issue05

EMUSIC-L Digest                                      Volume 43, Issue 05

This issue's topics: Sound reinforcement
	
	Help w/ Gates & Vocals (2 messages)
	How Many Speakers fit on an Amp? (3 messages)

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Date:         Tue, 18 Aug 1992 10:38:26 EDT
From:         MRLCC@CUNYVM.BITNET
Subject:      Help w/ Gates & Vocals

   Can anyone please give me the low down on compressers and/or gates? I am
using a non-digital studio and am having some trouble with vocalist blowing
the peak meters at times. I've heard that compressers can work for me and I
have access to them but have never used them. Can anyone enlighten me?

   I also heard that exciters can bring new life to vocals and since I am
starting to lay singing tracks (I can handle rap track with no problem) I
find that these seem to lack. Any tips from the pros out there?

   Please respond or email me some help.
-------

MARC(MRLCC@CUNYVM.CUNY.EDU)

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Date:         Tue, 18 Aug 1992 23:12:33 GMT
From:         Jon W{tte 
Subject:      Re: Help w/ Gates & Vocals

>  writes:

      Can anyone please give me the low down on compressers and/or gates? I am
   using a non-digital studio and am having some trouble with vocalist blowing
   the peak meters at times. I've heard that compressers can work for me and I
   have access to them but have never used them. Can anyone enlighten me?

A compressor can work in two modes: RMS or peak. RMS is cool for stuff
that varies in a sensible fashion (I would use it for auto-levelling
of a video track recording...) and Peak mode is what you want to cut
peaks (and to squeeze more punch into the final mix!)

What a compressor (in peak mode) does, is it reduces its gain when
the sound goes above a certain level. What that level is, and how
much it reduces the gain, are usually settable parameters. If you
want to, you can reduce the gain so that the output level never
exceeds the trigger level; that's eternity:1 compression, and
is also called limitting. Or you could opt to reduce only half of
the amount above the trigger level for a very soft compression.
I usually do 4:1 or 5:1 on vocals, and as a safeguard when sampling
since sampling clipping distortion sounds TERRIBLE.

The reduction also comes in an envelope: you set how fast the
compressor responds to a peak, and how long it takes for it to
return to its "normal" state. Use a very fast attack (.1 ms)
to get a good peak protection; use a longer attack to have
some of the peak "punch through". Adjust the decay to taste; a
fairly fast decay usually works OK (100 ms) - the wrong value will
result in a "pumping" sound - much like your favourite disco radio
station :-)

Compressors also come in stereo pairs, where a reduction in one
channel is reflected in the other as well, to avoid a skewed
stereo image.

Calculation example:

Compression 4:1, trig level -10 dB

	Input	Output
	-30 dB	-30 dB
	-20 dB	-20 dB
	-10 dB	-10 dB
	-5 dB	-8.75 dB
	0 dB	-7.5 dB
	5 dB	-6.25 dB
	10 dB	-5 dB
	20 dB	-2.5 dB
	30 dB	0 dB

Good compressors also have a little fuzz in the trig level
area ("soft knee") to make the compression sound smoother.
and start a little gentler, a little earlier than the actual
setting.

Looking at my Alesis 3630 from a recent vocal recording, it
says Trig -15dB, Comp 3:1, Attack .1ms, Release 180ms, and
Output +8dB

The last parameter is simply amplification; since a sound
usually will be compressed, it will lose in strength, and
that loss needs to be equalized with added amplification.
That also makes softer sounds come more to the front of the
mix.

Hope this helps (and I'm no mixing engineer, just a hobbyist...)

--
- I have decided that it is not boxes but my lack of skill that's the problem.
- Traitor! This kind of attitude will get you nowhere around here. If you must
  know, it's not your boxes that are the problem, it's your lack of a
  sufficient number of boxes. Go out and buy something.

------------------------------
Date:         Sat, 1 Aug 1992 08:23:48 GMT
From:         MuffinHead 
Subject:      Re: How Many Speakers fit on an Amp?

Here's something that's always eluded me:

   Say you have two 3-way cabinets and you hook them up in series. Um, lemme
draw a picture...

    X  = passive x-over (cap, coil, cap/coil)
    [< = driver

                      cab 1                        cab 2

                 ---X----[<----               ---X----[<----
          A     /              \      B      /              \    C
     ---------------X----[<----------------------X----[<----------------
     |          \              /             \              /          |
     |           ---X----[<----               ---X----[<----           |
     |                                                                 |
 amp +                                                                 - amp


   So, would the signal (frequencies) be the same at points A, B, & C or would
the first series of x-overs muck up the signal?


