ST92195D datasheet(157/249 Pages) STMICROELECTRONICS

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ST92195C/D - TELETEXT SLICER AND ACQUISITION UNIT

TELETEXT SLICER AND ACQUISITION UNIT (Cont’d)

7.7.6 Teletext Signal Quality Measure

In order to determine the quality level of the re-

ceived CVBS Teletext signal, the following infor-

mation is provided to the CPU through the Teletext

signal Quality Measure register, called “AC-

QTQM”.

7.7.6.1 Bad TV line criterion

During each VBI, “bad” TV lines are counted. The

“bad” TV line pulses issued by the Sync Extractor

macrocell are counted by a 4 bit counter. These

pulses are generated only during the VBI. This

“bad TV line” counter is reset by hardware at each

Vsync (extracted from the CVBS). If an overflow

occurs, then the counter value is locked to Fh.

7.7.6.2 Hamming Errors Code criterion

At the same time as each VBI, the number of Ham-

ming Errors detected by the “8/4 Hamming Decod-

ing” is counted. The detection is done only on the

two first bytes after the Framing Code Byte in each

Teletext packet during the whole VBI and when

the Framing Code is correct and recognized.

One “Hamming Error” pulse corresponds to one or

several errors in the same byte, corrected or not

by the “8/4 Hamming Decoding”. The Hamming

Errors are counted by a 4-bit counter, reset by

hardware at each Vsync (issued from the CVBS).

If an overflow occurs, then the counter value is

locked to Fh.

The ACQTQM register provides in a nibble format,

the 2 above described kinds of information. The

lower nibble of the ACQTQM register gives the

number of “bad TV lines” per VBI. The higher nib-

ble of the ACQTQM register gives the number of

“Hamming error codes” per VBI.

It is up to the application programmer to use this

information and issue an acceptance criterion on

the Teletext signal quality to enable or disable any

Data Slicing. The ACQTQM register is a read-only

7.7.6.3 Reduced Framing Code Criterion

The FR7B status bit in the ACQPRR register pro-

vides rough information about the Framing code

correctness.

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