How is threshold in compression measured in negative dB?

[marc32123]

I am just starting to learn about compression... I am using the Scarlett Plug in Suite. In the manual about it, it defines the threshold knob as follows:

Threshold Knob- Sets the level at which compression begins. The lower this value is set, the more of the signal will be
compressed as the audio will compress when the threshold is reached. Rotate the TRSHLD dial anticlockwise to lower the threshold
and so increase the compression.

So it is saying that the threshold knob sets the level at which the compression begins. What doesn't make sense to me is how the threshold knob's value on my compressor is from the range of 0 to -80 dB. How can I possibly set the level to start compressing at negative anything? Isn't sound measured in positive dB's, not negative?

This is how I imagine it should be. Let's say I sang a vocal part and there was a normal dB range of 70 dB. But there was a peak at 80 dB that I wanted to lower. So I would set the threshold knob at 75 dB, the level at which I didn't want it to go above. (with a ratio of 5/1 or whatever)

So yeah, basically it just makes no sense at all for me how it could be negative. Can someone who understands why it is please explain thoroughly why it is negative in a way that I can make sense of it???

 

[Steenamaroo]

Hi,
Yeah, it's confusing at first. Gain is generally measured as a positive figure as in your examples, but the recorded audio signal is generally measured as a negative figure.

0dB (I don't know the different scales) is taken as the maximum possible amplitude in modern DAWs, and by your converters - This is a line that you cannot cross.
Ever heard of clipping? This is what happens when a signal is amplified to go beyond 0. The converters simply clip the top of the waveforms flat.

Anyway, when you're tracking in a modern DAW, you'd usually aim to peak at a healthy minus figure. Let's say -10, for example.
If that was the case and you wanted compression, the threshold would be lower than that again...maybe -13? -15?..whatever

Make sense?

Just remember that gain applied isn't the same as recording level.

 

[marc32123]

Hi,
Yeah, it's confusing at first. Gain is generally measured as a positive figure as in your examples, but the recorded audio signal is generally measured as a negative figure.

0dB (I don't know the different scales) is taken as the maximum possible amplitude in modern DAWs, and by your converters - This is a line that you cannot cross.
Ever heard of clipping? This is what happens when a signal is amplified to go beyond 0. The converters simply clip the top of the waveforms flat.

Anyway, when you're tracking in a modern DAW, you'd usually aim to peak at a healthy minus figure. Let's say -10, for example.
If that was the case and you wanted compression, the threshold would be lower than that again...maybe -13? -15?..whatever

Make sense?

Just remember that gain applied isn't the same as recording level.

Yeah it does. So basically, when listening to a recorded audio signal through a compressor, the 0 on the threshold knob actually stands for 100 %. And than, -10 on the compressor would actually be like 90. -20 like 80. -30 like 70. Kind of like that right?

 

[RAMI]

Yeah it does. So basically, when listening to a recorded audio signal through a compressor, the 0 on the threshold knob actually stands for 100 %. And than, -10 on the compressor would actually be like 90. -20 like 80. -30 like 70. Kind of like that right?

Well sort of, but.......

No, it means what it says. -10db***, for example, is exactly -10db***.

There are different kinds of decibel measurements, which makes it confusing. A Decibel meter will meter in positive figures. "A plane is 120db***", etc......
That's not the same as what we're talking about in your compressor. The signal coming into your compressor will be in the "negative" range, like -6db***, or at least it better be or else you'll distort the crap out of it.

Also, I tried to avoid getting into this because I'm not good with which is which. But there's an "analog" scale which is called one thing (dbf?***) and then there's the digital scale, which is (dbfs?***). A digital signal at -15db*** to -18db*** (depending on the specs of what we're talking about) is equal to 0db*** on an analog system. This is why we longer try to record "as hot as possible without clipping". In digital, you want to stay away from going over 0db***.

