The latest technological advances in reverb design have yielded expensive hardware units that can 'sample' the sound of real rooms. Now Audio Ease's Altiverb brings the same convolving technology to the Mac — at a fraction of the cost.
You can get a lot of recording gear for comparatively little money these days, especially if you're prepared to commit to a software‑based setup. For example, most sequencers come bundled with dozens of plug‑ins, which could easily cost thousands of pounds to replace with hardware.
Some things never change though, like the search for really good, affordable reverb processors. That's principally because creating believable acoustic environments will always be a complicated business, requiring complex mathematics and lots of processing power. Things are, of course, better than they were 10 years ago, but good software reverb plug‑ins are few and far between, and even quite disappointing ones may well eat half your processor power. Going down the hardware route (as so many people still do) quite respectable multi‑effects units can be had for £250, but companies like Lexicon and TC Electronic still charge four‑figure sums for their most classy sounding products, reflecting the finely honed (and often jealously guarded) algorithms they use, and the R&D time spent getting them to sound as they do. For project studio owners the very best reverbs have always been aspirational and, more often than not, unaffordable items.
It's possible all that's about to change, though. Dutch company Audio Ease, who until recently have produced plug‑ins exclusively for MOTU's Digital Performer sequencer, are about to release their Altiverb in VST and (H)TDM formats to join the MAS‑format, G4‑only Altiverb that's been out for about nine months already. Altiverb is a so‑called convolving (or sampling) reverb, and as such joins the £4500 Sony DRE S777 and £5500 Yamaha SREV1 — both hardware units — at the top of the 'real reverb' tree.
If you go to a church or other reverberant space and clap your hands, you immediately become aware of the acoustic environment you're in. In convolution terms you're providing an impulse, and what you're hearing is the building's response to that impulse.
Acoustics specialists are able to analyse the response characteristics of reverberant spaces by making recordings in them of impulses that occupy a broad range of frequencies. A round from a starting pistol makes a pretty good impulse, but even better is a sine wave swept in pitch across the audible frequency spectrum. The resulting recordings consist, of course, of both the impulse plus the room response, and by comparing this with a reference 'impulse only' recording, a model of the room's reverb characteristics can be extracted. Convolving reverbs like Altiverb apply these response characteristics to the audio passing through them.
Intriguingly, Altiverb is able to sample things other than reverbs, such as delays, or response characteristics of equalisers and microphones, though its capabilities fall short of being able to convolve many other effects such as flanging or distortion. The distinction between audio treatments Altiverb can and can't sample is subtle and hard to understand, but suffice to say that whilst you can have the Sydney Opera House in your studio, the secrets of Brian May's guitar sound are safe for a little while longer.
Altiverb for MAS (the version I tested) is available both as a download and as a physical, shipped product on CD. The actual plug‑in installed in Digital Performer's MAS plug‑ins folder as might any other, but the first time I booted up DP with Altiverb installed I was invited to locate another folder containing the impulse response files that it uses for processing. There's a healthy selection on these on the installer CD, including some made in the Amsterdam Concertgebouw and several other well‑respected recording venues. They're also available from the Audio Ease web site, where new impulse responses are being posted all the time.
Whilst Altiverb's manual tends to be a bit rambling and lacks clarity in some places, both installation and first use of the plug‑in were very straightforward. Altiverb is activated in DP3 by selecting it in an audio track's insert slot, and depending on the type of track, a whole range of signal‑routing options is available. Altiverb can process mono‑to‑mono, mono‑to‑stereo, stereo‑to‑stereo, mono‑to‑quad and stereo‑to‑quad reverbs, although not all impulse responses come in all these varieties.
At a touch more than 800 pixels wide, Altiverb's window is not exactly compact (perhaps reflecting the 'heavyweight' nature of the processing it offers) but there are comparatively few controls. At far left is a large Reverb Time knob and accompanying digital display, while there's an information display window at the far right, with left/right and zoom buttons.
In the centre of the window there's a pop‑up menu for selecting impulse responses, along with three knobs for setting wet and dry signal levels and pre‑delay amount. An extra pop‑up menu shows up in anything‑to‑quad instances of the plug‑in, and there are also a couple of 'radio buttons' for switching Altiverb's operating mode between high latency with low processor load and no latency with high processor load. Processing audio with zero latency can easily max out even quite nippy G4 processors: running with high latency but lower processor load is an option while mixing, certainly, and very handy if all you ever use Altiverb for is adding a bit of 'bloom' to classical recordings or final mixes. Precise latency values are given in terms of samples in Altiverb's display window.
