On The Busses !!
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[dancetech database recompiled in 1999 - some articles listed April '99 are older]
We hear alot of talk about sends & feeds in studio work... it's all about direction of signal flow & matching up those routes & destinations... The same happens inside Computers.. data has to find it's way around the computer's innards.. whatever... I was looking at the mixer section today and though that so much of it depends on a newbie being familiar with the concept's of BUSSES.... Once they know this, it makes mentaly evaluating gear & other things alot easier.
So... The Buss! (I spell it with two ss's whatever).. BUSSES, they carry data... in mixing and recording you hear terms like, 'channels' & 'tracks'.. but tracks surely is the sound on the tape or on disk devided into different 'tracks' right?.. and channels are what then besides midi channels?... and do any of those terms describe the flow of the signals really at all?.. What is a 24 channel mixer?... what do BUSSES have to do with all this?...
Ok... lets go way way back to Ye Oldene Dayse, when 'mixers' were first developing commercialy. A small market almost exclusively to supply professional studios as there was no 'home-studio' market back then.... Let's say you want 4 channels of input, and want to mix all that down to a mono out.. Ok simple, you take the 4 outputs from the 4 Faders or Pot's on the 4 channels, and wire those to a final output with or without a volume control... Obviously it's not quite as simple as joining 4 wires, but you get the idea as far as SIGNAL FLOW is concerned?... 4 wires feeding into 1... and the '1' they all feed into is the 'BUSS'.
Ok, up from there we might want 8 channels, 12 channels?.. we might want to mix different types of input, we might want to send parts of, or all of the combined signals to various outputs
Now, regardless of the combinations, what about the actual channels of this ficticious simple mixer?... what if they breakdown?.. need cleaning?... updating or replacing?... They are HARD-WIRED - meaning they are physicaly soldered to the master BUSS's - fixed at one or both ends to a physical cable or length of steel wire - So this would mean having to turn off the mixer, take it to a workbench, de-solder and disconnect the faulty channel with it's own circuit board, and then remove it for repair or service... Not practical! A studio can't have downtime of the whole damned mixer just cos one channel is faulty!
So what happens is, you create a modular mixer - It still has a BUSS -But with this type of mixer you plug your channel boards into the BUSS using sockets... The BUSS is still the same - it has the wires which carry the signals, but previously the channels were soldered or 'hard-wired' to the buss's to merge them all to the final OUT... now, with a modular design, channels plug In and OUT of the main buss and mixer frame just like soundcards or graphics cards in a PC!
But... Ok, so in the simplest sense, a buss is a cable into which some channels feed a signal.... but why did I mention that stuff about modular mixers?... we'll see now... First - what is a simple buss again?
AGAIN - This is the simplest BUSS
4 channels feeding into a single mono BUSS OUT
Now... thats the most basic way a buss works... In the simplest sense, a buss allows multiple items to feed a signal into a cable and that cable carries the SUM of the inputs to some destination.
But why mention the modular mixer thingy? Well cos when you look at one it becomes more obvious how a mixer channel interfaces with a BUSS than if you looked at the same thing on a non-modular mixer... it's visualy just much more obvious... look...
|Here's an easily visible hard-wire BUSS running along the back-inside on a rack unit for Neve Modules. You can see the flexible cables connecting the Input & Output connectors on the back of the rack unit to the vertical grey channel Connector Blocks... |
See how those vertical connector blocks look like motherboard slots? well they are identical in fact to a motherboard slot - You'll notice also the horizontal lines of wires running along like telegraph-wires on poles...
The 'Poles' are the Buss Connector Blocks, and the horizontal 'wires' are the actual Busses themselves!! - That's them!!!... they are the actual buss's.. thats what they are, and you can see how this type of modular buss makes it very easy to grasp the principal of what a buss is...
|Here's the sort of thing that would plug into the the rack - This is a Neve 1095 module (pre-amp/eq module)... It's like adding a soundcard or other PC card into your PC slot... exactly the same principal... think of each Mixer channel as a PC card therefore. |
The front of this Neve module has all the controls.. behind that is the boards with the circuits.. and at the back is a slot with copper connectors just exactly like the edge of a soundcard... That slot plugs into the vertical Buss connector.
