31. Computer Audio Resources and Strategies
Real-time audio processing is a demanding task for general-purpose computers, which are usually designed to run spreadsheets and surf the Internet. An application like Live requires a powerful CPU and a fast hard disk. This section will provide some insight on these issues, and should help you avoid and solve problems with running audio on a computer.
31.1 Managing the CPU Load
To output a continuous stream of sound through the audio hardware, Live has to perform a huge number of calculations every second. If the processor can’t keep up with what needs to be calculated, the audio will have gaps or clicks. Factors that affect computational speed include processor clock rates (e.g., speed in MHz or GHz), architecture, memory cache performance (how efficiently a processor can grab data from memory) and system bus bandwidth — the computer’s “pipeline“ through which all data must pass. For this reason, many people involved with pro audio use computers that are optimized for musical applications.
Fortunately, Live supports multicore and multiprocessor systems, allowing the processing load from things like instruments, effects and I/O to be distributed among the available resources. Depending on the machine and the Live Set, the available processing power can be several times that of older systems.
If you are working on a multicore or multiprocessor system, you will want to enable support for it in the CPU tab of Live’s Preferences.
The Control Bar’s CPU meter displays how much of the computer’s computational potential is currently being used. For example, if the displayed percentage is 10 percent, the computer is just coasting along. If the percentage is 100 percent, the processing is being maxed out — it’s likely that you will hear gaps, clicks or other audio problems. Note that the CPU meter takes into account only the load from processing audio, not other tasks the computer performs (e.g., managing Live’s user interface).
Audio calculations have the highest priority in Live. Therefore, even if the CPU shows a high percentage of processor usage, the audio stream should remain uninterrupted. However, non-critical functions (such as screen redraws) might slow down because these tasks are handled only when the audio processing “lightens up“ a bit.
31.1.1 CPU Load from Multichannel Audio
One source of constant CPU drain is the process of moving data to and from the audio hardware. This drain can be minimized by disabling any inputs and outputs that are not required in a project. There are two buttons in the Audio Preferences to access the Input and Output Configuration dialogs, which allow activating or deactivating individual ins and outs.
Live does not automatically disable unused channels, because the audio hardware drivers usually produce an audible “hiccup“ when there is a request for an audio configuration change.
31.1.2 CPU Load from Tracks and Devices
Generally, every track and device being used in Live incurs some amount of CPU load. However, Live is “smart“ and avoids wasting CPU cycles on tracks and devices that do not contribute anything useful.
For example, dragging devices into a Live Set that is not running does not significantly increase the CPU load. The load increases only as you start playing clips or feed audio into the effects. When there is no incoming audio, the effects are deactivated until they are needed again. (If the effect produces a “tail,“ like reverbs and delays, deactivation occurs only after all calculations are complete.)
While this scheme is very effective at reducing the average CPU load of a Live Set, it cannot reduce the peak load. To make sure your Live Set plays back continuously, even under the most intense conditions, play back a clip in every track simultaneously, with all devices enabled.
31.1.3 Track Freeze
Live‘s Freeze Track command can greatly help in managing the CPU load incurred by devices and clip settings. When you select a track and execute the Freeze Track command, Live will create a sample file for each Session clip in the track, plus one for the Arrangement. Thereafter, clips in the track will simply play back their “freeze files“ rather than repeatedly calculating processor-intensive device and clip settings in real time. The Freeze Track command is available from Live‘s Edit menu and from the right-click (PC) / CTRL - click (Mac) context menu of tracks and clips. Be aware that it is not possible to freeze a Group Track (see 15.3); you can only freeze tracks that hold clips.
Normally, freezing happens very quickly. But if you freeze a track that contains an External Audio Effect (see 22.13) or External Instrument (see 24.4) that routes to a hardware effects device or synthesizer, the freezing process happens in real-time. Live will automatically detect if real-time freezing is necessary, and you‘ll be presented with several options for managing the process. Please see the section on real-time rendering (see “Previewing Files”) for an explanation of these options.
Once any processing demands have been addressed (or you have upgraded your machine!), you can always select a frozen track and choose Unfreeze Track from the Edit menu to change device or clip settings. On slower machines, you can unfreeze processor-intensive tracks one at a time to make edits, freezing them again when you are done.
Many editing functions remain available to tracks that are frozen. Launching clips can still be done freely, and mixer controls such as volume, pan and the sends are still available. Other possibilities include:
- Edit, cut, copy, paste, duplicate and trim clips;
- Draw and edit mixer automation and mixer clip envelopes;
- Record Session View clip launches into the Arrangement View;
- Create, move and duplicate Session View scenes;
- Drag frozen MIDI clips into audio tracks.
When performing edits on frozen tracks that contain time-based effects such as reverb, you should note that the audible result may be different once the track is again unfrozen, depending on the situation. This is because, if a track is frozen, the applied effects are not being calculated at all, and therefore cannot change their response to reflect edited input material. When the track is again unfrozen, all effects will be recalculated in real time.
Frozen Arrangement View tracks will play back any relevant material extending beyond the lengths of their clips (e.g., the “tails“ of Reverb effects). These frozen tails will appear in the Arrangement as crosshatched regions located adjacent to their corresponding clips. They are treated by Live as separate, “temporary“ clips that disappear when unfrozen, since the effect is then calculated in real time. Therefore, when moving a frozen clip in the Arrangement, you will usually want to select the second, frozen tail clip as well, so that the two remain together.
For frozen Session clips, only two loop cycles are included in the frozen clip, which means that clips with unlinked clip envelopes (see 20.5) may play back differently after two loop cycles when frozen.
The samples generated by the Freeze Track command are stored in your temporary recording folder until you save your Live Set, at which point they are moved to the following project folder sub-directory: Samples/Processed/Freeze. Please note that freeze files for tracks that contain an External Instrument or External Audio Effect will be discarded immediately when unfreezing.
You can also decide to flatten frozen tracks, which completely replaces the original clips and devices with their audible result. The Flatten command is available from the Edit menu.
Besides providing an opportunity to conserve CPU resources on tracks containing a large number of devices, the Track Freeze command simplifies sharing projects between computers. Computers that are a bit low on processing power can be used to run large Live Sets as long as any CPU-intensive tracks are frozen. This also means that computers lacking certain devices used in one Live Set can still play the Set when the relevant device tracks are frozen.
31.2 Managing the Disk Load
A hard drive’s access speed (which is related to, but not the same thing as rotational speed) can limit Live’s performance. Most audio-optimized computers use 7200 RPM or faster drives. Laptops, to save power, often use 5400 RPM or slower drives, which is why projects on laptops usually have lower track counts. The amount of disk traffic Live generates is roughly proportional to the number of audio channels being written or read simultaneously. A track playing a stereo sample causes more disk traffic than a track playing a mono sample.
The Disk Overload indicator flashes when the disk was unable to read or write audio quickly enough. When recording audio, this condition causes a gap in the recorded sample; when playing back, you will hear dropouts.
Do the following to avoid disk overload:
- Reduce the amount of audio channels being written by choosing mono inputs instead of stereo inputs in the Audio Preferences’ Channel Configuration dialog.
- Use RAM Mode (see 8.2.8) for selected clips.
- Reduce the number of audio channels playing by using mono samples instead of stereo samples when possible. You can convert stereo samples to mono using any standard digital audio editing program, which can be called up from within Live (see 5.5.5).