# CIF

Common Interleaved Frame

The CIF frame is a subdivision of the MSC frame into 4 equal parts. Understanding the transmission frame and its MSC part is key to understanding the CIF frame. The transmission frame represents how the antenna transmits bits.

The MSC, part of the transmission frame, consists of 72 times a modulation cycle of (D)QPSK modulation. Remember that a modulation cycle happens simultaneously on the 1536 carriers and that the (D)QPSK modulation uses symbols of 2 bits.

So we repeat that one modulation cycle sends 1536 carriers x 2 bits = 3072 bits into the aether.

(Aether is a physically non-existing material, a word used when people could not understand which medium transported light. It was considered the 5th element, “quintessence,” in medieval times, a material filling the whole universe).

The MSC consists of 72 consecutive modulation cycles carrying audio bits. Dividing the MSC by 4 delivers CIFs of 72/4 = 18 modulation cycles.

Figure 1 – division of the MSC into 4 CIF frames.

How many bits can a CIF transmit into the air?

Eighteen modulation cycles multiplied by 3072 bits per modulation cycle result in a total of 55269 bits per CIF.

Now we quantized the CIF as a unit of 55269 bits.

Let us look into the CIF timing.

A Transmission frame repeats every 96 ms, so each sub-part of the transmission frame is repeated every 96 ms. It is also applicable to the MSC. It should be clear that dividing the MSC into 4 CIFs can only result in CIFs of 24 ms duration.

Remember, 24 ms is a very important time lap in DAB(+) audio. Het is the duration of a DAB audio frame and 1/5 of the DAB+ superframe, which has a period of 120ms.

The story about the CIF continues with the CU (Capacity Unit), a further subdivision of the CIF (see glossary).

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