S/PDIF test generator
S/PDIF test generator
The generator is intended primarily for checking S/PDIF (Sony/Philips Digital Interface Format) receivers and any associated digital-to-analogue converters (DAC) and/or output filters. The externalclock – standard TTL level – enables 128 sample frequenciesto be generated. The clock may also be used for generating standardfrequencies with the remaining inverters serving as crystal oscillators(provided a 74HCU04 is used).
The sender is a Type CS8402A digital audio interface transmitterfrom Crystal. In this short article it is not possible to list allsettings that may be obtained with switch S1: the reader is referredto the data sheet of the IC or to the ‘sampling rate converter’ publishedin the October 1996 issue of this magazine. The connectionsto the switch are exactly as described in that article.There is an optical (IC4) as well as a coaxial output (K1, K2).Toroidal transformer Tr1 provides electrical isolation of the coaxialsockets and also serves to prevent earth loops. Capacitors C2and C3 provide the earth connections for the sockets.The transformer is wound on a TN13/7.5/5-3E25 core with atransformation ratio of 20:2:2 since TXP and TXN (on IC3) aredifferential outputs.
The primary voltage is 10 Vpp to give a signalacross the 75 coaxial outputs of 0.5 Vpp. After a reset, both outputsare low and are not short-circuited by Tr1. A coarse audio signalis added to prevent, for instance, muting of the outputs.Jumper JP1 enables either the left-hand or right-hand signal tocontain a rectangular signal at peak value and half the samplingfrequency.
This enables, for instance, the channel separation andthe combination of digital and analogue signals to be checked. Inmost DACs, filter action commences at half the sampling frequency.At that instance, there is hardly any attenuation by theanalogue filters, so that the level of the sinusoidal signal more or lesscoincides with that of a 0 dB signal. At this frequency, it is alsoclearly discernible whether de-emphasis correction is present (S1–4off: de-emphasis on) and, if so, whether this provides the requisiteattenuation of 10 dB.