572 Chapter 18
package. Each transformer costs about $2—a long way from the $20 or so required for
analogue transformers. Remember too that “ in digits ” only, one transformer is required
to couple both channels of the stereo signal. You’ll notice, looking at the circuit diagrams
( Figure 18.4 ), RS422 (RS485) receiver-chips buffer and reslice digital audio data. The
SN75173J is a quad receiver in a single 16 pin package costing a few dollars. The part
has the added advantage that, to adapt the interface between SPDIF and AES, all that is
required is to change the value of the terminating resistor on the secondary side of the
input transformer. SPDIF digital output can be derived by inverters driving in tandem. If
AES/EBU output is required, it is best performed by an RS422 driver IC.
18.1.5 TOSlink Optical Interface
In many ways an optical link seems to be the ideal solution for joining two pieces of
digital audio equipment together. Obviously a link that has no electrical contact cannot
introduce ground-loop, hum problems. Also, because the bandwidth of an optical link is
so high, it would appear from a superfi cial inspection that an optical link would provide
the very fastest (and therefore “ cleanest ” ) signal path possible. However, the optical
TOSLink is widely regarded as sounding a little less crisp than its coaxial, electrical
counterpart. There are a number of possible reasons for this. In the fi rst place, the speed
of the link is compromised by the relatively slow light-emitting diode transmitter and
phototransistor receiver housed within the connector shells. Second, inexpensive optical
fi bers, which allow the optical signal more than one direct path between transmitter
and receiver (the technical term is multimodes), cause a temporal smearing of the audio
pulses, resulting in an effect known as modal dispersion. This can cause a degree of
timing instability in digital audio circuits (jitter) and can affect sound quality. The only
advantage the optical link confers, therefore, is its inherent freedom from ground path-
induced interference signals such as hum and RF noise. Yet at digital audio frequencies,
ground isolation—if it is required—is obtained much better by means of a transformer.
If you want to modify a piece of equipment with an optical interface to include SPDIF
coaxial output, a modifi cation is shown in Figure 18.5.
18.1.6 Transmission of AES3-Formatted Data by Unbalanced Coaxial Cable
In October 1995, the AES produced an information document (AES-3id-1995) relating
to the transmission of digital audio information (utilizing the professional data format)