Mike Thornton from Production Expert shows in this 3 part series of free short videos how the growth of music streaming services and how they deliver music to the consumer is changing the way we need to mix and master content, especially for music streaming services. In part 3 Mike demonstrates the importance of True Peak metering and limiting and how they can help prevent the consumer getting distorted tracks.
What Is Different When Mastering For Online and Streaming Services
When we master for CD manufacture, the whole delivery process was lossless, usually 16bit 44.1k sample rate. However most online delivery systems using lossy codecs to create audio files to reduce the file size or bandwidth needed, typically mp3 or AAC for iTunes. Unfortunately these codecs do not handle high levels too well and so we need to be able to control what is going on near digital headroom with precision otherwise your music heard by your consumers could end up being distorted.
What Is Happening Between The Samples
Digital audio is like video or film, it is made up of a series of snapshots, or samples. Traditionally we have measured the peak level of the audio with a digital peak meter that is actually reading the samples, typically 44,100 per second, but what this doesn’t tell us is what is going on between the snapshots or samples.
It may seem very picky, but did you know that when you convert the digital audio back into analog, the reconstructed waveform between the samples, the intersample peaks, could be up to 6dB higher and as we are working very close to digital headroom and processing audio with codecs that are not very good at handling audio close to digital headroom we need both metering and limiters that work to something called true peak.
What is True Peak
True peak meters will display what is likely to happen between the samples when the audio is converted back into analog and so we can also be confident that we will not overload the lossy codecs and so end up with distorted tracks being delivered to the consumer. This is achieved using a technique called oversampling which enables us to measure the intersample peaks, what is going on between the peaks, and so make sure that we do not try and exceed digital headroom between the samples. We can then adjust our levels so that our carefully crafted tracks are not going to be distorted by the time the consumer listens to them.
Mastercheck from Nugen Audio has a true peak meter and it is recommended that if your mixes are going to be converted into mp3s that you make sure that your audio doesn’t go above -3dBTP and for AAC files you can go up to -1dBTP.
So How We Can Measure True Peak, What About The Limiter?
As well as having a meter like MasterCheck that can display true peak it is also very important that your mastering limiter is also working to true peak. Most limiters, either hardware or software are not working to true peak, and you can safely say that if they don’t shout their True Peak credentials from the roof tops then they are not true peak, and although there is no reason why you shouldn't carry on using your favourite dynamics processors in the production process, when it comes to the final mastering phase having a reliable and accurate true peak limiter is essential.
In this video we demonstrate this with an instance of MasterCheck from Nugen Audio and feeding it from a limiter plugin that isn’t a true peak limiter. Even though the threshold of the limiter is set to -6dBFS the true peak meter shows true peaks that are much higher.
Then we show that if we add an instance of ISL2 from Nugen Audio after the sample peak limiter with the ISL2 threshold set to -6dBTP there is some gain reduction. With a second instance of MasterCheck on the output of the ISL2 limiter and all is as you would expect. No peaks above -6dBTP.
Preparing Masters For Lossy Codecs
When it comes to preparing masters for lossy codecs, which all the music streaming services use to deliver bandwidth friendly content to the consumer, it is essential that your tracks are mastered so that these lossy codecs don’t distort.
In the mastering phase, before the tracks are converted, using a lossy codec like mp3 or AAC, you should use a true peak limiter to protect the codec stage. For example, if you know that your audio is going to be converted to mp3 you should set the threshold to -3dBTP as I have done here on my ISL2 from Nugen Audio.
Mike Uses The Nugen Audio ISL2
The Nugen Audio inter sample limiter is my tool of choice because it is so transparent, and unlike some limiters that are sold as true peak limiters don’t perform correctly and let through some peaks that they shouldn’t, the Nugen Audio ISL2 does exactly what it is meant to do and does it transparently.
I hope you have found this mini series of videos helpful in understanding how the workflows and techniques have changed now more and more music is being delivered to consumers using streaming services that use loudness normalisation to keep their customers happy so they don’t have to keep turning the volume up and down.