S/PDIF, or Sony/Philips Digital Interface is a protocol to transmit audio data over either coaxial or optical cables. The data transmission rate is determined by the transmitter, and the receiver has to recover the sample rate.
Important characteristics of S/PDIF software are the following:
This module can transmit S/PDIF signals at the following rates (assuming a 50 MIPS thread):
| Functionality provided | Resources required | Status | ||
|---|---|---|---|---|
| Channels | Sample Rate | 1-bit port | Memory | |
| 2 | up to 192 KHz | 1-2 | 3.5 KB | Implemented and tested |
It requires a single thread to run the transmit code. The number of 1-bit ports depends on whether the master clock is already available on a one-bit port. If available, then only a single 1-bit port is required to output S/PDIF. If not, then two ports are required, one for the signal output, and one for the master-clock input.
The jitter on the output-pin is within tolerances allowed by the spec provided a 500 MHz part is used. It is recommended to use an external flip-flop to resynchronise the data signal to the master-clock, which will eliminate the remaining jitter on the S/PDIF edges.
The precise transmission frequencies supported depend on the availability of an external clock (eg, a PLL or a crystal oscillator) that runs at a frequency of:
channels * sampleRate * 64
or a power-of-2 multiple. For example, for 2 channels at 192 Khz the external clock has to run at a frequency of 24.576 MHz. This same frequency also supports 2 channels at 48 KHz (which requires a minimum frequency of 6.144 MHz). If both 44,1 and 48 Khz frequencies are to be supported, both a 24.576 MHz and a 22.579 MHz master clock is required. This is normally not an issue since the same clocks can be used to drive the audio codecs.
Typical applications for this module include iPod docks, digital microphones, digital mixing desks, USB audio, and AVB.
This module can receive S/PDIF signals at three different rates. It automatically adjusts to the incoming rate, but for high rates a fast thread is required. The thread will fail silently if it does not have enough MIPS to parse the input stream.
The S/PDIF receiver is generated from a state machine description. The generated code requires a one bit buffered input port (transfer width of 4), and a clock block to work.
| Functionality provided | Resources required | Status | |||
|---|---|---|---|---|---|
| Channels | Sample Rate | 1-bit port | Memory | Thread rate | |
| 2 | up to 192 KHz | 1 | 3 KB | 80 MIPS | Implemented and tested |
The receiver does not require any external clock, but can only recover 44.1, 48, 88.2, 96, and 192 KHz sample rates.
Typical applications for this module include digital speakers, digital mixing desks, USB audio, and AVB.