Basic Encoding Techniques

Whether you encode your podcast by exporting directly from your editing platform or by using a stand-alone encoder, you can specify a number of parameters. You may have only a few choices if you're using encoding presets, or you may have the opportunity to specify exactly how you want your podcast encoded.

In the early days of low bit-rate encoding, back when people were connected to the Internet via slow modems, encoding technology was limited and required lots of tweaking to extract the best quality. Now, ten years later, codec technology and Internet connection speeds have improved so much that encoding high-quality podcasts should be within everyone's reach.

This is particularly true of audio podcasts. Modern codecs such as RealAudio and Windows Media Audio are capable of attaining FM-mono quality at a mere 32 kbps. The MP3 codec lags behind in quality, but because you can safely encode your podcast at 128 kbps, you should not have any quality issues.

Video is a little trickier. Assuming the majority of your audience is on a broadband connection, your video quality is limited by available bandwidth. Although you can't expect DVD quality at these bit rates, there's no reason why you can't create a perfectly acceptable video experience. This chapter helps you choose settings that should do the job. Let's start off with the easy stuff — audio encoding.

Audio Encoding

Audio encoding is easy, for a number of reasons. Raw audio files are large, but nowhere near as huge as video files. Therefore, the amount of compression that is needed to reduce them to a size that is suitable for Internet distribution is not excessive. Audio codec technology has progressed to a point where low bit rate encoding produces very good results. Podcasting reaps the benefits of ten years of cutthroat competition between RealNetworks and Microsoft, and the progress made by the MPEG organization with AAC encoding.

Because modern codecs sound so good, you really don't need to do much tweaking when you're encoding audio. You really have to decide only three things: whether to encode in stereo or mono, whether to use a speech or a music codec, and what bit rate to use.

Mono versus stereo
The first thing to decide is whether to encode your podcast in stereo or mono. If your program is predominantly interviews or spoken word, encode in mono. Mono encodings are always higher fidelity at a given bit rate, because only a single channel is encoded instead of two. If you're encoding in mono, you can use a lower bit rate and get the same quality or you can get better quality than a stereo encoding at the same bit rate.

If your content is predominantly music, you should encode in stereo, although it isn't strictly necessary. Even though music is recorded in stereo, most of the content is right in the center of the mix. The lead vocal, the snare drum, the bass drum, all will be right in the center of the speakers. And watch where you place your speakers. If you aren't sitting directly between the speakers, you aren't experiencing the full stereo effect anyway. However, one good reason to target stereo if you're playing music is that half your audience may be listening on headphones, which exaggerates the stereo effect.

Speech versus music
The next thing to decide is whether to use a speech codec or a music codec. If you're encoding an MP3 file, you don't have a choice. MP3 is a music codec. The good news is that MP3 is perfectly suitable as a speech codec as well, provided the bit rate is high enough.

Speech codecs can take special shortcuts during the encoding process due to the nature of speech content. With speech, the dynamic range tends to be very limited, as is the frequency range. After you start talking, the chances are good that you'll continue to speak at roughly the same volume and in the same register. Knowing this, a speech codec can make intelligent decisions about how to encode the audio.

Music content, on the other hand, has a wide dynamic and frequency range. There are bass drums and bass guitars, as well as crashing cymbals and violins. The shortcuts that a speech codec takes are completely unsuitable for encoding music content.

So the choice is fairly obvious: If you're encoding content that is speech only, you can encode at very low bit rates and still achieve high quality using a speech codec. However, for most applications, a music codec is perfectly appropriate.

Bit rates, sample rates, and quality equivalents
The most important decision to make about your audio podcast encoding is what bit rate to use. The bit rate determines the eventual file size of your podcast, which in turn determines how long it takes to download. The bit rate also determines the fidelity of your podcast. The higher the bit rate, the higher fidelity your podcast is.

