need to enable the AGC function ?


Thanks, Jamie C.

 

[djsoft]

With ReplayGain on RadioBOSS does it just auto-adjust by itself OR do we need to enable the AGC function ?

You need to enable ReplayGain in Settings->Playback->Miscellaneous. ReplayGain doesn't depend on AGC - it doesn't matter if AGC is on or off.

 

[radiodungog]

Dmitry,

Okay got it.  By default that setting seems to be +6dB on the screen for tracks with or without ReplayGain ... is that correct ?  Leave it at +6dB or change it to 0dB ?


Thanks, Jamie C.

 

[djsoft]

By default it should be +6dB for tracks with ReplayGain and 0dB for tracks without ReplayGain. Those settings work fine in most cases.

We add +6dB for tracks with ReplayGain because most ReplayGain scanners use target level of -6dB.

 

[radiodungog]

Dmitry,

Good, thanks ... would you consder putting a few tools on your website to help us like ReplayGain ?  This would make the learning process and support a bit easier for people.  Just a suggestion ...


Thanks, Jamie C.

 

[djsoft]

Yes, I think a guide will be published on how to use ReplayGain.