Digital sound is created by a process called “sampling.” Most digital sounds, whether on a CD or from the Web, were created by recording hundreds of very small snapshots of sounds as they were played. These snapshots build the waveform that is a digital sound. A 44.1 kHz file for example, uses more than 44,100 samples, or snapshots, per second. The quality, as you might expect, is fairly high – CD-quality, in fact. Four or five minutes of 44 KHz-sampled music creates a file of about 50MBs.
Digital Audio Basics
Here’s a quick chart that correlates the bitrate – or audio compression — of your files with how that file sounds:
| 7.5 kHz | sounds like | AM radio |
| 8 kHz | sounds like | a phone connection |
| 32 kHz | sounds like | FM radio |
| 44kHz | sounds like | a CD |
The human ear stops making major distinctions in sound quality at around 20 kHz. In general, finding an appropriate bitrate for files to be delivered over the Internet is about balancing the sound quality with the file’s size.
Though you will want to record and edit your files at a high quality, you will have to downgrade, or compress to a lower quality, to end up with a smaller file. Alternatively, you can stream the file (streaming is when a file begins playing before it has finished downloading). Be guided by what you know about your audience’s connection speeds when you decide how large a file can be before you need to think about streaming it.
Formats that have a reduced file size by eliminating data:
- MP3
- MP3 VBR
- MP3 Pro
- AAC
- WMA
File formats that don’t reduce file size at all are known as uncompressed. Here are some examples of uncompressed file formats:
- CDA
- PCM
- WAV
- AIFF
Each of these sound files is a stand-alone file. This means that the file must be downloaded in full before it can be played.
The common streaming formats are:
- Quicktime
- Windows Media
- Real Media
Streamed audio will begin to play before the entire file is downloaded and is very useful for delivering long audio segments and large files.
Audio can also be delivered using Flash.
Digitizing Audio
Both Macs and PCs come with extremely basic audio recording software, but you will need something more to edit your audio. Audacity (http://audacity.sourceforge.net/) is a free audio editing program available for both Macintosh and PC. There are many other good audio editing programs available, so use the one that works best for you and your system. In this tutorial we will be using Audacity.
You can also use Audacity to record audio, so if your field audio kit includes a laptop, you can use it in combination with Audacity as your recording device.
Besides audio editing software, you will need to have a computer with a lot of free space on the hard drive. Depending on the quality of the recordings you are making, a 10-minute interview could be as large as 100MB. Remember that you can always compress audio down to create a smaller file size, but you can never improve the quality of an original recording. So record your audio at as high a quality as you can manage and worry about compression later.
You can also record audio from your computer’s internal CD player, or you can use a sound editing software application to digitize audio from a tape player, microphone or other audio source.
To digitize from a device external to your computer, you’ll need equipment — the playback device (for instance, a digital audio tape player or cassette tape player) and the appropriate cable to attach that device to your computer’s microphone or line-in jack. The audio out on your playback device (source) will be connected to the microphone or line-in jack on the computer’s sound card. Microphones can be plugged directly into your computer’s microphone jack.