Mastering Techniques - Using A Compressor - Part 1

There are still more things I want to cover in the area of general DIY Mastering, but I've decided it's time to start posting some specific information up here to help people get to grips with common mastering techniques. So this is the first in a new series of posts headed "Mastering Techniques", which will run alongside the DIY Mastering posts. I'm going to cover things like level-matching, EQ, stereo width adjustment and so on, but I think the subject most people struggle with is compression. Often people have been told that the answer to any number of problems is compression, but don't know what they're trying to achieve, or how to get there.

This post started to get much too long, so I've decided to split it into several sections, this is Part 1...

Know What You Want

In my opinion, the most important thing when using compression is to have a clear goal. You'll see lots of explanations of compression saying things like "a compressor reduces the dynamic range of it's input", but I don't find these very intuitive, and prefer the explanation I was given as a trainee:

Compression is used to make things louder

There can be all kinds of positive side-effects of this process, like making things sound fuller, richer, more controlled or punchier, but at the end of the day, especially in mastering, it's all about loudness. NOT excessive loudness, but something musically beneficial.

Mastering engineers almost always use a limiter somewhere to boost level, but the problem with this is that mixes very quickly sound crushed or distorted using only limiting. Using some gentle compression first means the limiter doesn't have to be hit so hard, giving a more natural sound. More on limiters in a later post.

The ideas here apply equally to compressing individual instruments or voices in a mix, incidentally, but you'll need different settings, usually. First we need to recognise a harsh fact of life:

Not all compressors are created equal

The best analogue compressors cost thousands of pounds. However there are some really good software plugins these days which cost far less. For example if you're looking for emulation of a traditional analogue compressor in a mix, complete with "musical" pumping, I really like Sonic Timeworks Compressor X. Would I use it for mastering ? Probably not. However there are high-quality compressors available on a DIY budget which are suitable for mastering - the TC Electronics System 6000 is a mastering "industry standard", and many of it's algorithms are available as plugins for their Powercore system, for example. Many people also use plugins by Waves or Izotope, and discussions rage about whether it's really possible to master with something so cheap, and which is better, on the Sound On Sound mastering forums and elsewhere.

Regardless of which compressor you decide to use though, they all share similar controls and concepts, and at the end of the day it comes down to whether you're happy with the result you get. With that in mind, lets dive in:

Exploring Compressor Controls

Try this experiment - we're going to overdo everything to begin with, so you get a feeling of what the different parameters do:

• Choose an instrument to compress and solo it (We'll move on to compressing a whole mix later)
• Patch a compressor across the stereo master output channel of your system
• Start with a ratio of 2:1.
• Set attack and release times of 100ms, if possible. You may have to disable "automatic" options first.
• Gradually reduce the threshold until the meters show you 4 or 5 dB of gain reduction

One of two things will have happened. Either:

• The sound will have got quieter, because the compressor is holding back the louder peaks. In this case you need to add some make-up gain, sometimes called output gain. Adjust it until the sound is a similar volume to when you hit the bypass or disable switch

Or:

• The sound will already be at a similar level, in which case your compressor automatically boost the output (make-up) gain

Now toggle bypass on and off and listen to the difference. Depending on your material, you will hear more or less difference. If you can't pick anything out initially, increase the ratio to 4:1 or reduce the threshold until more gain reduction is happening. (If you can't hear 6dB gain reduction with a ratio of 4:1 you should probably get your mastering - and probably mixing too - done by someone else !)

If your compressor has automatic attack and release times, it will probably sound OK (but a bit squashed). Even with manual controls, it shouldn't sound too bad. Once you can clearly hear the difference between the bypassed and compressed signals, you can try and figure out what it is you're hearing.

All the controls interact to give different effects, but before we get to that lets look at each in turn. First, ratio and threshold.

Ratio

Try increasing and decreasing the ratio. Higher values (4:1 etc) cause a more exaggerated effect - the compression "hits harder". Lower values are more subtle. If your compressor doesn't have auto make-up gain, you'll need to adjust it to match the bypassed version for a clear comparison. As a rule of thumb, use a high threshold and high ratio for a hard-hitting sound, but watch out for unnatural results. Lower ratios give a softer, "warming" or "thickening" effect. I rarely use higher than a 2:1 ratio.

Threshold

The threshold control determines when the compressor starts working. Lower values will give more compression, higher values give less. A low threshold with a high ratio will give lots of hard compression and probably sound very squashed and lifeless, whereas a higher threshold and low ratio will be a much more subtle. Whatever values you decide on, there is one rule of thumb worth remembering:

If the gain reduction meter doesn't return to zero several times a bar, you're almost certainly using too much compression

- because this means that the signal is being compressed all the time, and will probably sound flattened as a result. Try a higher threshold, and then higher ratio if it's not doing enough.

