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Sonic Foundry Acid Pro v4

Audio + MIDI Loop Sequencer [Windows] By John Walden
Published January 2003

The user interface of Acid Pro v4 contains all the elements familiar from earlier versions of the software.The user interface of Acid Pro v4 contains all the elements familiar from earlier versions of the software.

With the addition in version 4 of surround sound mixing, ASIO and VST Instrument support and an expanding MIDI functionality, Sonic Foundry are attempting to move Acid beyond its original role in loop-based music creation.

SOS last looked at Sonic Foundry's Acid Pro in January 2002 when, in addition to the core functionality based around pitch- and time-stretching of audio loops, new features included MIDI tracks, a Video window and two new tools: the Beatmapper and the Chopper. While the MIDI features of version 3 could perhaps best be described as 'basic', all the new features were certainly very usable and, despite obvious competition from the likes of Sonar, Acid 3 is an excellent creative tool, either used on its own or sync'ed to a MIDI + Audio sequencer.

A year later, Acid Pro v4 is upon us. It's still a loop-based music creation tool first and foremost, but, aware of both increasing competition and new developments in computer audio (such as an increased interest in surround mixing), Sonic Foundry are continuing to develop the program further. Version 4 therefore includes an impressive array of new features. Topping the list of new capabilities are 5.1 surround sound mixing, step, piano-roll and event-list MIDI editing, ASIO driver support, VST Instrument support and plug-in automation.

Acid Basics

As with most modern music applications, installation of Acid Pro v4 is a largely automated process. The install application also offers the user the option to register the software via the Internet. On the test system this all worked without a hitch. Sonic Foundry's system requirements are pretty basic, a 300MHz processor and 128MB RAM being the key elements. Windows 98SE, ME, 2000 and XP are supported.

Given there are so many new features to consider in this release, it would be a shame to spend too much time here going over the basics of Acid. For those totally new to the software, the reviews of earlier versions are available on the SOS web site. In essence, however, Acid provides a means of adjusting the tempo and pitch of the audio loops in real time. For example, a drum loop recorded at 120bpm can be automatically time-stretched to play back at 100bpm while maintaining the original pitch. Conversely, a bass line recorded with a root note of A can be pitch-shifted to play back with a root of G but without altering its tempo. As described in the earlier reviews, the end result is a system that allows quite complex arrangements of audio loops to be constructed, all with the flexibility of instant pitch or tempo adjustment.

In addition, Acid includes the ability to record audio or MIDI tracks and, while the software's capabilities in these areas are not perhaps as strong as that of a dedicated MIDI + Audio sequencer, they are certainly good enough for more than just occasional use. Add to this an excellent user interface and a host of other features including DirectX effects, video playback and a versatile range of output options, and it is not difficult to see why Acid has carved itself a significant market with musicians and multimedia audio producers. Of course, all these features are still present in version 4... so what about all the new stuff?

You Are Surrounded

Surround sound mixing is becoming increasingly interesting to project and home studio owners. Acid now supports 5.1 surround sound, and new Projects can be specified as either stereo or 5.1 — although obviously the audio hardware in the host PC will need at least six physical outputs to feed whatever 5.1 playback system is in use. For 5.1 Projects, a low-pass filter can be enabled (with a selection of suitable cutoff frequencies) for tracks assigned to the subwoofer '.1' or LFE channel.

In a surround Project, the pan control in each track in the Project can be assigned to either the '5' or to the '.1' (LFE) but not both. This might be considered a restriction if you just want to send the bottom end of your bass or drum track to the subwoofer as well as all of its frequencies to the surround speakers. In the surround Project demos supplied with Acid, Sonic Foundry simply overcome this by having two identical tracks — one panned to the surround channels and the other to the LFE channel — with the volume sliders for each track used to balance them. An alternative approach is to insert a buss and assign the buss output to the LFE. Send levels to the buss from any of the tracks panned to the surround speakers can then be used to control the levels going to the LFE channel.

Double-clicking on the surround pan control for a particular track in the Track List opens a larger version of the surround panner for more detailed control. Individual speakers can be muted if required and, for more control over the use of the front centre speaker, a separate volume slider is provided (so there's plenty of scope for real or phantom centre placement of sounds). Pan position can be automated by using a key-frame system for individual tracks, for either creating smooth movements of sounds or bouncing material around from speaker to speaker. The panner also offers balance and constant-power options.

The whole panning system for 5.1 projects seems to be well implemented and is certainly very easy to use. When a 5.1 project is rendered, Acid can create six mono WAV (or AIFF) files. These could then be incorporated into whatever authoring application is being used to create the finished project, where the audio could be data-reduced if necessary using something like Dolby Digital AC3 encoding. By default, Acid does not provide any further encoding but, as we were going to press, Sonic Foundry announced a multi-channel encoding and DVD burning plug-in for Acid. Further details are provided in the Acid Burns box below, and the plug-in should be available by the time you read this.

Going Soft

A further significant new feature is support for VST Instrument plug-ins, and demos from Native Instruments and TC Works are provided with Acid Pro 4. A new VST Instruments dialogue (available via the Options / Preferences menu) allows the user to specify up to three drive locations where plug-ins are stored. Choosing Insert / Soft Synth allows a virtual instrument to be added to the project and creates a new channel strip for the instrument within the Mixer window. A MIDI track can then have its output assigned to the soft synth via the Track List. On the test PC, both under Windows 98SE and XP, I had no problems running the various demo instruments supplied with Acid.

NI's Absynth running happily within Acid under Windows 98SE.NI's Absynth running happily within Acid under Windows 98SE.

