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An even greater challenge for pushing the audio envelope of browser and mobile games is file size. In these titles we are dealing with a very finite amount of space in which all data must reside; an amount that is minuscule compared to a Blu-ray, or even a DVD. So how do we pile on the variations and high quality assets that will make our game pop and sound great given this limitation?
Perhaps the single most revolutionary breakthrough in digital audio in the past 20 years was the invention of high compression, high quality codecs. Being able to compress a PCM file to one tenth or one fifteenth its normal size without noticeable artifacting and acceptable frequency response is really what makes console quality audio possible on smaller platforms (and even consoles for that matter!)
While compression is a modern miracle for cramming sounds into your game, it is still a matter of weighing quantity versus quality. Most game engines with support for browser and mobile platforms use either ogg or mp3 as the compression format of choice.
In both of these codecs, the lower the bitrate, the lower the quality and the smaller the file. It is the sound designer's job to figure out where that sweet spot is on the bit depth/file size matrix for their sounds.
For better or worse, when dealing with browsers, cell phones, and tablets we are not talking about audiophile 5.1 systems, so it is a bit easier to err on the side of lower bitrates without overly compromising sound quality.
For example, in Unity, I compress most of my sounds at 56kbps ogg (roughly quality 0) which translates to 14:1 compression and gives me acceptable quality on every set of speakers I've played through, from Dynaudios, to laptop speakers, to headphones.
Another tip to remember when dealing with ogg and mp3 compression is that from a file size perspective, stereo sfx are virtually "free." There are a couple different ways compressed audio handles stereo files, all of which keep the file size very close if not identical to a mono file of the same length.
The best quality stereo files in these compressed formats are Joint Stereo, which is a means for the codec to selectively compress in a way that maximizes the stereo effects of the sound without reproducing extraneous data. I like to use stereo sounds for my UI sound effects to give menus a bit more audio character, and am able to do so without fear of weighing down the overall audio footprint.
Another important point in regards to compression is to always keep your source files at the highest possible sampling rate. These codecs handle downsampling themselves, so a 12kHz WAV file compressed to a specific bitrate will be the same size as a 48Hz file post-compression, but with one quarter of the sonic data. Downsampling before compressing is a sure fire way to make your game sound muddy and should be avoided at all costs. As a rule of thumb, I design all sounds at 48kHz and import them into the engine at this rate, letting the compression parameters I set within the engine dictate how small my sounds will get.
Optimization is another very important factor when it comes to making great sounds for smaller platforms with smaller memory allotments. Being cognizant of file size throughout development can help you be judicious in your optimizations. Simple tasks like cutting unnecessary tails from your wave files, limiting variations, or reducing the bitrate/quality for lower frequency sounds all seem mundane, but can make a cumulative difference in the end.
A final note should be made in regards to compression and optimization when dealing with looping mp3 sounds on mobile platforms. This can be a source of endless frustration for most sound designers in that your engine MUST be capable of compensating for the sample padding generated as a result of compressing to mp3.
When an uncompressed file is converted to mp3, the compression algorithm adds samples to the sound file which make seamless loops impossible. Many engines offer their own way to circumvent this issue (Flash, Unreal and Unity 3.2 and later all do.) However, it can be a very frustrating technical hurdle to get around if you are not aware of it and your engine does not support gapless looping. For more information on this issue see PJ Belcher's article on iOS Audio Design.
Similar to consoles, if we want to get longer sounds, such as music, dialog or long ambiences playing in our smaller games, we need to stream them. Streaming has historically been off the physical media of a console or PC (hard drive, CD-ROM/DVD/console disc, etc.). We still use the same methodology, except now our common sources to stream from are either the physical media (a hard drive on PC and SD-cards or other removable memory sources on mobile devices) or from the internet.
When thinking about what sounds are appropriate to stream, there are several factors to consider. Perhaps most importantly is, where are we streaming from? Streaming from physical media is quicker as far as response time if latency is a concern, and also significantly more stable since network traffic can be disrupted much easier than the communication between a device and its storage medium. Online games can fight for bandwidth with both networking and streaming going on. Streams can drop if there's a network hiccup, or not buffer in time to begin when they need to.
On the other hand, streaming from the internet allows for a smaller initial (or shorter incremental) download of your game. It also allows a smaller overall footprint and the potential ability to dynamically update or alter content by uploading audio files to the internet and patching a single script rather than needing to append whole compressed audio files or package files.
Some engines even allow for multiple streams to play simultaneously opening up the possibility of sophisticated multi-stream scenarios for dynamic dialogue, surround ambience, interactive/dynamic music or other creative uses. In short, you have much greater flexibility with online streams, but greater stability from local streams.
Streaming can be a complicated scripting affair or a matter of checking a box depending on how you want to stream and what options your engine offers. The methodology often differs if you will be streaming from disc versus from the internet, and some engines only offer one solution or the other.
There is a time and a place and special content ripe for streaming. It is up to you to decide the most prudent way to stream and what content should be streamed. Regardless, streaming can be an excellent means to showcase long dramatic sounds in your game that would not otherwise be possible.