Attack & Release Time

Since I specified long attack and release times, you will probably hear the sound being "snatched" away right after a new note comes in, and perhaps "pumping" back up afterwards. Sustained notes are a good way to hear this. If you increase the attack and release times you'll certainly hear these effects. (For a deliberate example of this classic "thump-suck-relax" pumping listen to "One More Time" by Daft Punk). Automatic or "intelligent" settings reduce this effect, but increase the risk of using too much compression without realising it.

Now try reducing the attack time to it's minimum value. The sound will probably duck very quickly away to begin with - perhaps too fast to hear it happen, followed by the longer release back up to full volume. The chances are it will have lost any impact it used to have, especially with a higher ratio, and sound quite unnatural. Reducing the release time decreases the "pumping" effect, but as you get too short will start to sound "crunchy" or distorted.

Now your ears are focused on what the settings are actually doing, where should you set them ?

Answers in the next post.

What is Mastering ?

The art of mastering for CD and DVD evolved directly from the process of "cutting" a lacquer for vinyl pressing, and involves many of the same concepts. But what actually is it ?

Here are some possible definitions of mastering:

• Creating a production master for replication
• Achieving an optimal balance of tone and level
  
• Gaining the benefit of an experienced, impartial ear
• Top/tailing and sequencing
  
• Fixing any outstanding problems from a mix
  
• Making your music sound the best it can be
  
• Making it f-ing loud !
  

So, which one is right? Well, all of them, to varying degrees - except the last one. My favourite explanation is this one:

Mastering is the art of making a collection of tracks into an album (*)

(*) or single, or compilation, or podcast, or catalogue...

This is achieved by one or more of several techniques. As well as technical tasks like ensuring the best transfer from the source and creation of a suitable master, these may include:

• Level adjustments, often using a limiter or compressor
• Equalisation (EQ) to achieve a natural, balanced sound - broadly speaking the right amount of bass and treble, but also much more detailed adjustments, for example to remove unnatural resonance or build-up at certain frequencies, or perceptually improve limitations of the mix. Common examples might include adding "air", or "punch", or adding "edge" to guitars, for example
• Correction of faults - for example removal of clicks, pops or thumps, buzz, hum etc.
  
• Detailed sound restoration - usually only necessary on "vintage" sources, can include removal of vinyl clicks, hiss & distortion etc.
• Stereo image adjustments - this is less common, but may involve widening the stereo image, or (occasionally) adding reverb

Exactly which of these is needed varies from album to album, and even track to track. Some jobs need major surgery, a very few I've ended copying flat from the source. In the later case, did I actually master them? Yes - because I listened carefully, in a dedicated studio using exceptional monitoring, and used my ears and experience to determine that nothing extra was needed. (Actually, more often than not we can spot a source this good within a few minutes of beginning to listen, in which case we contact the artist and offer them the chance of a Direct Transfer instead, so they don't pay for something they don't need.) Deciding to do nothing at all on one or two tracks is just as valid a mastering decision as any other.

However there is still lots of room for confusion and debate. Should mastering engineers:

• Use the minimum possible processing, and keep everything as close as possible to the original material
• Preserve the artist's original vision
• Pull out all the stops to transform a source into what it always "should have been"
• Preserve the original's dynamic range and impact
• Use EQ and compression to achieve major increases in level
• Make everything fit their "trademark sound" ?

Once again my answer is "all of the above" except the last one. This may seem contradictory - surely some of them are in direct opposition to each other ? Not really - because an element of the mastering engineer's skill-set which doesn't often get talked about is intuition. With almost every source I play, I can hear where the artist or engineer was "trying to get" within minutes or seconds. I can hear what they're trying to achieve, and I see my job as trying to help them get there. Using minimal processing, if possible but if not by throwing the kitchen sink at it, and all combinations in between - always staying true to the original mix.

The issue of levels and compression is a good example of this - there are lots of easily available limiter and compressor plugins now, but many of people complain that they ruin the sound. Mastering engineers use very similar tools to achieve their results, and claim that they make things sound better. How can both be true? Partly because there is still a distinction between the tools - mastering studios typically spend thousands of pounds on a single compressor, whereas for the same money you can buy an entire suite of plugins - but also because we are constantly honing our skills to use the tools transparently, and our listening environment to be able to hear when it's working. To some extent, the skill of a mastering engineer is to achieve an appropriate level for every track , sometimes reducing the dynamic range in the process, but make it sound as if the final result is actually more dynamic. Or to make major EQ adjustments, without changing the essential qualities of the original.

