Rendering  Podcasts

Riley Byrne, Owner, Podigy.co

26 October 2017

In our Guide to Podcast Editing, we briefly went over a set up that would fit most podcasters for rendering. However, in our quest to be as comprehensive as possible, today we’re going to dive into what we’re trying to emphasize when we render our podcasts and different methods that might be more appropriate for different styles of shows.

First and foremost, when we’re rendering our podcast we’re essentially making a copy of our show that is going to be easily accessible to people on the internet. That’s why we don’t send out lossless .wav files that are hundreds of megabytes, or .opus files that would be unplayable by most devices. When we’re rendering out our episode, we’re trying to make our file as high quality as possible while still accessible to as many people as possible. These two goals are often at odds with each other, so as podcasters we need to decide what best fits the needs of our listeners.

You might be wondering why not simply render out each podcast at 320 kbps and just provide listeners pristine audio and be done with it, and that is a fair question. The answer is twofold. One, almost all podcast hosting services will limit your uploads on a per-episode or per-month basis. The smaller you can make your podcast, the less you have to pay for hosting, and the more money you have to invest in other aspects of your show. If you’re self-hosting your podcast, you’re paying for the bandwidth of all your downloads so if you can halve the size of the episodes, you’ll pay half as much for the same amount of listeners.

And secondly, it’s important to think about how people are listening to your podcast. Anyone who has tried to download a podcast at the cottage can attest to the pain of waiting for a 100 mb file to download over a spotty 3G connection. And as podcasts become more popular, a lot more people from around the globe will be looking to tune in and those who are making their shows small enough to fit low bandwidth budgets will reap those rewards.

For example, in our episode of The Podcaster’s Podcast about sample rates, we briefly touched on how the Stuff You Should Know show renders its episode at 22 050 Hz and 48 kbps. Why would a company that has recently taken on investors to expand their podcast range send out their episodes at such a low quality? It certainly isn’t for lack of resources or access to equipment. What is much more likely is that they noticed that a large percentage of their listeners are from areas without access to large amounts of bandwidth and thus adjusted their product to fit their listeners needs and sends out hour long episodes that are routinely under 20 megabytes.

But what should our takeaways be from this? Most importantly, I think it shows us podcasters without the download numbers or budget of the How Stuff Works crew what the minimum viable product is in terms of podcast fidelity. I have no doubt that quite a bit of thought went into the decision to render some of the most popular podcasts in the world with half the normal sample and bitrate. Beyond that, and looking at the style of HSW’s roster, we can probably surmise that educational content is popular in areas with low bandwidth needs and that a simple, two person set up without fancy processing is an effective way to deliver a message to listeners.

Mono, Stereo and Joint Stereo

But we might be getting ahead of ourselves. The first thing you need to know about rendering your podcast is that for 90% of podcasts, rendering out in mono is advised. Of course, there are exceptions to this rule, but if your podcast is simply a few people all talking to each other, rendering out in mono allows you to immediately halve your file size (a stereo track is just two mono tracks).

More than halving the file size, rendering in mono also eliminates a problem I’ve been hearing more and more often in podcasts that is completely unnecessary: panning voices across the stereo spectrum. I first noticed it on The Adam Buxton Podcast, but have noticed it with more and more regularity, which is concerning.

The idea behind it is benign enough. When you’re chatting with friends around a table, people on your left will sound louder to your left ear and vice versa. It’s part of how our brain processes spatial awareness. However, when podcasts try to replicate this by panning the host a little more left and the guest a little more right, they’re only mixing for people who hear perfectly in both ears and are listening on a stereo device with both speakers. Anyone who is deaf in one ear, listening on a mono device, or only listening with a single earbud in is getting a subpar experience where one person always sounds louder than the other. Mixing your podcast to mono ensures that everyone can have a perfect experience, no matter how they’re listening.

The only reason to be rendering your podcast in anything other than mono is when your show features a lot of music or your audio has quite a few effects in it, like Radiolab or 99pi. By “a lot of music”, I mean more than just your intro/outro and ad music. If you’re buying professionally mixed and mastered music from one of those royalty free sites, it should be already prepped to sound great in mono. I’m talking more music discussion podcasts, or podcasts where musicians perform live.

If you absolutely need to render your podcast in the stereo field, then the best way to do so and still keep file sizes small is using the “Joint Stereo” option. This allows any information that is identical on both the left and right channels (people talking) to be in mono, and anything not identical (songs, vocal effects etc) to be in stereo.

Rendering in Mono is the best way to immediately decrease your file size, and has no ill-effect on sound quality. I cannot overstate this enough.

Constant vs Variable Bitrates

This is oddly, one of the most contentious debates across in the podcasting community (other thin “how to get more listeners?”) is Constant vs Variable Bitrates.

