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Much more about 720p24 HDV, Part 2

Mar 27, 2006 7:12 PM

In the last Newsletter, we calculated that 720p24 HDV has an effective 46.8Mbps data rate vs. the 36.9Mbps rate of 720p24 DVCPRO HD. This calculation, which may surprise some, is a reminder that:

1) When an intraframe codec writes a fraction of 50/60fps to a non-tape media, the format’s specified data rate must be reduced by this fraction.

2) You cannot compare inter- and intraframe rate codecs on the basis of their data rates. You must consider interframe rate codec compression efficiency.

3) Intraframe codecs are compared by their compression ratios.

4) Interframe codecs are compared by their bit-rate reduction ratios.

5) The image quality of interframe codecs (HDV) cannot be inferred from their data rates unless you also take into account both frame size and frame rate.

6) You cannot compare CBR and VBR interframe codecs on the basis of data rates.

7) You cannot compare MPEG-2 4:2:0 encoding and MPEG-2 4:2:2 encoding on the basis of data rates.

8) You cannot compare MPEG-2 to MPEG-4 (or MPEG-4 AVC) on the basis of data rates. Nor can MPEG-4 be compared to MPEG-4 AVC on the basis of data rates.

By looking at the GOP structure of 24p HDV video, we can confirm the 46.8Mbps data rate. Within a six-frame GOP sequence, a “B” frame is typically half the size of “P,” which is typically half that of an “I” frame. The efficiency of MPEG-2 is, therefore, achieved by P and B frames — not I frames.

In JVC's six-frame GOP (IBBPBB), the average bit rate for an "I" frame can be determined by substituting the P and B ratios, as follows:

I + B + B + P + B + B
I + 0.25I + 0.25I + 0.50I + 0.25I + 0.25I
2.5I = I + 0.25I + 0.25I + 0.50I + 0.25I + 0.25I

Because there are four GOPs for 24p:

10I = 2.5I x 4 = 18.7Mbps
I = 1.87Mbps

Remember that an I-frame carries one frame of video that is interframe-compressed approximately as much as one frame of video compressed using intraframe compression. (The intraframe compression ratio for 24PA is approximately 7:1.)

Given equal frames sizes (number of pixels), the compressed data carried by interframe- and intraframe-compressed video frames are equal. Therefore, six frames of DVCPRO HD have an aggregate data rate that is equivalent to 6I, which means these six frames have an aggregate data-rate of 11.22Mbps. (6 x 1.87Mbps.) Since there are four sets of six frames for every 24 frames, the aggregate data rate for 24 frames is 4 x 11.22, or approximately 45Mbps. This value nicely confirms the 46.8Mbps obtained in the last Newsletter.

If you remember algebra, since six frames of DVCPRO HD are equivalent to one HDV GOP, here is a very simple confirmation:

I' = a DVCPRO HD frame
I = an HDV I-frame
6I' = 2.5I = one 6-frame GOP
2.4 ratio = 6I' x 2.5I

The 2.4 ratio result confirms the 2.5X Data Rate Multiplier in the second Table in the last Newsletter.

In the next Newsletter, we’ll take a closer look at HDV Color Sampling.

© 2008 Penton Media, Inc.