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Difference between CIRC Encoding and CIRC Decoding | Advantages of CIRC Encoder

This page compares CIRC Encoding vs CIRC Decoding and mentions difference between CIRC Encoding and CIRC Decoding including advantages of CIRC encoder.

Introduction: CIRC stands for Cross-Interleaved Reed-Solomon Code. It is used in Compact Disc (CD) system to provide error detection and correction. CD digital audio system was defined for digital storage and reproduction by Philips Corp.

There are several sources of channel errors viz. small unwanted particles or air bubbles in plastic material, scratches during handling and so on.

CIRC Encoder and CIRC Decoder

CIRC encoding adds one redundant parity byte to every three data bytes. CIRC is concatenated error control scheme which contributes to high fidelity performance in which data are rearranged in time. Due to this arrangement error bursts are made to appear as single random events. The figure depicts CIRC encoder and CIRC decoder block diagram.

CIRC Encoding using CIRC Encoder

CIRC Encoding

Let us understand CIRC encoding step by step on system frame consisting of 24 symbols and frame size of 6 sampling periods.
• Step-1 : Even no. sample are separated from odd no. samples by two frames time in order to detect byte errors.
• Step-2 : Four R-S parity bytes are added to interleaved 24 byte frame. (28,24) code is outer code.
• Step-3 : Each byte is delayed a different lengths which spread errors over several codewords. C2 encoding and D* interleaving correct the burst errors and error patterns.
• Step-4 :Four R-S parity bytes are added to Δ interleaved 28 byte frame. (32, 28) is inner code.
• Step-5 :The purpose is to cross-interleave even bytes of frame with odd bytes of next frame. D interleaving and C1 decoding correct most random single errors and detect longer burst errors.

CIRC Decoding using CIRC Decoder

The inner and outer R-S codes with (n,k) -> (32,28) and (28,24) each use four parity bytes. Code rate of CIRC is (k1/n1)(k2/n2) = 24/32 = 3/4.

C1 and C2 decoder can correct maximum of 2 symbols errors or 4 symbol erasures per codeword.

CIRC Decoding Equations

Let us understand the CIRC decoding process step by step.

CIRC Decoding

• Step-1 : D Deinterleaver : Alternate delay lines and each delay is equal to 1 byte.
• Step-2 : C1 Decoder : If multiple error occur, the C1 decoder attaches an erasure flag.
• Step-3 : D* Deinterleaver : Error at the output of C1 are spread over a number of words at the input of C2 decoder. Reducing the number of errors enables C2 decoder to correct these errors.
• Step-4 : C2 Decoder : Correct the burst errors which C1 could not correct, but if C2 decoder can not correct, the 24 byte codeword is passed on unchanged to the deinterleaver and are given an erasure flag via.
• Δ Deinterleaver : Detect the byte errors using interpolation.

Advantages of CIRC Encoder

From the above, we can summarize benefits or advantages of CIRC Encoder as follows.
➨It is useful to correct combination of both random error and burst error. It offers higher random error correctability.
➨It can correct error bursts with about 3500bits in sequence. This corresponds to about 2.5 mm length on CD surface.
➨It can compensate long error bursts with 12Kbits (i.e. 8.5 mm) occurred due to minor scratches.
➨It offers very high efficiency.

The only disadvantages of CIRC technique is its addition of redundant bytes. The samples which can not be corrected by C2 decoder could cause disturbances. To avoid this condition, interpolation process inserts new samples in place of unreliable ones. Also refer benefits or advantages of interleaving >> and difference between Block Interleaver and Convolutional Interleaver >>.

References : Presentation by Diana B. Llacza Sosaya on Digital Communications at Chosun University which is available at following link.

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