Actually using Altiverb is child's play. From the pop‑up menu you select the impulse response you want, and away you go. The Reverb Time control can make any IR's tail shorter, but not longer, and it doesn't work in real time: every change of reverb time setting is followed by a period of recalculation.
Many IRs, by Audio Ease and others, come with JPEG, GIF, PICT or TIFF files which show up in the display window when the IR is selected. Using the window's forward and backwards buttons it's possible to view these graphics files one by one. Many of the bundled IRs include diagrams of the mic and speaker placement used in the creation of the IR, interior and exterior shots of the location, equipment lists, and copyright/credit information. Any graphics file associated with an impulse response in this way is automatically scaled to fit into Altiverb's front‑panel window, but clicking on the zoom button opens another window showing the file at its original resolution. Seeing the building a particular IR was produced in can help to put an acoustic in context, but it's a tiny bit gimmicky, and I imagine seasoned Altiverb users won't bother too much with it.
The process of recording impulse responses is a little convoluted (no pun intended) and I imagine many Altiverb users will simply never bother when there are so many good ones available for free. In essence, though, it's quite easy to do, and I made several during my first few weeks with Altiverb, including some of various bright, live rooms which sound great for drums and guitar.
An ideal setup for recording IRs at a venue would be a laptop computer running your preferred multitrack audio software, a multi‑channel 24‑bit audio interface, a single full‑range monitor speaker, some good mic preamps, and the best mics you can get your hands on. To give an example, Audio Ease's own IRs were produced with a G4 running Digital Performer, a MOTU 1224 interface, a Genelec S30 monitor speaker, a Yamaha HA8 mic preamp box and Bruel & Kjaer 4006 and 4011 microphones. This sort of setup is not much fun to carry around but for various reasons makes stereo‑to‑stereo or stereo‑to‑quad impulse responses much easier to make.
It is possible to use more portable equipment, though, and I made a whole range of IRs using a Tascam DA302 twin‑deck DAT machine, an Alesis M1 Active monitor and an M‑Audio DMP2 mic preamp, together with Rode NT1, Sennheiser MKH20 and MB P648/100DK microphones. Anything‑to‑quad responses aren't possible with this setup, and stereo‑to‑stereo ones require a bit of post‑production, but I got great results all the same.
Different mics and mic setups, along with mic‑to‑monitor distance, have a huge bearing on the final outcome. Whilst firing off starting pistol rounds might seem like a fast and easy way of providing impulses, the sine‑wave sweep method yields much better results, particularly in terms of signal‑to‑noise ratio. There are various tricks to producing better quality IRs, like mixing down several sine sweep recordings before feeding them to the IR Pre‑Processor, and making multiple sine sweep recordings with the monitor speaker pointing in different directions. The bottom line, though, is that home‑grown IRs aren't difficult to make, and it's actually quite a satisfying process.
One thing that excited me about Altiverb even before I got it was the idea of producing some 'creative' reverbs. What, I wondered, would happen if you tried to create an impulse response from something quite different from a starting pistol shot or sine sweep? Well, for starters you don't need IR Pre‑Processor for this sort of thing because, as I mentioned earlier, Altiverb IRs are really just audio files. With that in mind you could record literally anything and feed it directly to Altiverb, though to say the results of doing this are uneven is a pretty huge understatement. I tried piano glissandos, sections from CDs, drum loops and speech, and the results were sometimes exquisite, though more often than not ended up being bizarre contortions of the IR audio file. Not really what Altiverb was designed for, perhaps, but fascinating nonetheless.
Altiverb IR Pre‑Processor is a separate application, bundled with Altiverb, which facilitates producing impulse responses from recordings made using starting pistol shots or sine‑wave sweeps. There are various Trim options for dealing with the correct handling of beginnings and ends of files, and buttons for specifying the input and output file location. A Process button starts the analysis process, during which a little progress window appears, and in anything upwards of a few seconds you end up with some usable impulse responses. It's all very simple.