The listed audio bit rates range from 20 kbps to 256 kbps. If you're producing audio-only podcasts, you should target somewhere between 64 kbps and 128 kbps. If you're encoding predominantly speech, you can safely stay at the low end of that; if you're encoding music, you may want to stick to the higher end of the spectrum.

Note At the end of the day, you know best how you want the podcast to sound. Try encoding at a couple of different bit rates, and see which one sounds best to you.

The other thing you may be able to set is the sampling rate. The sampling rate determines how much high-frequency information is encoded. For example, CD-quality audio uses a sample rate of 44.1 KHz, to capture the full 20–20,000 Hz frequency range. The sampling rate has to be at least double the highest frequency you're trying to capture. Depending on what bit rate you're targeting, you may be offered a few different sampling rates.

The interesting thing about sampling rates is that a higher sampling rate isn't necessarily better. The sampling rate determines how often the incoming audio signal is sampled, so it determines how much audio the encoder has to try to encode. If you set a higher sampling rate, you're telling the encoder to try to encode more high-frequency information, but the encoder may have to sacrifice the overall quality of the encoding. Essentially, the sampling rate determines the trade-off between the frequency range and the fidelity of the encoding. At a given bit rate, an encoder can offer higher fidelity with a reduced frequency range or reduced fidelity with a higher frequency range.

We suggest that you choose a lower sampling rate, thereby allowing the encoder to create a higher fidelity version of your podcast. There is very little information above 16 KHz in most audio programming, and most people don't have speakers that reproduce it faithfully anyway. Therefore, choosing a 32 KHz or 22 KHz sampling rate should provide more than enough high-frequency information.

Basic Audio Production

Now that you've spent time drooling over the latest and greatest audio gear, and invested some of your hard-earned cash in decent equipment, you need to figure out how to hook it all up and produce professional sounding podcasts. The great thing about working with audio is that for a minimal investment, you should be able to produce your podcast to a very high standard. The powerful technological leaps we've seen in the world of computers have also brought great advances (and price drops) in the world of home recording. What once required thousands of dollars worth of equipment now costs hundreds, or less.

We will start off showing you how to connect your equipment to get the best sound, and then talks about some general recording techniques. Audio production may seem daunting at first, but by setting up some simple procedures and sticking to them, you'll find it to be pretty simple, and more important, lots of fun.

After that, we cover editing, where much of the power of audio production actually lies. Good editing can transform your podcast from mundane to professional. The techniques described are all standard operating procedure in radio, television, and recoding studios around the world. Though we can't hope to turn you into an audio engineer in a few short pages, we can at least point you in the right direction. Let's start by setting up your equipment — the right way.

Setting Up Your Equipment
Different kinds of equipment you need to produce your podcast to a high standard. Ideally, you took the plunge and bought some equipment to fit your budget and the scale of your production. Now it's time to unpack everything and connect everything together. This is actually a critical step in podcast production. If you set up your equipment incorrectly, you'll leave yourself vulnerable to noise, interference, and distortion, which will compromise the sound quality of the final production. If set up correctly, your hardware will have you on the road to creating broadcast-quality programming. To understand why this step is important, you have to understand the concept of gain.

Setting your levels
Gain, also known as level, is the measure of the power of your audio signal. When using analog audio equipment, such as microphones and mixing desks, the signal is a continuously varying voltage. The higher the voltage is, the higher the gain and the louder the audio. All audio equipment is designed to work within a certain known range of voltages. To obtain the best possible quality out of your audio equipment, without adding any noise or distortion, you want to work within the optimal range for that piece of equipment, known as its dynamic range.

Dynamic range
The dynamic range of a piece of audio equipment is the difference between the loudest sound it can handle without distortion and the internal noise floor of the equipment. For example, when you turn a portable radio up too loud, you'll hear the sound crackle and buzz; that's distortion. You've just exceeded the dynamic range of the radio. The noise floor lies at the other extreme of the spectrum.

