Over the past couple of years, we've heard a lot about reverbs that work by a process called 'convolution' (and more recently other processors, too — see page 216 for details of another one from Focusrite). Unlike conventional artificial reverb, convolution-based reverberators need a so-called 'impulse response' (or IR) of a real acoustic space to work. This can be measured by firing off a special test signal into the space and recording the result using an accurate microphone (or two mics for stereo). The convolving reverb can then 'cross-multiply' the impulse response with the dry audio you want processed, and the end result is that the audio sounds as though it was originally recorded in the real space that you've previously 'sampled'. You can read more about the details of the process in the preview of the first hardware convolving reverb (see SOS June '99, or look up www.soundonsound.com/sos/jun99/articles/sonydres.htm on the SOS web site). Since the release of that first (very expensive) Sony unit, convolving reverbs have also been made available as software plug-ins (Audio Ease's Altiverb is probably the best known so far), but now Emagic have come up with their own interpretation of the process under the name of Space Designer.
Space Designer is a Logic plug-in and can only be used within Logic v6.3 or later. Like most convolution processes, it is fairly CPU-hungry when compared with conventional reverbs, but it has been optimised to keep the CPU load as sensible as possible. Though the simplest theoretical model of convolution starts with a test impulse only as long as single sample, this is difficult to do in practice, as a single sample isn't loud enough, and doesn't contain enough frequencies. You can create an impulse by bursting balloons or firing starting pistols in the acoustic space you want to 'sample', but you get a better result if you record the result of very accurate loudspeakers playing sine-wave signals that sweep through a wide range of frequencies out into the space you want to capture. Finally, Space Designer has to carry out further processing on these recordings to convert them into what would have been recorded had a pure impulse had been used. The result is then stored as the impulse response of the acoustic space.
As you can guess from this description, sampling your own real spaces is no mean feat, and requires a lot of care and patience, even though Space Designer includes its own built-in swept sine-wave test signal. You need to set up microphones, speakers and a recorder, as well as some means of playing the test signal, and that's assuming you can find suitable-sounding rooms to sample. The recordings also have to be made in the almost total absence of background noise to be effective. However, the process may also be used to sample other hardware and software reverbs, which is considerably simpler. The only proviso is that effects you're sampling contain no modulation, such as chorus or flanging. This precludes sampling those reverb patches that use internal modulation to help randomise the sound.
If sampling existing reverbs doesn't appeal to you either, I should point out that Space Designer is ready-supplied with over 1000 high-quality IRs on CD-ROM, not only based on rooms and halls but also more off-the-wall spaces and natural venues, such as forests and the calderas of (hopefully) extinct volcanos! Because convolution also works for sampling digital reverb units and plug-ins, there are also many IRs included which are based on patches from some of the best-known reverb units.
As already stated, Space Designer only works with Logic v6.3 and above, which means its not available to PC users working with older versions of Logic Audio. Updates to version 6.3 are included on the Space Designer CD-ROM for existing v6.x users of Logic Audio, Logic Gold and Logic Platinum, though users can also download these from the Emagic web site. The installer disk also includes updaters for OS 9 and OS X, so OS 9 users aren't being left out in the cold. Having said that, most third-party products have now been 'Audio Unitised', so most users would probably be better off switching to OS X by the end of the year.
Once Logic v6.3 and Space Designer have been installed using the now familiar automatic installers, you have to go through another installation process to load the IR library from the second CD-ROM. Once this is done, the IRs appear within Space Designer's Settings menu. Authorisation for the plug-in is via the XS key as usual using the included type B code, after which the user has 84 days in which to obtain the necessary permanent type A code from Emagic, either by registering on-line at their web site, or by returning the included registration card.
Using Space Designer is as easy as using any other digital reverb plug-in — in fact it's easier in some respects. Unlike traditional synthetic digital reverberation, which has lots of user-adjustable parameters, convolving reverbs are only accurate when exactly emulating the room that has been 'sampled', so there are fewer user parameters to adjust, and automation is limited to things like level and mix. Nevertheless, Emagic have built in a welcome degree of user adjustability by providing volume envelopes and tonal filtering, plus further filtering (with variable resonance) linked to a graphical filter envelope that can be used to change such characteristics as low- or high-frequency damping. Reverbs may also be created based on synthesized early reflections rather than measured IRs, and in this case, density, length, envelope and stereo-spread parameters are also available. Pressing the button again randomly generates a new variation on the synthesized IR based on the current length, filter and spread parameters, and when you find one you like, you can save the patch.