So, you might ask - what's the point of paying for something that can be so subtle ? There are several answers to this:

• Often it's not that subtle ! If I'm doing my job, the mastered version will simply be better than the original, while retaining everything that was good about it.
• Where the difference is less obvious, it will only be difficult to hear when level-matched. The importance of increasing the level of a track to it's own particular "sweet spot", without pushing it over the top, is hard to over-emphasise. Doing this for all of the tracks in an album, and getting them balanced perfectly against each other in their final sequence is even more important and valuable. This needs to be done with great care and skill though - it's almost impossible to achieve this simply by pushing the level up into a plugin.
• Ideally, the differences should be subtle. A truly great mix only needs the slightest of tweaks, but even these minor adjustments, over the course of a whole album, add up until the sum is greater than the parts.

As I type this I'm struck again by the inherent contradictions of the job. It requires you to be entirely humble - I start every session by listening carefully to the source and thinking "what's good about this ?"; but also supremely arrogant, making changes to a mix that someone has sweated blood over for perhaps weeks or months. It requires deep technical knowledge, but many of the judgements made are largely aesthetic and artistic. It means knowing when something ain't broke and not trying to fix it, but also knowing when to roll up your sleeves and get your hands dirty...

What is mastering ? Has this post helped explain it at all ? Who knows...

Mastering The Orb - Complete BBC Sessions

Please forgive me a self-indulgent nostalgia post:

I love my job !

I vividly remember hearing "Little Fluffy Clouds" by The Orb for the first time - it was in 1991, and coincided with the beginning of one of my favourite times in UK music. I was in my second year at university, and it was the same time that Orbital's Brown Album came out, U2 released Achtung Baby, Screamadelica by Primal Scream was huge, Resevoir Dogs and the Director's Cut of Blade Runner were released, I was listening to "Copper Blue" by Sugar, Bowie's "Hunky Dory" and ""Mixed Up" by the Cure, and They Might Be Giants released "Flood". My friend Dan (nowadays I master his albums) introduced me to "My Life In The Bush Of Ghosts" by Byrne and Eno - and with hindsight, it was the year I started mastering.

That's not strictly true of course - I actually started work as a trainee tape-op at SRT roughly two years later, and mastered my first album for paying customers a few months after that. But I was mastering, nonetheless - or rather, re-mastering - in my own fashion. Bootlegs, to be exact - purely for my own listening - of Prince, no less, at various gigs on the Lovesexy tour and numerous after-show parties. I bought these in dodgy indie record shops for a pittance, on analogue cassette (remember those?) and then discovered I couldn't listen to them, because they were usually dull, distorted, off-azimuth and copied at such a ridiculously low level with no noise reduction that they were bathed in hiss as a result !

So, I would borrow a friend's ghetto blaster and use it's primitive 3-band graphic equaliser to copy the crappy transparent plastic TDK D90 original (remember them?!) across to a lovely new silver SAX90, making improvements as I went. Boosting bass, correcting L/R balance, probably rolling out the top end during quiet sections to hide the hiss and boosting it again in the loud bits to counter the inevitable distorted mush that came with them...

Those were the days. And in fact, nowadays I do pretty much the same things, but with gear I could never even have dreamt of back then. Samplers were the new big thing - I remember tearing my hair out my with best friend Ben (I master his albums now, too) trying to get a multi-patch set up in the cutting-edge Yamaha ____ - and it would always crash solid after about two hours work. I recorded albums for friends on a Tascam 8-track cassette machine, and thought myself lucky to actually have access to a DAT machine to mix down to.

"Little Fluffly Clouds" conjures up all of that for me, in a heartbeat.

Anyway, here I am, nearly twenty years later, actually mastering The Orb for real - listening to a version of "Little Fluffy Clouds" from the the 1998 Steve Lamacq sessions - and even better, "A Huge Ever Growing Pulsating Brain That Rules from the Centre of the Ultraworld" from the original 1989 Peel Sessions (and everything in between) for a new double-album release. It's not distorted, it's not bathed in hiss, it doesn't need much bass and it's certainly not on analogue cassette, but it's still benefitting from some TLC from me. Which feels good.

