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Q: "I can see red lights on my master track. Can my songs ever be recovered?" With Audio!

A post by David Mellor
Thursday November 30, 2006
An RP visitor is seeing red - at the top of the meters on his master track. Does this mean his music is clipped, or can it be saved from dreaded odd-order harmonic distortion?
Q: "I can see red lights on my master track. Can my songs ever be recovered?" With Audio!

Question from a Audio Masterclass visitor: " I've been mixing some of my songs in my home studio with Pro Tools. My concern is that the lights on top of most of my tracks turn red and I have to reduce the master volume sometimes to about -7 dB to avoid the red light on my master track. Is it possible for my songs to recover a normal volume after the mastering? Thanks in advance for your help." - Jean-Baptiste

I'll ask a Q myself - "What are meters for?"

The answer to that is so that you can see that your recording is achieving a healthy level above the noise that is present in any medium, analog or digital. And also so that you can see that you are not driving the medium into distortion or clipping.

So generally you want to see the lights going high up on the scale, but you don't want red, as that means that the tops of the peaks are sheared off, or 'clipped' as we call it, causing intense distortion, largely of the exceedingly undesirable odd-order kind.

But does a red light always mean that clipping has occurred? Well... sometimes it does, sometimes it doesn't.

Let me generalize to the jobs that all hard disk workstation softwares have to perform.

When you are recording a new track, that track must not be clipped. So the red light on the track that you are recording means that clipping is occurring. Red, in this case, means bad.

When you are mixing, you might be mixing several or many high-level signals. So although none of the signals themselves are clipped or cause red lights, the summation of them does cause the red lights to show in the master track. (I hate calling the master track a 'track' because it isn't a track and it causes confusion, but that has become the generally accepted term.)

Now, if you can see red lights in the master track and you bounce this to disk, your bounce will be clipped. The red lights indicate that you have reached, and attempted to exceed, the maximum possible level in the digital file.

So what do you do? You could lower all the faders by a few dB. If your software allows you to move all of the faders by the same amount, that would be a handy tool.

Or you could lower the master fader.

But hang on a moment. If the red light is showing on the master track, surely the audio is already clipped. Lowering the master fader will lower the level, but the signal will still be clipped.

Actually, on a properly designed system, this is not so. And that is because of the wonderful feature of headroom.

The good old analog consoles, on which DAW design is based, had headroom too.

In the analog world, although you might regard scale point 6 on your peak program meters as the absolute peak you could send to your recorder, the console itself would have more than ten decibels of headroom in hand.

So if you had the master faders at 0 dB (no change in level) and your mix was peaking above the maximum allowable, then you could lower the master faders. Your signal was not clipped in the console, and would record cleanly.

This is true of DAWs too - except a properly designed DAW will have a massive amount of headroom - more than you could possibly ever need. Even if you have to pull down the master fader by 30 dB, your mix will still not be clipped. And I just can't imagine that situation would occur.

Here's what Digidesign has to say...

"This brings up another important point: while you can sum hundreds of 'real world' tracks in the Pro Tools mixer without clipping the 'input side' of the mix bus, you can easily clip the “output” side of the bus (when you go to a 24-bit DAC or 24-bit digital output, or submix signals to an Aux Input). That’s why it’s so important and useful to utilize Master Faders - they utilize no DSP, provide metering that lets you know if you’re clipping on the way to your destination, and allow you to trim the output level of your sub- or final-mix without losing any resolution (because resolution is preserved by the 48-bit mixer). Because Master Faders preserve resolution when trimming, you don’t have to trim down all of the contributing faders to the mix to obtain the optimal output level. This is just like a well-designed analog console - you trim down the master fader to avoid clipping the output circuitry in the console while the mix bus headroom allows the input faders to stay
in their sweet spot."

So, the answer to the question is that if the software is properly designed, then make absolutely sure that no red lights show when you are recording. If red lights show when you mix, you can pull down the master fader without fear of your mix being clipped.

Oh... you want proof? OK here it is.

Here's what I did...

I opened a new file in cheap 'n' cheerful Pro Tools LE using an Mbox 2 interface. I created thirty-two tracks and filled each one with a 400 Hz sine wave at 0 dBFS (full level before clipping).

Then I mixed them together.

Because all the tracks are identical and in phase, then when you double the number of tracks, the level also doubles, which is the same as rising by 6 dB.

So one track on its own is at 0 dBFS, two tracks mixed would be at +6 dBFS, if they were not clipped. Four tracks would be +12 dBFS, eight tracks +18 dBFS, sixteen tracks +24 dBFS and thirty-two tracks a massive +30 dBFS!

That should be so clipped that possibly half of the known universe would be destroyed.

But if I lower all of the fader levels of the tracks by just over 30 dB, I get this...

[Warning: High level sine and square waves can kill speakers, headphones and ears. Turn it down!]

Depending on your browser, you may have to click twice.

That is thirty-two tracks, each at -30 dB mixed together. It's very close to 0 dBFS and it's clean!

OK, you would expect that. Now let's put the tracks back up to 0 dB and mix them. This is the result...

Ouch! That is now so clipped it's almost a square wave.

But if I bring the master fader down by just over 30 dB...

It's clean again! This proves that Pro Tools LE has at least 30 dB of headroom and you are perfectly free to pull down the master fader by as much as this, should you feel the need. If there are no red lights in the master track after you have pulled the master fader down, there will be no clipping in your mix.

But... does anyone know of any software for which this is not true? That would be interesting to find out.

A post by David Mellor
Thursday November 30, 2006 ARCHIVE
David Mellor has been creating music and recording in professional and home studios for more than 30 years. This website is all about learning how to improve and have more fun with music and recording. If you enjoy creating music and recording it, then you're definitely in the right place :-)
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