Constant Bitrate,  or CBR, takes the bitrate you assign the file and applies it across every second of audio. If you choose 96 kbps as your bitrate, every second of your podcast will be rendered out at 96 kilobits.

Variable Bitrate, or VBR, works by dynamically changing the bitrate of the file as needed. In most programs, you set an upper threshold for bitrate, and the program will determine the most effective bitrate for the podcast dynamically. For instance, if you set 192 kbps as the upper limit for an episode of your podcast, the intro music might be rendered at 160 kbps, the conversation at 80 kbps, and long pauses or silences at 10 kbps. The great thing about this system is it has the capacity to sound better and be as much as 40% smaller than CBR rendered files.

However, VBR files has a single fatal flaw that make them unusable by most podcasts, and it isn’t even a flaw of the format per se, but rather how audio players choose to read mp3 files. Almost every audio playback device uses a simple “Bitrate x Number of Bits” formula to determine the length of an audio file. This is okay for CBR files because they use the same amount of bits per second regardless of the content. VBR files, conversely, fluctuate based on the bandwidth needs of the track and thus audio players can report playhead positions that are inaccurate by up to a minute or two per hour. As many shownotes rely on accurate timecodes to point out important events, this makes the VBR files problematic.

While there is a workaround to this issue that has been around for years, which basically writes the bitrate changes into the metadata of the track, no podcast players currently take advantage of this, making VBR a highly efficient, well developed, patent free way to make podcasts more accessible that podcasters can’t use. Like I said before, rendering out podcasts for listeners is all about making little sacrifices to make the best product for the most amount of people.

Choosing a Bitrate

Now that we’ve decided on a mono (or joint stereo), CBR file, we need to decide what that constant bitrate is going to be. And even this isn’t as simple as it should be, as there are two different mp3 encoders that work virtually the same except in low bitrate cases. There is the Fraunhofer encoder, which is made by the the people who invented the mp3 codec and is said to work better at lower bitrates than the free, open source LAME encoder. For our purposes, remember that Reaper renders mp3 with the LAME encoder, and iTunes uses the Fraunhofer encoder.  

Note: While Reaper allows you to change the mono bitrate of the mp3 directly, iTunes is a little odd in that you need to choose the bitrate you want stereo files to be, and then also select a preference of mono. So a bitrate of 64 kbps would be a Stereo Bitrate of 128, 96 would be 192, etc.  

So instead of setting any hard and fast rules, here are 4 of the most common bitrates (48, 64, 80 and 96) run through both Reaper’s LAME encoder and iTunes’ Fraunhofer encoder.

Based on these examples, you can surmise that much more is going to be made by your bitrate choice rather than encoder choice. Generally we recommend using 96 kbps, as below this some noticeable artifacts start to appear, however, you should choose whatever seems right for you and your listeners!

AAC, The Dark, Patented Horse of Podcasting

Our guide up to this point has all been about the best way to render .mp3 files, but there is another way that is seldom talked about, and that is rendering to AAC.

AAC was initially designed to be the successor to mp3, developed by Apple to be more efficient at lower bitrates. And it is! Listen to these examples of .m4a files and their equivalent .mp3s:

There is no doubt that AAC is much more efficient than mp3, and yet very few podcasters use the format, for, from what I can see, are very outdated reasons. You see, in the mid-2000s, AAC files were unplayable on Blackberry devices, and many guides written at the time suggested that mp3 files would allow for greater compatibility across devices. However, since then, two things have changed:

Blackberry started supporting AAC files

Everyone stopped supporting Blackberry

This is a little cheeky, but sufficed to say AAC has seen adoption across virtually all platforms that podcasts come out on, including Android and Windows Mobile. The eagle-eyed among you may have also noticed that this podcast is in AAC. The fears of the mid-2000s are no longer relevant, and yet podcasters still seem reluctant to adopt AAC files.

To the best that I can find, it seems people are reluctant to use AAC for the reason that Apple owns the patent to it and could at some point revoke the rights to use it in the future. While it is true that Apple owns the patent, they own it so that they can license it and make money and if they were to suddenly to stop allowing others to use it, they would be affecting not just podcasters, but also Facebook, Youtube, Netflix, Amazon Prime and Twitter, who all seem like companies that would take the future of their audio codecs pretty seriously. Furthermore, up until a few months ago, the same could’ve been said of Fraunhofer and the mp3.

So, for those of you adventurous or forward thinking enough to render their podcasts in AAC, here a few tips. Firstly, to do it from Reaper change Output Format to MPEG-4/MOV, and then underneath that Format to MPEG-4 Audio. Ignore the rest of the settings, except for the bitrate next to Audio codec. Once again, decide for yourself what you think is best, but this podcast was rendered out at 54 kpbs and sounds comparable to  a 96 kbps mp3. So judge for yourself.

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