Perhaps it's a sign of the times (hah!) that although I still remaster Prince bootlegs at home - purely for my own listening - nowadays it's with with ProTools running on a MacBook Pro, from where they go directly to iPod Touch and sound fantastic - but oddly enough, it's a CD of the same gig.

I love my job.

DIY Mastering Part 4 - Mastering Speakers and Monitors

The most important tool a mastering engineer has (apart from his ears and expertise) is the monitoring, meaning the speakers, amps and converters. We debate all of these at great length between ourselves, in online forums and down the pub, but without question the speakers have the greatest impact on the sound. (They need to be well-placed in a decent treated room, but I already covered that in my third DIY post.)

This post won't enable you to choose a set of speakers by reading specifications or manufacturer's blurb, but hopefully it will give you an idea what you should be looking for, and why.

So, what are suitable speakers for mastering ? Well for example we have B&W Matrix 801s in Studio 1 at SRT, Genelec S30Ds in Studio 2 (In general I don't rate Genelecs, but the S30s have a ribbon tweeter and are in a class of their own). Other common names include ATC, PMC and Dunlavy. What all these monitors have in common (apart from costing at least as much as a large family car !) is that they have:

1. Flat, or neutral frequency response - meaning their response is very even over the whole audible range. So, for example, B&W 801s come with a graph measuring their response - it only varies by +/- 1 dB from 20 Hz to 20 kHz.
2. Acurate phase response - meaning that the stereo image is very detailed, giving an absolutely solid mono center image, but also a deep, convincing "three-dimensional" soundstage - at least, on a decent recording. The stereo imaging in our studios is so good that it's common for people to ask if some of the sound is coming out of the center speaker (used for surround work), even though they are listening to pure stereo. And on the occasions where a mix element is accidentally phase-reversed, it leaps out and draws attention to itself.
3. Clean, controlled dynamics - with only minimal distortion introduced, even at high listening levels. (Using a suitable amp is important for this, too, of course.)

Of course there are many other aspects of speaker's capabilities I could list, but it's almost more important to realise that these characteristics mean mastering speakers should be:

1. Revealing - allowing you to hear details you'll miss on other speakers - faults, like clicks, hiss and distortion; but also the positive sides of the recording - accurate stereo imaging, "depth" (meaning the impression of a three-dimensional sound field), impact and dynamics. Listening on a mastering system should almost be like wearing headphones, in this respect.
2. Clinical - Mastering speakers don't make things sound good. Unlike speakers for tracking or mixing, they shouldn't add "vibe", or "feel". Often they might be described as "bland", "boring", or at best "civilized". In fact, they are reliable and acurate. Only the very best recordings will sound good on them ! In fact that leads us neatly to:
3. Unforgiving - It's often shocking to hear how bad many recordings sound on a mastering rig - the uncompromising accuracy uncovers limitations masked by lesser reproduction systems. While this won't necessarily make for a very satisfying listening experience, it's essential for the mastering engineer to hear and correct any problems or limitations in the original mix.

Together, all these requirements add up to help the engineer achive translation - when it sounds it's best on the mastering system, it will sound great everywhere else, too.

So, if you've got this far, you're probably thinking "how the hell do I choose speakers which meet all those criteria ?". If you can't rely on printed specifications alone, this is a tough question. Until you're learnt exactly what a recording should sound like, it's hard to recognise a pair of speakers that make it sound that way.

Probably the first piece of advice is to get some recommendations. This post was inspired by a thread on the Gearslutz mastering forum asking about mastering speakers, and places like this or the SOS forums are good places to start - perhaps less than £500 isn't really realistic, though! Several of the speakers I mention above are available second-hand for "reasonable" amounts of money. My personal favourites, B&W, have ~35 staff working constantly on research and development, and the advances they make for their most expensive speakers always filter down to the cheaper models. I have Series 2 601s at home and am frequently amazed by how good they sound. Not that I'm suggesting you use them for mastering, but considering the difference in size and price, there's almost certainly something in the 600 range to suit.

Next - listen ! Find a showroom where you can test a few different speakers, ideally in the same room, and listen to various different tracks you know well. Even better, get some on demo and try them in your room. Remember you're looking for accuracy, not excitement - mastering monitors reveal limitations that others don't. Choose a range of recordings and be objective. "Sledgehammer" by Peter Gabriel is a great-sounding track, but it has very tight, controlled bass, so if the speakers give it a thundering bottom end, they aren't flat. "Dummy" by Portishead is a classic album, but it's almost entirely in mono - that should leap out at you. Maybe even take a CD with a few test-tones on it - for example a sine wave sweeping up from 20Hz to 20kHz will not only give you an idea about the speakers frequency response, but may also reveal a few buzzes and rattles along the way. (Don't play it too loud, though, or you may find yourself paying for some blown tweeters...)

