6.9 Analysing the Digital Audio Interface

6.9 Analysing the Digital Audio Interface

Because of the wide variety of implementations possible in the standard interfaces and because of the need to test digital interface signals to determine their 'health' as electrical signals, there have arisen a number of items of test equipment which may be used to analyse the characteristics of the signal. The following is just a short summary of testing techniques, more detailed coverage of which may be found in Cabot 14 , Blair et al . 15 , Mornington West 16 , and Stone 17 .

6.9.1 Eye Pattern and Pulse-Width Testing

The eye pattern of a two-channel interface signal is a rough guide to its electrical 'health' and gives a clue to the likelihood of it being decoded correctly. In a test set-up described by Cabot, pictured in Figure 6.16, it is possible to vary the attenuation and high-frequency roll-off over a digital link so as to 'close the eye' of a random digital audio signal to the limits of the AES3 specification. Having done this one may then verify whether a receiver correctly decodes the data and thus whether it is within the specification. The testing of eye patterns requires a high quality oscillo-scope whose trigger input is derived from a stable clock locked to the source clock rate, preferably at a submultiple of it. Alternatively, provided that the clock recovery in the receiver is of high quality and rejects interconnect jitter it may be possible to use this, with the proviso that the eye pattern's reliability will be affected by any instability in the trigger signal.

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Figure 6.16: This test arrangement is suggested by Cabot for measuring the ability of a device to decode digital input signals at the limits of the eye pattern.

An alternative to eye pattern testing on an oscilloscope is the use of a stand-alone interface analyser such as that described by Blair et al ., capable of displaying the amplitude and 'pulse smear' of the signal on an LCD display with relation to either an internal or external reference signal.

It has been suggested by Kondakor 18 of the BBC Designs and Equipment Department that eye patterns may not always indicate the 'decodability' of a signal, since he has found examples of seemingly good eye patterns which still give problems at the receiver. Consequently a device has been built that measures the variations in pulse width of received signals a parameter which becomes greater on poor electrical links displaying the result on a simple visual display which indicates variations in the three possible bit- cell timings (the '1', '0' and preamble wide pulses ) in the form of bar lengths. It is claimed that this gives quick and easy verification of the reliability of received signals, and correlates well with the likelihood that a signal will be decoded.

6.9.2 Security Margin Estimation

Often it is necessary to get a rough idea of how close an interface is to failure and one way of doing this is to introduce a broadband attenuator into the link at the receiving end (with sufficient bandwidth to accommodate the digital audio signal). The attenuation is gradually increased until the receiver fails to decode the signal. The amount of attenuation that can be introduced without the link failing is then a rough guide to the margin of security. An alternative to this method is for test equipment to examine the narrow pulses that follow the extra wide pulses in the subframe preambles of the data. Due to intersymbol interference these are usually the first to be reduced in width and level, or even disappear in cases of poor links and limited link bandwidth. Some interface receiver ICs use this criterion as a measure of the security margin in hand.

6.9.3 Error Checking

Digital interfaces are designed to be used in an error-free environment in other words, errors are not anticipated over digital links and there is no means of correcting them. This is an achievable situation in well-designed systems and above a certain signal-to-noise ratio one may expect no errors, but below this ratio the error rate will rise quite quickly. The error rate of a digital interface can be checked by a number of means. Common to all the techniques is that a certain bit pattern is generated by the test equipment and the received pattern is then compared against the generated pattern to check for data errors. It is then possible to attempt such things as increasing the noise level on the interface by injecting controlled levels and bandwidths of artificially generated noise to see the effect on error rates. A novel means of checking for errors also described by Cabot is to use a sine-wave test signal and to monitor the digital THD + N (distortion plus noise) reading at the receiver on suitable test equipment. Any error gives rise to a spike in the signal, resulting in a momentary increase in distortion which can be monitored at the output of the notch filter. He points out that errors in LSBs will produce less of an effect on the THD + N reading than errors in MSBs. By correlating errors with the data pattern at the time of the error it is possible to determine whether they are data dependent.

6.9.4 Other Tests

Further checks on the interface may be performed on commercial test equipment, such as the accuracy of the sample clock (compared against either an external or calibrated internal reference), measurements of data cell jitter and common mode signal level. Test equipment can also show the states of all the information in channel status and other auxiliary bits if required, many systems converting these into useful human-readable indication. Examples exist of systems that will analyse the incoming channel status and other data and modify it to suit the established requirements of the receiver in order to set up proper communications in problem situations.



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

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