Audio Loops

Now we get to the most specific and proper definition of a loop in ACID. Not only are loops audio files, they are audio files that are automatically inserted into a Loop track in ACID. Most, but not all, loops are fairly short, with durations measured in seconds and, much more rarely, tens of seconds. Loop tracks are identified by a loop icon in the track header and are designed to repeat, often seamlessly. 

Using AQD as your primary loop audio editor has significant advantages. While it isn t formally an audio editing enviromnent, there is no reason why you shouldn t use AQD to trim loops. Ttimming AQD events is a very simple process, and you can listen to the media file and future loop against the destination project. AQD can also be used to determine the all-important tempo of a loop. See the next section for the specific procedure. 

Higher order architectures have been successfully used to build converters for digital audio applications [Adams, 1986][Hauser, 1991], However, the complexity of modulator and demodulator increases with the order. Higher order architectures also exhibit stability problems [van de Plassche, 1994], Dithering is a remedy to many of the problems of low level signals (see the previous section) and has been used in sigma delta converters as part of the noise shaping loop [Vanderkooy and Lipshitz, 1989]. 

Massie, 1986] Massie, D. (1986). A survey of looping algorithms for sampled data musical instruments. In Proc. IEEE Workshop Appl. of Signal Processing to Audio and Acoustics, Mohonk Mountain House, New Paltz, NY. 

Of primary importance in any converter design is not what happens to the output when we perturb the reference (which is what the closed loop transfer function really is), but what happens at the output when there is a line disturbance. This is often referred to as audio susceptibility (probably because early converters switching at around 20 kHz would emit audible noise under this condition). 

The two most popular classic detector circuits for FM are the Foster-Seeley discriminator and the ratio detector. Other types of FM detection used today include phase-locked loop circuitry. The basic function of the discriminator is to convert the frequency swings in the FM signal back into amplitude variations for further audio processing. The discriminator is, therefore, susceptible to both amplitude and frequency variations. For this reason, the Foster-Seeley detector is always preceded by a limiter stage. The ratio detector on the other hand, acts like a limiter, and so there is no need for the use of a separate limiter stage. 

Home "stereo" audio systems, as well as 5.1 channel DVD "home theaters," sometimes develop problems from ground loops. Usually the symptoms are low... 

ACID Pro is the high-end version of ACID that is appropriate for studios, professionals, and prosumers. As with other versions, stepping up to this version when you are ready is a simple matter. Uncompromising quality and the technical tools you need to get the job done make this an awesome addition to any multimedia professional s audio rack. ACID Pro also comes with a CD of basic loops and includes an extensive set of audio effects plug-ins. 

The timeline forms the hulk of the AQD workspace. This is the area where the project is laid out, left to right, in chronological order. The various sounds or loops that are used to create a song are mixed together in tracks from top to bottom. There is no hierarchy to the tracks all loops that overlap vertically are mixed together to form the audio that is played back from ACID. Track order is therefore left entirely up to you and is for your own benefit. The timeline is made up of many separate parts (see Figure 1.3). 

Loops are specially recorded audio files that are usually fairly short in duration. Loops are standard audio files that can be played back using just about any Windows-compatible media player (even the Windows Media Player). What makes a loop a loop is that it is recorded and structured so that when it is played back twice in a row, the end of the loop immediately and seamlessly leads back into the beginning of the loop. This means that most loops can be played over and over again infinitely, sounding as if it were a much longer continuous recording. Loops are normal. wav audio files and are commercially available from multimedia companies for use in loop-based music tools. ACID loops from Sonic Foundry contain a little more information than a typical. wav file (such as key and tempo information) that make them particularly easy to use in ACID. Sonic Foundry loop libraries can be used with many other multimedia applications (for example, video editing applications or Flash animation tools) and loops from other companies loop libraries can also be used in ACID. 

Tracks in ACID (see Figure 2.2) are analogous to tracks in a studio or on an audiotape. A track in ACID contains one media file or part. The controls on an ACID track can be used to adjust, mix, and otherwise modify an audio loop with special effects. In the ACID world, the number of tracks is unlimited and can contain MIDI songs or even video. 

All audio events, video events, and MIDI events are contained in tracks on the timeline. Therefore, you need to create a new track before you can create an event To create a track, browse for and preview media files (loops) in AQD Explorer and then double-click one to create a track. 

While all of the methods for changing the pitch are fundamentally the same, changing the root note of a loop as it occurs in a project is probably the last method you should use. The default root note information saved with an ACIDized loop was saved there by the artist who recorded the loop and provides important information both to you, the composer, and to AQD itself about the inherent key of the audio file. In most cases, it is probably better to change the key of the track that contains the loop instead of changing the root note of the loop in the properties dialog box, even though the results are identical. See Chapter 10 for more information about the root notes of loops. 

While recording an audio source is a fundamentally different operation firom recording MIDI data, the procedure for doing so is very similar. Audio is recorded as sounds, and MIDI is recorded as data that can be used to recreate the performance (for example, what note was played, how hard the key was pressed, and how long the note lasted). MIDI can be recorded into individual tracks or multiple takes can be recorded into a single file by looping the playback. MIDI setup and configuration are dealt with in much more detail in Chapter 10. 

Every effect has its own array of controls that are used to modify the parameters of the plug-in. These are discussed in detail later in the chapter. There are some general procedures that will allow you to effectively audition and modify effects. Effects can be modified as the project is playing back, which means that you can change the effects variables in real-time while you listen to the results. With the Audio Plug-In window open to the plug-in that you want to work on, click on the timeline (or press Alt-1-0 [zero]) to set the focus to the timeline and press Play. It is often useful to use a looped playback of a shorter section of a project to hear the results of an effect. While playback continues, go back to the Audio Plug-In window and modify the effect. 

Up until this point, the word loop has heen used in a very general way, referring to any audio file on your computer that is used in ACID. Loop originally meant a physical loop of audiotape that had the two ends spliced together to create an infinitely repeating audio clip. In much the same way, the archetypical loop file on a computer is a short file that will play hack seamlessly end to end— and perhaps most commercial loop files are designed to work that way. But the definition has definitely expanded to include material that will not endlessly and seamlessly loop, and there are a large number of loops that don t actually loop. 

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