I have a question about SD Closed Captioning assignments of ADV7392.
According to FCC Code of Federal Regulations（CFR Title 47 Section 15.119 and EIA-608,Closed Captioning data 8bit indicates 1 character.ADV7392 has 4bytes SD closed captioning registers (Subaddress 0x91 to Subaddress 0x94).
Could you tell me how we should assign the registers.May we set the binary data we want to display to the registers?Are there any sequence which it should be careful of or any preference?
In response to questions one and two maybe the following will help. I will repsond later to questions 3 and 4 later.
Closed Captioning is the display of text over video pixels on a display. It is used mostly as a method of displaying subtitles. In NTSC, the data is located on Line 21 and 284 of the video stream. Sometimes Line 284 is used for Extended Data Services (XDS). XDS indicates such information as the programme name, time into the show, time remaining to end etc. It should be noted that if Line 21 only is being used then no VBI data need appear on Line 284. However if line 284 is being used, then Line 21 must be present, though again if not to be used should be composed of Null ASCII characters.
These VBI lines are composed of:
· A clock run-in signal: A 7-cycle sinusoidal burst that is frequency locked and phase locked to the caption data and is used to provide synchronization for the decoder.
· A start bit: Following the clock run-in signal there is a blanking interval for two data bits; this is followed by a high start bit.
· Two 7bit words (with a parity bit on each): The start bit is followed by 16-bits of data, composed of two 7-bit and odd parity ascii characters. Closed Captions are transmitted using a Non-Return to Zero (NRZ) code.
Figure 2: Composition of the Closed Captioning Information on Line 21 and 284 of NTSC.
For YPbPr the VBI information is only on the Y component, for RGB it is contained on all three data streams.
There are two operational modes for closed captions: 1) Text Mode, 2) Caption Mode. In text mode rows 7-15 of the display are used. With each carriage return received the top most line of the captions are removed from the screen and each line then moves up one. This gives the text a scrolling effect. Each line of text can have 32 characters, with the left and right-most character being a space character.
In Caption Mode there are several display options: 1) roll-up, 2) pop-on and 3) paint-on. Roll-up is similar to that used in text mode; however the number of text lines used can be specified. Pop-on is where the captions pops up anywhere on the screen. Paint-on is similar to pop-up; differing in the way the memory is stored in an internal buffer.
Control codes are used to specify the format, location and attributes of the characters. Each control code is two bytes. They are sent at the beginning of each row in the form of a preamble. They can be also sent mid-row, say for example when the colour of the text was to be changed.
Figure 3: CCAP Preamble Format (Figure taken from Video Demystified, 4th Edition)
The requirement for CCAP was to take a NTSC (480i) signal with CCAP embedded in the VBI and input it into a ADV7850, take the video output of the ADV7850 and input this into the ADV8002, with the VBI data transferred over SPI. The ADV8002 is then required to output a NTSC signal with the CCAP embedded.