As a further new feature of Acid is its ability to work with ASIO drivers, it is also possible to achieve the sort of low latency values required for real-time playing/recording of soft synths. I tested this using the Absynth demo in combination with the ASIO drivers for my Echo Mia 24 card. Two buttons at the top of the window Acid places around the VSTi can be used the enable real-time MIDI playback and select the required MIDI input. I was able select the smallest buffer size (128 bytes) and play Absynth live without glitching while Acid was playing back about a dozen stereo audio tracks. The synth felt both very responsive and solid in operation.

More MIDI Please

Although MIDI made a welcome entry into the Acid feature list in v3, what was on offer was pretty limited. Things have certainly improved in this release. While MIDI tracks can, of course, be recorded and played back as before, step recording and both piano-roll and event-list editing are provided. The latter two are achieved via Yamaha's new OPT (Open Plug-in Technology) standard, so it is conceivable that further improvements to Acid's MIDI facilities could be added in between major version updates.

The piano-roll editor, in particular, is a welcome addition; as well as for editing parts created in real time, it's great for the creation of new parts such as bass lines. As with audio tracks, once a MIDI phrase has been created, it can be looped as required along the timeline. Perhaps the most significant shortcoming of this editor is the quantise function. MIDI events can be quantised to a range of resolutions, but at present, there are no options for groove quantising or 'humanising' a MIDI part — this would be an obvious candidate for enhancement, via OPT or otherwise.

This comment aside, while Acid's MIDI specification is not going to challenge the likes of Cubase, Logic or Sonar, it is very easy to use and there is enough MIDI functionality to avoid having to sync up a full-blown MIDI sequencer on every occasion a MIDI part is needed. Add a couple of decent soft synths to your setup and Acid 4 will now allow you to easily mix and match these with your palette of audio loops.

What Else?

Amongst a range of other additions and improvements, four are worth a brief mention here. While Acid already provided automation of parameters such as track volume, pan and send levels, version 4 introduces plug-in effects automation. This is not available for all DirectX audio effects, but the v4 package includes a resonant filter, track EQ and flanger/wah/phaser that can be automated. The automation is achieved via the usual envelopes drawn into the Track View. Between them, these three new effects provide some excellent creative options, including a few filter-sweep cliches if required!

The Chopper now allows loops selected from larger audio files to be saved as a new audio file. This is an excellent tool for extracting drum loops from all those audio sample CDs!The Chopper now allows loops selected from larger audio files to be saved as a new audio file. This is an excellent tool for extracting drum loops from all those audio sample CDs!Another very useful addition is the option for loop cloning within the Chopper. This allows any selected loop region to be 'Chopped to a New Track' and, as part of this process, the loop is saved as a new audio file. This has a number of applications, but one thing it proved very useful for during testing was isolating each drum loop within a track ripped from an audio sample CD. While this could be done in any decent audio editor (including Sound Forge), it is very convenient to have this functionality within Acid itself.

The Video window was an excellent addition to v3, and has undergone some further improvements in v4. Support is now provided for WMV files, while two new rendering options are provided for projects that include video: stretch to fill frame and fast video resizing. The former automatically adapts the frame size of the original video if it is different from the selected output size, while the latter produces faster rendering times for video resizing when top-quality output is not critical.

Acid Pro v4 now also includes support for time signatures other than 4/4, and time-signature changes can be added in the same way as tempo or key changes using markers. While there are a few loop libraries out there that include 3/4 loops, if you are a fond of a little syncopated jazz, you may well have to roll your own. This said, the feature is a welcome addition and very easy to use.


I've always liked Acid as a tool for rapidly kick-starting the creative process even if, on occasion, I then sync up Logic for straight audio or MIDI work. The new MIDI and VSTi support in v4 makes Acid much more self-contained — indeed, during the period of the review I completed a couple of small projects using just Acid, where previously I'd have had to use a combination of Acid and Logic. If you are already an Acid user, then version 4 is a very worthy upgrade.

If you don't currently own Acid, is it a 'must-have' product? This is a more difficult question to answer, as there are now a number of alternative products offering real-time pitch- and tempo-matching of loops. If you also need a well-specified MIDI + Audio sequencer, Cakewalk's Sonar is the obvious competition here, but Bitshift's pHATmatik Pro VST plug-in is another possibility for those using Logic or Cubase. This is a real case of 'horses for courses' but if the majority of your music production is done with audio loops, Acid Pro 4 offers a first-class working environment — powerful, easy to use and, in the right hands, very creative.

Acid Burns

As we went to press, Sonic Foundry announced a 5.1 Surround plug-in pack for Acid Pro v4, which will provide two functions. First it will implement Dolby Digital AC3 encoding with many user-customisable options over settings such as data rate, sample rate and dialogue normalisation. Second, it will provide DVD burning allowing users to burn 5.1 or stereo AC3 files to DVD (given suitable DVD-R, DVD+R, DVD-RW or DVD+RW hardware). The plug-in pack will be available during November as a downloadable product only, at a suggested selling price of US$399.


  • Still a first-class environment for working with audio loops.
  • Excellent new features including 5.1 surround mixing, VSTi and ASIO support.
  • Considerable improvements in MIDI support.


  • MIDI quantise functions limited.


Version 4 brings some significant new features to Acid Pro. For those whose music production is dominated by audio loops, Acid Pro v4 offers a first-class working environment with enough MIDI functionality to cope with many tasks. Surround, VSTi and ASIO support are also very welcome additions.


£299 including VAT.

SCV +44 (0)3301 222500.

test spec

  • Sonic Foundry Acid Pro version 4.0a build 237.
  • Pentium 4 1.6GHz PC with 1GB RAM, dual-booting into Windows 98SE and Windows XP Professional.
  • Tested with Echo Mia 24, Yamaha DSP Factory and Yamaha SW1000XG soundcards.