This is a guest post by Barry Gardner, the mastering engineer who operates SAS audio mastering services.
The first stereo records were made in the 1930′s and yet it took a long time to popularize the new format and release such recordings for mass consumption. Mono recordings were the standard for many years whilst various attempts were made to advance stereophony. In music production itself there has almost always been a mono button built into the master section of large format stereo or quadraphonic mixing consoles. It was there for good reason as stereo recordings had to maintain sonic compatibility with AM sound systems and monophonic broadcasts amongst other reasons.
The word compatibility is partly subjective and partly technical and the ratio of technical to subjective balance has become somewhat of a moving target. When we consider mono compatibility we must fundamentally consider the phase relationships throughout the audio spectrum between the left and right channels of a stereo recording.
By summing the left and right channels of a stereo two-track mix or recording we are collapsing the stereo image which can be generated either by two microphones (or a complex combination of microphones) or ‘stereo effects’ generated image by artificial means (pseudo stereo).
The nature of stereo means that there are time-arrival differences between the L and R channels of a stereo signal, this in part accounts for some of the spacious effects that make stereo music an enhanced listening experience over and above mono signals. Two microphones can sample the sound field from two positions in space and produce an effect which gives a sense of at least two dimensions when reproduced on two loudspeakers/amplifiers each fed a discrete left and right signal.
These timing differences mean that the signals between the left and right have a different and complex phase relationship relative to each other. Phase is a very important concept to understand and I point you to my short, easy to understand article giving a primer on phase for those to whom this is a new concept.
Why is mono compatibility important?
Mono compatibility of a stereo mix is important because not every audio reproduction/transmission system is stereo. A few examples of potentially mono systems are national and commercial FM radio in weak reception areas (a receiver can sometimes automatically sum to mono to give a clearer less noisey signal), FM pirate radio, AM broadcasters, some PA systems, powered mini single loudspeakers for MP3 players.
Whats the worst that can happen?
If you have sound sources in your mix that are completely out of phase between the left and right channels or significantly out of phase complete loss of that sound source from the music mix could be the worst case scenario. This is a compelling reason to make friends with the mono button on your music production system, be it a mixer or digital audio work station.
The wider picture
Maintaining a mono compatible stereo image is closely linked to the wider goal of a subjectively articulate and/or believable stereo image. A good stereo image is well spread between the L/R and phantom centre image, has nice width and balance between mono and stereo components.
There is an aperture for taste in this area and what might be deemed appropriate for a classical music recording may not be the same for a dubstep track. However it is important that both musical styles will work effectively in mono without too large a compromise to the mix balance and tone of instrumentation, however disparate they may be as genres.
A mix engineer has numerous tools available to make adjustments to instruments within the stereo field, fundamentally – pan controls for mono sources and balance controls for stereo interleaved/ganged sources. This allows positioning within the left and right and phantom centre image of a stereo reproduction system. These controls can also be applied effects returns adding further depth and control.
Identifying phase problems by ear
Whenever a mix is being produced it is vital that the engineer periodically checks the compatibility of the mix in mono. For an experienced engineer this is normally an aural check as the engineer will have been trained to identify phase problems by ear.
It is worth mentioning that this is much easier to do using well placed loudspeakers as opposed to using headphones. Mixing on headphones means it is much more difficult to spot a phase problem. This is because the headphones ‘couple’ all sound from the left ear cup into only the left ear and all the sound from the right ear cup into only the right ear. With loudspeakers some sound from the right speaker enters your left ear. This makes listening for phase problems on speakers psychoacoustically easier than headphones.
There are two ways to listen for phase compatibility problems – namely in stereo… and in mono. I recommend training yourself to hear these problems in stereo as this tends to stop problems before they start in a mix, even before you hit the mono button.
Some digital audio workstations will have the option to mono the stereo master output bus. For those who do not, you can get hold of the free Brainworx bx_solo plug in and put it in an insert point on the stereo output bus.
