# [time-nuts] Why these Crystal Frequencies?

Magnus Danielson magnus at rubidium.dyndns.org
Thu Dec 15 14:12:27 EST 2011

```On 12/15/2011 11:54 AM, Heinzmann, Stefan (ALC NetworX GmbH) wrote:
> 13.5 and 27 MHz are usually associated with digital video. SD video with 720 x 576 has a pixel clock of 13.5 MHz, and the corresponding SDI bit clock is 270 MHz.

18 MHz is another digital video frequency.

13,5 MHz is the sampling frequency for luminance samples for SD-SDI 3:4
video, ITU-R (formerly CCIR) BT.601. The chrominance difference samples
goes at 6,75 MHz sampling frequency. These samples are 10 bit, so you
get a 27 MHz rate of 10 bit samples or 270 Mb/s rate of the full SD-SDI
signal (then only called SDI signal).

18 MHz then relates to that in the 16:9 format variant producing
according to the same logic a 360 Mb/s rate SDI signal, but it's

18 MHz is also used in analog video synthesis as it relates well to many
signals.

27 MHz is a magic frequency as both PAL and NTSC relates in an easy
relationship to it.

PAL:
25       * 625 * 432 * 4 = 27 MHz

NTSC:
30/1.001 * 525 * 429 * 4 = 27 MHz

The factor of 4 is for the 4 samples of luminance and chrominance
differences.

Related frequencies for HD-SDI is:

European
25 * 1125 * 1320 * 4 = 148,5 MHz = 11/2 * 27 MHz

US
30/1.001 * 1125 * 1100 * 4 = 148,35 MHz = 148,5/1.001 MHz = 11/2.002 *
27 MHz

(Let me tell you that I hate the 1.001 factor for HD-SDI rates)

Cheers,
Magnus

```