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Understanding signal strength

Discussion in 'F.O.H.' started by H-evolve, Apr 19, 2018.

  1. H-evolve

    H-evolve Member

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    I think I am confusing things when it comes to signal strength, especially when it comes to DI signals, say, of a guitar recorded directly in the interface.

    First off, though I'm neither an audio engineer, nor an electrical engineer (I'm a mechanical engineer :(... ), I still think I remember that dBs are actually a "Reference" unit system. It's the ratio of something over a reference, then converted to a logarithmic scale (very badly explained I think, but it should be close enough).

    Now, when talking about audio, it seems people are either talking about dbV or dBu. The reference for a dBV is 1 Vrms, while it is 0.7746Vrms for a dBu. So far so good?

    First question: in a DAW, when looking at the faders, are those dBu or dBV or something else?

    Second question: If I take my DI signal for example. I send my guitar signal directly into my interface... So I have a signal coming out of my guitar at "X Vrms", or if you prefer, X dBV or X dBu, whichever... I turn my input gain on my interface so that I get a signal in Reaper that peaks at maximum, say, -6dB...

    Now, I do I know if I "amplified" the guitar signal or if I "reduced" it? Where is "unity" gain on an interface? Is it at noon?

    To better illustrate my point, if I used said recorded signal to Reamp, I will get different results depending on how strong the recorded DI is. Suppose I recorded my signal at -30dB (as seen in Reaper fadder)... the Reamped sound will be very soft (even with Reamper at max gain), like if I would have lower the volume knob on my guitar.

    But if it is strong enough (suppose -1 dB in Reaper), then I won't have to max the gain of my Reamper and still reproduce quite correctly my true guitar sound, as if I'd be playing directly in the amp.

    Maybe this is a complex topic, but it really interests me to understand this.
     
  2. MrBongo

    MrBongo idiot at work

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    faders are control units. Analog faders are adjustable resistors with an additional amplification stage that can do, say, +10dB.
    Since dBu and dBV have the same logarithmic factors, the values would be the same.
    In a DAW you have dBFS (Full Scale), where 0dBFS is the maximum. This is shown on the meters. DAW faders still can do like +24dB, because it´s just a control unit.

    An interface or preamp can have numbers on the knobs that show the amount of amplification or attenuation. Line level units often start with -10 ( = 10dB of attenuation), many mic pres start at +6 or higher ( = at least 6dB of amplification all the time), because mic signals are weaker.
    The converter in your interface sets the point of reference and defines which signal level turns out to be 0dBFS in your DAW. Some interfaces have switchable reference points, so you can adjust them to consumer or pro signal levels.
    Depending on the reference level, you may either need amplification or attenuation for your desired signal level to be recorded.

    That is an issue of the gear used afterwards. Digitally recorded audio is nearly the same at 0dBFS and -30dBFS, except for minimal rounding difference and converter noise.
    Maybe an ampsim models an amplifier´s input response that is not the same at all gain settings. Maybe your real amp reacts differently at max gain than at half gain.
     

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