Choosing Your Encoding Settings


Authoring streaming media requires choosing certain encoding settings. The settings decisions you make are based on limitations of, among other things, the current state of software engineering, computer processing and the subjective nature of human hearing. Thankfully, how you make these encoding settings decisions is generally similar across all formats.

NOTE

Don't encode files that are already encoded. An encoded file sounds better if you begin with uncompressed source audio.


Any audio file that can be opened in a waveform editor can be saved (converted) as an MP3 file.

Whether your source audio is spoken word, sound effects, or music, you must choose the bit rate, codec, and channel (mono or stereo) settings that are appropriate to your own and to your projected audience's needs prior to encoding. It's all about file size versus audio quality. Obviously you want your streaming audio file to sound as good as the available technology and bandwidth will allow. But you must also take steps to ensure that the stream reaches the user intact. Not everyone has broadband connectivity, so it's recommended that you choose the lowest bit rate setting you can get away with that still sounds acceptable. For instance: If you're authoring content that contains only a human voice (mono) speaking under normal conversational conditions, it isn't necessary to encode the streaming file at anything over 56Kbps (28Kbps is also recognizable). The voice will be heard and understood at 28- or 56Kbps, and you won't consume unnecessary bandwidth. If you're authoring encoded music files in stereo, however, listeners will have a more enjoyable sonic experience if you offer the streaming file at a higher bit rate (56Kbps minimum and above).

In these tutorials you'll use a 32 Kbps (mono) encode setting for 56Kbps users and a 192 Kbps (stereo) encode setting for DSL (or any other broadband format) users. It's possible to encode at higher bitrates but the sound quality isn't all that much better and, by keeping the high quality setting at 192 Kbps, broadband users have headroom to use their connection for other tasks.

The Step-by-Step tutorials in this chapter will take advantage of a piece of technology called variable bit rate (VBR) that is used to optimize MP3 encoding (both on-demand files and live streams). A brief explanation of VBR follows.

Standard MP3 files are encoded at a single (or "constant") bit rate (CBR) through the entire file. That means high-quality, high bit rate streaming files are at the mercy of available bandwidth, potentially dropping out or disconnecting altogether when data rates fall below acceptable levels. This inconsistent connectivity is a typical happenstance for anyone who is subscribed to an ISP that is overselling bandwidth. You know the scenario: Everybody goes online at a certain time, nobody gets anything, and 3 a.m. becomes the optimal time for listening to uninterrupted music on the Internet.

VBR encoding takes advantage of every little bit of bandwidth by looking at an audio file and automatically choosing at what bit rate to encode the file based on how much audio information is present at any given moment. A song that begins quietly or with a single musical instrument will, during that section, be encoded at a lower bit rate than say, the middle of the same song (when all the instruments might be playing together and the volume and frequency range might be greater). Songs with many fluctuations in dynamic range will be encoded at several bit rates.

The key conceptual difference between CBR and VBR might be expressed this way: CBR encoding specifies compression by space, and VBR encoding specifies compression by quality. With CBR, quality might be dropped in the process of maintaining the specified bit rate. With VBR, the bit rate changes to meet the quality level desired. Ten seconds of silence encoded at a CBR of 128Kbps uses the same disk space as ten seconds of full-on stereo opera. VBR encoded files of the same audio would result in a low bit rate for quiet sections and higher bit rates for louder sections. VBR encoding, like CBR, is set at the time the audio is encoded.

The MP3 standard is designed so that information about bit rate encode levels is included throughout the file. This makes it easy for VBR-enabled MP3 players to seamlessly decode and play VBR files just like CBR files. In fact, users are likely to have a more consistent listening experience with the frequent bit rate changes in a VBR-encoded file than with a CBR-encoded file. In a CBR file, when you suddenly reach a more full-range portion of a song, only the same number of bits are available. This can result in unwanted audio artifacts (a "swooshy" sound that is most noticeable when encoding to lower bit rates).

Upgrade your player if you haven't in the past two years. You can choose not to enable VBR if you think your listeners might not benefit, but that's not recommended.



Streaming Audio. The FezGuys' Guide
Streaming Audio: The FezGuys Guide
ISBN: B000H2N1T8
EAN: N/A
Year: 2001
Pages: 119

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