Nowadays Mass produced mixers of the type we have in our home studio's and production suites are usualy NOT modular but the BUSSES still exist inside the mixer of course, a modular or non-modular mixer must has busses; it can't function without them.
Busses in modern mixers tend to use flexible ribbon-cable like IDE harddrive cables and these plug into printed circuit board modules using connector plugs as we've seen above. Because these types pf mixer console are not modular, you dont need a fixed buss running along the inside into which channel cards can be plugged... the channels boards are usualy fixed inside the frame/chassis, and the busses are then plugged into the boards as a strip of ribbon cable.. Like in this open A&H mixwizz from a soundonsound article:
You can see the BUSS ribbon cable running along the front connecting on each channel with a black connector block!.. This BUSS cable is actualy almost identical to a floppy-drive ribbon-cable, with the same type of black plastic connector blocks with tiny brass pins inside.
One place you can still see the modular approach working today is with API's 'Lunchbox' system, where API (and 3rd party) modules 'plug in' to an API 'Lunchbox' rack unit... The 'Lunchbox' acts as a host supplying a frame, power-supply and in/out connections for any module you plug in to the rack - Here's how it works:
1. The frame has connector blocks running along the back, and modules such as this Purple module slot IN to the rack...
2. A Lunchbox filled with API modules
3. The rear of the Lunchbox where the modules connect to the outside world for input/output
In this case the BUSS connecting the modules will be for power only, because the modules have no common output or send destination - Each module added to the rack ONLY has an IN and OUT... so any BUSS in this instance is for supplying voltage/power only... Look at the image 1. - You can see the Purple module has a connecting card just like a PC card with copper tabs... It's that 'tab' which (like a pc card) slots into the vertical green connectors inside the back of the Lunchbox. - Those connectors carry the signal in/out and power connections.
Y'see BUSSES are signal pathways which are used by several items, not a dedicated path used by only one thing such as the IN/OUT path for each added module in the Lunchbox rack... If however, this Lunchbox had a master L/R output and could be filled with say 6 eq channels (so you could mix 6 module 'channels' down to a stereo output); then yes.. there would then need to be a master Left/Right BUSS inside the Lunchbox to carry the outputs from the 6 channels to the common master L/R OUT destination.
So, again for noobs... a signal-path is referred to as a Buss only when it can be fed by signals FROM several items or when it carries signals TO/FROM multiple items/destinations - EG: 8 Mixer channels mixed IN to a master L/R buss - or - a PCI buss carrying digital data to/from more than one source/destination on the same pci buss.
You can therefore think of the main Busses on a mixer like as typical 'blocks' part of a US city... all the people in the houses on 'Mix buss road' use the same routes (Busses) to get to the different common places they can visit. The houses are the mixer channels.
Yeah this subject comes up so much on forums, so some more thoughts on it....
In the images above in part-1, there's a NEVE rack... it's got buss's. In reality in this rack, the bus's are mostly for carrying power to the modules.. but, power also travels along busses.. but then... 'power' or electrical current is actualy what we are mixing on our mixer so there ya go!... these busses are in fact carrying elecrtical currents... some may be to provide power, and some may be the actual electrical signals of the music.. it's all electrical flow tho.
Back in your PC or mac you have plenty of busses... the whole motherboard is a mass of them... Front Side Buss (FSB), PCI buss, IDE buss, etc etc... well they do exactly the same thing in principal.. Millions of cycles of very small electrical current per second is carrying data bits around the busses on the back of the individual electrical cycle waves.... You have a soundcard in your PCI Slot 3... You assign the soundcard L/R OUT as master OUT in Cubase... You hit play... the PC routes the electrical data down the PCI BUSS to the correct card slot (identified by it's ID and address), the digital audio data arrives at the correct PCI slot... flows into the correct socket pins, the soundcard pins carry it via a curcuit-board path to the DA (digital-to-analog) converters, and the D/A's convert the digital data to an analog signal and send it to the output socket!
