Multiband compression

Multi-Band Compression Today I’d like to spend just a little bit of time talking about multi-band compression. Multi-band compression is something that’s been at play in audio for decades. It first came into play in radio station broadcasts. More recently, beginning in the early 1990’s as DSP became cheaper and more affordable, we were able to deploy multi-band compression affordably so that recording, mixing and mastering engineers could use it. What’s good about multi-band compression? Probably the main thing that multi-band compression allows for and that gives you an advantage as opposed to using one single band across the spectrum of compression is that you can set independent attack and release times for different parts of the spectrum. The reason that’s important is that a bass waveform has a much longer period to it. The fundamental frequency of a kick drum in a hip-hop tune is going to take 50–70ms for the low-frequency transient to get through. If you don’t want your compressor to be chomping at the very beginning of that low-frequency transient, you need to make sure to give the attack time enough time before the compressor kicks in so it doesn’t restrain the low end. Of course, 50-70ms is going to be a very long period of time for a compressor to wait if you are thinking about trying to also compress a jangly, bright acoustic guitar or something like that. You might have a tune that has a deep kick drum and a jangly guitar and it’s hard to find a single compression setting that’s going to work equally well in both those parts of the spectrum. A multi-band compressor allows you to divide the spectrum up into typically three or four different sections, apply different time constants, different attacks, different release to each part of the spectrum and to optimize the performance of the different segments of the compressor to the various instruments in a mix. What does this mean for your audio? In theory, it means you can get a compressed signal that’s tailored more to the program, and also because you can restrict the dynamics a little bit more effectively without hearing the compression per se, it theoretically gives the possibility of getting a louder sounding result in your mastering work. Potential problems with multi-band compression Multi-band compression is problematic for a couple of reasons that most people don’t really think about. First of all in order to divide the audio, you have to run through a series of filters – hi-pass, low-pass etc. Every time you run audio through a filter, you lose something in terms of fidelity. That goes a little bit against the credo of the mastering engineer. The idea of mastering is to always make something come out sounding better than how it went in, at least from one perspective. Running through the crossovers that are adding possible a little bit of ringing, a little bit of distortion and a little bit of noise flies in the face of trying to make stuff sound better. The minute you turn on a multi-band compressor, even if you have linear phase filters that are well designed you’re still going to get some change to the audio and it’s usually not a flattering change. That’s problem number one to my way of thinking. Problem number two is that I believe there’s something important about the proportion of harmonics in the sound of any given instrument. For a moment, I’d like to segregate this conversation so I’m making it clear – if you’re talking about dance music or music concrete – electronic music that doesn’t refer to an instrument that you might hear played acoustically then all bets are off in terms of maintaining the integrity of the original instrument. But let’s say you’ve got a recording that’s got a bass, and a guitar, and a vocal, and assume that I’m correct in saying that there’s something important about the proportion of harmonics to the fundamental frequency of any instrument. The minute you put a crossover in a compressor and start doing something different with one range of an instrument and another range of that same instrument, you’re going to start to skew the relationship the fundamental and the early harmonics of that instrument which give it the warm, full, clear part of its sound and the higher harmonics which give it some edge or brilliance. At first it can be a very seductive phenomenon that you get when you get this increased sense of brilliance and control and so on, but to my ear usually what comes out of a multi-band compressor – the instruments themselves – don’t sound as good as how they sounded going in to the multi-band compressor. So keeping that in mind, I think most mastering engineers who work at the top of the craft use multi-band compression sparingly if at all. It’s just something to keep in mind when you turn on a multi-band compressor, just for a moment focus your attention on the individual instruments in the mix. Listen to the bass, listen to the guitar before and after, listen to the vocal and make sure you aren’t harming that instrument in a certain way that ultimately means the thing isn’t going to sound as good as when it went in.

Stereo Processing - Twice as nice?

Stereo Processing

I’d like to talk a little bit about stereo processing and mastering.

So what does it mean, ‘stereo processing’?

Maybe the place to start is to talk about what does it mean to think about stereo – there are really two different ways of thinking about stereo. One is left channel / right channel. You have two mono channels that are feeding two channels in an amplifier, two speakers, and the arrival time coming out of one speaker or the other or both at once will determine the perceived direction of any given sound. If you delay one speaker a little bit, everything will sound as if it is coming from the other speaker because of the precedence effect, or the Haas effect as we know it. So that’s one way of thinking about it – as dual mono.

The second way of thinking about it is something called M/S or Mid-Side processing. It is a style of processing audio that’s coming a little bit more into vogue in some plug-ins, compressors, equalizers and analog equipment that is being introduced to the market over the last several years. Going way back, it is best known as a microphone pickup technique.

