A Single Transistor S/PDIF LED Driver

Mar 21, 2013

Introduction :

This article demonstrates a simple S/PDIF LED driver circuit using a single transistor. To support up to 96 kHz sampling rates on a S/PDIF stream, the combined LED and driver circuit must have a modulation bandwidth of at very least 6 MHz. Suitable LEDs include some high-radiance red (9MHz) and infrared (20MHz) LEDs. With a matching optical receiver, it is possible to assemble an optical wireless free-space digital audio link working over distances in excess of 100'.

About S/PDIF:

S/PDIF (Sony / Philips Digital Interconnect Format) describes a consumer format for transmission of digital audio, either uncompressed stereo or compressed multi-channel formats. Many consumer audio DVD and Blu-ray players are equipped with either 75 ohm digital coaxial, or digital fibre-optic (TOSLINK) S/PDIF output jacks. These are wired connections to amplifiers or receivers equipped with digital audio inputs with high-definition audio support up to at least 24bit/96kHz. The fairly low complexity and bandwidth of the S/PDIF format, compared to high-bandwidth standards such as HDMI, allows for fairly easy assembly of DIY circuits such as that described here. The S/PDIF digital audio interconnect format is described by a sampling rate of the 64-bit audio data frames. Each 64 bit frame contains a single audio sample for both the left and right channels along with various status bits. Therefore for sampling rates of 48kHz and 96 kHz, the information bit rates are 3.072 and 6.144 Mbit/sec respectively. The LED modulation circuit should have a bandwidth equal to or greater than this bit rate to achieve adequate resolution of the bits. The scope trace below shows a S/PDIF coaxial output data stream at a 96kHz sampling rate measured with a 2' 75 Ω coax cable terminated with a 75 Ω resistor. The data bit rate is exactly 6,144,000 bps. Each bit interval is ~ 163 ns in duration. "0" bits have a transistion at the beginning and end of a bit interval. "1" bits have a transition at the start and in the middle of a bit interval. For the driver and LED to clearly create the "1" bits of a 96kHz S/PDIF stream, the 10/90% risetime of the combined circuit and LED should be 1/4(bit interval) or ~ 40ns which corresponds to a circuit bandwidth of ~ 9MHz with 6MHz being a lower limit. For 48kHz S/PDIF, the required bandwidth is about half or 3 - 5 MHz. The pattern below shows part of a frame of a 96kHz S/PDIF stream. There are 8 "0" bits at the end preceeded by a single "1" bit. The longer pulse preceeding this is a slightly longer marker bit which signals the start of a frame. The coaxial S/PDIF standard specifies a nominal electrical level of 0.5Vpp at a termination impedance of 75 ohm. However some S/PDIF output levels have been measured at almost twice this level. For the stream shown below, the signal level is almost 1Vpp:



The pattern below shows a 192kHz S/PDIF stream, the highest rate supported:



LED Driver Design:

The schematic diagram and protoboard version of a single-transistor driver design is shown below. The circuit is a swamped-emitter current source with the LED in the collector circuit. It has a bandwidth similar to an emitter-follower circuit. The bandwidth of an emitter-follower circuit is strongly dependent on the emitter resistor Re and the total resistance to the left of the transistor base as well as the cutoff frequency of the transistor. For the circuit shown and with a 2N2222a transistor with fT = 300 MHz, the circuit electrical bandwidth is ~ 60MHz which is higher than the LED response bandwidths considered here (< 25 MHz). The quiescent LED current which is almost exactly the emitter current is determined by the biasing resistors Rb1, Rb2 and the emitter resistor Re and targets a value of ~ 12-15mA with a modulation amplitude of ~ +/-10mA from a typical S/PDIF input signal. This current modulation range is within the current-drive specification of most typical LEDs. The transistor is a 2N2222a NPN switching device but other high-speed common transistors can be substituted. The circuit can be easily modified to work with lower supply voltages. The emitter resistor Re can be lowered to 22 Ω for S/PDIF input signals below 350 mVpeak with the supply voltage ~ 9 VDC. Most simple AD/DC adapters can be used to power the modulator although the actual output voltage should be checked at the fairly low load current of ~ 20mA. An optional input shunt capacitor Cs of 33 - 70pF can be added to reduce high-frequency ringing in the collector current although slower LEDs discussed here will optically filter out this ringing. The input termination resistor Rt, in parallel with the biasing resistors and reflected emitter resistor, is chosen to provide a net input termination resistance of ~ 75 Ω :





Performance:

Current rise and fall times of < 15ns were measured in the circuit across Re using a fast pulse generator as input source. A typical emitter current response is shown below. The response time of ~ 10ns is limited by the Analog Discovery oscilloscope used here:



The scope traces below show the response to a S/PDIF input signal. The upper blue trace is the current-monitor voltage (across Re) and the lower trace is the input voltage at the transistor base demonstrating the same tr/tf ~ 35 ns as the S/PDIF source measured directly. Since this is a swamped emitter-follower circuit controlled by Re, the amplitudes are almost identical with tr/tf ~ 35ns:



The LED optical output response, as measured with a fast photodetection circuit (BW ~ 100MHz) is shown below for high-radiance red and infrared LEDs. The "1" bits (narrowest pulse feature) is resolved adequately with the red LED. The infrared LED, with a higher bandwidth, produces the data stream more accurately. The tr/tf of the photoresponse is a combination of the S/PDIF input signal (~ 35ns) and LED speed. Both LEDs will function properly in an optical wireless S/PDIF link. The line Y1 represents the optical dark-signal level:





It is easy to build a complete transmitter/receiver optical-wireless S/PDIF link capable of perfect transmission over distances in excess of 100 feet.

References