Traditional uses for main busses on a mixerOk... We already have seen how a mixer contains busses for aux sends, monitor busses, main master L/R OUT buss etc, and you will often hear people talk about a mixer and they say; "Oh it's an 8 buss mixer" - What do they mean by that? - Did they count up all the individual busses for every send and feed and signal and it comes to a total of just 8 busses??! - No!.. they don't mean literally that the mixer has only 8 busses inside, but the problem is that this can confuse noobs.
When people say; "An 8 buss mixer", they mean it has 8 master busses... let's call them 'master busses' for now although they have various names. They are master output busses because quite simply, you can switch ANY of the mixer channels IN so they are feeding ANY or ALL of the busses!... Also they are like copies of the master L/R buss because they are all fed by the input channels' pans and faders just like the main master L/R buss is. Also, each buss goes to an output socket, just like the main master L/R buss... So for now we'll call them 'master busses' or as people usualy refer to them, as just plain 'busses'.
So, on the input channel of an '8-buss' mixer there are buttons to switch that channel so it sends it's fader output to buss: 1, 2, 3, 4, 5, 6, 7 or 8 as well as to the master L/R buss... A channel can send to any combination of busses or even to all of them at once!... so you can think of 'busses' in this context as extra master outputs that channels can be switched to send to... checkout this picture...
Ok... so now, traditional uses for 'master busses' or just plain 'busses' on a mixer...
Well it's really about organisation and management... the 'master busses' or just plain 'busses' in this scenario really are about making your life easier and avoiding problems, but mixer 'busses' can also be used for creativity and greater control.
A typical mix strip...
- vox main
The chaos of 16-24 channels isn't massive but what we'd do with the busses is for example, send the 3 BV's to a stereo buss (a pair of master busses, maybe buss 1/2 or buss 3/4 etc)..
Now all your BV's are on a stereo fader.. ok, now after you individualy balance the 3 BV's by adjusting their individual channel eq's and faders, you can then easily adjust the OVERALL master level of all 3 BV's as a GROUP with just the single buss fader or a pair of buss faders. Much easier to quickly reach over and raise/lower the BV's as a group on one pair of faders than to scrabble about trying to raise/lower the 3 individual BV channel-faders AND keep their relative fader positions the same!
Same with things like the toms and/or the stereo kit... You could buss the whole kit apart from the kik, snare and hats to a stereo buss and then easily adjust the whole 'Rest of the kit' relative to the kik/snare/hats.
Master or 'sub-group' busses V other mixer busses!
Ok, so that's busses - and particularly master-busses or as they are often called 'sub-groups' or just plain 'groups' - The busses people refer to when they say "4 buss mixer" or "8 buss mixer" - But remember there are other types of buss both in mixers and outside of mixers as we've seen.... There is a fundamental difference between 'master busses' or 'sub-group' busses on a mixer and the OTHER types of buss on the mixer, such as the Aux-Send busses, and I want to just explain that cos it is important and there is a very distinct difference between these 2 types of buss which effects how they can be used for what types of task.
Checkout this image...
As we've seen, the mixer input channels can be switched into the master L/R buss using the L/R switch on each channel - Each channel can also be sent to any or all of the sub-group Busses - That is buss 1/2, 3/4, 5/6 and 7/8 - The important thing to grasp here is that the master L/R Buss and the sub-group or group Busses all take their feed from the input channels fader and pan positions.
So I can switch the 4 input-channels like in this image; sending the same mix of the 4 channels to both the 1/2 buss AND to the Master L/R Buss... but the 'mix' of the relative volume/pan positions of those 4 channels will be identical at the master L/R buss AND at the 1/2 buss - The only thing we can do is adjust the buss 1/2 and master L/R level faders to be different, but the relative mix of the 4 channel's fader/pan levels will be identical arriving at the master L/R buss and any sub-group buss that we also send the 4 channels to.
