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What is Audio Jitter? Learn How It Affects Sound Quality 2024

Jitter is when a digital sound is not played or recorded exactly as it should be. It happens because the clock that controls the sound is not perfect. Some people who have digital sound systems are concerned about jitter. Other people think jitter is not a real problem, and you cannot hear it. In this article, you will learn why jitter happens, how to measure it, how to reduce it, and how to tell if you can hear it or not. The article has pictures to help you understand.
Do you want to know more about jitter and how it affects your sound quality? Then keep reading and discover the secrets of digital audio!
updated

Author: Yuri Korzunov,
Audiophile Inventory's developer with 25+ year experience in digital signal processing,
author of the articles that make audio easy for beginners

 

What is jitter audio?

 

 

 

 

 

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What is jitter audio

Digital-analog converter (DAC) transforms sample sequence (digital values) to analog voltage level sequence. In the ideal case, the same time distance between the samples should be provided. The distance is defined by clock. However, clocking is not ideal and the restored signal is distorted like the one that is shown in the picture.

Jitter is a deviation of time between samples (deviation of sampling rate).

 

Jitter audio explained on example
Jitter audio explained on example

 

To understand jitter, let’s look at an example.

In the picture, see the top row of ladybugs. They are all moving at the same speed and distance from each other. They cross the white lines together.

In the picture, see the bottom row of ladybugs. Some of them are moving faster or slower than others. They cross the white lines at different times.

This difference in timing is called jitter.

 

Jitter time distortions of playing back digital signal
Jitter time distortions of playing back digital signal

 

In the upper left part of the picture, we can see an original musical signal (green) captured in digital form.

In the bottom left part of the picture, we can see a restoration of the captured digital signal back to analog form.

The samples (vertical lines with dots) have unstable time positions (jitter) at the horizontal axis. And the restored signal is distorted. We can see it in the right part of the picture.

 

 

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How does jitter affect sound quality? Can we hear jitter? What does jitter sound like?

 

Jitter is a non-linear distortion. Theoretically, we can artificially boost jitter deviation and listen to more noise and artifacts.

In real life, the jitter always impacts analog signal at DAC output. But it's too small for modern systems. The author thinks, it is impossible or almost impossible to listen to jitter. Because jitter distortions compete with quantization noise, non-linear distortions and own noise of electronic components. I'm not sure, that we can separate real-system-jitter noise and other noise.

 

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What causes jitter audio?

Jitter is a sample clock issue (clock deviation).

Jitter appears in a moment when digital signal in analog input is transformed into binary sequence.

 

How the jitter appears
[bit value (0/1) detection]
How the jitter appears

 

To transmit music data thru a line (cable), the bit sequence is converted to an electrical form. Coding in the electrical form may be implemented in different ways (voltage level values or other).

Here we consider simple amplitude coding:

  • signal voltage level above threshold (dot line at the picture) is binary value 1,
  • voltage below threshold is binary value 0.

The line transmitter device converts binary data to electrical levels. The line receiver device converts electrical levels back to the binary sequence.

And clock reference moments are the points, where voltage levels get higher than the threshold.

The line transmitter generates a signal close to a rectangle.
In the line signal "lose" form due to noise, frequency and non-linear distortions.


Thus, clock reference moments are detected in the line receiver. And, offset from the initial time position (time deviation at the picture) is probable.

 

Below we will consider where the jitter penetrates the audio system.

 

 

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How to measure jitter

Jitter is a nonlinear distortion. To measure jitter, it needs to put pure sine to input of a learned system.

 

Scheme of the jitter measurement
Scheme of the jitter measurement

 

At the system output, we check artifacts (harmonics) and noise.

 

Jitter spectrum
Jitter spectrum

Jitter products (artifacts and noise) depend on the input signal. To check the dependency we can take spectrum for different input signal levels and frequencies.

In the picture, we do see not a real spectrum. It is an only illustration for better understanding.

We can measure:

  1. full system,
  2. ADC's digital output by a spectrum analyzer,
  3. DAC's analog output by mathematically modeled sine.

Measurements #2 and #3 allow the separation of errors in system parts.

 

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Jitter vs delay

Jitter is a deviation time between samples. Delay is shifting of full signal waveform with keeping time distance between samples.

 

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Jitter vs latency

Latency is a delay in processing inside an audio device (read below about FIFO buffer). Delay is constant.

Above we are considered, that clock deviation is an effect of digital signal distortions.

Several factors impact clock deviation:

  • clock generator instability,
  • noise in digital audio signal line.

Clock generator has electronic elements, that define frequency and its deviations.

Changing of power DC voltage can cause frequency deviations in the clock generator.

A power supply unit can generate noise into electrical lines. This noise can modulate the generated clock impulses.

Modified clock signal causes the time deviation (see the picture).

 

Noise, that impacts to digital audio signal line, penetrates from electrical circuits and the air.

 

 

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Total system jitter issue

 

Unfortunately, jitter is not a pure DAC issue. When we use recorded stuff, we have jittered sequence of digital samples.

To understand fully jitter issue, it's need to learn full recording-playback system.

  1. The first jitter source is clock generator of analog-to-digital converter (ADC),
  2. After it, digital signal pass thru the digital audio system without time errors,
  3. A digital music signal comes to a DAC and is converted to analog with time errors (read details below).

