display | more...

NTSC television scans 525 lines about 29.97* times a second. Multiplying these gives 15,734 Hz, the horizontal scanning frequency of NTSC television. (PAL television uses 625 lines * 25 Hz = 15,625 Hz.) I've seen lots of CD rips that include significant energy at 15,734 Hz. The worst offenders are TV soundtracks such as "Schoolhouse Rock," but I've seen it on all sorts of discs. In fact, somebody on Slashdot has reported having to discard a Kylie Minogue CD because eir hearing was so sensitive to the frequency.

I deal with it by first using the 15.7 kHz signal as a clock to remove wow and flutter and then using a simple antiresonant notch filter to remove the clock signal. (Because the signal is close to a sawtooth wave, containing both odd and even harmonics falling off at 6 dB/octave, you also need notches at 31,468 Hz and 47,203 Hz if you're sampling at 96 kHz.)

* The exact figure is 30000/1001. It was set this way so as not to cause beats with the 4.5 MHz audio carrier.

Log in or register to write something here or to contact authors.