Prism
Operation Manual Issue 2.00
Page 8.8
Correct operation of the Valid bit should be checked in the same way although, again,
a variety of 'correct' responses are possible since the Valid bit may have different
meaning for different source devices. In general, one would expect receiving
equipment to either mute or sample-hold in response to an 'Invalid' flag. Some
equipment implements a sample-hold which gradually decays to a mute - this is a
clever idea since it allows concealment of single-sample errors as well as muting of
prolonged 'Invalid' periods.
Finally, the DSA-1 can be used to inject precise line-up tones into the input under test,
to check gain settings, level meters etc. GEN1 also features a 'Pulse' waveform which
allows digital overload indicators to be tested with periodic full-scale pulses of one, two,
three or four samples duration. The pulses can be positive-going, negative-going or
both.
8.4. Evaluating 'in-line' performance
It is useful to test digital-to-digital equipment 'in-line' since this may provide more
information than testing the inputs and outputs separately as described above.
8.4.1.
'In-line' carrier tests
It is especially important to perform the following tests if equipment is 'cascaded', i.e.
if devices are connected 'in-line' and each is synchronised from its data input. In this
situation the tests can be usefully applied to the whole chain at once, since carrier
degradation in cascade is normally progressive. The following tests are made by
connecting the DSA-1's appropriate GEN2 output to the input of the system under test,
and the output of the system under test back to the appropriate DI input of the DSA-1.
The DSA-1 generator would normally be set to Internal Sync. By selecting the fs/4
GEN2 function and monitoring using headphones or the built-in loudspeaker it is easy
to hear sample-slipping or interpolated errors in the equipment or cascade.
Jitter attenuation is the amount by which jitter entering a device is attenuated at the
device output. Jitter attenuation is not a simple value since it always varies with jitter
frequency. The interface standard does not cover the subject of jitter attenuation (see
the footnote at the end of this chapter). Most equipment may be categorised as a jitter
attenuator, or not. A jitter attenuator reduces incoming jitter even at very low
frequencies (sometimes down to a few Hz). Equipment which is not specifically a jitter
attenuator normally reduces jitter outside the audio band (say above 50kHz) since this
is usually a characteristic of the AES3 receiver device itself. To characterise jitter
attenuation precisely, by applying constant jitter amplitude at any desired frequency,
a Prism Sound JM-1 Jitter Modulator is required. However, the DSA-1 can make a
general assessment of jitter attenuation by applying jitter to the input under test and
measuring the output fs jitter. The two useful jitter functions (waveforms) generated by