The principle of M/S, or Mid-Side is to think about everything that’s coming from both speakers at one (that’s common to both channels) as being one component of the sound and everything that’s different, that’s arriving at a different time from one speaker or the other, or the ‘difference channel’ as being the other component of the sound. This is sometimes known as ‘A minus B’. if you want to listen to the difference signal in a mix or a recording, you can take the two channels in a recording, pan them both to mono, flip one of them out of phase and what you end up hearing is the ‘difference signal’ – everything that’s not common to both channels in a recording.

So that’s a different way of thinking about the stereo field and the way that we thinking about processing stereo – what’s important, what’s interesting, what’s meaningful about this?

If you think about it, when you are listening to two speakers, in order to determine direction you are relying very heavily on this concept of phase or arrival time shall we say. If something shifts in the arrival time between two speakers then suddenly you’ll have a very very different sense of its location. If we’re processing the left channel and the right channel separately and we introduce an equalizer that’s not a linear phase equalizer for instance, there’s a strong possibility that that instrument or that part of the spectrum is going to change in it’s perspective in the stereo image. If that’s what’s desired – great – maybe that’s a desirable effect. But more often than not, we want to do the same thing to both channels at the same time in order to keep the phase coherency between the two channels and maintain that sense of stereo image. What’s important about stereo image is not just the localization of an instrument – not just that the trumpet is coming from right field - but also that the integrity of the reverb stays the same, that the width of the stereo field and the sense of openness and space and top end remains the same as what was present in the mix when it came into the mastering studio.

M/S Paradigm

Processing in the mid-side paradigm has a similar kind of concern about it. In other words if you change the arrival time, the phase relationship between the mid and the side channel, you are likely to drastically change the sense of space in a recording. If you exaggerate the side component too much, you are also likely to find that everything that was in the center of the mix might begin to recede for the listener. I think most people would agree that the stuff that appears in the center of the stereo image is very often the most important stuff. For a Pop mix, it’s usually the kick drum, the snare drum, the vocal, the bass – the main players in a mix. So you have to be careful about how much you change things in the mid channel and the side channel with respect to each other.

What to watch for with stereo processing

As you start to think about mid-side, it’s important to note that there some other arenas of what happens to audio that is playing in the M/S sandbox if you will - where they become important.

The first thing I want to point to is when you are using a stereo compressor. If you are feeding the left channel and the right channel separately into two sides of a stereo compressor and you allow the detector circuit for each side to behave independently, you could end up with some very strange behavior when you are listening to the output of the speakers (listening to a stereo mix) when for instance, the drummer hits a rack tom that appears in one speaker that jumps out of the mix a little bit, one channel of the compressor might be compressing heavily whereas the other doesn’t compress hardly at all and the whole stereo image can begin to steer in one direction or another. So most of the time when we are compressing a stereo mix, we’ll work in a mode where the two channels are linked together and the compressor is really paying attention primarily to the mid component of a mix and not so much to the sides.

How codecs affect stereo processing

In a similar sense, MP3 encoding is based on the idea that you want to try to do as little processing as possible to the most important part of the signal, namely the mid, the thing that is common to both signals and throw away as much of the information that is involved in the quote “less important part of the signal” namely low level signals, out of phase signals, very high frequency, very low frequency signals that are out of phase and so if you start to create a mix where you exaggerate the side information and de-accentuate the center channel, when you pass this audio through an MP3 encoder you may find that you are hearing a much more pronounced effect from the MP3 encoder on your mix.

So there again, you want to be careful about exaggerating the side channel too much, exaggerating the sense of space too much because given that we are all having to deliver things with lossy codecs (MP3, AAC), you may end up with some unintended consequences of your processing.

Metadata, ISRC, UP and QC

What is metadata?

Any information that is included within a program, whether it is for a download or creating a disc, that is not the program itself. It is embedded within the digital file (as a download or when burned to disc). Examples are track IDs, start and stop IDs, ISRCs, UPCs, and CD Text information. One of the things that the mastering engineer is responsible for is understanding what these are and including all this information in a master.

What is an ISRC?

It stands for International Standard Recording Code. It is a number that is allocated to any publisher (record label, artist, or anybody that is owns a catalogue of music). It is registered in the USA with the RIAA (Recording Industry Association of America) and in Europe with GEMA (a performance rights organisation) . The code is a unique identifier that gets attached to every single piece of music (each song within a record would have its own ISRC code). Any time that the music is played over the air, downloaded or streamed, the identifier is logged. This is vital in the payment process .

http://www.usisrc.org/

What is a UPC?

It stands for Universal Product Code. Is a number assigned to a product. Traditionally it has been a physical item, such as a cereal box in a grocery store, which has a bar code (and correlating number) to scan at the checkout to identify what that product is. The same is true of CDs or DVDs. But they are also used to track downloads in some cases, so you should register and include it in your product.

What is QC?

