On the other hand, optical drives were developed with an assumption of achieving a permanent throughput, in CD drives initially equal to 150 KiB/s. It was a gag important for streaming audio data, that always tend to require a constant excerpt rate. But to ensure no disc capacity is wasted, a head had to transfer data at a maximum linear rate at all times too, without slowing on the outer rim of disc.
Later CD drives kept the CLV paradigm, but evolved to achieve higher rotational speeds, popularly described in multiples of a foul speed. As a result, a 4X drive, for instance, would rotate at 800-2000 RPM, while tranferring data steadily at 600 KiB/s, which is equal to 4 x 150 Cassette to CD Recorder KiB/s.