Digital Audio Files and Music Distribution


Topics This Week


Distribution Formats

Currently, music is distributed as a physical product on a CD, or as a file to be downloaded from the Internet. The process of preparing a final mixed file for distribution will vary slightly depending on the method of distribution. Effective distribution depends on standards which allow music to be played by a broad range of hardware and software applications.

The standard file format for CD distribution is called Red Book audio, which specifies exactly how uncompressed audio files will play on consumer devices. Once a file has been mastered, the process of making an audio CD is for the most part transparent, since software such as iTunes, which is bundled with any computer, will be capable of writing this format.

When distributing audio files on the Internet, typically files are either downloaded and played or streamed from a special server. In either case, audio files, because of their size, need to be compressed so they can be downloaded in a reasonable amount of time. They easiest way to distribute music on the Internet is to post compressed MP3 files on a Web page for download. Since any Web browser and a wide variety of stand-alone applications can play MP3 files, just about anyone can download and play music in this format. The format used for streaming audio will depend on the player. Currently, the most common forms of streaming media are Quicktime, Real Audio, and Windows streaming media. Each has its own method of compression and special server requirements.


Preparing Files for Distribution

Mastering is the final step in the production process where a final, mixed version of a song is created. The goal is to determine the overall sound of a song, and to make sure it sounds exactly like you want it. The steps involved in creating a master will vary slightly, depending on the equipment and software you're using. The following will give you a good starting point:

  1. Create a final stereo mix file. Regardless of the software you use, the result will be a final stereo mix file. The process of combining all the elements of a mix into this format is often called bounce-to-disk. Follow this link for a tutorial in bouncing a mix to disk in Reason.
  2. Edit the files. Use destructive editing to trim the beginning and end of the song, removing any unwanted noise or count offs. You can edit together sections from different mix takes to assemble final versions of any songs. Add fade-out endings where needed.
  3. Make final EQ, compression, and level settings. Make the overall level of a song is as high as possible without clipping. Here you can also make some final EQ adjustments as well. Make sure you save the original, unedited or processed file in case you want to start this process over again. Document all settings used to create the final versions so you can easily return to those settings if further adjustment is needed. Follow this link for a tutorial in editing and processing a file in Audacity.

Making a CD Master

If you're creating a CD with more than one song on it, follow these additional steps:

  1. Assemble songs in a final order. If you are assembling a number of songs on a CD, think how songs will flow from one to the next. Pace the songs so that selections are well matched according to mood, tempo, and even key.
  2. Determine the amount of time between songs. How do songs flow from one to the next? Ideally the amount of time between songs should comfortably lead the listener from one song to the next. For example, you won't need much time after a song that ends with a long slow fade. Or, you might want to add more time between a long fast song and a ballad to give the listener time to adjust.
  3. Listen to the entire CD and make any adjustments needed. Listen to the entire CD from beginning to end. Make notes as you go. You'll probably want to change some of the time between songs and perhaps the levels. This shouldn't be a problem if you've saved the original, unedited files.
  4. Create a disk-at-once image file of the final list of songs. A disk image is a single file that contains all the audio data as well as the table of contents settings that CD players need to access the songs. To do this, the software and CD recorder you use must be able to write in disk-at-once mode.
  5. Burn a master CD and a backup. Although your CD recorder may be able to make a CD at 8, 16, or even 24 times the speed of playback, if you're making a master for duplication, burning a CD at slower speeds will minimize the chance of errors in writing to disk. Take your time and burn at 2x.

For this course you'll be completing one song as your final project, so we'll take a closer look at the process of getting a mixed song from Reason to a CD and an MP3.


Making CDs

Currently, the most common way of archiving and distributing music and data is on a CDR-RW (recordable CD-write once). Although your computer is equipped to write both CDRs and DVDs, we'll focus on CDs. There are two types of CDRs and it's important to understand the difference between them.

The process for making each is different. On your computer, you can make music CDs with iTunes and data CDs from the desktop.

Follow this link for a tutorial on burning data CDs.

Follow this link for a tutorial on using iTunes for burning audio CDs and making MP3 files.


Music on the Web - MP3 files

Digital audio files are generally quite large. Full CD-quality files will occupy about 10MB of disk space per stereo minute. So a three-minute song will take 30MB. Therefore, distributing full CD-quality music via the Internet can be painstakingly slow. The following chart illustrates data transfer rates with different types of internet connections. We need to reduce the size of these files to make distribution more manageable. MP3 files use a data reduction scheme to reduce the amount of data needed to represent audio information.


 

Transfer Rates

bps = bits per second
300 bps = 37.5 bytes per second. Or about 30 characters a second. (good typist does 40 words a minute)
2400 bps = 300 bytes per second
Kbps = kilobits per second
56 Kbps = 7K or 7000 bytes a second 
Mpbs = megabits per second
• DSL = 1.5 Mbps or 187.5K per second
• WiFi (airport) 11.5 Mbps
• WiFi 802.11g (airport extreme) 54 Mbps

 

 

Connection Speed and Download Times
Type of Connection Connection Speed Data Per Minute* Transfer Time for a 3 MB file*
Typical dial-up
28.8 kbps
216 KB
13 minutes, 55 seconds
Fast DSL/Cable
3 Mbps
22.5 MB
8 seconds

* under optimal conditions

Sound Quality

The sound quality of an MP3 file is determined by the bit rate used to encode it. The greater the bit rate, the better the performance. In addition to reduced frequency response, low bit rates produce a garbled, warbling sound.

Typical performance of MP3
sound quality bandwidth mode bit rate reduction ratio
telephone sound 2.5 kHz mono 8 kbps * 96:1
better than short wave 4.5 kHz mono 16 kbps 48:1
better than AM radio 7.5 kHz mono 32 kbps 24:1
similar to FM radio 11 kHz stereo 56...64 kbps 26...24:1
near-CD 15 kHz stereo 96 kbps 16:1
CD >15 kHz stereo 112..128kbps 14..12:1

 

For best results, an MP3 file should be encoded using a bit rate of 128kbps. Although the amount of data reduction will vary, you can expect a 10:1 reduction ratio. With this, a 4 minute song that takes up 40MB as a stereo, uncompressed audio file can be reduced to a 4MB MP3.

Your computer is equipped with Quicktime which can play files encoded with the MP4, AAC compression used in iTunes. In general, this will provide better sound and increased data reduction, however it is not nearly as common a standard on the WWW as MP3. When in doubt, use the MP3 option in iTunes when encoding files for distribution. .


Standard MIDI Files

Although a Reason file can only be opened using Reason, any MIDI sequence we make can be opened by virtually any other MIDI, DAW or notation software if we export it as a Standard MIDI file (SMF). An SMF can store any MIDI Channel Voice message, the tracks they're recorded on and the names of those tracks. To export tracks from Reason, choose Export MIDI File from the File Menu. Although you can import SMFs into Reason, playing and saving them is not practical using Reason Adapted.


This Week's Assignment

Review the tutorials from this week to produce an MP3 of your final project. Check the guidelines in the My Assignments section of Course Companion for details on how these should be submitted.

Suggested Additional Reading:


Review Questions


Distributing Music Vocabulary

MP3
Red Book audio
Streaming audio
Quicktime
Real Audio
Windows Media
Bounce-to-disk
Mastering
CDR-RW
File compression
Data reduction scheme
CODEC
Bitrate
MP4
AAC

SMF