Altiverb promises much, and I'm pleased to say it doesn't disappoint. In the obvious area where many cheaper, conventional reverbs can sound poor — producing long reverb tails in vast spaces — Altiverb simply excels. Most of the supplied impulse responses of churches, for example, offer a stunning level of realism and musical results that are 100 percent believable. The 'St Joseph Church' IR is a good example: a rich, warm acoustic with a wonderfully complex eight‑second tail. In fact, I recently used this IR to add a much‑needed sense of space to some choral recordings I'd made in a disappointingly dry, flat‑sounding acoustic. When run through Altiverb the entire recorded sound seemed to change. It had more warmth, apparently greater transparency and separation, and the reverb tail seemed to flow out of the original recording rather than being superimposed upon it. Easing the reverb time back a little didn't affect the character or quality of the reverb at all.
Of course, for a huge amount of music production really long reverbs are of no use at all, and it's often for their plate and small room algorithms that many engineers are prepared to shell out on pricey conventional reverbs. Here too, Altiverb produced results that eclipsed anything I'd heard before. Dry drum kit and percussion tracks sound tremendous played through some of the shorter IRs, such as 'School Building, piano classroom', and again it's the way the original recording seems to merge with and become part of the applied acoustic that makes the final result so compelling.
It has no doubt already occurred to many reading this review that if Altiverb can sample real acoustic spaces, it should also be able to copy artificial ones. I'm not sure anyone has yet worked out the full legal implications of sampling, say, a Lexicon PCM91's reverbs, but that's just the kind of thing that's possible. Already floating around on the web are IRs produced from a TC Electronic M3000, a Lexicon PCM90 and Kind Of Loud's RealVerb plug‑in. I've no doubt that some users who fancy having a high‑end Lexicon will hire one for a day and get busy with the Altiverb IR Pre‑Processor. All the IRs of conventional reverbs I've heard have been superb, and being able to use Altiverb in this way obviously extends its usefulness massively.
I ought to point out that, perhaps not surprisingly, audio played through a hardware reverb doesn't sound exactly the same through an Altiverb sample of the same reverb, though it often takes repeated listenings to hear the difference. Also, Altiverb has a little trouble sampling reverbs which use chaotic modulators to produce more believable tails — it is possible, but you have fewer options during the sampling phase.
It's no exaggeration to say that it's hard to go back to a conventional digital reverb after hearing Altiverb. The natural‑sounding tails are one thing, but Altiverb does more than that: it creates entire acoustic environments, mimicking the tonal character of a space in a way that few other reverbs I've heard have ever managed to do. Altiverb doesn't just sound real — you believe, utterly, that the reverbs it creates are real!
If there's a down side, it's undoubtedly processor usage. My G4 is getting a bit long in the tooth, certainly, but it can quite easily manage a 24‑track mix with a couple of conventional reverbs and dozens of other plug‑ins like EQs, dynamics, delays and even soft synths. It's not really up to the task of running Altiverb, though. A mono‑to‑stereo zero‑latency instance used up about 60 percent of the CPU power with nothing else going on, and I could run stereo‑to‑stereo instances only in high‑latency mode. Even on a dual 1GHz G4 a single stereo‑to‑stereo Altiverb in zero‑latency mode can use up to half the processing power, so a stereo‑to‑quad IR could, presumably, use it all up. Perhaps the forthcoming G5 chip, with its rumoured multiple Altivec engines, will make using Altiverb a more realistic proposition in an otherwise busy mix.
Altiverb represents a major advance in host‑based signal processing, and whilst 495 dollars might seem a lot for a single plug‑in, just think what sort of hardware reverb you'd get for the same money. By any standards this is a super‑high‑end bit of audio processing gear, and completely redefines price‑to‑performance ratio for reverb. An outstanding achievement.
It would be remiss of me not to mention the only other software‑based convolving reverb I'm aware of: Acoustic Mirror by Sonic Foundry. Formerly available as a separate application, it's now part of the two‑track editor Sound Forge, and offers pretty much all that Altiverb does but in a noticeably more complex form. There are a lot of impulse responses available for Acoustic Mirror, ranging from vast reverbs to Shure microphones. It's surely only a matter of time before someone re‑convolves them (if that's a word) for Altiverb.