ZR36060PQC datasheet(26/46 Pages) ETC

26 / 46 page

Integrated JPEG CODEC

The compression pass is always executed with Bit Rate Control

(BRC). At the end of every compression process the ZR36060

calculates the new scale factor, and, if the FSF (Fixed Scale

Factor) bit equals zero, it writes the new scale factor in the Scale

Factor register. It uses this new value in the register as the scale

factor for compressing the next image field. The host only has to

program the initial scale factor to be used for compressing the

first field. Otherwise, if FSF=1, the ZR36060 uses a fixed scale

factor for all incoming fields.

In a compression pass the ZR36060 does not update the Alloca-

tion Factor (AF). During encoding of each block the ZR36060

calculates a measure of the spatial “activity” in this block,

denoted BACT. If the Block Accumulated Code Volume (BACV)

exceeds the specified allocation (given by BACT*AF), or if it

exceeds the Maximum Block Code Volume (MBCV), the code for

the block is truncated accordingly. Note that both means of bit

rate controlling (namely truncation due to the Allocation Factor,

or truncation due to the MBCV) cannot be set “off” directly, but

they can be practically eliminated by setting MBCV and/or AF to

their maximum values.

Note: Use of the Allocation Factor is only valid if the compression

pass follows a Statistical Pass. If only Compression Passes are

executed, the mode normally used for motion JPEG compres-

sion, the Allocation Factor must be set to its maximum value to

avoid corruption of the image.

6.7.1 Data Corruption during Compression

If during the compression of a field the ZR36060 senses a data

corruption event, it immediately asserts the DATERR output.

However, the ZR36060 continues the process until it finishes

compressing the field. At this time it asserts END and enters its

IDLE state.

1) If DATERR is not enabled as an interrupt requesting event

(i.e., it is cleared in the Interrupt Mask register), then the

ZR36060 samples START again, and if START is asserted a

new process begins.

2) If DATERR is enabled as an interrupt requesting event, JIRQ

is asserted together with the assertion of the DATERR output,

and when the ZR36060 completes the current process it enters

the WAIT-ISR state and remains “frozen” (ignoring START) until

the host reads the Interrupt Status register. At this time the

ZR36060 goes back to its IDLE state and is again ready to start

a new process, depending on START.

In both cases, DATERR is deasserted at the beginning of the

next process, i.e. simultaneously with the deassertion of END.

In compression the ZR36060 identifies a data corruption condi-

tion if:

a) the strip buffer overflows, or

b) VSYNC (leading or trailing edge controlled by the FIVedge

parameter) of the next video field arrives before END of the

current field is asserted.

6.8 Statistical Compression Pass

In this mode the ZR36060 goes through all the calculations

involved in encoding the given video, but without writing any

code to the code FIFO. That is, the ZR36060 does not assert

CCS, COE, CWE as a code master, or CBUSY as a code slave.

At the end of the process the ZR36060 writes the new scale

factor into the SF register, and the calculated AF, ACV, and ACT

values into their respective registers. Then it asserts the END

signal and returns to the IDLE state.

6.9 Auto Two-Pass Compression

In this mode the ZR36060 first executes a Statistical Compres-

sion Pass, then, without asserting END, it immediately starts a

Compression Pass. Activation of START (and FRAME if

needed) is required only for the first pass. The second pass

follows immediately with the next VSYNC, without regard for

START and FRAME. (Note that START and FRAME are only

sampled when the ZR36060 is in IDLE, and in this mode the

ZR36060 does not go into IDLE between the two passes).

6.10 Tables-Only Compression Pass

In this mode the ZR36060 produces only the abbreviated format

for table specification, that is, its code output contains no frame,

scan, or Huffman-coded segments. The output code includes

the SOI marker, table marker segment(s), optional APP and/or

COM marker segments, and the EOI marker. The process is

activated by START (possibly with FRAME), and upon comple-

tion END is asserted and the ZR36060 returns to IDLE.

Figure 36. Compression with Pulsed START

START

FRAME

state

VSYNC

IDLE

END

WAIT

ACTIVE

CMP

IDLE

2 Processed Fields

IDLE

WAIT

ACTIVE

Host decide to

compress next

field only

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