All audio equipment produces some amount of noise; there's no such thing as a perfectly quiet piece of equipment. That's because they're imperfect by definition. Every piece of audio equipment has all kinds of electronic components, each one adding a minute amount of noise, which taken in total is the noise floor. You can hear this noise — just turn your stereo up really loud while you're not playing anything. You'll hear a hissing and possibly a buzzing noise. This is the system noise that is being amplified. If you were actually playing a CD, you wouldn't hear this noise, because the music would be much louder than the noise.

More expensive equipment uses better components, which produce less noise. Consequently better equipment has a greater dynamic range. Cheaper equipment, well, you get the idea. This is the argument for investing in decent audio production equipment. If you produce audio with no audible noise, your podcast sounds much better. Noise is a dead giveaway that an amateur is behind the controls. Another giveaway is distortion. After your signal distorts, you can't remove the distortion. It can't be edited out of the signal, and it compromises the quality of your podcast.

Dynamic range is measured in decibels (dB). The human ear is capable of perceiving up to 120-130dB of dynamic range, before the pain threshold kicks in. We can hear a faucet dripping down the hall in the middle of the night, and endure hours in front of our favorite rock band. Our ears are extremely sensitive, which is not necessarily the case with the equipment and/or transmission methods used to produce audio.

Different audio transmission methods have different dynamic ranges. For example, compact discs have about 96dB of potential dynamic range, whereas FM radio has only about 70dB of dynamic range and AM radio has only about 48dB of dynamic range. This is because of the noise inherent in each system. If you think about it for a second, the quality differences between these systems is obvious. The larger the dynamic range is, the higher the quality of the audio signal and the less apparent any noise is.

Using meters to monitor levels
To control your levels, you need to keep an eye on your meters. Virtually every piece of audio equipment comes with some type of meter to indicate the level of the signal. Meters fall into three main categories: VU meters, LED Peak meters, and software VU/Peak meters, shown in Figure 1.


Figure 1: A software VU/Peak meter Courtesy Sony Sound Forge

The first is VU or Volume Unit meters, which are common on older equipment (and new equipment going for that hip retro look). The needle indicates the overall power of the signal, represented as an average. They're very good for comparing the volume or power of a signal, but not good at registering quick peaks. VU meters usually have two scales, one that runs from 0 percent to 100 percent, and another that has zero where the 100 percent mark is, with negative numbers below 100 percent and positive numbers above 100 percent.

The next type of meter is the LED (Light Emitting Diode) Peak meter. LED meters are very fast, so they are generally used to indicate the peak values of the audio signal. LED meters generally have a single scale, measured in dB, running from approximately -40dB, up through zero, and on to +10 or +20dB.

Note Decibels are a relative measure of power. The decibels used to measure the +20dB measurement on a meter aren't the same as the 120dB pain threshold. One is a measure of sound pressure, while the other is a measure of voltage. It can get kind of confusing

Finally, we have the software meter. Software meters can operate as VU or LED meters, and sometimes as both concurrently. In the image on the right of Figure 1, the meter indicates both VU level (the bulk of the display) and peak level (indicated by the thin line hovering above the VU level). The critical difference between analog meters and software meters is that analog meters have headroom, which means that the signal is allowed to go above zero, and digital meters have no headroom: Go above 0dB, and you'll hear awful square-wave distortion, which is very unpleasant. Digital meters usually include some sort of clip or peak light that indicates when you have peaks above zero. These must be avoided at all costs.

The point of all this is that you need to be careful when setting up your equipment Set your input level too low, and you're liable to hear some of the internal noise of your equipment. Set your level too high, and you'll get distortion. What you want to do is set a level that is high enough so that you don't hear equipment noise and conservative enough that you don't ever get distortion. It's a fairly broad range, so spend the time to learn how to set levels correctly. The result will be a much higher quality podcast.

