The Duende Bus Compressor can be used on sub-groups, master faders and even channels and is generally employed to compress groups of instruments or a whole mix.
When the Bus Comp is inserted over the master fader in Pro Tools it appears post-fader in the signal path. What this means in use is that if you move the master fader level (i.e. to perform a fade out at the end of a song) then the level into the Bus Comp is changed. So if the master fader level is lowered the input to the Bus Comp is also lowered, so by the end if the song there will be no compression taking place as the input would be well below the threshold. In this scenario it may be preferable to use the Pro Tools trim plugin placed after the Bus Comp to perform the fade out.
In Cubase, Nuendo and Logic the insert is pre-fader so the level of the fader does not affect the input level into the Bus Comp.
A bus is referred to in different ways in the various DAWs. Traditionally a bus is a path over which a signal travels in order for to reach a certain destination. This destination is a channel which can accept various inputs from various buses and is referred to as a sub-group. Pro Tools and Digital Performer use this definition and the signal will be sent to an Aux Input or Aux Track via a specified bus. In this case the Aux Input or Track is equivalent to what we refer to as a sub-group. Logic refers to a bus as both the path over which the signals travel and the destination sub-group. Cubase and Nuendo handle this in a similar way but refers to them as groups.
Refer to the diagrams below which attempt to make this clearer!
The Bus Comp found in SSL consoles has been most famously used over the mix output of the desk, and it has been said that strapping the Bus Comp over your mix is the final ingredient to make it sound like a record. Try placing a Bus Comp over main output quite early on in the mixing process, and using about 1-3db of gain reduction.
Bus compressors are generally used over signals which exhibit a variation in frequencies and transients, i.e. a drum kit, backing vocals or any group of signals.
It is often desirable when mixing a drum kit to send all the tracks to a bus before they reach the mix output. A compressor can be inserted over this bus, which in the case of the Duende Bus Comp provides a way of bringing all the separate drum tracks together so they all start to sound like a whole kit again. It also gives an overall control over the dynamics of the drum kit and lets you govern the interaction between the tracks.
You can see that the 3 mono and 2 stereo drum tracks have been assigned to a subgroup. Inserted over this sub-group is a Duende Bus Comp.
Play the track and open the Bus Comp.
Listen to the drums with the COMP IN button switched out to get used to the dynamic in them. Now hit the COMP IN button and notice how both the density and punch of the kit is increased.
- The THRESHOLD is set at such a point so as to control the higher level elements of the drum track. By decreasing this threshold, more compression takes place which can be clearly heard and is reflected in the VU meter.
- The MAKE UP GAIN is set at the point where the apparent loudness of the kit is about the same whether or not the COMP IN button is down. As mentioned in the compression and expansion tutorial this is a good way to A/B as louder signals generally sound better to us humans.
- The RATIO is at the lowest setting of 2:1. At this ratio the Duende Bus Comp features an extremely soft knee characteristic, meaning even the very low level portions of the signal are affected by the gain reduction circuitry, but less so than the high level parts. At 4:1 and 20:1 the knee is harder and behavior is similar to a regular compressor.
- A long ATTACK time of 10ms is used. This allows the initial attack of the drums to come through before the compressor kicks in.
Gradually lower the attack time and notice how the kit starts to sound squashed as the compressor reacts more quickly. The main elements of the kit (kick and snare) become increasingly compressed but the rest of it (hats, crash and ambiance) stay about the same. The attack time for percussive signals is often set to be quite long to allow these transients to ‘breathe’.
- The RELEASE is set to 600ms. Use fast release times to make the compressor ‘pump’, or longer times to provide a smoother response.
The release time is a control which can be set according to the tempo of the track. A track at 140bpm will require the release to be set faster than one at 90bpm. At 140bpm the compressor must recover more quickly, and the release should be set to it’s fastest setting without causing the pumping that is generally avoided.
See the diagrams below where the thick black lines show the gain reduction:
With the release set too slow, you can see that the compressor does not have chance to recover before it reduces the gain on the next peak. Because the compressor does not have time to bring the audio back up to full level, the level of the next transient is compromised. This will results in the first beat being at a high level and the following ones being lower. It also means the average signal level is lowered because there is always some gain reduction happening.
The second diagram shows a shorter release time which gives the compressor time to get back to its no gain reduction state. This release time will probably sound most natural (which may not be what you are after!), and will yield the highest signal level without a pumping effect being heard.
The Bus Comp can easily be made to produce a pumping by setting a fast release time, 0.1 or 0.3s. This can be heard quite clearly over the drum kit by pushing the ratio up to 20 and bringing the threshold down. A denser, more exciting sound can be achieved like this and can be a very desirable effect. Notice how it sounds like the drummer is hitting the kit twice as hard and he’s put a few pounds on!