has a field rate of 50 Hz, their least common multiple is 300 Hz, and with 3 samples per line, this yields a sample rate that is a multiple of 900 Hz. For monochrome NTSC the sample rate is 5m × 60 × 3, where 5m is the number of active lines per field, which must be a multiple of 5 (the rest used for synchronization), and for PAL the sample rate is 6n × 50 × 3, where 6n is the number of active lines per field, which must be a multiple of 6. The sampling rates that satisfy these requirements – at least 40 kHz (to encode up to 20 kHz sounds), no more than 46.875 kHz (requiring no more than 3 samples per line in PAL), and a multiple of 900 Hz (to allow encoding in both NTSC and PAL), are thus 40.5, 41.4, 42.3, 43.2, 44.1, 45, 45.9, and 46.8 kHz. The lower ones are eliminated due to low-pass filters requiring a transition band, while the higher ones are eliminated due to some lines being required for vertical blanking interval; 44.1 kHz was the higher usable rate, and was eventually chosen. The sampling frequencies of 44.1 and 44.056 kHz were thus the result of a need for compatibility with the 25-frame (PAL countries) and 30-frame black and white (NTSC countries) video formats used for audio storage at the time. Video format Audio samples are recorded as if they were on the lines of a raster scan of video, as follows: analog video standards represent video at a field rate of 60 Hz (NTSC, North America – or 60/1.001 Hz ≈ 59.94 Hz for color NTSC) or 50 Hz (PAL, Europe), which corresponds to a frame rate of 30 frames per second (frame/s) or 25 frame/s – each field is half the lines of an interlaced image (alternating the odd lines and the even lines). Each of these fields is in turn composed of lines – a frame of 625 lines for PAL and 525 lines for NTSC, though some of the "lines" are actually for synchronizing the signal, and a field comprises half the visible lines in one vertical scan. Digital audio samples are then encoded along each line, thus allowing reuse of the existing synchronization circuitry – as video, the resulting images look like lines of binary black and white (rather, gray) dots along each