The reverb decay envelope is displayed in a graphical window which can also be switched to show the filter envelope. The early reflections pattern is always clearly visible as a separate image within the overall decay (see page 192), and the decay shape can be adjusted using a couple of grab handles, much as you would use the Bezier curve tools in a drawing program. As with conventional reverb, you can add pre-delay. The only thing you can't do is make the reverb time longer than the original Impulse Response file (unless you drop the sample rate — more on this in a moment). Note that some control changes take a few seconds to take effect as they process the IR before it is convolved with the audio passing through the unit.
For special effects, you can reverse the IR file at the touch of a button, which reverses the entire reverb effect. It's also possible to offset the timing of the IR so that its start gets truncated, and using the envelope controls, you can also create slow reverb build-up from an IR that originally had a fast build-up. In fact, given that convolution reverbs have a reputation for being inflexible, Space Designer allows a lot of adjustment before it starts to sound in any way unnatural. Where the input is stereo, you can also adjust the width of the stereo signal feeding the reverb, from mono to extra wide.
Convolution takes a lot of CPU horsepower, but there are a couple of ways you can conserve this. Firstly, you can pick a shorter reverb time (by shortening the IR time), because with convolution-based reverb, the amount of CPU overhead is proportional to the reverb decay time. Secondly, you can switch to a lower sample rate — half, a quarter or even one eighth of the original, which reduces the high-end response accordingly and doubles the reverb decay time every time you halve the sample rate, unless you activate the 'Preserve Length' function. With most natural-sounding reverbs, you can get a good-sounding result at half sample rate, but if you go any lower, the loss of top end becomes noticeable, so extreme settings are best reserved for special effects. Note, however, that halving the sample rate also doubles the plug-in's inherent throughput delay, which is around 128 samples. A Latency Compensation button on the plug-in compensates for the extra delay by also delaying the dry sound to match the reverbed sound and by getting Logic to apply extra plug-in delay compensation on other channels. However, if Space Designer is being used in a Buss and set to 100 percent wet, there should be no need to activate this, as 128 samples at 44.1/48kHz only equates to around 3ms, and it's traditional to use far more pre-delay than this.
Roll Your Own Rooms
Sampling a real room is conceptually similar to sampling a piece of hardware, though you need a fairly accurate speaker and two flat microphones if you're going to do the job with any degree of accuracy. Placement of the speakers and microphones in the space is also important, as not only does this affect the reverb characteristics, it also changes the amount of direct sound from the speaker picked up by the mics. Ideally, the geometry of the mics and speakers should be chosen so as to minimise the level of direct sound captured.
Sampling other reverb hardware is much simpler than sampling rooms, though the principle is much the same. The CD-ROM includes a swept sine-wave file specifically designed for recording room impulses and this is simply fed through the device in question with the appropriate program active and the audio output set to 100 percent wet. The resulting short audio file can be recorded into Logic (ideally at 24-bit resolution) and then saved. The manual includes practical advice concerning normalising, trimming and fading the swept sine audio files, but there's nothing complicated about the procedure. After this, you access the file from within Space Designer, and there's an automatic routine for converting the swept sine recording into an IR file, known as 'Deconvolving'. The IR can be called up from the settings library whenever you want to recreate the sound of the original room or reverb unit, and it may be saved with the preset IRs and categorised any way you wish.
Sampling software reverb plug-ins is even simpler; you simply load the swept sine file into an audio track, insert the plug-in you wish to clone and then bounce the track to a new audio file. Once normalised, this audio file can be called up within Space Designer and turned into an IR file.
As far as Logic is concerned, IRs are simply specially recorded audio files and Project Manager sees them as such. What's more, there's no reason not to use other short audio files instead of swept sine recordings and when you're asked to locate the original test signal, just to see what weirdness comes out when you turn your files into an IR!
I tested the practical concept of 'DIY' convolution (see the 'Roll Your Own Rooms' box on the right for more on the theory) by 'sampling' a rather nice vocal studio reverb from my TC Powercore's repertoire, a process that starts off by inserting the sine sweep test signal into a stereo audio track, then feeding it through the plug-in being plundered with the latter set to 100 percent wet and with the levels set as high as possible, but avoiding clipping. If the plug-in has no input gain control, you can insert Logic's Gainer plug-in. Once set up, you can bounce the processed sine sweep to a new interleaved stereo AIFF audio file (making sure you capture all the reverb decay) and then trim the starts and ends. You then call this file up in Space Designer by pressing the 'Deconvolve' button there and locating the audio file in the browser window.