--
_____________________________________________________________________________
$ smills@umaxc.weeg.uiowa.edu $ That's the way it crumbles... cookie-wise.  $
$ MuffinHed@aol.com           $          -Jack Lemmon, "The Apartment"      $
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Date:         Mon, 3 Aug 1992 08:51:40 -0400
From:         RAY BROHINSKY 
Subject:      Re: How Many Speakers fit on an Amp?

in response to smills' drawing:

although the voltage seen by each speaker is influenced by its crossover
network, the volltage applied to the system from amp+/- is what the
amp applies. This means that the current in any series point (like A,
B, or C) is the same. It is either allowed to pass, or not.

Thus, if the three crossover networks in cab 1 have a different set
of characteristics, and none of them will allow current related
to a certain frequency range to pass, cab 2 will suffer from the
same `dead spot', simply because no current can pass one point in the
string if it has that frequency characteristic. However, if the crossovers
are chosen to allow current of all frequencies of interest to pass,
then both speakers will have current available to them at whatever
frequencies the amp pumps into the string.

If this seems counterintuitive, reduce it to a simplest example: since
all characteristics of a parallel network can be reduced to a single
series object, consider when all the crossovers pass AC above 10hz,
but block DC and freqs below 10hz. This is equvalent to a capacitor-
resistor string with a (theoretical) corner at 10hz. Two of these
in series will look like just as much an open circuit below 10hz,
and current will flow above that.

Allowing for the fact that current may be reduced, rather than just let
flow or blocked, by the characteristics in the strings, you already have
an initial limitation on what can come out of the speakers.

Add to this the two networks in series seen from a voltage perspective.
If one has a higher impedance at (say) 10Khz than the other, it will drop
more voltage, and an o'scope measurement across its coils will show this.
(although a spectrum analyzer might show it better!). Depending on the
efficiencies and makeup of the drivers, this may be insignificant
in the greater scheme of what comes out of the boxes (ie, coil speakers
develope output by moving a cone: the cone is moved by an electromagnet
in a magnetic field, the electromagnet's field is current-related; piezo
drivers are a purly-voltage-driven [nearly so, anyway] phenomena, and
are more effected by drops in voltage).

Finally, within each speaker cab, you have three different elements,
and passive crossovers. The passive crossovers are each made slightly
differently, and although they have a characteristic that can be
measured, it rarely is: you are told what the average characteristic
of the `average' crossover network from this line, or of this type, or
maybe even some other network's characteristic, in its specs. This
variance can make the contents of the two boxes quite different.
Likewise, the driver elements may or may not be similar(even if they
come from the same manufacturer!).

The most noticeable effect, if things are close enough, will be what
kind of drivers are used, and how the band-edges of the crossovers
are matched. In the first case, you generally have current refused
for the coil-driven speakers that is `out of their range', and
the crossover for the piezo tweeter (if such is present) will worry
only about filtering low freqs and perhaps even re-shaping the
bandpass to make up for the piezo's frequency characteristic.

Then, of course, you are dealing with fairly large cabinets, here. The effect
of the baffling and placement of the speakers in a non-symmetrical world
will have a very large effect on the sound coming out of them.

In answer to your original question is that the frequencies would be
the same at points A,B,and C, regardless of how you measured them
(voltage or current, although the voltages at B would be different
by being half the voltage of A referred to C or C referred to A, with
appropriate sign changes, if the cabs are miraculously matched, and
could be wildly different if they are very unmatched!). However,
this measurement does not directly or indirectly tell you what will
come out of the cabs.

The answer to your second question is, no the first set of crossovers
will not muck up the second, in a directional order: both will equally
or unequally muck up the overall current.

The reason for this is because electricity is blatently unfair about direction,
for the most part. In fact, scientists haven't really agreed on which
way it flows, + to - or - to +, yet!

raybro

------------------------------
Date:         Mon, 3 Aug 1992 12:59:54 +0200
From:         Adam Mirowski 
Subject:      Re: How Many Speakers fit on an Amp?

>From: MuffinHead 
>
>Here's something that's always eluded me:
>
>   Say you have two 3-way cabinets and you hook them up in series. Um, lemme
>draw a picture...
>
>    X  = passive x-over (cap, coil, cap/coil)
>    [< = driver
>
>                      cab 1                        cab 2
>
>                 ---X----[<----               ---X----[<----
>          A     /              \      B      /              \    C
>     ---------------X----[<----------------------X----[<----------------
>     |          \              /             \              /          |
>     |           ---X----[<----               ---X----[<----           |
>     |                                                                 |
> amp +                                                                 - amp
>
>
>   So, would the signal (frequencies) be the same at points A, B, & C or would
>the first series of x-overs muck up the signal?

You have elements in series, so unless you go into fairly high radio
frequencies, the current is the same at any time in every point of the
circuit (A, B or C).

--
Adam Mirowski,  mir@chorus.fr (FRANCE),  tel. +33 (1) 30-64-82-00 or 74
Chorus systemes, 6, av.Gustave Eiffel, 78182 Saint-Quentin-en-Yvelines CEDEX

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