Digital Audio
If you're running a computer in the sound room, you have a great opportunity right in front of you.
Part I - Why digital audio?
Most churches still use the venerable cassette tape for
recording audio. While this works fine for many churches, it has a few
shortcomings.
- It's very hard to duplicate cassettes - the equipment is slow, it's expensive, and the sound quality is marginal at best, short of professional equipment
- It's hard to share easily - other than a cassette duplicator, copying a cassette is a real-time process
- It's hard to archive - cassettes degrade over time, and they take up quite a bit of space
- Fast-forward and rewind are slow and tedious
Consider digital audio for a moment.
- It never degrades with time
- It can be compactly stored - from 10 hours up to hundreds of hours on a single CD, and thousands of hours on a hard disk
- Duplication is easy with most modern computers
- Non-professional CD duplicators are fairly inexpensive
- Instant access to any part of the recording
Part II - Types of Digital Audio Storage
Digital audio is very flexible. Some examples:- CD burners are available that act just like a cassette tape recorder, accepting a line-level audio input (such as from a mixing board) and producing a ready-to-use CD.
- Most computers now include CD writers, that can produce either audio CDs or data CDs.
- Computer sound cards allow the recording of digital audio, and numerous programs are available for this purpose
- Computer hard drives can store digital files once recorded
- Compact discs can be written in data mode, with the files containing compressed digital audio
- Web sites can offer digital audio for playback or download
Part III - Digital Audio File Formats And Sizes
There are a few basic properties of digital audio data that should be understood.Sample rate: this refers to how frequently the audio is measured when it is recorded. The higher the rate, the more accurately high-frequency sound will be reproduced. The laws of nature dictate that to reproduce a certain frequency, the sample rate must be at least twice as high. Since human hearing ranges up to around 22,000 cycles per second in perfect-hearing individuals, audio engineers long ago selected 44,100 samples per second as the standard for compact disc digital audio. As computer audio matured, this sample rate was retained as the standard. Some systems use 48,000 samples per second. If high-frequency sound is not important, some systems use 22,050 samples per second to reduce file size.
Bit depth: this refers roughly to how many possible steps exist between the quietest and loudest sounds. The higher the bit depth, the more accurately the audio will be reproduced. A binary (computer-based) number is represented in terms of "bits", essentially a one-or-zero value. One bit can store a single on/off value. Two bits can store four values (0 and 0, 0 and 1, 1 and 0, or 1 and 1). Three bits can store eight values, and so forth. The compact disc digital audio group selected 16 bits as the standard. Many computer sound cards now reproduce audio with higher bit depths, sometimes up to 96 bits. Of course, more bits mean more data, which means larger digital files.
Compression ratio: audio data can be compressed many different ways. Compression ratio is the ratio of the original versus final file size. There are two basic types of compression - lossy, and loss-less. "Loss-less" compression means that when the data is un-compressed, it is returned to exactly as it started before compression. There is no loss of information. This form of compression is of course desired for text information, or software, where changing the information would be disasterous. However, it is also not very efficient for small file sizes. "Lossy" compression means that some approximations are made when the data is compressed - the original information can never be exactly recovered. So some information is lost permanently.
While lossy compression sounds bad, it is in fact not significant in some situations. The human brain is very good at some tasks, and one of these is filling in missing data. Most digital photographs are compressed with the "JPEG" method, which approximates the information as closely as possible. Depending on how much the picture is compressed, it will begin to look worse and worse as it it uncompressed. The same basic situation exists with digital audio compression - several lossy methods are available which, depending on the compression ratio, begin to sound worse than the original, but which can result in astoundingly small file sizes. Essentially, the compression software analyzes the sound, determines what parts of it cannot be heard, and discards those.
Aside from the compact disc digital audio standard, which is specific to CDs, several common formats of digital audio data files exist, that are relevant to this discussion. Each one is discussed in terms of its common Windows file extension.
- WAV (wave) files: this is the largest format of digital file, but also the most perfect representation. The data is not compressed. Several sample rates and bit depths are possible for smaller file sizes.
- WMA (Windows Media Player audio) files: this compression method is used by the widely available Windows Media Player, bundled with every copy of Windows sold today. It uses various compression methods, both lossless and lossy, depending on how much compression is requested. It also can use various bit depths and sample rates.
- MP3 files: this very common audio file format is typically based on lossy compression, and like the Windows Media Player files, can also use various bit depths, sample rates, and compression ratios.
Other uncommon audio file formats include AIF, AU, and OGG files. Which file format you use is going to be dependent on a number of factors - one of which is what sounds best to you.
A WAV file from a CD, at 44,100 samples per second, 16 bits, uncompressed, will typically take about 650 megabytes per hour. WMA and MP3 files vary widely in file size, but for CD-quality audio they typically require about 60 megabytes per hour. However, with maximum compression, WMA and MP3 files can be as low as 3 megabytes per hour, although this begins to sound like a scratchy AM radio - perhaps adequate for archival purposes but not for duplication. Still, consider that at 3 megabytes per hour, two years' worth of Sunday sermons can be stored on a single CD-ROM - perfect for archival.
For the purposes of this discussion, let's assume that compressed audio is your choice - you don't want to fill up your entire hard drive in a matter of weeks.
Part IV - Digital Audio Programs
The list of available digital audio programs is long and complex. For most Windows users, there are two primary choices - Windows Media Player (which comes with Microsoft Windows) and Musicmatch Jukebox. Each is quite capable, and offers a complete range of recording compression ratios and file sizes, and each offers a fairly broad set of playlist tools.
Part V - What To Do With Digital Audio
Okay, so digital audio is cool. No doubt about that; we all have stacks of shiny, colorful CDs. But what can YOU do with it? Here are some basic ideas to get you thinking.- Record your worship and/or sermon direct to digital. Skip the whole cassette thing.
- Record audio CDs for duplication and distribution.
- Duplicate CDs for less than the cost of cassettes. Save your audiovisual department's money.
- Compress the audio files and archive them. Fit your last two years' worth of sermons on a single CD-ROM.
- Upload the compressed audio to the church web site, so anyone can listen to the sermons without picking up a CD.
- Get an advanced digital audio system, and put together your own church worship CD!
Why Digital Audio?
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