most used systems. An effort to harmonize European broadcasts on the 625-line system started in the 1950s and was first implemented in Ireland in 1962. SECAM thus had the added issue of having to be compatible both with their existing 819-line system as well as their future broadcasts on the 625-line system. As the latter used much less bandwidth, it was this standard that defined the amount of color information that could be carried. In the 8 MHz standard, the signal is split into two parts, the video signal, and the audio signal, each with its own carrier frequency. For any given channel, one carrier is located 1.25 MHz above the channel's listed frequency and indicates the location of the luminance portion of the signal. A second carrier is located 6 MHz above the luma carrier, indicating the center of the audio signal. To add color to the signal, SECAM adds another carrier located 4.4336... MHz above the luma carrier. The chroma signal is centered on this carrier, overlapping the upper part of the luma frequency range. Because the information of most scan lines differ little from their immediate neighbors, both luma and chroma signals are close to being periodic on the horizontal scan frequency, and thus their power spectra tends to be concentrated on multiples of such frequency. The specific color carrier frequency of SECAM results from carefully choosing it so that the higher-powered harmonics of the modulated chroma and luma signals are apart from each other and from the sound carrier, thereby minimizing crosstalk between the three signals. The color space perceived by humans is three-dimensional because of the nature of their retinas, which include specific detectors for red, green and blue light. So in addition to luminance, which is already carried by the existing monochrome signal, color requires sending two additional signals. The human retina is more sensitive to green light than to red (3:1) or blue (9:1) light. Because of this, the red () and blue () signals are usually chosen to be sent along luma but with comparably less resolution, to be able to save bandwidth while impacting the perceived image quality the least. (Also, the green signal is on average more closely correlated to luma, making them a poor choice of signal to send separately). To minimize crosstalk with luma and increase compatibility with existing monochrome TV sets, the and signals are usually sent