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
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SPDIF: Data & Cabling Details
Multi-channel Audio
In 1998, IEC958 was renamed IEC60958 or SPDIF. Normal audio as well as IEC61937 data-streams can be carried by SPDIF. MPEG2, AC3 or DTS which are all multi-channel sounds are contained in IEC61937 data-streams. During transfer of IEC61937 data-streams, audio sample carrying bits are replaced with the data bits from data-stream and SPDIF signal headers.
Channel Status
There is a bit in channel status information that indicates the nature of data in SPDIF, which is, whether the data is digital audio or other data like DTS, MPEG audio or AC3. Equipments handling both normal audio and IEC61937 can determine what to do with the data received, from SPDIF signal header bits.
Cabling Details
SPDIF or IEC958 uses 75 ohm co-axial cable with RCA connectors. A typical SPDIF COAX connection work distances of at least 10-15 metres. 75 ohm COAX is same as used in video transmission and is economical. 110 ohm output impedance and symmetrical connections with transformer isolation are used in AES/EBU interfaces. The signal level in this interface is considerably higher than consumer version at more than 10v. This is because transmission of AES/EBU digital audio signals is at video like high frequencies (around 6Mhz) making its handling not the same as that for standard analog audio lines. XLR-3 microphone cables perform poor digital transmission with impedance rating typically varying from 30-90 ohm. The impedance mismatch causes drop in signal level and reduced cable length. A good cable can make AES/EBU signal transmission work for more than ten metres. 1 mm plastic fibre is the media for transmission of TOSLINK optical signals used in SPDIF interface. Red transmitting LED which is visible light is used to transmit signals. Formats for SPDIF electrical and optical signals are same except for conversion to light signals in case of optical signals. Transmission distance for optical cable may be as low as a few metres because of using light as media.