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Monday, August 31, 2009

Audio for Podcasting: Hard Disk Recording

When digital audio first appeared, tape-based digital recorders using Digital Audio Tape (DAT) were the only viable system for recording DAT_logo the relatively large amount of data required by a digital audio recording system. This was because tape was less expensive to manufacture than hard drives, and tape was easy to store for later reuse. As hardware became more sophisticated, disk drives started to grow in capacity and performance, to a point where by the mid-1980's digital disk recorders started showing up in professional recording studios.

However, storage limitations and exorbitant costs meant that a 10MB drive could cost over $2000, and could hold only about a minute of audio. This worked great for MIDI and music sequencer applications like Cubase, running on the Atari ST computer (see Figure 1) but was insufficient for recording and editing high-quality voice-over narration.

atari_st_cubase

Figure 1. Start of an audio revolution: Cubase 1.0 for Atari ST
[Click to enlarge]

However, as computers became more powerful, so the hard disk capacity used to store data on a computer grew. Its now common to see disk drives that store 100, 200, or even 500 gigabytes (GB) of data. Storing digital audio on hard disk makes a lot more sense and in fact now the most affordable alternative. Today you can buy a one terabyte hard disk for about $300 that gives you hundreds of hours of stereo, 16-bit recording at a 44.1K sample rate.

Advantages of Hard Disk Recording
There are a variety of reasons why hard disk recorders are more versatile and powerful than their tape-based cousins. Advantages include:

  1. Instant access time. The most expensive part of most tape machines is the motors that control the motion of the tape, called the transport. Transports are often very complex mechanical devices with very precise tolerances. Since a tape machine has to physically move tape linearly across the tape heads, and shuttle the tape from beginning to end, a powerful and reliable transport is required. The better the transport, the faster you can get from one portion of the tape to another, which saves time.

    A hard disk drive does not have this limitation. Hard drives are now fast enough so that any data stored on a hard disk drive can be accessed in an instant of time. This means that locating any portion of a song can be done almost instantaneously since there is no tape to wind. The elimination of rewind and fast-forward locate time is a huge benefit of hard disk recording. The less time spent winding tape, the more time can be spent getting the best take recorded instead.
  2. Flexible and powerful editing. In tape-based systems, there are two methods of editing the audio on tape: re-recording over it, or physically cutting the tape. You can record over existing material on a tape deck or record between two tape machines insert new portions of audio or move audio into a new location. The problem with editing with tape this way is that in order to perform the edit you must wipe the original material by recording over it; this is sometimes referred to as "destructive editing". In addition, since you are recording, it takes at least as long as the length of the edit to perform it, that is, inserting a one-minute audio segment will take one minute.

    With hard disk based recording, the data that is on the disk can be re-arranged without having to be deleted. This is often called "non-destructive editing". The audio data on disk can be also be manipulated in many different ways. The power that a disk recorder provides over tape recorders in terms of editing is substantial.
  3. Processing the audio. In addition to editing the digital audio data stored on a hard disk (by cutting, copying and pasting), audio can be processed using software algorithms called digital signal processing or DSP. Using DSP, audio stored on hard disk can be stretched and squeezed, raised and lowered in amplitude, put on its head and turned inside out just by re-arranging the zeros and ones. Additionally, a DSP-based hard disk recorder provides one of the most substantial benefits of digital audio in the first place because audio can be manipulated and processed while remaining in the digital domain. The only practical way to perform DSP operations on digital audio is when it is stored on hard disk. Tape-based systems, with their linear recording processes makes advanced DSP features impractical to implement, regardless of tape format used.

Next time: Authoring your podcast using an audio editor.

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