This is what is known as Quality Check. Mastering is the final process before distribution. As a result, it is the mastering engineer’s job to make sure that there are absolutely no flaws in the program (a dropout or a click for example). The mastering engineer should give the client assurance that there is no problem with the audio.

On Stereo-Imaging

Stereo-imaging tools are often included in DIY mastering packages or equalisers with stereo spreading facilities (most commonly offering some kind of mid-side processing options), so it is important to acknowledge their purpose and their limits.

As with many of the specialist tools we have for processing audio, they are great at solving specific problems. If you have something mono or largely in mono, for example, and you need to try and widen it, you can add reverb or perhaps exaggerate the little stereo information that already exists in the track.

But what exactly is the ‘stereo information’?

Well, it has to do with the relationship between the ‘out-of-phase’ information, and the ‘in-phase’ information. Anything that is in-phase happens at exactly the same time in both channels, and that information will appear to be centered. If anything is slightly delayed off to one side or the other, compared to the center of the image, it is ‘out-of-phase’ and is one of the things that creates a stereo sense of spread.

So when you are mixing, you are using pan pots, delays and reverbs (etc) to create a stereo image of individual elements within an overall stereo mix. However, when you go in during mastering and increase the out-of-phase component, you are changing the relationship between the out-of-phase and in-phase parts of the signal of the entire mix. You therefore are able to radically change the sense of the stereo image and the placement of each individual instrument in a song, which can be very dangerous if not treated with care. And although you may increase the perceived wideness, this is at the expense of the in-phase components in the song. That is to say, the elements that are dead-center, which also tend to be the most important elements of most productions – vocals, snare, kick, bass – are weakened.

So sometimes – in its various forms – stereo-imaging can be used to good effect. However, one should err on the side of caution.

CDR Quality - Or Lack Thereof

While we're on the subject of good sound (how's THAT for a non-sequitur?!) there's something that's slowly creeping into the world of music production, namely poor quality CDR's.

While I don't want to get into a long discussion of the CRC (error correction) that's built into the playback of a CDR, you should know that the CD format was built to tolerate errors. In some cases it will fix errors on playback perfectly. In other cases, if it can't reconstruct the data, it will 'approximate' the data. The implication is, when you play a CD you don't know if you are hearing exactly what was recorded to it. The difference is usually very subtle, and arguably fine for most consumers, but not so fine for those of us that work hard to craft recordings.

This problem is not so prevalent with replicated (pressed) CD's, but moreso with duplicated (burned) CD-R's....and it's getting worse. At my studio we routinely check every disc that's intended as a master, and in the last 6 months we have noticed a significant decline in the quality of the burns to disc. We can still get a workable master, but sometimes we run into a batch of discs that are unusable. We wouldn't know it if we didn't test, and that makes me wonder how many discs people make that are malfunctioning in ways they might find unacceptable....if they knew.

THe answer is probably a move away from redbook audio to full resolution data transfer from local servers for consumers and studios alike....but for now we check our discs carefully, and make ddpi masters when we can.

Should I (the Artist) Attend The Mastering Session

Should I Attend The Mastering Session

Having the artist attend the session can often be useful for both parties, and make for effective and fast communication. However, it is not absolutely essential that the artist attends the session and it is commonplace that mastering sessions and communications are dealt with via FTP (file transfer protocol) and email/telephone respectively.

What If I Can’t Attend?

As mentioned, it is not essential that artist attends the mastering session. There are certain things that should be provided to the mastering engineer, however:

· The sequence – This is the song order of your album. There are many possible orders and this should be decided and given to the mastering engineer before the session.

· Notes – any questions and concerns you may have. For example, perhaps you feel one song is too quiet, or the vocal is not quite bright enough, or you want a warm and darker master. Let the mastering engineer know this, as he will otherwise take lead from what he is hearing and assume that it is the creative intention of the artist and producer.

Learning From The Artist

Learning From The Artist

When working on a science experiment, conducting a poll or doing anything creative there is always a danger in beginning the process with the conclusion already decided. Not only will initial prejudice skew the results but it will often get in the way of discovering something more meaningful than you could have imagined or anticipated.

Where art is concerned, we usually look for a strong sense of identity and conviction in the creators or participants, and there lies a paradox. My clients want me to have a strong idea of how I think their recording could sound (or in some cases should sound), yet there have been instances where my clients have a strong idea and I learn from them. Just last week a client instructed me to NOT add any high frequency equalisation. He didn't want his project to be articulated, or 'opened up'. The texture he was after was murky and purposely distant. This is certainly not a choice I would have made, however the result was rather stunning. It evoked a feeling of warmth and melancholy, and allowed for a sort of intrigue that wouldn't have been achieved without his instruction.

It behooves us to always keep our ears and minds open, and to listen for those little nuggets that might indicate meaning or serve as a catalyst for new ideas. It allows the producer or engineer to do something different and out of the ordinary... and when has creating really been a place for the ordinary?