The point is, using the additional 'master busses' ('sub-group' or 'group' busses) like this they will always receive a COPY of the same relative-levels mix of the 4 channels that is also going to the master L/R buss... So we cannot setup a DIFFERENT 'relative-levels' mix of the 4 channels on any of the busses because they only take a copy of the L/R master-buss mix from the 4 channel faders/pans...
Any combination of input channels sent to the sub-group Busses will be identical in relative-levels to each other as the same combination of channels being sent to the Master L/R buss
If we want to create a different mix of the 4 channels (with different relative volume-levels to each other) then we need to use a completely new spare BUSS, and it needs to be a BUSS into which we can SEND a completely different amount of each channel than we are sending to the master L/R buss and Sub-Group busses... So we need a new buss that has a new and completely seperate volume control for each channel so that we can SEND 4 completely different relative-levels of the 4 channels into that new buss!... That way we get a new mix of the 4 channels, each with different relative volume levels to those set by their faders... For that we use AUX (auxilliary) BUSSES!
Auxilliary (or 'extra') Busses are usualy MONO meaning you can send some of each channel IN to the AUX BUSS but you cannot assign a pan position for each channel - You can get stereo aux-busses - they do exists - and then of course you have a volume AND a pan control to send any channel into the AUX BUSS at any volume and pan position - Sometimes this extra stereo AUX BUSS is called a MONITOR BUSS.... But, lets not confuse outselves here by getting into that too much cos a MONITOR BUSS like that is not the same as the monitor buss we referred to earlier and which takes a copy of the main L/R mix - This type of monitor buss is perhaps more accurately called a SECOND MONITOR BUSS... and such a stereo buss allows you to create a completely new stereo mix of all the channels which can be sent to stage sidefills or studio playing-room monitors or whatever.
The main L/R mix contains all the instruments & vocals in the band and is sent out to the PA for the audience to hear! - Using for example 4 Aux-Sends on the mixer you can create 4 additional completely different MONO mixes of any combinations of the channels - MONO Auxillary-send Busses are used to fullfil their usual tasks which on a 4-aux-send mixer would be typicaly like this:
Sending any channel OUT to a common/shared effect-unit such as a reverb or delay
- AUX-SEND-1 is used to send a copy of the main-vocal, two backing-vocals and snare-drum to a reverb unit - To do this you simply go to each VOCAL channel and the SNARE channel and turn UP the AUX-SEND-1 pot! - The MONO Aux-send-1 mix of vocals/snare then goes OUT of the AUX-SEND-1 SOCKET on the back of the mixer to the reverb unit - the vocals/snare pass thru the reverb and then the output from the reverb-unit is sent back into to the main L/R mix adding reverb to the vocals/snare...
The more reverb you want on ANY of the 3 vocals or the snare-drum, then the more you turn UP the AUX-SEND-1 pot on those mixer channels. This allows us to send exactly the right amount of the 3 vocal channels and the snare-drum channel to the reverb, and those relative-send-levels between the 3 vocals and snare can be completely different to their relative VOLUME levels set by their channel-faders in the main L/R mix.
- AUX-SEND-2 is used to send a copy of the main-vocal, two backing-vocals & the rhythm-guitar to an echo/delay unit to add some slight delay to these channels - To do this you simply go to the voclas/guitar channels, and turn UP the AUX-SEND-2 pot! - The MONO Aux-send-2 from the vocals/guitar then goes OUT of the AUX-SEND-2 SOCKET on the back of the mixer to the echo/delay unit - the vocals/guitar signals pass thru the echo/delay and then the output from the echo/delay-unit is sent back into to the main L/R mix adding echo/delay to the vocals/guitar!...
The more echo/delay you want on ANY of the 3 vocals or the guitar, then the more you turn UP the AUX-SEND-2 pot on any of those mixer channels. This allows us to send exactly the right amount of the 3 vocal channels and the guitar channel to the echo/delay-unit, and those relative-send-levels between the 3 vocals and guitar can be completely different to their relative VOLUME levels set by their channel-faders in the main L/R mix.
