Digital Audio Mastering for Your Podcast
When producing the podcast file that you will put on your website, you need to convert the original, “mastered” audio into a smaller format. There are two main audio formats that you should use for podcasts: MPEG3 and AAC. Both are referred to as Lossy codecs – they basically remove data to make a smaller file.
MPEG3: Motion Picture Experts Group – Layer 3
The standard digital audio format for podcasts is MPEG 1 Layer 3, commonly referred to as MP3. MPEG stands for Motion Picture Experts Group and was developed for the first multimedia content. There are several other audio formats: Layer 1, Layer 2 as well as the most common Layer 3. There are different measures of quality within an MP3 file. As a podcaster, you will want to select a quality that both sounds great but also downloads quickly for listeners.
If your podcast is mostly voice with no background music or musical segments you may want to consider using a mono file as this will reduce the MP3’s file size and as a result, minimize your podcast’s download time even more.
AAC: Advanced Audio Codec
The Advanced Audio Codec was designed as a replacement for MP3 with the main focus being to reduce the file sizes of lower bit-rate recordings. It was first adopted by Apple who standardised their iTunes software around the format. At lower bit-rates an AAC encoded file will sound better than an equivalent MP3.
Unlike MP3, AAC allows for extensions including Digital Rights Management and linked media. However: support is not as wide spread as MP3 and many mobile devices may not support it or the extensions that Apple have added.
Apple’s iTunes software enables AAC encoded podcasts to use chapters, bookmarks, external links, and synchronized images displayed on iPod screens or in the iTunes artwork viewer.
If AAC isn’t one of your export options in your software, you can convert an MP3 into an AAC file by using Apple’s iTunes program, but there may be some additional loss in quality.
Check Your Mix – Review your Podcast
See if your mixed down MP3 sounds good on a variety of stereo systems such as your computer speakers, headphones, portable stereo and car stereo. If your mix translates well from system to system, you know that you have created an excellent work of art.
File Conversion – Converting to MP3
Whatever recording program you are using, you will have the option to save or export your recording as an MP3. If the only option is to export as a WAV file, that’s OK too. You’ll just have to complete one extra step to convert the WAV file to an MP3 file.
Structure for Saving Podcast Episodes
Here’s a tip: organize your podcast episodes by creating a simple file folder structure
Save time by getting organized right from the start. Create a file folder in “My Documents” called “My Podcasts”. In your “My Podcasts” folder create new folders for each episode in advance such as “Episode 01” , “Episode 02” , “Episode 03” etc… Having a defined structure for your podcasts will not only keep you organized but will help you plan shows in advance. Whenever you find an interesting story, come across a press release, or think of a great idea, you can save it directly into the folder for that specific podcast. When it comes to planning and recording future episodes, you will already have some content that you can use as a starting point for your next episode.
When saving your podcast episodes, export your podcast recording as an MP3 file. Save the file as ‘Podcast_Episode_01.mp3’ and be sure to export into the appropriate folder.
Digital Audio File Sizes – File Tips
WAV or AIFF files are uncompressed audio formats used by PCs and MACs. The formats have huge file sizes with great quality – typically referred to as CD, studio or master quality.
MP3 or AAC are compressed audio formats that are much smaller (typically a tenth the size of WAV) and are faster to download.
To encode audio you need to set:
Measured in kbps, this determines the overall file size and quality. High bitrates produce larger files with better quality, lower bitrates smaller files but for more complex music may produce distortion.
Measured in kHz, sample rate determines the accuracy of the recording. If you recorded your audio at 44.1kHz, you should encode the MP3 at the same value. Higher sample rates will produce larger files.
codec quality (optional)
This is measured in a numeric scale and varies depending on the codec used e.g. the MP3 LAME library uses the values 1-12. Higher values produce larger, higher quality files preserving more detail.
A note about the sample rate: for the best playback on the web you should use multiples of 11.025kHz (11,025 Hz) and use no more than 44.1 kHz, otherwise you may experience the “chipmunk” effect in Flash based players. Generally for podcasts you shouldn’t need more than 44.1kHz recordings.
Summary – Key points about Mastering Digital Audio
It’s easy to make your podcast sound exactly the way that you envisioned it to. After mixing and mastering your podcast, you will be rewarded with your very own podcast-ready MP3 file.