Setting levels
When setting a level with a VU meter, be careful because VU meters don't register peaks. Those peaks may be loud enough to exceed the equipment's dynamic range and therefore cause distortion. It's best to set your levels between -10dB and -6dB on a VU meter. This leaves quite a bit of headroom for transient peaks, and any unexpected jumps in level. When you're setting levels with a peak meter, you can be a bit more aggressive, because you can see pretty much exactly where your peaks are. You should set your level so that the indicated level is in the -6dB to -3dB range. These settings should get you a good, clean, loud signal, with very little perceivable noise. If your levels occasionally peak above zero, don't worry; most audio equipment has sufficient headroom to handle momentary peaks without distortion.

Setting levels in the digital domain is a whole different matter. Any signal above 0dB causes distortion, because 0dB is considered an absolute maximum. As long as the signal remains above 0dB, you keep getting the same maximum value. The result is a sound wave with the top squared off, which sounds horrible. This is known as square-wave distortion.

You must be conservative when setting levels on digital equipment. Digital meters are almost always peak meters for precisely this reason. When setting your digital levels, you should target -10dB to -6dB. This should leave you plenty of headroom. It's better to be a little conservative and maximize your level later on using signal processing rather than set it too high and end up with distortion.

Exploring the World of Podcasting

We show you how to find, subscribe, and listen to thousands of podcasts available on the Net. The steps for getting audio files onto your portable audio player, such as an iPod, or burning a CD and listening on a desktop computer are few and easy to understand. We also introduce you to the client software used to subscribe to and download podcasts, including the wide variety available for the Windows, Macintosh, and Linux operating systems.

A variety of podcast portals offer lists of podcasts; we walk you through some of the better neighborhoods in podcastland, explaining the basic mechanics that make podcasting work. Finally, as part of that technical introduction, we explain the technical origins and personal conflicts that color the podcast landscape. After this, if you just want to listen, you're ready to go. But we bet a big bag of fish that, after you've tried listening, you're going to want to start to speak with a podcast of your own.

The Basics of Listening
Listening is easy. Managing your subscriptions is easy. Keeping up with everything you can download with such ease is harder. That's because the listener is in control. Unlike broadcast media, where every listening choice is a zero-sum game, where choosing one program means you can't listen to the others, podcasting gives you the power to stack up a full schedule of listening and more.

First, you need to get an application commonly referred to as a "podcatcher," news reader, or aggregator. All these applications do the same thing; they visit a list of servers to check for newly posted files. A push client was locked to a particular server.

If you had multiple push services, it meant running several different applications. With podcast and RSS, your subscriptions are handled by one application. Later, we introduce you to the choices in podcatchers; here, we focus on what a podcatcher does.

As shown in Figure below, a podcatcher running on your computer maintains a list of subscriptions in the form of uniform resource identifiers (URIs, also called URLs) that tell the application the name of each server and where subscription files are stored on a regular schedule that you specify. Each subscription is referred to as a "feed," which is the Web address of a file that describes the catalog of shows stored in a particular directory on a server. You may have feeds for several different programs on the server, each with a unique URL for the XML file for each show.


The podcatching process

For example, let's say you've subscribed to a podcast called Big Blue's Beer Show, which is stored on a server named http://www.bigblueBeer.com in a file called "podxml.xml." The full address of the file is http://www.bigbluebeer.com/site/feeds/podxml.com, and you've set your podcatcher to visit the site every day at 6:00 AM to check for new shows listed in that XML file. When the application finds a new show listed in the podxml.xml, the full audio file is downloaded to your PC and stored in a directory where you can open it and listen, or the podcatcher application can identify new audio or video files and move it to your computer or portable audio player. The podcatcher then moves to the next subscription on the list, in this case, a http://PodcastBible.com podcast, and checks that server for a new show. At the end of the update process, your podcatcher has a list of programs that are downloaded and ready for listening.

Of course, you can have your podcatcher visit many servers, collecting programs all day long, but remember that audio files take lots of space on the hard drive. An hour of MP3 audio is typically about 30 MB in size. Like a digital video recorder for your television, podcatchers require some tending, with frequent weeding to keep space available for new programs.