Confusing timing diagram for QCIF format in OV7670 datasheet.

Any reply is greatly appreciated. I wanted to confirm about the QCIF timing diagram from datasheet(shown below). The ON time of the HREF for both vga and QCIF is shown as 640tp (tp=2tpclk). So does that mean each line of QCIF (174x2 bytes) take 640tp seconds? So in that case, bytes of a QCIF line arrive at the rate of 6.66Mhz (when HREF is high and for pclk frequency of 24Mhz) and not at 24Mhz?Hence if i were to sample bytes(of QCIF format), belonging to a line(when HREF is high),i must not sample at every PCLK rising edge?