EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: VEGETA on July 09, 2023, 07:43:01 am
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Hello,
I have ADV7181D which converts analog video to digital, and the chosen output format is 20-bit YCbCr 422.
I plant to hook THS8135 after it to convert it to analog YPbPr, however I am confused from sync insertion stuff.
for YPbPr output, I think I should have these controls:
sync and sync_5 -> 0 for "Video DAC mode. Negative sync inserted".
M1 = 1 and M2 = 0 -> for YCbCr 422 2x10b. Here M1 is latched as "M1_INT" since sync input control pin is 0.
so my question, where does sync signal comes from?
I know in 2x10b ycbrc sync code is embedded but the same controls can be used for rgb 444 and ycbcr 444 digital inputs which do not have sync signal.
my guess is that in order for sync to be inserted properly, I should have a composite sync signal on the "sync" control pin, this Csync signal can be taken from LMH1980 IC... this is when I need THS8135 to insert the sync on Y or G pin but if I don't want that then I should choose "sync" and "sync_t" to be 1 all the time.
is my understanding correct?
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If you choose to use a composite sync signal, you can obtain it from an IC like the LMH1980. The LMH1980 can extract the composite sync signal from the incoming video signal and provide it to the sync input of the THS8135.
Alternatively, if you don't want to insert sync externally, you can keep the "sync" and "sync_t" controls set to 1 all the time. This configuration implies that the ADV7181D will generate the sync signals internally, which will be passed to the THS8135 without the need for external sync insertion.
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If you choose to use a composite sync signal, you can obtain it from an IC like the LMH1980. The LMH1980 can extract the composite sync signal from the incoming video signal and provide it to the sync input of the THS8135.
Alternatively, if you don't want to insert sync externally, you can keep the "sync" and "sync_t" controls set to 1 all the time. This configuration implies that the ADV7181D will generate the sync signals internally, which will be passed to the THS8135 without the need for external sync insertion.
hmm this is interesting.
adv7181d output is ycbcr 422 2x10b which has sync signals embedded within it... do you mean it will be put on the output Y signals even if i didn't insert it externally?
i say this because ths8135 have a mode where it takes ycbrc or rgb 444 parallel bits input which does not have sync embedded. what will happen if i didn't insert the sync there? for rgb it can be ok but ypbpr analog video must have sync on y.
please inform me how did you know the ths8135 will output the embedded sync without me putting the external sync.
EDIT:
I found this in datasheet:
The input data format can be either 3x10 bit 4:4:4, 2x10 bit 4:2:2, or 1x10 bit 4:2:2. This enables a direct
interface to a wide range of video DSP/ASICs including parts generating ITU-R.BT656 formatted output data.
However, the THS8135 needs specific input synchronization signals to properly insert a composite sync onto its
outputs as it does not extract embedded SAV/EAV synchronization codes from the ITU-R.BT656 input. Along
with other extra functionality, this feature is available on a derivative device (THS8200).
thus, it requires csync input on the sync pin since it cannot extract it from embedded codes.