13-11-2023, 11:03 AM
525 video is not 15750Hz line scan frequency. It's about 15734Hz due to the change made when NTSC colour was introduced. There was a potential problem with the beat between the FM sound carrier and colour subcarrier. Rather than risk having to retune the sound IF in millions of TVs they shifted the whole scan structure by 1000/1001. Some years later when timecode was invented this caused no end of trouble and still does.
The CCCIR/SMPTE 601 standard exploits the 1000/1001 to allow a common Y sampling frequency of 13.5MHz. This gave non-square pixels, different shape on 625 and 525. Another nuisance when interfacing with computer systems. Subsequent HD standards have 2 different clock frequencies. One for 50Hz and exact 60Hz systems, another for 59.94Hz systems. Having designed multistandard digital video kit I can assure you that all the possible standards are a PITA.
Fortunately 48kHz sound works OK with all of them, though in a slightly odd-legged way with 59.94Hz. 44.1kHz sound in a TV environment can best be called a nuisance. Thank heavens for good, low-cost audio sample rate converters.
The CCCIR/SMPTE 601 standard exploits the 1000/1001 to allow a common Y sampling frequency of 13.5MHz. This gave non-square pixels, different shape on 625 and 525. Another nuisance when interfacing with computer systems. Subsequent HD standards have 2 different clock frequencies. One for 50Hz and exact 60Hz systems, another for 59.94Hz systems. Having designed multistandard digital video kit I can assure you that all the possible standards are a PITA.
Fortunately 48kHz sound works OK with all of them, though in a slightly odd-legged way with 59.94Hz. 44.1kHz sound in a TV environment can best be called a nuisance. Thank heavens for good, low-cost audio sample rate converters.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