***
Guys, please forgive me about the DB and DBFS, etc.....And please correct me, too, because I'm pretty sure I fucked it up. But the jist of what I'm saying is right, I think. I'm just not sure about the "DB" terms.

 

[mixsit]

... Isn't sound measured in positive dB's, not negative?

This is how I imagine it should be. Let's say I sang a vocal part and there was a normal dB range of 70 dB. But there was a peak at 80 dB that I wanted to lower. So I would set the threshold knob at 75 dB, the level at which I didn't want it to go above. ...

This is what's setting it up for not computing. Sound is rated in positive dBSPL, and a range difference can be in dB. But it's SPL' (at least) is dependent on, well, something as unrelated as how close you are to a sound source, i.e. simply how loud it is in the air. But that has nothing at all to do with the 'level of a signal scaled on it's meter.

http://hux.com.au/Soapbox Items/World Audio level Reference.pdf
Threshold' could have been expressed on any one of these scales for example.

..the threshold knob's value on my compressor is from the range of 0 to -80 dB.

BTW, check this one. Compressor Threshold Range -40dBu to +20dBu
160A | dbx Professional Audio

 

[Steenamaroo]

Yeah it does. So basically, when listening to a recorded audio signal through a compressor, the 0 on the threshold knob actually stands for 100 %. And than, -10 on the compressor would actually be like 90. -20 like 80. -30 like 70. Kind of like that right?

Yeah, as long 100% means no compression... ^^

I wouldn't think of it in percentages, though, because it's all relative to the dynamic range of your recorded audio.

If you record constant distorted guitar that ranges from -15 to -5 on your scale, your compressor wont have any effect unless the threshold is at -6 or lower.

 

[marc32123]

Yeah, as long 100% means no compression... ^^

I wouldn't think of it in percentages, though, because it's all relative to the dynamic range of your recorded audio.

If you record constant distorted guitar that ranges from -15 to -5 on your scale, your compressor wont have any effect unless the threshold is at -6 or lower.

Okay I got you. Another thing. Is there anyway I can check what the actual dB's are on the waveform in my audio recordings?

For example, if I record a track in my DAW, I can see the track I recorded, and there is a waveform that shows the amplitude across the recorded track. When I go to open up the track to view the waveform even closer, there is no scale so I can see what the exact dB's are at different points on the waveform.

 

[Steenamaroo]

Most Daws will have a fader with a little meter beside it.
The meter should give you a rough idea of what's going on, but it should also record peak values too.

I don't know about checking specific points on a waveform, but really you shouldn't have to go in that deep.

You can also get some great metering plugins for free, like this one.

 

[Bobbsy]

It's worth saying that we tend to get sloppy about the use of "dB". As an absolute measure, it's pretty much useless unless there are some fancy initials after it. A dB reading is a comparison between an artitray "zero point" and another level. For most computer based stuff, what's meant is "dB(FS)" which stands for "Full Scale" and means as system where the 0dB point is the maximum the system can use...basically at 0dB(FS) the binary number that describes a sample is at 111111111111111111111111 in a 24 bit system and can go no farther.

Trouble is, there are all sorts of other scales used for sound, particularly in the analogue world. There re thing like dBu, dBV, dBVU and so on. For measuring live sound levels, you're into dB(SPL) which stand for sound pressure level. Heck, the satellite that feeds the dish on your roof is measured in dBW which means that the power is specified in relation to 1 Watt.

The only time you can leave off the extra initials is when you're talking about a change of level.

I agree it can get confusing.

 

[Farview]

The numbers on the threshold knob relate to the numbers on the meters. If you set the threshold at -10, the signal has to get higher than -10 for the compressor to kick in.

It does get confusing because there are several db scales that all measure different things differently. They are not interchangeable.

 

[Steenamaroo]

The only time you can leave off the extra initials is when you're talking about a change of level.
I agree it can get confusing.

I think a simple explanation of how a compressor works is another perfect place to leave of extra initials, but the info is good to know.