NICAM service in L-SECAM

The terrestrial broadcasting system for color television in France complies with the L-SECAM standard. This system uses two modulated carriers, one image and the other sound, which are transmitted in the same multiplex and modulated respectively in MABLR (reduced-sideband amplitude modulation) and AM (amplitude modulation). Taking into account the necessary precautions in sound broadcasting and demodulation, the resulting signal is a high-quality monophonic sound. In order to enhance the system to transmit high-quality stereophonic (or dual) sound, it is necessary to add an additional sound carrier to the television channel. In its draft recommendation appended to Report 795, the CCIR (Comité Consultatif International de Radiocommunications) recommends the use of the NICAM 728 digital system (or the lower-quality German FM dual-carrier system) for most television systems in use in Europe; in France, studies have therefore focused on the NICAM 728 system.

According to the EBU (European Broadcasting Union), the NICAM (Near Instantaneous Compression of the Audio Modulation) system was invented by the BBC's Research and Development department. This discovery was subsequently standardized: European standard ETSI (document: ETS 300 163) for systems B, G, H and I.

NICAM digital audio has a bit rate of 732 kbit/s (hence the name NICAM 728) with 32 kHz sampling and 14-bit quantization compressed to 10 bits. This gives a bit rate of 704 bits, plus the few extra bits needed to transmit control information and other data. In the L-SECAM standard, the width of the sidebands (main and residual or "heel") and the protection ratios on adjacent channels preclude the insertion of this additional carrier above the sound carrier, as is the case in the other B, G, H and I standards with 5.5 and 6 MHz intercarrier instead of 6.5 MHz in L. It is therefore advisable to place the digital carrier below the sound carrier, taking into account the interference it may cause to the image and analog sound, as well as to out-of-band signals in other channels and co-channels.

The characteristics of radiated television signals are set out in CCIR report 624-4 (1990) volume XI; the characteristics specified for the L standard are to be adapted for a new service corresponding to the L-SECAM/NICAM standard, in particular the nominal width of the video band defined at 6 MHz must be reduced so as not to interfere with the band occupied by NICAM, i.e.: 5.85 MHz ± 255 kHz. The residual RF image modulation must no longer be zero, to enable receivers to demodulate NICAM correctly, and the NICAM carrier level must be defined.

In order to analyze the interference caused by the NICAM carrier, and to define the parameters applicable to the addition of this new...