SingMai Electronics
PT9 User Manual Revision 0.3
Page 15 of 38
7.
Cross-colour reduction
When colour NTSC and PAL broadcasts were first introduced, to maintain compatibility with
monochrome TV transmissions and because of the restriction in the bandwidth available for
transmission, the chroma had to occupy the same frequency as the higher luma frequencies (see
Figure 9).
Figure 9 PAL frequency spectrum
The luma information occupies from 0 - 5.75MHz (in a studio environment – i.e. not for RF
broadcast) whereas the chroma information is modulated onto a subcarrier at approximately
4.43MHz with a 1.3MHz bandwidth (PAL standard). Therefore, at frequencies from approximately
3 – 5.7MHz, the chroma and luma occupy the same spectrum.
It is the job of the video decoder to separate these two components.
To separate the luma from the composite signal we could employ a low pass filter that blocks all
frequencies above 3MHz. This will result in ‘clean’ luma (i.e. with no chroma crosstalk) but we have
reduced the luma bandwidth of the signal by nearly one-half. This approach is even more
detrimental for NTSC where the subcarrier is at 3.58MHz, still with a 1.3MHz bandwidth, so the
resulting luma bandwidth would be only 2.3MHz (although we do also get a small amount of the
very high frequencies, above 4.8MHz). We can improve the luma bandwidth by using a narrow
notch filter, centred on the subcarrier frequency, and allow a small amount of cross-talk at the
extremes of the chroma bandwidth but we are still removing a large part of the luma detail.
The chroma, once demodulated, will always contain high frequency luma. To reduce the
interference of the chroma information for monochrome TV receivers, the chroma subcarrier
frequency has a fixed phase relationship with respect to the horizontal line frequency. We can
