Computer Audio


With today's desktop computers, multitrack audio recording can be done without the use of tape recorders, mixing consoles, and outboard signal processors. Hardware devices facilitate the input of several simultaneous audio signals, while software programs can record these signals to the computer's hard disc. Editing and signal processing can be performed with a degree of precision and fidelity unobtainable with analog technologies. Digital audio tracks can be either individually routed via hardware to a traditional mixing console, or mixed internally using software.

It would be misleading, however, to endorse digital audio without some reservations. As mentioned in a previous section, digital audio technology operates with a finite set of possible values to which the audio signals it processes must conform, and thus forces quantization of the audio information to certain values of frequency and amplitude, as determined, respectively, by the sampling rate and the bit depth. 16-bit resolution, for instance, is in principle not adequate to faithfully reproduce all the possible dynamic nuances inherent in many analog tape formats. Additionally, digital audio recording requires conversions of most signals from analog to digital and vice-versa. These processes employ hardware that can vary widely in the quality of workmanship and performance, and thus can appreciably degrade the original signals.


MIDI (Musical Instrument Digital Interface) is a digital language enabling communication between hardware and software. MIDI instructions can be transmitted to computer hardware from an external device, such as a keyboard, or from a computer input device. Alternatively, MIDI data can be read from a computer file. The data instruct the hardware to, for instance, trigger playback of a sound from a sound bank, which is a file containing a number of sounds grouped together as instruments. Such sound banks can be obtained commercially, or can, in some formats, be customized by the insertion of user samples. MIDI instructions can represent information concerning numerous parameters, including the sequence and duration of notes, amplitude (velocity), timbre, envelope, and others. In addition, clock pulses can be transmitted in the form of MIDI time code (MTC) to effect changes in tempo.


MIDI time code can be used to synchronize the activity of multiple hardware devices, software programs, and even tape recorders. For the purpose of synchronization, one device serves as master, and the others as slaves. The master device transmits the data to which the slaves conform. When a tape recorder is used, it will most often serve as the master, although its transport functions can be slaved to time code using rather expensive synchronization devices. In either case, a synchronizer is used to convert time code data to an analog audio signal, which is recorded to a track on the multitrack tape. This track then becomes the "stripe", which when played back through the synchronizer is converted to digital data that can be read by the computer hardware. Using this method, it is possible, for example, to synchronize analog tracks from a tape recorder, MIDI tracks, and digital audio tracks.

SMPTE (Society of Motion Picture and Television Engineers) time code, besides providing the basic clock pulses, can contain information about the specific position of a track, with an accuracy of up to 1/30 of a second. This enables tasks such as overdubs to be performed from any point in a recording. When a striped tape is cued to a given point, hardware and software "chase" the code, and maintain synchronization.