Finding the right speaker in your price-range is a major challenge for a would-be DIY mastering engineer, but once you've worked with truly acurate monitoring for a while, you won't want to go back.

CD & DVD on the same disc

Richard Durrant's DAVD discs arrived today - CD audio on one side, DVD on the other of a single disc. This is a format I've been wanting to promote for years, back when the only option was DualDisc, but manufacturing costs plus licensing and compatibility issues put paid to that.

But all that is history now, and you can get a DAVD pressed up for only 60% more than a normal CD. That's incredible value, compared to a 2-disc CD and DVD package which costs more than twice as much, including the packaging. And as a format it beats Enhanced CDs hands down, if you ask me.

So, what are you waiting for ?!

And, speaking of Richard, here is a video of his single-handed live performance of one of my favourite things, Steve Reich's Electric Counterpoint, a piece for 12 guitars... prepare to boggle.

Next post DIY Mastering #4 - Speakers

Christine Tobin - The Secret Life of a Girl

An excellent (and well-deserved) review of Christine's new album which we mastered back in March, as I mentioned in my first post. It's always good to see one of your artists doing well - I love this record, and I wish Christine every success with it. Maybe this will be the breakthrough she deserves !

And while I think of it, Phil got a great review too !

Christine Tobin, Secret Life of a Girl

 (Babel) 

John L Walters
Friday May 16, 2008
The Guardian

Dublin-born singer Christine Tobin is a jewel of the London jazz scene, streets ahead of the pack, but she deserves better. She should be on a global stage, rubbing shoulders with fellow troubadours like Cave, Mitchell and Cohen. And while her current 15-date tour should help broaden her UK fan base, this is an album that could break through to a bigger, international audience. Tobin's eloquent songs are framed by masterful arrangements and beautifully recorded to make the most of the content. There's no self-indulgence: it's all about the songs. Dave Whitford's sonorous bass and Phil Robson's self-effacing guitar are the perfect match for Tobin's rich, dark-hued vocals.

Highlights include the swampy Bye Bye; the Corner of an Eye, with its archaic "spring reverb"; and a delicious cover of Cohen and Robinson's Everybody Knows. Yet it's a tribute to the quality of Tobin's writing that there are no weak spots in this excellent collection.

Mastering Software Part 1 - Metering Levels and Loudness

I mentioned the Orban Loudness Meter on the SOS forums, and thought it would be good to start a series of posts here on useful mastering software. So, first up - metering apps. Several of the best editing packages include decent metering, for example Wavelab, and Audacity, but here are a few personal favourites:

•  Orban Loudness Meter Great (free!) utility includes a PPM meter, VU meter, ITU BS.1770 meter and CBS Technology Center loudness meter - it's fantastic to have access to these less common metering systems, which offer measurements of perceived loudness, which can be more useful than pure RMS. (Plus it runs well under Parallels on my MacBook Pro ;-) 
• Pinguin Audio Meter Not free but comes in several flavours, the Pro version includes an incredibly detailed spectrum analyser, audio phase correlator (more in a later post), PPM & VU monitoring plus a specrogram. Interestingly, Pinguin have done research to suggest that RMS is the most reliable way of measuring perceived loudness.
• Speaking of Spectrograms, there's Spectrogram - and Audacity has this capability, too.
• Finally, the fantastic (Mac-only) AudioLeak - an Leq (Long-term Equivalent Level) Analyzer for audio files and sound input streams. Swiftly discover the average RMS of an audio file, and it's A-weighted counterpart. Just watch out - by default it displays a combined mono value which reads higher than other applications like Wavelab - I usually use the average of the left & right values - they should be almost identical if the L/R balance is right, anyway. Also great for DVD Producers wanting to find the right dialnorm setting for a Dolby Digital encode.
• Update - here's another one - the SSL X-ISM meter. This is useful because it includes checking for inter-sample peaks. This is well-explained on the page I linked to, but briefly: strange as it may seem, when audio has been heavily limited and is peaking at (or very near to) zero, it is possible that the reconstructed analogue output actually peaks above zero. (In theory, up to 6 dB higher !) In cheaper players, MP3 encoders and other utilities, there is often no headroom allowed for this situation, and unwanted clipping can result. This meter will help you watch for this effect and avoid it. As a rule of thumb, don't allow your audio to peak higher that -0.3 dBFS.

Coming soon - how to choose a mastering studio.