Analyze This

Dec 1, 2007 12:00 PM, By Larry the O

Can You Correlate?

Correlation meters measure the similarity between two signals (see Fig. 7). A fully correlated signal is the same in each channel — that is, the signal is mono. But correlation is not the same as phase: if two channels are extremely dissimilar, they may have a lot of out-of-phase information, or they may have a number of hard-panned mono instruments. However, uncorrelated material is usually perceived as stereo material, and highly correlated material as mono, so a correlation meter can give some idea of the overall width of the signal. It is important to look at trends in the meter more than its instantaneous behavior, because moments with a lot of uncorrelated material between channels are common.

Using mid-side meters is another way of contrasting mono and stereo information in a signal. A mid-side meter typically has two displays: one showing material that appears equally in both channels (mid), the other showing material that is different between them (side). A healthy mix generally has near-equal amounts of both.

Interchannel phase can also be displayed on a polar plot, but two polar-phase plots might represent very different information. For instance, SpectraFoo's Phase Torch shows a vector whose length represents frequency and whose angle represents phase angle. Inspector XL's Phase Scope, on the other hand, plots amplitude as the distance from the circumference of the scope and perceived direction of the sound as the angle.

Transfer Functions

A transfer function is generated with a differential FFT analyzer: that is, FFTs are computed for two signals, and then one FFT is divided by the other, leaving only the difference between them. If one signal is the input to a system and the other its output, the display shows how the signal was altered going through the system. Put a vocal track into one input and an EQ'd version of the same vocal on the output, and you will see the actual EQ being applied (regardless of what the EQ display says).

Transfer functions have several powerful properties. First, since they are a comparison, the source material could be anything, including music. Second, it is possible to show phase differences as well as spectral and level differences, and even to calculate coherence, which is a good indicator of signal-to-noise ratio.

Transfer functions can be used for any kind of comparison, but they are most commonly used for acoustical measurements, especially in tuning sound systems. You can also check the performance of pieces of equipment or software and compare the outputs of two microphones, among other things. As powerful as transfer functions are, they are rarely used in studio applications. One reason for that is they are CPU intensive, making them difficult to release in plug-in format. Another is that test applications often call for comparison, but music recording and mixing production deal more with what is happening in real time and, in some cases, the recent past.

Code Tools and Statistics

There is a class of information in digital recording that relates to the behavior of the digital audio medium rather than of the source signal, and these are code-related tools. Test packages such as SpectraFoo have a more extensive set of code tools than do most production-oriented analysis packages.

Clipping counters let you know the frequency with which digital clipping has occurred. Some analysis tools refer to “overs” rather than “clipping,” and Inspector XL distinguishes the two, defining clipping as levels exceeding a user-defined clip threshold, and overs as signals that exceed full-scale. In many cases, one or two clipped samples won't be audible, so if a clip counter shows only a few clips for an entire song, there may not be reason for concern. On the other hand, having dozens of clips might be a concern. Clip counters can generally be programmed to respond to a specified number of consecutive clips.

Bit scopes are real-time indicators of the instantaneous use of each bit in a digital word. If, for example, there were a plug-in or device that operated at 16-bit resolution but claimed to use 24-bit, it would be obvious on a bit scope, because bits 17 through 24 would never light up to indicate use. Another example: a bit in the middle of the word that never lights up to show that it is used could indicate a problem in a digital audio converter.

Usage Challenges

The strength of analysis tools is that they give visualizations of quantitative data, which makes understanding the data more intuitive. One limitation of analysis tools is that using more than two or three at a time creates visual clutter. Small analysis windows are not very useful, so with four or five of them open at the same time, even a dual-monitor system must devote substantial space to meters. Constantly updating multiple, real-time graphic readouts is also taxing on a CPU.

There is also the human problem of paying attention. With too many dancing displays going at the same time, the analytical focus that was the impetus for using a meter in the first place becomes dissipated, and you can end up glancing from one meter to the next, trying to catch events that require attention. One way around this is to let the program watch the meters for you, alerting you when specified conditions are met. However, the only package I have found that implements alarms is Inspector XL.

Analysis tools give us useful data, but it is only data, not knowledge about the audio. So, in the final analysis, the ears remain the best and most important source of knowledge about whether something sounds good. But data can be seductive, and people sometimes come to rely more on what they think meters are telling them than on what their ears tell them. That becomes a problem if someone is not metering the appropriate information or if the quantitative data supplied by a meter does not map well to the most closely related perceptual attribute. For instance, level meters mostly give representations that are of the power in a signal. But loudness is a perceptual attribute that does not map directly to signal power as shown on a typical meter. Or the problem may be as simple as not metering the right parameter.

In any event, meters are best treated as supplements to what we hear. If there is a discrepancy between the two, further inquiry may be in order. But it is foolish to assume that the meters must be “right” and that you aren't hearing correctly.

Be a Meter Reader

Meters and analysis tools present various ways of looking at our audio. Getting good results from them depends on three things: understanding the working principles of each tool well enough to know when to use it, being able to read each tool's display, and having the context to interpret what the display is showing and to extrapolate useful knowledge from it.

A large and growing number of analysis packages are priced affordably and are available for all of the major operating systems and plug-in formats (see the sidebar “Some Metering and Analysis Tools”). For a creative artist, making informed decisions isn't always necessary, but for an engineer at any level, making informed decisions can help lead to good results.

Larry the O is a musician, producer, engineer, sound designer, writer, and master soup stirrer in the San Francisco Bay Area. Special thanks to Bob Katz, Joe and B. J. Buchalter of Metric Halo, and Roger Nichols Digital. Tips of the hat to BIAS, Steinberg, iZotope, and Audiofile Engineering.

SOME METERING AND ANALYSIS TOOLS

Many metering and analysis tools are available. Below is a list of the major ones, along with their Web addresses, which will lead you to more information about them.

Audiofile Engineering Spectre www.audiofile-engineering.com/spectre.php

BIAS Reveal (Master Perfection Suite) www.bias-inc.com/products/masterPerfectionSuite

Blue Cat Audio Analysis Pack www.bluecataudio.com/Products/Category_Analysis

iZotope Ozone www.izotope.com/products/audio/ozone

Metric Halo SpectraFoo www.mhlabs.com/metric_halo/products/foo

Programmable Analysis Software (PAS) Surround Meter (shareware) www.audio-software.com

Roger Nichols Digital Inspector XL and Inspector (freeware) www.rogernicholsdigital.com/inspectorXL.html, www.rogernicholsdigital.com/inspector.html

Sony Creative Software Sound Forge www.sonycreativesoftware.com/products/product.asp?pid=431

Steinberg WaveLab www.steinberg.net/128_1.html

SuperMegaUltraGroovy FuzzMeasure Pro www.supermegaultragroovy.com

Troodon Technologies TrooTrace www.troodontechnologies.com/products.htm

Waves PAZ Analyzer www.waves.com/Content.aspx?id=233

BONUS MATERIAL
Learn more about FFTs (Fast Fourier Transforms) and their impact on data parameters