So, you’ve put your brilliant plan for a podcast in place. You’ve scheduled for guests, recorded your episode, and mixed it all together after hours of work. But, now you have to export it. Now you know that you’re going to export it as a MP3 and you’ve put a lot of work into this, right? Why don’t you just export it as the best quality possible: 320kbps at 96000Hz with a bit depth of 24. Wait! Let’s take some time to discuss what bitrates and sample rates to choose and why they’re so important to the success of your podcast.
What is Bit Rate, Sample Rate and Bit Depth?
Bit rate is how much date is stored in your podcast per second. This number is entirely separate from bit rate, sample rate and bit depth but, if you increase either your sample rate and bit depth while retaining the same exact bitrate you’ll have a lower perceived quality. Of course, you can achieve lower bit rates and achieve comparable perceived quality with more advanced codecs but, today we’ll focus on MP3. If you say increase your sample rate, the codec has to squeeze more information into your chosen bit rate and your perceived quality will suffer as a result.
Sample rate is an entirely different concept to bit rate. When you record your audio something called a DAC (digital audio converter) is taking note of the amplitude of your audio at a given rate. The faster you set this rate the more accurately higher frequency sounds can be rendered. Also, if you decide to slow down your audio without adjusting the pitch (so that your pitch will lower) the slower you can lower your audio before you are unable to convey higher pitch sounds.
Bit depth is similar to sample rate but, it exists on an entirely different axis. When you’re recording the DAC mentioned previously is recording loudness of the amplitude in addition to when that amplitude was recorded. This provides you with a 2D plot of sound over time which you can perceive as a waveform inside of your DAW (digital audio workstation). Whereas the sample rate is the accuracy of the X axis of this plot the bit depth is the accuracy of the Y axis of this plot. The higher your bit depth the more accurately you can render your dynamic range of your audio. This is important when you consider that the equipment you’re recording with has a certain level of equipment noise. When recording you want the dynamic range of your audio to be separated from dynamic range of your noise floor. So, when you record at lower bit depths your DAC will have a harder time perceiving your signal to your noise and you’ll have progressively more “crunchy” sounding audio the more you lower your bit depth.
What Bit Rate, Sample Rate and Bit Depth Should I Choose?
Of course there a bit rate, sample rate and bit depth for every situation but, today I’m focused on podcasts using the MP3 codec. The BBC uses 64kbps for their monophonic podcasts and if that’s good enough for them then it good enough for me! If you have stereo sound effects or have a situation where you want to preserve your stereo mix in your final product I recommend doubling this bit rate to 128kbps. This is because, generally speaking, your bit rate is divided among all of the channels in your audio. If you make use of extensive music tracks in your podcast then I’d go for 256kbps. There isn’t a need to go up to 320kbps (which some people regard as “CD quality”) in your podcast.
When considering sample rate for the delivery of your podcast you want to consider the source which your podcast came from. Most all video has its audio stored as 48000Hz so, if you a pulling audio from a video to turn it into a podcast its best to keep the audio at 48000Hz. However, if you’re recording a podcast that is purely audio use 44100Hz as your sample rate. There is a couple of reasons for this. First of all, CDs have had a sample rate as 44100Hz. Till this day this sample rate has remained as a standard sample rate for most—if not all—DACs. 44100Hz was chosen because the range of human hearing has been estimated at around 22000Hz. Therefore, a sample rate of 44000Hz can perfectly replicate a tone at 22000Hz. The engineers decided to add an extra 100Hz to CDs so they could then easily fade out higher pitch tones to reduce artifacting at higher pitched sounds. As a side effect of video having a sample rate of 48000Hz if the source of your podcast is a video call then, you’d want your podcast to stay at 48000Hz—even if you turn off the video in your video call! You do not want to re-render audio between 44100Hz and 48000Hz because this process may result in a incredibly high pitch but low amplitude tone. This may not be perceived by most—if not all—people but, it’s just a fact of sample rate conversions that can easily be avoided.
Choosing what bit depth you want is pretty simple. CDs have always been 16-bit and I recommend that you release your podcast as 16-bit as well. I haven’t noticed any further clarity to be had when going beyond 16-bit and going any lower then 16-bit may result in incompatibility with certain DACs. Of course, you could record in higher bit depths to give yourself more freedom when editing but, its defiantly not required. Re-rendering between higher bitrates down to 16-bit does not have the same problem as switching sample rates and won’t result in any artifacting.
Why Shouldn’t I Choose the Best Possible Quality?
I’m sure you’re itching to have your audience to have the best quality version of the podcast you’ve worked so hard to produce. However, you have to keep in mind one critically important aspect to your podcast that many people seem to overlook: time to download. You see, most people—myself included—like to download their podcast so they can listen to it when out and about. I know it’s just become second nature to me as a long-time podcast listener to download everything to my phone’s SD card. Whether you are a long-time fan or a fist time listener it can be a pain to watch the download bar ever so slowly crawl to 100%. This is especially so if you are a first-time listener and want to get a taste of a podcast. Therefore, its important to keep the customer experience as smooth as possible and even more important when you’re first starting out and you might still be building a listener base and your server bandwidth isn’t the best. With my recommendations above, I feel like a good middle ground can be reached between quality and speed.
I’ve written this blog post as I’ve been producing podcast for a while now and I’ve been listening to podcast even longer! I want to help people just starting out their podcast to avoid the logical fallacy that their podcast needs to be the highest of bitrates possible to be a good podcast. If you have any questions please reach me at my “Contact” page and happy filming!
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