When audio theory really affects your recordings and you don’t know why, you know it’s time to study. When music is stored digitally, it needs to be reproduced digitally, and if the clock timing is not 100% accurate, then a quantization error occurs. Jitter is one of the forms of audio distortion which can stem from digital file storage and conversion. It occurs with variations in clock frequency. These distortions can occur in the recording and playback of audio. A lot of equipment for recording and listening to music advertises its low numbers as a selling point, as manufacturers account for the issue.
How it Happens
Digital to analog converters are designed to transform the digital values to a voltage level in order to reproduce sound. Ideally, the same time distance should exist between the samples. This distance is called ‘clocking’. It isn’t perfect, and if the clocking doesn’t boast metronomic accuracy then jitter can occur. Technically speaking, jitter is an alteration in time between the digital data and analog sample rate. Jitter is an issue with clocking, which is related to the sample rate. It affects the sample rate frequency and does not relate to the bit depth.
A simple metaphor may help. If you think of the audio data as ticks on an analog clock or wristwatch, jitter can occur if the ticks on the clock don’t take place at the right timings. It may sometimes ticks faster (less than a second between ticks) or slower (more than a second between each click). This is jitter. You may still get 60 ticks in the minute, but they may be at the wrong intervals.
Jitter vs Dither
It is easy to assume jitter with dither. The two are similar in some ways. Both relate to digital audio processing.
Dither is intentional. It is a noise frequency used to handle quantization error. In a PCM system, the signal can only be amplified based on a fixed set of values. This is what we refer to as quantization. If the signal doesn’t use dither, this can cause distortion. If you use ‘dither’ it will remove harmonics and get rid of distortions you don’t want in your sound. It does so by replacing the unwanted harmonics and distortion with a fixed noise frequency. It’s often used during the Mastering process and there is usually no need (it’s not recommended in fact) to apply it during the Mix.
How Jitter Can Affect Your Recordings
You will never want this phenomenon in recordings. It is always unpleasant and though changing the harmonics may not be as damaging, the clicks and annoying sounds it can add means that if you record with jitter then the sound will inevitably be damaged. This is irretrievably bad. There is no way you can get rid of it once it is on a recording.
It is tough to imagine jitter without hearing it. This example can show you how it may sound. It can vary, the sound is not always identical, but this is a good way to understand how it can actually sound.
Remember that jitter can occur with both the recording or the reproduction of sound. Both use digital conversion. But recording-wise, only the inbound analog-to-digital will be stored in the recording, the other will only affect when you’re hearing. So it’s a lesser issue, since you’ll often bounce digitally your tracks.
The rate that jitter occurs does not scale to the sample rate proportionally. However, as we increase the sample rate then more jitter can crop up. Many more samples will be affected at a higher sample rate. The time shift in higher sample rates can result in more critical jitter.
When recording at high rates, sound cards can be put under a huge amount of processing pressure. If you are recording at a very high sample rate then you are increasing your chances of jitter. Your sound card will need to be excellent quality, and have a brilliant A/D converter and quartz crystal to keep the clocking accurate at these high sample rates.
An audio interface will often be sufficiently good for making home recordings. Cheap interfaces with cheap chips can let you down though. The conversions which are made including A/D and D/A conversions are done in many different stages as the audio travels from the soundcard to the PC. The fewer conversions you have in the audio recording chain, the lower the chances of jitter.
Avoiding Jitter when Recording
Avoiding jitter is the goal when you are recording audio. If you record music and it has any form of distortion in that recording, then the quality has been permanently degraded.
Jitter is a bit like digital clipping or ‘peaking’. Once it is on the recording, you can’t get rid of it. Make sure that the clock signal which is controlling your AD converter (often within your audio interface) is good enough quality. When buying an interface, the jitter will be one of the things audiophiles tend to review.
If you can avoid sample rate conversions throughout the audio signal, you should do this. The real time sample rate conversion that is inevitable, make sure that the converter has low jitter clocking for both input and output. Equipment for home recording is expensive anyway. However, the best way to avoid jitter is to make sure you have high-quality recording equipment which has been designed with avoiding jitter in mind. Use the very best equipment available. When purchasing an audio interface or other recording equipment, check that the manufacturer has considered A/D or D/A conversions.
It can be frustrating if you have started to hear annoying clicks and unwanted sounds on recordings you’ve made. It may be that jitter is getting in the way. There are certain tips you can follow to avoid the phenomenon from damaging your recordings. Making the right choice when buying equipment is vital, and minimizing the number of conversions made within your audio chain can also help a huge amount.