Page 2: Monitor Controllers

Inputs & Outputs

A basic requirement of a monitor controller is to allow comparison between a current work‑in‑progress and a reference: assuming you want to work with an external reference source like a CD player, that means at least two stereo inputs. Most studio equipment has balanced analogue line‑level outputs, but the ability to accept signals from unbalanced consumer equipment is often important too. A mini‑jack socket, for example, allows many smartphones to be hooked up. Many more advanced monitor controllers also include digital and/or USB inputs and a reference‑grade D‑A converter, and some models now cater for Bluetooth streaming. When comparing sources, it’s usually important that they have the same perceived volume, so the better‑equipped monitor controllers include facilities to adjust or ‘trim’ the level of selected inputs.

Some models, such Heritage Audio’s RAM System 2000, include a Bluetooth receiver, which could prove handy given that most new smartphones lack mini‑jacks!

It’s often helpful to be able to audition signals on a couple of different monitoring systems. The primary reference would typically be a set of full‑range, high‑quality speakers, perhaps with headphones as a secondary option. In addition, checking a mix on a set of compact and limited‑bandwidth speakers gives a useful impression of what someone listening on a TV or laptop computer might hear. Where multiple speaker outputs are provided, the monitor controller usually incorporates facilities to adjust the level of the additional outputs relative to the primary set, so that the SPL (Sound Pressure Level) in the room doesn’t change significantly when switching between systems.

In installations involving a subwoofer, it can be helpful if the sub can be switched on and off from the monitoring controller (that may require bass‑management facilities in the controller rather than the sub), and in multi‑channel rooms it’s generally necessary to be able to switch between the surround speaker set and a stereo set, to check downmixes. It’s preferable to check mono compatibility on a single dedicated mono speaker too, so an output configurable for that purpose can be useful.

Soloing the Sides channel makes it quick and easy to align channel gains on stereo sources or stereo mic arrays, as well as to assess what’s being lost in the summed‑mono signal.

On The Level

It’s important not to disturb the main volume control, because we automatically acquire an acoustic frame of reference when working at a consistent listening volume; if that volume changes, our frame of reference changes too, and mixing decisions made with the different monitor levels come out differently. More sophisticated monitor controllers usually allow a ‘reference’ listening level to be recalled at the press of a button. With less elaborate systems, we rely on using a calibration mark around the volume control — I often use a wax pencil mark. Ideally, this would be around the 1 or 2 o’clock positions, where ganged rotary pot controls offer the most precise gain adjustment and accurate tracking between channels, leaving an extra 10‑12 dB of gain for checking low‑level sounds.

After the volume control and source/speaker selection, I find that the most useful controls are the Dim and Mute buttons. Mute kills sound whereas Dim reduces the level, typically by 20dB, and they’re useful if you need to quickly quash loud sounds (such as someone moving or unplugging a mic) or have a discussion in the control room, without shouting over the music or changing the volume setting. Importantly, a Dim button allows you to check a mix at low levels — which can be very helpful in identifying balance problems — without disturbing the volume control. But it also neatly side‑steps the problem that most ganged potentiometers suffer from very poor channel tracking at low volume settings, causing the image balance to be offset and/or move around wildly with small changes in volume.

Signal‑check Facilities

Most monitor controllers cover the basic signal‑switching and conditioning facilities described above, but when auditioning music or other recorded signals, other tools can help you identify common problems. The most important is undoubtedly the ability to check mono compatibility, which requires listening to the sum of the left and right channels. This summed signal is likely to be louder than either channel individually, so most (not all) mono‑summation circuits introduce an overall attenuation of 3‑6 dB, to maintain a consistent listening level. This summed‑mono signal is typically routed to both speakers, which is useful for checking the location and focus of the phantom image. In a correctly setup system that should manifest as a narrow, sharply‑focused sound source precisely mid‑way between the two speakers. If not, one speaker may be defective or set up incorrectly, or there could be problems with local reflective surfaces. Some monitor controllers also include a balance control to shift the image centre left or right, to compensate for the individual listener’s hearing; different people have different sensitivity in each ear, and so perceive ‘the middle’ differently.

The control panel of the author’s Crookwood mastering console, which features all of the signal‑check facilities discussed here.

Listening to summed mono on two speakers is the default condition, but it creates a very different impression, particularly of the low end, from listening to mono on a single speaker. More capable monitor controllers allow the summed‑mono signal to be routed to a single speaker in a stereo pair — traditionally the left one — and some can be configured to route the summed‑mono signal to a separate dedicated ‘mono‑check’ loudspeaker. If this facility isn’t provided, an acceptable alternative is to send summed mono to both speakers, then mute the right speaker, which requires individual channel mute controls in addition to the overall mute function. Usefully, though, this facility doubles as a channel solo: auditioning each channel in isolation can help you identify problems on a single channel, which can be difficult to detect with both channels running.

The next most useful feature is a polarity reverse (often mislabelled ‘phase’). It’s usually applied to the right channel but occasionally each channel can be inverted individually. Flipping the polarity of one channel provides a helpful ‘sanity check’ when auditioning very wide or ‘phasey’ mixes: if the mix becomes more stable and focused with the polarity reversed, there’s probably an unwanted polarity inversion in the recorded signal.

Most acoustic signals have a distinct natural polarity, typically with more positive energy (compression) than negative (rarefaction). Ideally, the monitor system should reproduce sound in the same polarity, often called absolute phase, and flipping the polarity of both channels simultaneously allows that condition to be checked. Many monitor systems will sound quite different if, for example, kick drum transients cause the woofers to move inwards instead of out!

Polarity reverse functions should be implemented before the mono sum circuitry, so that when the right‑channel polarity reverse and summed‑mono facilities are both employed the output is L‑R instead of L+R. This is the ‘stereo difference’ signal, or Sides channel. Soloing the Sides channel makes it quick and easy to align channel gains on stereo sources or stereo mic arrays, as well as to assess what’s being lost in the summed‑mono signal. The stereo difference signal is very revealing of the damage imposed by lossy codec formats such as MP3: lower bit‑rates affect stereo imaging and reverberation character significantly.

Other useful facilities are left‑right swap and mono left/mono right: the mono left/right modes route the selected channel to both monitor speakers, while the left‑right swap exchanges the two channels. The former is helpful when working with single‑channel material or assessing the quality of each channel independently. The latter is useful for identifying where different elements are in the stereo image, and confirming whether a channel swap has occurred somewhere in the plugging. Taken together with the polarity inversion functions, these facilities allow individual auditioning of the Mid, Sides, Left and Right elements of a stereo programme.

Many monitor controllers are intended to serve as the central control hub of a project studio, with facilities for artist headphone monitoring (with independent source selection and talkback). Metering is sometimes included internally too, but more often, a dedicated output is provided for an external metering system. Sometimes the DAW input is made available at buffered outputs to feed external hardware recorders.

I would prioritise the ability to set a reliable reference listening level above all else, followed by having dim and mute buttons easily to hand, being able to check summed mono on a single speaker, and access to the stereo difference signal...

Decisions...

Different virtual or hardware monitor controllers include different subsets of the features and facilities described above. Selection of an appropriate model depends on the physical installation requirements, and your normal workflow patterns and personal preferences. I would prioritise the ability to set a reliable reference listening level above all else, followed by having dim and mute buttons easily to hand, being able to check summed mono on a single speaker, and access to the stereo difference signal — these are the functions I use all the time.

If your budget is tight and most of what you want to check emanates from the DAW, you could consider a hybrid approach whereby your DAW provides sophisticated signal‑check facilities, and feeds a simple monitor controller offering external inputs, a level control and mute/dim.