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IR–S/PDIF Transmitter

Posted Apr 22, 2013 at
10:01 am

The best-known ways to transmit a digital audio signal (S/PDIF)
are to use a standard 75Ω coaxial cable or Toslink optical modules with matching
optical cable. Naturally, it can happen that for whatever reason, you cannot (or
don’t wish to) run a cable between the equipment items in question. With a
wireless solution, you have the choice of a wideband RF transmitter or an
optical variant. Here we describe a simple optical transmitter. The matching
IR-S/PDIF receiver is described else-where in this website. Although designing
such an IR transmitter/receiver system does not have to be particularly
difficult, in practice there are still several obstacles to be overcome. For one
thing, the LEDs must have sufficient optical switching
speed to properly pass the high frequencies of the S/PDIF signal, and they must
also produce sufficient light intensity to deliver a noise-free signal at the
receiver over a reasonable distance.

IR–S/PDIF Transmitter Circuit Diagram

At a sampling frequency of 48 kHz, it’s necessary to be able to transfer
pulses only 163 ns wide! The LEDs selected here
(Agilent HSDL-4230) have optical rise and fall times
of 40 ns, which proved to be fast enough in practice. With a beam angle of only
17°, they can also provide high light intensity. The downside is that the
combination of transmitter and receiver is highly directional, but the small
beam angle also has its advantages. It means that fewer LEDs are necessary, and there is less risk of continuously
looking into an intense infrared source. The circuit is essentially built
according to a standard design. The S/PDIF signal received on K1 is ampliﬁed by
IC1a to a level that is adequate for further use. JP1 allows you to use a
Toslink module as the signal source if desired. JP1 is followed by a voltage
divider, which biases IC1b at just below half of the supply voltage.

This causes the output level of the buffer stage driving switching transistor
T1 to be low in the absence of a signal, which in turn causes IR LEDs
D1 and D2 to remain off. The buffer stage is formed by the remaining gates of
IC1. This has primarily been done with an eye to elevated capacitive loading, in
the unlikely event that you decide to use more LEDs. A
small DMOS transistor (BS170) is used for T1; it is
highly suitable for fast switching applications. Its maximum switching time is
only 10 ns (typically 4 ns). Getting D1 and D2 to conduct is not a problem.
However, stopping D1 and D2 from conducting requires a small addition to what is
otherwise a rather standard IR transmitter stage, due to the presence of
parasitic capacitances.

Power supply circuit diagram

This consists of R7 and R8, which are connected in parallel with the LEDs to quickly discharge the parasitic capacitors. The
drawback of this addition is naturally that it somewhat increases the current
consumption, but with the prototype this proved to be only around 10 percent.
With no signal, the circuit consumes only 25 mA. With a signal, the output stage
is responsible for nearly all of the current consumption, which rises to
approximately 170 mA. In order to prevent possible interference at such high
currents and avoid degrading the signal handling of the input stage, everything
must be well decoupled. For instance, the combination of L2, C4 and C5 is used
to decouple IC1.

The circuit around T1 must be kept as compact as possible and placed as close
as possible to the voltage regulator, in order to prevent the generation of
external interference or input interference. If necessary, place a
noise-suppression choke (with a decoupling capacitor to ground) in series with
R9. Note that this choke must be able to handle 0.3 A, and if you use additional
stages, this rating must be increased proportionally. The circuit should
preferably be ﬁtted into a well-screened enclosure, and it is recommended to
provide a mains ﬁlter for the 230-V input of the power supply. For the sake of
completeness, we have included a standard power supply in the schematic diagram,
but any other stabilised 5-V supply could be used as well. LED D3 serves as the obligatory mains power indicator.

Author: T. Giesberts
Copyright: Elektor Electronics

Tags: 74HCU04 RF Connectivity SPDIF

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