bitmap image Fig. 5 Subframe signal filtered with time constant of lOOns (top) and 400ns (bottom).

ing to a -3dB frequency of 1.6MHz. (For all of the simulations and measurements presented in this paper the audio sampling rate is 44.1kHz.) Although the filtered subframe now has edges with finite rise and fall times, no transitions are missed; hence, no errors will occur as long as the receiver can latch the data correctly following transitions.

The lower section of indicates the same subframe, but filtered more severely with a 400ns time constant, corresponding to a -3d13 frequency of 400kHz. In this simulation, the transition at the edge of cell 3 is missed; this will definite!y result in a bit error in the decoded subframe. This example also indicates that receiver bit errors are most likely to occur during preambles where the largest variation in transition times occur (one half-cell width to three half-cell widths). This is interesting: many practical ADIC ICs indicate full lock to the received signal when the preambles are correctly detected; hence, if lock is achieved, bit errors are not likely.

However, most practical interface decoders will exhibit an upper time-constant lock limit considerably less than 400ns, due to the finite "time aperture" about the average zero-crossing point, during which a transition is allowed without latched data errors. In "Jitter in the Digital Audio Interface" (below), we derive an expression for the peak-to- peak variation in zero-crossing times tx in terms of the range of interface signal pulse widths. If we let the large pulse width be 3tc/2 and the smaller be tc/2 (where tc is the cell width in the biphase-mark coding), then (cf Equation.11):

Equation 1:

tx = RC ln [ (1 + e - tc/2RC) / (1 + e - 3tc/2RC) ]

Using the experimental ADIC circuit shown in fig.4, full signal lock was achieved for interface time constants up to RC = 170ns, indicating that for this particular example the maximum zero-crossing-time aperture tx is approximately 45ns. In practice, time constants greater than lOOns are excessive for digital links, which should be designed with bandwidths well above 6MHz. We have measured 2-meter interface links with a characteristic impedance of 75 ohms, correctly terminated both at the transmitter and receiver, with 10-90% rise and fall times of<10ns. This performance level corresponds to a 35MHz bandwidth.

In this section we have shown that bit errors in the received subframe occur when transitions in the interface signal are incorrectly latched. This will not occur in most interface receivers for interface time constants of 100ns. Bit errors that do occur will most likely be in the preamble, · and will usually result in the receiver failing to lock onto the incoming signal.

Audio bit errors due to band limitation alone are extremely unlikely. Of course, this simple analysis does not consider the effects of noise on the bandwidth-limited link. As the bandwidth of the link decreases, the eye-pattern rep- resentation of the received signal suffers from a decreasing opening, This results in more time spent in the threshold region, and the probability of noise-induced errors increases. Nevertheless, Cabot4 presents an interface example in which bit errors are negligible for noise levels up to 20dB below the interface signal level with RC filtering up to 160ns, and claims to achieved zero error-rate transmissions over an unmatched digital audio link of 100m length.

JITTER IN THE DIGITAL AUDIO INTERFACE

The second mechanism to consider is that of the recovered clock frequency's modulation. If the clock signal fed to an ideal DAC varies in frequency, the reconstructed analog output from the DAC will include error artifacts, even if the sample values fed to the DAC are correct. The easiest way to analyze such an effect is by examining the jitter on the recovered clock. (In this article, we define jitter as the instantaneous timing deviation of dock transitions from their correct positions.)

bitmap image
Fig.6 Subframe signal unfiltered (top) and filtered with time constant of 200ns (bottom).

bitmap image
Fig.7 Subframe signal unfiltered (top) and filtered with time constant, of 200ns (bottom) (expanded time scale).


4 R.C. Cabot, "Measuring AES/EBU Digital Audio Interfaces," presented at the 87th AES Convention, New York, October 1989, Preprint 2819.


to previous page        Stereophile, March 1996        to next page