Convergent evolution

Digidesign has always pushed the technology envelope and has not been afraid to produce its own hardware to give the software a performance edge. Sometimes this left the company looking somewhat silly, as was the case when it launched a product in London during the mid-1990s to a room full of high-powered music-technology journalists. We all watched the system crash and burn with a certain amount of wry amusement. This is a problem that any company working at the bleeding edge of technology will experience from time to time.

Pro Tools, like a lot of the software packages developed for audio post applications, uses a non-destructive editing scheme, which means it does not directly alter the audio when you edit it onscreen – it simply creates a new EDL (Edit Decision List) entry that describes what you have done to the audio. This EDL is just a list of instructions that tells the DAE (Digital Audio Engine) what to do when you decide to play back the edited audio. This means the software does not have to copy large chunks of audio data around the hard disk whenever you perform an edit, so the actual work is performed only once you hit the playback button. The upshot of this is that the Digidesign software can wring the last ounce of performance out of your computer’s hardware. This technique of using EDLs was borrowed from the analog film and video-editing worlds, where the online video-editing suite was controlled by the contents of the EDL, performing the edits on the original video masters to ensure that the highest audio and video quality was maintained.

The internal data structure of the EDL simply notes the start point and location of the audio data for each edit. I know this because I had to transfer a Pro Tools session between an Apple Mac system and my PC-based Soundscape recently. This wasn’t a problem, as the Soundscape SSHDR-1 software can import Pro Tools sessions – at least for sessions created by versions 3, 4 and 5 of the Digidesign software. It turned out that the Pro Tools software on this particular Apple system had saved all the audio as 32-bit files, rather than the studio standard (24-bit) or the CD standard (16-bit). In fact, all I needed for my application was 16-bit quality, so I wanted to ‘down-sample’ the source audio to the lower bit depth (or sample size). Considering the way that Pro Tools operates, I simply located the source audio files and used Cool Edit Pro to batch convert the audio files to the lower resolution, which worked a treat. The EDL must just reference the edits by counting the samples, and since I hadn’t changed the sample rate that didn’t affect the edit points one jot.

Not Fade Away

In fact, Pro Tools takes a hybrid approach to non-linear editing. While competing audio post systems such as Studio Audio’s SADiE and Sydec’s Soundscape perform all the edits in real-time, Digidesign software creates separate files whenever you do a fade or a cross-fade. This obviously stresses the machine more during the editing process, but unless you are doing an outrageously long fade you probably will not notice the performance hit. The additional storage overhead of keeping the fades as separate data files is also going to be minimal for virtually all sensible applications. The advantage of this approach is that the computer does not have to perform the calculations to implement the fade during playback, simply to reel off the pre-calculated fade segment.

While the audio fade calculation is relatively trivial, if you have a lot of tracks fading at the same time it will cause the processor loading to spike, leaving less computer power available for running audio effects processing plug-ins such as reverberation, equalisation and so on. Without this approach you could get into the situation of mixing a track where you are pushing the computer nearly to its limit, only to find that the whole thing falls apart when the playback hits the final fade, which might involve a large number of separate audio tracks, and hence fade calcu-lations. The Digidesign approach neatly avoids this by doing these fade calculations in advance.