Site Contents
- Introduction
- Audio Data Format
- Interface Applications
- Hardware Specifications
- Data Transmission
- Digital-Analog Converter
- Resetting & Auto-Switching
- History & Characteristics
- Data & Cabling Details
- Coding Format
- Conversion Circuits
- Internal Connection
- External Connection
- Optical Output Circuit
- Transmission Line
Sponsors
SPDIF: Data Transmission
Data Format
SPDIF or IEC958 Type-II is basically a data format that transmits PCM (Pulse Code Modulated) digital audio signals between devices and stereo components like CD/DVD players. The data from SPDIF uses DC free bi-phase mark or Manchester coding. This coding is self-clocking in nature and can work with changing sampling rate.
Transistor-Transistor Logic (TTL)
All digital logic circuits use this. Typically TTL is 5 volt (on) and 0 volt (off). It finds use in almost all digital devices and the TTL signal levels are compatible with nearly all logic ICs. Many sound cards are provided with TTL SPDIF outputs. Add-on units are also there for conversion of TTL to COAX or TOSLINK.
Data Transmission
There are several ways in which SPDIF data can be transmitted but data remains the same whatever the way. Lack of absolute polarity in SPDIF signal makes data transmission lot easier. SPDIF digital signalling standard has 75 ohms characteristic impedance terminated at both ends. This indicates that the Transport source driver must possess output impedance of 75 ohms and the DAC receiver must have parallel 75 ohms resistive termination. The signal can propagate from Transport to DAC if ideally both the terminations are set at 75 ohms. Assuming all components of the transmission line are also set at 75 ohms, there will be no reflection on the transmission line connecting the Transport to the DAC. If any component is without 75 ohms impedance, reflection will be caused in the transmission line. The transition takes certain period of time to be transmitted to the other end of the transmission line. Normally about 2 nanoseconds per foot, the transition time is slower than the speed of light. There is scope for speeding up the transmission with correspondingly matching impedance. An accurately matched impedance ensures reduction in transmission time and a transport less sensitive to different digital cables.