digital audio measurement

8/10/2019 Digital Audio Measurement

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K.Murugan,DDE

STI(T), Delhi.



Measurements for digitalinterfaces can include:FOR AES3 TRANSMITTERS(WITH OR WITHOUT LONGCABLES)

Carrier amplitude .

Common Mode amplitude (if

differential) Sampling frequency accuracy or

phase-lock to reference sync.

Number of active bits.

Channel Status (compliance)

Fs Jitter .

Data Jitter.

Eye Narrowing (at 200mV eye-height)

It is also desirable to display theEye Pattern.

FOR AES3 RECEIVERS(LONG CABLES WILL BESIMULATED)

Lock to desired samplingfrequencies & lock range.

Tolerance of: reduced carriervoltage, slow rise time, phase

offset from reference sync,source jitter (low and highfrequency), common-modeinterference, differential wide- band noise .

Behaviour in response to specificchannel status test patterns andvalid bit.

For D-D systems, it is also usefulto check if the User Data channelin AES3 is transparent.



Unlike analogue systems, digital transmission

systems do not degrade the audio gracefully or

progressively.

As the carrier becomes progressively degraded,

audio transmission will suddenly fail.

Symptoms are sometimes heard as pops and

glitches or dropouts in the audio.

A degraded signal may be received correctly most

of the time, but "tick" infuriatingly at irregular and

infrequent intervals or even have intermittent

errors in the metadata.



Handling audio in the digital domain offers many

advantages over analog methods.

An analog signal studios had a large investment in

signal-transmission infrastructure, specifically in

two-conductor, shielded cables interconnectingsystems, equipment and studios.

So the standard uses a self-clocking, polarity-insensitive technique to allow studios to transmit

digital audio over these existing cables.



What can go wrong?

Some common errors include incorrect sample rate, excessive jitter,insufficient amplitude, poor data integrity caused by incorrecttermination, or the use of poor cable.

There can be problems in the analog -to-digital conversion processitself, although the quality of converters has progressed substantially.

The most common design implementation used for converters thesedays uses an over-sampling technology.

This method can substantially reduce residual noise and distortion byusing a noise-shaping technique that shifts the noise upwards in thespectrum beyond the audio band.

But, in return, it does produce substantial “out-of-band noise” that cancause problems in some areas.

Good practice suggests that these out of- band components be filteredout at the source before they can pollute subsequent devices.

Being aware of possible artifacts can help when troubleshootingobscure problems.



Jitter is perhaps the biggest problem in digital audiotransmission.

As mentioned earlier, the AES3 bit stream is selfclocking.

The AES3 receiver derives its clock from the transitionsof the data stream itself.

If the interface pulses received were perfect rectangles,the time of the fast-rising vertical transitions would beclearly defined.

But because the cable has capacitance and the AES3transmitter has finite source impedance, the leveltransitions have a finite rise time.



Jitter can also be caused by noise or cross talk added tothe DataStream.

The noise or cross talk will cause ambiguity in the zero-cross transition of the data pulses, again causing jitter.In a broadcast facility, program synchronization is veryimportant.

If there is jitter on this clock signal, it can be transferredto any audio device that uses it in the process of tryingto maintain audio/video synchronization.

Another phenomenon, “jitter accumulation,” occurswhen several digital devices are cascaded — a commonsituation in a broadcast facility.

Each device can pass on the jitter it receives whileadding its own accumulation of jitter.



As the DC value moves up and down with time, thetransition time, and thus the embedded clock edges,vary.

This variation from cycle to cycle results in aconstantly varying phase shift called cable-inducedclock jitter.

This problem can be reduced by using a cablespecifically designed for AES3 transmission,



Most cable manufacturersmake such a cable with thecorrect 110 impedance.

It‟s important to have the cable properly terminated at the farend.

Data-pulse integrity, unlikeanalog-signal transmission, isaffected by source, cable andtermination impedances.

Black-With proper source,cable and terminationimpedance.



The digital-to-analog converter usually uses theclock signal extracted from the digital datastreamas its sampling clock.

In such cases, the jitter will modulate theconversion process.

This can raise the noise floor or add unwantedfrequencies to the audio signal.



More seriously, if jitter reaches too high a level,some data receivers will begin to malfunction,

eventually losing lock.

Often, this situation may occur in a large facilitydue to a particular interconnection of several

subsystems with the resulting jitter-accumulation effect.

The trouble report may describe an intermittentsignal loss, but, when the trouble-shooterassesses the final device in the chain, he canfind no fault because the complete path is nolonger intact.



It is not possible to see jitter by looking at atime-domain waveform.