 

Jitter audio into full audio system
Jitter audio into full audio system

 

Music capturing is periodical measurements of analog signal. If the periods will vary, it causes time distortions in recorded audio samples.

 

Jitter of audio recording
Jitter of audio recording
.
Let's compare capturing with jitter and without

 
Jitter in audio recording

 

In the upper-left part of the picture, a signal is captured without jitter.

In the lower-left part of the picture, a signal is captured with jitter.

We can see that samples for both these cases have different values.

I.e., for both signals, captured with and without jitter, after restoration to analog form (playback), different waveforms of the analog signal are happens.

If music was recorded with jitter errors we can't compensate it further. Because the jitter is random.
As rule, recording studios use professional apparatus, including dedicated clock sources (in instance, Word Clock [1]). Therefore, engineers in the studios try maximally decrease losses due to bad clock.

 

 

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Digital domain between ADC and DAC

 

Let's look at point #2 (digital signal pass thru the digital audio system). When signal is placed into digital domain (signal in digital form) jitter does not matter. Because time scores for binary signal form have pure mathematical values with infinite precision.
Any delays in processing between samples are not mattered for restored signal.

However, processing delays into digital domain can cause real-time interruptions of data stream, which feed DAC. But it is not a jitter issue. Read details below.

 

 

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Clock schemes and jitter

 

Now let's look to point #3 (signal comes to DAC from pure digital part of the audio system).

 

Points of audio jitter impact
Points of audio jitter impact

 

Noise, that cause jitter, penetrate to digital signal several ways from:

  • computer power supply unit (PSU) to digital audio interface clock generator,
  • computer power supply unit thru digital audio interface to digital audio cable,
  • air to digital audio interface of DAC device,
  • DAC PSU to DAC's digital interface,
  • DAC PSU to DAC-chip,
  • DAC clock generator,
  • DAC PSU to DAC's clock generator.

However, FIFO buffers provide "jitter isolation" between any "jittered" segment.

 

FIFO ("first input, first output") kind of buffer (array of number samples) when samples out from buffer on a first-come.

In the article putting and getting of samples to/from buffer are asynchronous. Clocks of buffer writing and reading are independent.

FIFO buffer causes time delay between sample input and output (latency). Latency is measured in seconds, milliseconds, microseconds.

Short buffers cause a small delay value. Short latency is important for real-time audio systems, used for music live performance and production. For home audio big latency value almost is not matter, except real-time sound adjusting.

 

FIFO buffer with asynchronous writing (input) and reading (output).
Sample move thru buffer step-by-step
FIFO buffer with asynchronous writing (input) and reading (output). Sample move thru buffer step-by-step

 
FIFO buffer with asynchronous writing and reading

 

Jitter before FIFO-buffer doesn't impact to clock after FIFO. Because the buffer is asynchronous.

Therefore, there are no reasons to worry about jitter before FIFO into DAC for the scheme considered in the picture above.

For an audio system, before suggesting of effective jitter suppression, learning of the system scheme is recommended.

In most cases, asynchronous FIFO buffer allows fixing interruptions in a binary data stream. Of course, significant interruptions can't be fixed by FIFO. The non-fixed interruptions cause pauses, clicks or other sound damages. It happens when the buffer is empty and no binary data is ready for conversion to analog form.

 

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DAC clock and jitter

DAC clock synchronization has 3 options:

  1. by DAC digital interface (in instance, SPDIF);
  2. by internal clock generator;
  3. by external dedicated clock generator.

 

Below we consider cases for DAC with internal FIFO of input audio data.

 

Synchronization by digital interface has jitter sources:

  • clock generator and PSU of the digital line transmitter;
  • air noise impact to signal during transmitting thru cable;
  • PSU of DAC.

 

Synchronization by DAC's internal clock generator have primary jitter sources:

  • this generator;
  • PSU of DAC.
  •  

Synchronization by external dedicated clock generator have jitter sources:

  • the external clock generator and its PSU;
  • air noise impact to clock signal during transmitting thru cable;
  • PSU of DAC.

 

For home applications, I'd recommend using DAC's internal clock generator as a simplest way. It may be chosen in the DAC's settings.

However, it does not guarantee the best result. Each setup should be analyzed (measured) to choose the minimal jitter configuration.

 

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Conclusions

What is jitter audio. Conclusions

 

  1. Jitter only happens when you change digital signals to analog signals or the other way around.
  2. Jitter does not change the signal in digital or analog form.
  3. To measure jitter of your music system, you need to compare two signals: the output signal and the original sine wave.
  4. A FIFO asynchronous buffer is a device that separates the parts of the system from jitter.
  5. Usually, if your DAC has its own clock or uses an external clock to sync, you don’t need to worry about jitter before the DAC’s digital input.
  6. If your DAC syncs by the digital audio input, you may need to reduce jitter before the DAC inputs.
  7. The best way to reduce jitter is to study the audio system design (including the devices) and do some tests.

 

 

 

 

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Frequently Asked Questions

 

What is jitter noise?

Jitter noise is noise, that is a result of time instability of timing (ADC or DAC clock). Read details...

 

What is jitter in USB?

USB can't cause jitter for transmitted audio data. Audio data get jitter only on border between analog and digital domain. Read details...

 

 

References
  1. About clock in recording studios

 

 

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