SPDIF digital interface
SPDIF stands for Sony/Philips digital interface format, also known as IEC 958 type II, part of IEC-60958. It is a collection of hardware and low-level protocol specifications for carrying digital audio signals between devices and stereo components. It is a consumer version of the standard known as AES/EBU. SPDIF, as the name suggest, is a purely digital system used to transmit audio signals in a number of formats, the most common being the 48 kHz sample rate format used in DAT and the 44.1 kHz format used in CD audio. Each data bit is transformed into a 2-bit code (10 or 01 representing bit at logic 1 and 00 or 11 representing a bit at logic 0) using biphase mark coding technique in which clock speed is twice the bit rate of the original data. SPDIF which has no defined data rate is designed for transmitting 20-bit audio data streams plus other related information.
Where the source data is less than 20 bits per sample, the superfluous bits are set to zero.
UHF modulator
The UHF modulator receives CVBS and audio signals from the demux/ MPEG decoder chip and uses it to modulate a UHF carrier of an unused channel to provide a facility for UHF output to a TV receiver. The UHF modulator (Figure 18.15) has five components:
● An I2C-controlled phase-locked loop (PLL) frequency synthesiser
● An amplitude modulator
● An audio oscillator for the sound sub-carrier
● A video clamp to ensure correct modulation index
● A loop-through amplifier
Tuning of the modulator is carried out by a d.c. voltage (0–30 V) derived from the PLL frequency synthesiser. The audio signal is used to frequency modulate the sound sub-carrier. The modulated sound carrier is added to the clamped video signal and used to amplitude modulate a UHF carrier. The modulated UHF is then fed into the loop-through amplifier, which mixes it with the UHF signal from a normal external antenna. When the digital decoder is on standby, the UHF synthesised oscillator is switched off by a command from the microcontroller via the I2C bus. But the loop- through amplifier operates normally so that the RF signal from the exter- nal antenna may loop through to the RF output socket.