On 2014-03-30, Dave Malham wrote:
48 kHz is pretty well the international standard sample rate for broadcast organisation and has been since they started upgrading from the 32kHz used (by the Beeb) for distributing audio to FM transmitters back in the late 60's.
...and many others in their wake, since that rate was enshrined in NICAM and standardized as such for broadcast TV.
Interestingly 32kHz is the only one of the standard rates with a history directly traceable to audio work: it's a distant relative of Bell Corporation's bandwidth requirements for analogue telephone circuit passband. Those were something like .3-3.5kHz, which given imperfect analogue anti-alias filters became a Nyquist frequency of 4kHz and so a sampling frequency of 8kHz in the early digital work. Later when higher multiplex rates were set, the European E hierarchy dropped the inelegant, inband, bitstealing utilized in the American T system, yielding a digital hierarchy with a clean 8 bits by 8kHz basic utility band per circuit. The British GPO circuits BBC designed their PCM system to be compatible with was an early instance of that sort of reasoning, so taking BBC's requirements for FM broadcast quality, the Nyquist frequency had to be at least 15kHz, and so the lowest suitable multiple became 32kHz.
All of the other standards trace back to video line rates, which via the NTSC and PAL/SECAM frame rates in case trace back to the 60Hz and 50Hz mains frequencies used on the two sides of the pond. The same in fact could have happened with NICAM: BBC did entertain a version of their system where the audio frames were time division multiplexed into the horizontal blanking interval of PAL I, and the sample clock was locked to twice the horizontal scan rate of 15625Hz, i.e. 31.25kHz. However, given that BBC did radio work too (primarily?) and ended up building a separate backbone for that using NICAM, eventually they just dumped the bits onto an independent subcarrier at the higher channel edge.
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