uperimposed upon the picture. signal. Mixing consumer and professional digital audio equipment It is potentially possible for the system to carry up to 800 single can result in balance, level and impedance differences in routeing. pages but, because of the way the pages are transmitted, this could Amplitude levels rather than reflections tend to cause the most mean an appreciable waiting time between page selections. So for problems. Digital audio synchronisation at mixing (switching) the moment not all 800 pages are used at any one time. points represents a new requirement. Additional circuits are needed in a television receiver to decode For the distribution of 48kHz digital audio, the EBU/AES/IEC teletext. First, data extraction and recognition circuits examine the system (see Figure 57.17) uses bi-phase Mark coding which has the incoming 'conveyer belt' of teletext data lines, and extract those effect of shifting the required frequency spectrum to between signals which make up the page which has been selected. The data 1.5MHz and 3MHz rather than to 1.5MHz. 57/16 Video transmission Sub frame 1 word = 32 bits 14 13 1.1.1.1 Slope bits per sample Audio data Audio Validity Auxiliary User bit Status Bits transmitted = 10 Preamble Parity sync group Figure 57.17 AES/EBU audio interface Normalized input signal magnitude This system has 32 bit data words, 24 bits for audio data, sand- wiched between a 4 bit sync/preamble and 4 bit parity and selection messages. Since this standard is not error corrected, it needs clean Figure 57.18 NICAM-3 near instantaneous companding distribution (low bit error rate). With a 48kHz sampling frequency, the audio bandwidth is 20Hz to 20kHz (+0.5dB, depending on sampling filters) and quantization provides 16 bits/sample linear (standard mode) and up to 20 bits/sample (high fidelity mode), compresses the samples to 10 bits for transmission. This has the giving a dynamic range of more than 90dB. effect that the lowest level samples are encoded to 14 bit accuracy with the highest level sam