These media usually move through popular platforms such such as iTunes , SoundCloud or streaming services such as Spotify or ShoutCast . When digital audio is compressed to upload to these platforms and use their distribution facilities, it is inevitable that its quality can be affected and undesirable noise will be produced when listening.But let's not panic! Some of these problems can be minimized and even prevented by following some good practices during the mastering process.
It is possible to hear many definitions of what audio mastering is . However, the most common and common refers to the final process of taking an audio mix and preparing it for distribution. There are several considerations in this process:
- Unify or adjust the sound of a register to correct any mix balance problems, or improve particular sonic characteristics.
- Maintain consistency and coherence throughout an album, making each track fit and sit on the entire tracklist .
- Preparation for distribution, which traditionally leads to its duplication or replication on CD, vinyl or some digital format for download, depending on the intended distribution format.
This article will help you prepare your audio for digital downloads.
What do MP3 and AAC formats do to your audio?
Lossy compression refers to a data encoding method that uses inaccurate approximations (or partial discards of data) to represent content that has been encoded ( more info on Wikipedia ). In simple terms, lossy compression uses psychoacoustic modeling in an attempt to eliminate audio information that our ears will not miss .
This often means that information from the higher frequencies or from the farthest elements in the stereo image can be removed. Any encoder with losses introduces approximation errors, a noise that can increase the maximum levels and cause distortion peaks in the audio signal, even if the original source to be compressed was at levels below 0dB . It can be difficult for the middle ear to isolate and detect these noises, so consider the following sound examples:
NOTE: SoundCloud outputs all its audio in MP3 format with a 128kbps bitrate , which makes this comparison redundant. For a valid comparison, the original files are available for download .
Here's an uncompressed file: 44.1kHz WAV , 16bit -CD quality.
The same file at 320kbps in MP3 - the highest bitrate of the MP3 standard ...
Here we have the same MP3 file, but with the 'side' channel isolated . These same noises were present in the previous example, but are now much more apparent when heard in isolation.
There are a wide variety of ways that lossy encoders work and their resulting quality. As always, use your ears to determine which encoder and CODEC sound best for you.
A mastering engineer would take steps to anticipate and prevent such noises, regardless of the encoder they end up using.
Mastering for iTunes
Since Apple switched to iTunes Plus in 2007, iTunes downloads are at 256kbps in the AAC format. ITunes uploads require 24bit WAV files that have been previously encoded at 256kbps AAC.
Here are recommended settings when mastering audio for iTunes :
- Use true peak limiting , like Ozone 7's , to make sure the margin is set to -1dBFS . Apple recommends leaving 1dB of headroom to prevent any spikes that may occur due to noise added by the AAC encoder.
- Forget about the "war of the volume" ( Loudness War ), and not complicate the dynamic processing, such as compression or limiting. Compress a song to improve its quality, not to increase its volume. The sound check in iTunes uses an advanced algorithm to determine the volume (not just simple peak / RMS values ), the level of each track is set to -16dB and adds this information in the metadata of each audio file. A "competitive" track with no dynamic range will sound much worse on iTunes when played after a track with a large dynamic range.
- Use iTunes as a tool to compare your masters with reference tracks from other artists that you would like to bring your sound closer to . With 'SoundCheck' turned on, listening to your tracks and comparing them to the same perceived level as those of other artists will help you determine where to take your masters for iTunes listeners . Tweak your tracks based on those listens, then finish encoding your masters in full WAV or AIFF quality in iTunes .
Mastering for SoundCloud
SoundCloud converts uploaded audio to MP3 at 128kbps for streaming from its web platform. If any audio file on SoundCloud is also available as a download, that version will be in the original format in which it was uploaded.
Uploading an MP3 file to SoundCloud is somewhat redundant , as the same site will transcode it later. That will also introduce more noise into the already compressed audio. So it's best to upload an uncompressed 24bit WAV version and let SoundCloud process it.
Here are recommendations when mastering audio for SoundCloud :
- Use the Ozone Real Peak Limiter , make sure the margin is -0.3dBFS . This is an acceptable threshold to mitigate the majority of distortion peaks that can occur in the encoding process. Anyway, it will depend on the source material. Perhaps a margin of -0.5, -0.7, -1.0, or -1.5 dBFS sounds much better to you and with less distortion. In these cases, only the classic method of trial and error will suffice. You may have to do several versions until you get rid of all of them and stick with the one that sounds best.
- SoundCloud does not have a 'Soundcheck' option like Apple's. So a Master earmarked for SoundCloud has much more freedom to increase its overall level and compete in volume . Consider this option as a practical and aesthetic choice. Be sure to match the volume with the tool 'Automatically match effective gain' of Ozone to equalize and objectively evaluate the increased volume.
- Use a tool for stereo width . Opens the stereo field in the treble band by 5-20% . 128kbps MP3 audio is the commonly accepted minimum for quality. As a result, a lot of information is lost during encoding and a mix with a very wide stereo field is susceptible to noticeable noise. Ironically, broadening your field as a prevention will help you avoid the feeling of loss of power and stereo width.
Mastering for YouTube
YouTube encodes all uploaded video (and the audio it contains), in order to offer different streaming qualities at 360, 480, 720, 1,080, 1,440 (2K) and 2,160 (4K). YouTube uses the H.264 video CODEC in conjunction with the ACC audio CODEC . The quality of stereo audio playback will depend on the user's choice of streaming quality :
- 360 and 480p video will play audio at 128kbps
- Video at 720, 1,080, 1,440 (2K), 2,160 (4K) will play audio at 384kbps
YouTube can only compress video to the same or lower quality, so it is better to upload the best file you can do with the H.264 CODEC. And why not upload a MOV with uncompressed audio? For best results, YouTube recommends uploading already encoded media files, rather than uploading a MOV file containing high-quality WAV audio.
Here are some recommendations when mastering audio for YouTube :
- It uses Ozone's peak limiting , and ensures the level headroom to no more than -1dBFS .
- Not all encoders are the same. Dump from the video editor in maximum quality and without compressing neither video nor audio, and after seeing and hearing the result, try different encoders until you get the best result.
The 'True Peak Limiting' option is just below the 'Threshold' meter in the Ozone Maximizer .