‘In stereo’ listen for…
Listen for sounds that produce overly wide spatial effects. Sounds coming beyond the width of the speakers. In extreme cases sounds appearing to come from behind the listening position. Check for comb filtering effects when the head is moved from left to right in space, across the stereo field.
‘In mono’ listen for…
If you mono sum the stereo mix you will need to quickly A/B mono and stereo and listen for any elements in the mix that drastically change tone or change their level/mix balance. So if a synthesizer drops in level and thins out then it could well have a mono compatibility problem with that source.
For a novice engineer this may not be initially easy to hear so there are a few tools that I can recommend which can give some visual indication. The visual tools are ideally used in tandem with listening as it is a good idea to become familiar with the sound of overly wide or out of phase mix elements.
Visual aids : L / R phase metering
Span is primarily a spectrum analyzer which has a basic phase correlation meter built in. Flux stereo tool is a vector scope with phase correlation meter.
In essence a phase correlation meter has a left to right swing and is labeled: -1 0 +1
When a stereo mix passes through the meter it should largely be positioned towards +1 on the right hand side of the meter if it is very mono compatible. If the signal gravitates largely towards the left hand side it will mean the signal does not have good phase compatibility. A typical mix with good mono compatibility would hover between 0 and +1.
As an experiment just to show the working of the meter you could generate some mono sine waves of exactly the same frequency, pan them hard left and hard right and nudge one of them in your DAW timeline by a few milliseconds. Watch what happens to the phase scope and correlate it’s readings by mono summing the sine waves and listening to the aural effect.
This is purely for experimentation and gaining familiarity witha phase scope. A stereo mix will have a very complex phase relationship between left and right channels as there will be multiple frequencies playing at any one time.
Visual tools can be of assistance when learning about mono compatibility but ultimately an experienced engineer will always decide based on what is heard rather than what is seen.
Further manipulation of the stereo image
So we now know that mono compatibility is important and plays a wider role in a good sounding mix. A good stereo image will have a balance of power and immediacy whilst providing true or pseudo stereophonic interest, acoustic believability and a sense of depth and space.
Another tool that can be used to manipulate stereo width is a M/S plug in. Such software is inserted on your tracks and allows for adjustments to the mid and sides of a stereo signal. The mid signal contains all the information that is identical in both channels (mono sum) of the stereo field and the side signal contains all the information that is different between the left and right signals. Being able to control this allows for manipulation of the perceived width of various stereo sound sources.
Another common tool is generically known as a stereo imagizer, this generally takes some of the left channel signal and feeds it at low level into the right hand channel and vice versa. The effect is to produce a wider and more diffuse sounding image, often when over used the phantom centre image loses solidity and with extreme use you can end up with a ‘hole in the middle’ effect. This type of tool has to be used with care as it is possible to create phase incompatibilities with already ‘wide’ sound sources. Beginners should use it with care and use it very minimally if at all and always in conjunction with listening and/or a phase meter.
Additionally, the engineer will have various delay based effects such as reverb, delay, echo, doubling with time delay, de-tuning with time delay and chorus to further embellish the mix with short delay based textures across the stereo image.
Stereo width and sound sources
Different sources obtain their width from different means. Acoustic recordings may be recorded with stereo microphone techniques so mono compatibility will ideally be checked at the time of recording and not at the mix stage. Again this can be done aurally or using a phase meter.
Some synthesizers use multiple oscillators, unison oscillators and built in effects to give a very wide and impressive sound. Beware these ‘presets’ as they can be set with rather extreme settings that may not work well in mono. If you are programming your own sounds you will know your synthesizer architecture well enough to be able to avoid such issues but with presets you may have to edit the existing effects and oscillator panning positions to get the preset to work well in mono. Synthesizers can be very complex and it is really a matter of ensuring you know the synthesizer architecture sufficiently well to be able to program your way into mono compatibility.
Mono compatibility remains important today and the first goal is to become an expert at hearing overly wide images and close to, or, out of phase sound sources. This will be key in maintaining good mono compatibility and producing good mixes that have a well balanced stereo spread.