Spectra Plus

The technology that lets your PC speak can equally well make it listen. Brian Heywood gets all analytical on us...

While the musical uses of a MPC‑based soundcard are pretty obvious, it is worth considering what other applications could take advantage of its ability to 'listen' to the world using the audio input, or 'speak' to it using the audio out. The most obvious tasks are voice control of the PC — which could be useful in the studio, when your hands are full of musical instrument, or getting the computer to read out instructions — a boon to those with a visual impairment. The next level of complexity is to get the PC's processor to analyse the audio, so for instance the G‑Vox guitar system allows you to tune the guitar by analysing the notes' frequency.

One sophisticated audio application I've been using recently is Spectra Plus from Pioneer Hill Software in the US. This package is a powerful frequency analysis tool that uses FFT (Fast Fourier Transforms) to process sound — and essentially turns your soundcard‑equipped PC into a high‑specification audio spectrum analyser. This kind of audio analyser is very useful in virtually any kind of musical environment, especially if your room is less than ideal. While some people can afford to design their studio/music room from scratch, most of us have to convert a room (or rooms) that we already have. An audio analyser can therefore help to make the best of the audio environment that fate has dealt us.

Spectral Analysis

In case you are not familiar with the device, a spectrum analyser allows you to display the frequency components of an audio signal, and display them in a number of ways. At its simplest, the analyser will display a real‑time frequency response diagram showing what frequency components are present in a particular audio signal. Since we often process a sound in terms of its frequency — for instance by equalisation or 'tone' controls — the spectral analysis of the signal actually contains a lot more useful information than the simple waveform plot known as a 'time domain' representation (ie. amplitude versus time).

Simple as it is to get the PC to display audio data in the time domain — almost all Windows‑based waveform editors will do it as part of their editing interface — the ability to analyse the frequency components of a signal is less common. There are editors (like Turtle Beach's Wave for Windows) that give simple frequency displays, but nowhere near the quality required for serious analysis. Obviously, you need to see the waveform to perform basic edits, but once you get past the 'razor blade' stage, then you need a more sophisticated way of looking at your audio data.

Spectra Plus gives facilities that are only found in hardware analysers that cost around ten times the price of the software, plus some features only possible on a computer‑based system. It has five display modes, which show various views of the current audio data. These are: Time Series (amplitude vs time), Spectrum (amplitude vs frequency), Phase (phase vs frequency), Spectrogram and 3‑D Surface (time vs frequency vs amplitude). The audio signal can be captured in real‑time or can be recorded as a WAV file and then processed.

All views allow you to capture and hold the peak values for any frequency, so you can build up an overall picture of either the audio programme material, or (by using a white or pink noise source) the audio environment through which the signal passes (ie. speaker * room * microphone * mixing desk * PC soundcard). One neat feature is that the software allows you to calibrate the displays, to compensate for any effects that the microphone, speaker or mixer may have on the frequency make‑up of the audio signal you are testing.

Cosmetic Surgery

Spectral Plus has a wide range of uses in any situation where you need to process audio for music applications. For instance, it can be used to determine the frequency response of a recording area or live venue — say for a studio recording area or a live gig. In this case, the spectrum analyser view can be used in real‑time mode to directly monitor a pink noise source, allowing you to use a 1/3‑octave graphic equaliser to compensate for the room's acoustic characteristic. You could use a portable PC with built‑in MPC sound facilities, or record the audio onto DAT, and then process the audio later.

Another use is to analyse a WAV file that is to have its resolution downgraded for release: say from 16‑bit stereo sampled at 44.1 kHz to 8 bit/22.05 kHz sound (eg. for a game or multimedia soundtrack). The 3D or Spectrogram view can be used to check for high‑frequency components that can then be removed by the filtering function. Since I've had the software, I've used it to analyse the acoustics of my studio's recording area, as well as track down an annoying frequency artefact in an old recording that was used on the Worlds Apart CD by the American artist Keope.

The software allows you to use the mouse to directly read the values off the displays in the various views, although you will need to calibrate the PC's soundcard to take absolute amplitude readings. There is an overlay facility that allows you to compare two frequency characteristics, so you can perform before/after tests, and also generate the compensation files used in the aforementioned calibration feature. These overlay files are stored as text files, so they can be imported to a spreadsheet program, or even generated by hand from the device's specification, using your favourite text editor.

While the ability to look at the frequency components of a piece of audio is one of those facilities that you are unlikely to need on a daily basis, it can be terribly useful when you do need it. The software will operate on any Windows PC equipped with a soundcard, although if you just want to analyse WAV files, you can dispense with the soundcard. The software is available from Broadleaf Engineering and Design (0181 466 6211) and costs £260 (plus VAT) — which is considerably cheaper than a hardware analyser of equivalent functionality.