Audio Codecs : Speech optimized codecs & Music optimized codecs

Now that you know how codecs work, it's time to see what codecs are available to podcasters, how they differ, and why you might want to use them. The first thing to consider is whether you're planning on using music in your podcast. If you are, then you definitely want to use a codec that is suited for music. If your podcast is just speech, then you may want to consider using a speech codec, because you'll be able to get very good quality at ridiculously low bit rates, thereby saving you money on bandwidth.

Choosing a codec is a tricky business. Newer codecs offer better quality, and some offer advanced functionality such as book marking and embedding images. However, many of the newer codecs play back only on a limited number of portable devices. If you want the latest and greatest features, but also want to cater to the widest possible audience, you may want to consider encoding to multiple formats.

Music-optimized codecs
As mentioned previously, if you're going to include any music at all in your podcast, you must use a music codec. Luckily, you're spoiled for choice. Here's a list of possible candidates:

MP3: The granddaddy of them all. MP3 wasn't initially designed as a low bit rate codec, so other codecs sound much better at low bit rates. It also does not support book marking. But just about every computer and portable media device in the world will play back an MP3 file.

Advanced Audio Coding (AAC): The new and improved MPEG audio codec, meant to replace MP3. The only problem is that it isn't supported on some portable players. AAC enables advanced features such as book marking and embedded images.

Windows Media Audio (WMA): The standard on Microsoft PCs. It has many advanced features such as markers, script commands, and embedded links. WMA is not supported on iPods, though it is supported on the "Plays for Sure" family of portable media devices.

RealAudio (RA): The default audio codec of the RealPlayer, which offers embedded links and script commands. It is supported on a number of cell phones.

OGG Vorbis: An open source audio codec offering excellent quality. Unfortunately, Vorbis isn't supported by many of the proprietary players, nor by the iPod.

Speech-optimized codecs
If your podcast doesn't include music, you should consider using a speech codec. They provide better quality at the same bit rate as a music codec, or the same quality at a reduced bit rate.

Audible Audio (AA): Developed for the first portable digital media player, which was released by Audible and designed to play back audio books. AA supports a number of advanced features such as book marking. Unfortunately, Audible doesn't make an AA encoder publicly available.

ACELP.net: The granddaddy of voice codecs. In fact, the AA format is based on the ACELP.net codec. ACELP.net is supported by both the Windows Media and Real players.

OGG Speex: Another branch of the OGG open source project, specializing in low bit rate speech compression.

Windows Media Audio Voice Codec: During Windows Media encoding you can specify that you're encoding voice content, and the Windows Media encoder will use a voiceoptimized codec.

How perceptual codecs & audio codecs work

Perceptual codecs take advantage of how we actually perceive audio and video, and use this information to make intelligent decisions about what information can safely be discarded. Perceptual codecs are by definition lossy because of this. The original cannot be recreated from the encoded file. Instead, an approximation that attempts to retain as much fidelity as possible is constructed. The idea is that we won't notice what has been discarded.

Our ears are extremely sensitive. We can hear from 20Hz to 20,000Hz and sounds over a wide dynamic range, from a whisper to a scream. We can pick out a conversation at the next table in a crowded restaurant if the topic happens to catch our ear. We can do this because our brains filter out the information that is not of interest and focus on the rest. Our brains effectively prioritize incoming sound information.

For example, even a quiet classroom has plenty of sounds, such as the hum of air conditioning, people shuffling papers, and the teacher lecturing at the front. If someone sneezes in the room, for that split second, everyone notices the sneeze and nothing else. The sneeze is the loudest thing in the room and takes precedence over everything else.

Similarly, our eyes can take in a wide range of visual information, the entire color spectrum from red all the way through purple, and from very dim environments to very bright environments. Our field of vision is approximately 180 degrees from left to right. What we actually pay attention to, though, is much more focused. In general, we pay more attention to things that are brightly colored and things that are moving.

Perceptual codecs use this information to make better decisions about what information in audio and video files can be discarded or encoded with less detail. Perceptual codecs prioritize the loudest frequencies in an audio file, knowing that's what our ears pay most attention to. When encoding video, perceptual codecs prioritize bright colors and any motion in the frame.

At higher bit rates, perceptual codecs are extremely effective. A 128 kbps MP3 file is considered to be the same apparent quality as a CD and is only one-tenth the size of the original, which is pretty incredible if you think about it. Some of the savings is encoding efficiency, but the majority of it is perceptual encoding. As the bit rate is lowered and the codec is forced to discard more and more of the original information, the fidelity is reduced and the effects of perceptual encoding are more audible. Still, you should always balance the required fidelity of your podcast with the realities of bandwidth and throughput.

How audio codecs work
Audio codec technology has made spectacular advances in the last few years. It's now possible for FM quality to be encoded in as little as 32 kbps (in mono, that is). Modern codecs such as Windows Media, Real, and QuickTime AAC can achieve CD quality in approximately 64 Kbps. How do they do it?

The idea is to capture as much of the frequency and dynamic range as possible and to capture the entire stereo image. However, given the target bit rate, the codec usually determines what a reasonable frequency range is. Files that are encoded in mono are always slightly higher fidelity, because the encoder worries about only one channel, not two.

Another economy can be made if the codec knows that it will be encoding speech. Speech tends to stay in a very limited frequency and dynamic range. If someone is talking, it's unlikely that her voice will suddenly drop down an octave, or that she'll start screaming for no reason. Knowing this, a codec can take shortcuts when encoding the frequency and dynamics information.

Caution Don't try to encode music using a speech codec. The shortcuts a speech codec uses are totally unsuitable for music, because music uses a very wide frequency range and is generally very dynamic. If you encode using a speech codec, it sounds awful. So don't do it.

After the frequency range has been determined, the codec must somehow squeeze as much information as possible into the encoded file and decide what can be discarded. Perceptual audio codecs use the concept of masking to help make that decision. If one frequency is very loud, it masks other frequencies, so the codec can safely discard them because we wouldn't perceive them.

This is why all background noise must be minimized in your original recordings and your programming must be nice and loud. This ensures that the codec doesn't discard any of the programming information.