Sending selected channels OUT to stage monitors for musicians to hear onstage
- AUX-SEND-3 mix containing only the main-vocal, two backing-vocals & the rhythm-guitar... To do this you simply go to each VOCAL channel and the rhythm-guitar channel and turn UP the AUX-SEND-3 pot!
The MONO Aux-send-3 mix of vocals & guitar ONLY then goes OUT of the AUX-SEND-3 SOCKET on the back of the mixer to the front three stage wedges for the vocalists and guitarist.
- AUX-SEND-4 mix containing only the main-vocal, rhythm-guitar, keyboards & bass... To do this you simply go to the main-vocal channel, the bass channel, the rhythm-guitar channel & the keyboards channel and turn UP the AUX-SEND-4 pot!
The MONO Aux-send-2 mix of main-vocal, rhythm-guitar, keyboards & bass ONLY then goes OUT of the AUX-SEND-4 SOCKET on the back of the mixer to the drummers stage wedge so the drummer only gets a mix of the main-vocal (so he knows where he is in the song) and the main other band instruments so he can hear the bass/guitar/keys vocalists.
Hang on tho, there's one last thing... what's that button marked 'PRE' on the mixer next to the AUX SEND 4 pots, and why does it say POST next to Aux-5 and Aux-6?
PRE and POST fader auxilliary sends/busses
Ok this one is fairly easy for noobs once you get it, but a nice diagram will help alot here.. hang on....
Study the image and think about it.... SOME mixer-channel PRE/POST Aux-Sends feeding the AUX BUSSES can be switched to take their signal from the either the PRE or POST fader channel signal... But what's the point of that?
PRE FADER AUX SEND: When the channel fader is lowered/raised the signal going out of a PRE-FADER Aux-Send retains the same constant level. This is because the PRE-FADER AUX-SEND is taking it's input from BEFORE the channel fader - The only way you can alter a PRE-FADER AUX-SEND output-signal-level once it's been set is to adjust the channel EQ or the channel-input pre-amp/gain level.
POST FADER AUX SEND: When the channel fader is lowered/raised the signal going out of a POST-FADER Aux-Send changes by the same relative level as the main channel-fader is moved by. This is because the POST-FADER AUX-SEND is taking it's input from AFTER the channel fader - If the main channel fader is set to for example: -20dB, an the fader is dropped by 6dB, then the POST-FADER Aux-Send will also drop by 6dB from whatever level it was set to.
When to use PRE or POST type Auxilliary Sends
PRE FADER AUX SEND: Use to send out individual channels or a unique combinations of channels to destinations where you want that combination of Aux-Sends to stay fixed and NOT change the amount of signal they send out regardless of what you do to the channel faders for the main L/R FOH (Front Of House) mix! - So you'd use this type of Aux-Send to feed a live on-stage foldback/monitor-mix... If you alter any levels using the channel-faders to change the main L/R FOH (Front Of House) mix that the audience is listening to, the levels going out of the PRE-FADER Aux-Sends to the on-stage musicians does NOT change.
POST FADER AUX SEND: Use to send out individual channels or a unique combinations of channels to destinations where you want that combination of channels to follow the volume of their own channel faders! - SO for example you use this type of send to feed effects units such as reverb because if you lower the channel-fader to lower perhaps the main-vocal, you ALSO want the send to the reverb to drop in level too - If the send to the reverb does NOT drop in level to match the channel-fader drop in level, then the channel-signal will drop in volume but the Aux-Send from that channel to the reverb will not!!... If you alter any of the channel-faders to change the level of a channel, the levels going out of the POST-FADER Aux-Sends to the effect unit DOES also change to match!!
Anyways... busses are everywhere... not just on mixers!... Hopefuly this'll keep 'buss curious' noobs busy and out of mischief for a while!
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