as differences from luma (): and . This way, for an image that contains little color, its color difference signals tend to zero and its color-encoded signal converges to its equivalent monochrome signal. SECAM colorimetry was similar to PAL, as defined by the ITU on REC-BT.470. Yet the same document indicates that for existing (at the time of revision, 1998) SECAM sets, the following parameters (similar to the original 1953 color NTSC specification) could be allowed: The assumed display gamma was also defined as 2.8. Luma () is derived from red, green, and blue () gamma pre-corrected primary signals: and are red and blue color difference signals, used to calculate chrominance: SECAM differs significantly from the other color systems by the way the color difference signals are carried. In NTSC and PAL, each line carries color difference signals encoded using quadrature amplitude modulation (QAM). To demodulate such a signal, knowledge of the phase of the carrier signal is needed. This information is sent along the video signal at the start of every scan line in the form of a short burst of the color carrier itself, called a "colorburst". A phase error during QAM demodulation produces crosstalk between the color difference signals. On NTSC this creates Hue and Saturation errors, manually corrected for with a "tint" control on the receiving TV set; while PAL only suffers from Saturation errors. SECAM is free of this problem. SECAM uses frequency modulation (FM) to encode chrominance information on the color carrier, which does not require knowledge of the carrier phase to demodulate. However, the simple FM scheme used allows the transmission of only one signal, not the two required for color. To address this, SECAM broadcasts and separately on alternating scan lines. To produce full color, the color information on one scan line is briefly stored in an analog delay line adjusted so the signal exits the delay at the precise start of the next line. This allows the television to combine the signal transmitted on one line with the on the next and thereby produce a full color gamut on every line. Because SECAM transmits only one chrominance component at a time, it is free of the color artifacts ("dot crawl") present in NTSC and PAL that result from the combined transmission of color difference signals. This means that the vertical color resolution of a field is halved compared to NTSC. However,