Next, you're asked to find the original test signal used to make the recording, after which Space Designer thinks for a few seconds while it creates an IR file that you can save in your Impulse Response folder. This can be loaded into Space Designer using the IR Sample button, but I found the initial IR file was far longer than necessary, and so I shortened it using the Length control on the Space Designer panel to reduce it to around four seconds. This was long enough to capture all the decay of the reverb I'd sampled, but cut the processing load down to a sensible level — the original 16-second IR file was longer than my computer was prepared to entertain! Any variations you create can be saved as presets in the usual way.
Subjectively, the reverb I got back was near-enough identical to the original as far as I could tell, but things got interesting when I started using Space Designer's parameters to adjust it. I found it sounded best without filtering, which isn't surprising, as the original reverb also sounded fine without it, but the effect of reducing the IR length to less than one second was noteworthy, as all of the complex early-reflections stuff was retained, but the reverb decay tail was shortened, changing what was a room reverb into something more like a room ambience. This dropped the CPU loading even further, so I thought I'd 'cheat' and try to synthesize a reverb tail by inserting a Platinumverb plug-in after Space Designer and setting its ER/Reverb balance to all reverb tail. This worked surprisingly well in replacing the shortened tail, although when I scrutinised the sound, I realised how coloured and ringy even Platinumverb is compared with the original Powercore reverb. Still, with the tail added at a suitably low level, the illusion was so good that I feel Emagic should consider adding a synthetic tail generator to Space Designer so that those with limited CPU power could still use it to create long decay times. After all, the designers have included a sophisticated synthetic early reflections generator. Furthermore, adding a synthetic tail option would allow modulation parameters to be introduced to compensate for convolution's inability to deal with these in the original signal.
Initially, I looked at the lists of IRs supplied with Space Designer (which you may as well think of as reverb presets) and wondered why so few were designed for use on instruments or voices compared with all the emulations of natural spaces or special effects. However, because the included library comes with around 1000 IRs, including many taken from just about every big-name hardware reverb you can think of, there are enough good vocal and instrument treatments spread around the place. Many of the less obvious sampled spaces also work well with conventional voices and instruments. Furthermore, the supplied CD-ROM is entitled Volume 1, so I would also imagine that more IR libraries are planned.
A direct comparison between Space Designer and a decent 'traditional' reverb plug-in made it clear that they sound hugely different, especially when you listen to the library IRs made in real acoustic environments. The early-reflection patterns that give sound spaces their identity are very much more complex and contain much more vivid auditory cues than those used in conventional digital reverb, and this really comes across with Space Designer. You can place a singer in a rocky gorge or a forest (or indeed, the caldera of a volcano!) and really believe that's where they are singing. Even the quirkier effects have an organic sense of reality about them. Having said that, you don't dismiss the samples of artificial reverbs when you listen to those after the 'real' examples — after all, we're used to hearing that sound on records, so it still sounds 'right' — and the ones in Space Designer compare very favourably with the originals.
Processing load will always be a concern with a plug-in like this, and on my ageing single-processor 800MHz Mac G4, a useable Space Designer reverb could take anything from 25 percent of the available CPU power to 100 percent of it, with most of the practical examples taking around or just under half. Of course we live in the era of the Apple Mac G5, and tests by Emagic using a dual-processor, 2GHz G5 machine suggest that you should be able to run about 16 six-second Cathedral reverbs or up to 36 Plates at once — although sadly, I'm unable to corroborate these tests yet, as my G5 is just about to arrive at SOS Towers at the time of writing! The bottom line in practice is that anyone with a machine less well specified than my G4 800MHz machine would probably find Space Designer too processor-intensive, but those with more powerful machines should be able to spare the horsepower, especially given that Logic now includes the Freeze function to lower the CPU burden when you're using virtual instruments and other plug-ins.
I was initially sceptical of Space Designer simply because it offered so many reverb types that I felt were only going to be of use to those working in film post-production, and I didn't see how this could justify the heavy processing load on the computer. However, after playing with it for a while and trying out some of its reverb examples on voices and instruments, I've come away very impressed by its quality and realism. Having the ability to sample your own reverb hardware or rooms is also fantastic, and taking impulse responses from hardware or plug-ins proved to be relatively simple. I was also impressed by the ease with which the reverb presets could be customised for specific applications. Space Designer isn't cheap, and it does demand a significant share of the available CPU power, but if you want the ultimate in quality and have a fast enough computer, it is pretty irresistible.