AES3 data analyzers are available, as well ascomplete audio-measurement systems thatinclude sophisticated data-integrity-measurement functions.

Such instruments measure the level of jitterdirectly, usually expressing the result in UI ortime.

They also usually measure the more obvious, but nonetheless, important, parameters likedata signal level and sample rate.



A particularly valuable test is an “eye pattern” display.

This function averages multiple pulses and displays astatistical average.

The measurement system extracts the clock from thedata stream but regenerates it using a phase-locked loopto produce a “perfect” clock reference — free of jitter

but of the same frequency and phase as the embeddedclock.

This reference synchronizes the display, but the actualdata stream is displayed without correction and therebyshows its actual jitter.

If the signal were perfect, the display would be arectangle with thin traces.



The rise time gives a

triangular appearance to the

display, which is where the

term “eye pattern” comesfrom.

The size of the opening in thecenter of the eye directly

measures the integrity of the

signal.

Slow rise times and high jitter

make the opening smaller.



Digital audio signal transmission and storage offers theadvantage of higher initial quality, more robust delivery,

virtually no progressive degradation with successivegenerations of storage or transmission, and more predictable quality at the far end.

But these advantages can only be realized if the AES3

digital data transmitters and receivers in the individualdevices in the chain are well-designed and if thetransmission techniques follow good digital data

practices.

Knowledge of the mechanisms of degradations and howthey play out with the equipment in your facility will

prevent unpleasant surprises and ensure high-qualityaudio delivery.



AES17 recommends methods for testing digital

audio equipment.



AES17 aims to provide a consistent methodology formanufacturers and system builders.

Instead of stating all of the test conditions and settings,

it is convenient to state 'measured in accordance withAES17'. AES17 covers A-A, A-D, D-D and D-Amodes.

There are several common considerations: inputcharacteristics, output characteristics, linear response,amplitude non-linearity, signal-to-noise ratio and crosstalk and channel separation.



It is advisable to obtain a good digital audio testsystem.

It is worth considering not only themeasurement capability, but also whethertesting can be automated.

Automation provides many benefits:consistency, ease of use, ease of documentation,

and speed, to name but a few.Prism Sound offer AES17-compliant

automation scripts.



The key differentiator is the ability not only to

measure interface jitter, but to be able to

separate the Fs-jitter (sampling-rate jitter,

source-related) and data-jitter (data-rate jitter,

cable-related) components.



Note what each of the bytes handles. All are maintained todetermine a smooth process under the AES/EBUregulations.

A professional serial interface for transferring digitalaudio from CD and DVD players to amplifiers and TVs.

AES/EBU is typically used to transmit PCM and DolbyDigital 5.1, but is not tied to any sampling rate or audio

standard.

AES3 and AES3id - Short and Long Distances.

AES3 uses 110 ohm shielded twisted pair (STP) cablewith XLR connectors up to a distance of 100 meters.

AES3id uses 75 ohm coaxial cable and BNC connectorsfor up to 1,000 meters. "Unbalanced" coax is better for

long distances than "balanced" twisted pairs.



Analog audio has given way to the AES3 and

Sony/ Phillips Digital Interface Format

(SPDIF). AES3 data stream are also embedded

in SDI television signals.



Two sample rates are in common use:48kHz in the professional environment and 44.1kHz in the consumerworld.

One scenario that can cause problems is if a faultytransmitting device has a sample rate sufficientlydifferent from what it should be.

This could prevent a subsequent device from lockingonto the signal. Or perhaps someone is using theincorrect sample rate, perhaps 44.1kHz instead of therequired 48kHz.

Sample-rate converters can correct the latter problem.



UI is the unit interval

of the short pulse.

32 bits/sub frame=64

bits/frame.

Bit period is1/64.

1 bit has a transition.

Shortestpulse=1/128=1 UI.



Eye pattern test is designed to show a pictorial

view of the edge amplitude variance of digital

transmission over a transmission line for one

unit interval.

Jitter is the variation in time of an event such as

a regular clock signal from nominal.

Jitter can affect the digital signal in two ways.

In the sampling process.

And in the digital interface.



177 ns-44.1KHz.

163 ns-48KHz.

41 ns-192KHz.



Sample Rate Clock Jitter.

When sampling does not occur at precise regular

interval, the reconstructed analog signal will not bethe same as the original.

Interfering Signal Jitter.

Noise and interfering signal may add or subtract

amplitude from the pulse train, causing late or early

zero crossings. This problem is magnified with

long cable runs.



Data Induced Jitter.

The combination of slow rise and fall times

(due to long cable) and random „1s‟ and „0s‟ inthe audio data can cause data induced jitter .

digital audio measurement

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