Bit Perfect Jitter
Digital audio is a series of samples made with a fixed interval.
We have two components, the signal and the time.
A good digital audio stream is a bit perfect stream with perfect timing.
Bit perfect is an easy criteria to meet, computers and networks are build with bit perfection in mind. If not, internet banking would have been far more exiting.
Timing is a matter of a stable clock. As absolute perfection don't exist, there is always some fluctuation in clock speed. This fluctuation is called clock jitter.
Interface jitter is jitter introduced in the transmission of digital signals.
Jitter could be induced by noisy power supplies, improper ground path and electromagnetic interference.
Crucial is the sampling jitter, deviations in the sampling interval in the DA conversion stage
What is Jitter?
Jitter is time-base error. It is caused by varying time delays in the circuit paths from component to component in the signal path. The two most common causes of jitter are poorly-designed Phase Locked Loops and/or reflections in the signal path.
Here is how waveform distortion can cause time-base distortion:
The top waveform represents a theoretically perfect digital signal. Its value is 101010, occurring at equal slices of time, represented by the equally-spaced dashed vertical lines. When the first waveform passes through long cables of incorrect impedance, or when a source impedance is incorrectly matched at the load, the square wave can become rounded, fast risetimes become slow, also reflections in the cable can cause misinterpretation of the actual zero crossing point of the waveform. The second waveform shows some of the ways the first might change; depending on the severity of the mismatch you might see a triangle wave, a squarewave with ringing, or simply rounded edges. Note that the new transitions (measured at the Zero Line) in the second waveform occur at unequal slices of time. Even so, the numeric interpretation of the second waveform is still 101010! There would have to be very severe waveform distortion for the value of the new waveform to be misinterpreted, which usually shows up as audible errors--clicks or tics in the sound. If you hear tics, then you really have something to worry about.
If the jitter becomes to high, it becomes audible. The sound is often described as 'coarse'.
Severe jitter will cause timing errors to such an extend that the bit flipping occurs. Even flipping the LSB (the least significant bit) in 16 bit PCM (CD audio) is audible (clicks).
Jitter can raise the noise floor.
Red: a Gaussian jitter with an RMS value of 1 ns. Green: without.
Jitter can create sidebands, a bit like how acoustic instruments create harmonics
A sinusoidal jitter with 300ps peak value applied to a 10KHz sinusoidal tone.
Source: http://www.tnt-audio.com/clinica/diginterf1_e.html
Audibility of jitter
In 1974 the BBC research deparmentent concluded:
For jitter having a random, white noise, spectrum extending from 30 Hz to 16 kHz, it is estimated that impairment on critical programme would be perceptible to less than 5% of listeners provided the jitter amplitude is no more than 50 ns r.m.s.
Source: BBC
Ashihara (2005) did an experiment indicating that random jitter is not audible below the 250 ns
Adams (1994) states that jitter threshold is dependent on the combination of components used in the DAC and concludes that phase jitter has to be as low 20 ps – 1 ns to obtain signals of 16 bit quality.
Dunn, J.: “Jitter: Specification and Assessment in Digital Audio Equipement”,
AES Convention Paper 3361, October 1992.
We can see that the audibility threshold decreases from 500ns at low frequencies to as little as 20ps at 20kHz. Especially when using formats or converters with high sample-rate this will be a major issue.
The digital equivalent of flutter is periodic jitter, which is caused by instabilities in the sample clock of the converter (Rumsey & Watkinson 1995). The sensitivity of the converter to periodic jitter depends on the design of the converter. Periodic jitter produces modulation noise. Practical research by Benjamin and Gannon involving listening tests found that the lowest level of jitter to be audible on test signals was 10 ns (rms). With music, no listeners in the tests found jitter audible at levels lower than 20 ns (Dunn 2003:34).
Source: http://en.wikipedia.org/wiki/Analog_sound_vs._digital_sound
As long as you stay in the digital domain, jitter is not a problem. Computers are designed with jitter in mind and it should be low enough to avoid bit flipping due to timing errors.
Copy a audio file from one device to another is a matter of a bit perfect copy not a matter of a jitter free transmission. Jitter in digital audio only counts when it is converted to analogue by the DAC. A DAC which is input jitter immune saves a lot of problems. In this case the only source of jitter are the components of the DAC.
Bit perfect and a perfect timing are the two components that make a perfect digital audio stream.
Bit perfect is almost guaranteed, that’s the way computers are working. If not it is clearly audible. It is typically digital, it is a one or a zero, it is perfect or it is clicks and ticks.
Jitter is unavoidable, there is no clock with infinite cycle to cycle time precision.
The quality of the source, the ability of jitter rejection of the DAC and the architecture of the DAC will decide if it becomes audible.
The audibility of jitter depends also on the usual suspects: your ears, your sensitivity and the rest of your audio gear.
However the thresholds are remarkably low.
1 ns is 0.000 000 001 of a second,
1 ps is 0.000 000 000 001 of a second.
Compared with excellent turntables wow and flutter values (0.05% variation in rotation speed) one might wonder why DA conversion is so sensitive to jitter.
But sample rate fluctuations doesn’t translate into variations in pitch, it is mapped as noise and distortion in the analogue domain.
A bit more or less jitter might make the difference.
The sound of jitter
In general I find jitter to cause a loss in "inner detail" which usually relates to a "flatter" sound. It looses the "liveness", it becomes boring. Sometimes I find jitter also effects bass significantly. This is strange because I would expect high frequencies to be more susceptible, but I frequently hear a significant improvement in bass articulation when decreasing jitter.
Its hard to listen to a piece of gear and say "thats got high jitter" just by listening, because many other things can cause similar sonic effects. By building my own gear I've been able to do quite a few tests where I can hold everything equal except change jitter and can definately hear major improvements in sound by lowering jitter.
But frequently similar changes can be had (for example) by upgrading the power supply of the preamp.
Another issue is all jitter is not the same. The spectrum has a lot to do with it. I have one receiver with 200 ps of jitter that sounds significantly better than another with 50ps, BUT the spectrum is radically different between them. Thus just picking the lowest published "jitter number" will not gurantee the best sound.
