Video overlay generator

[Frank Raffaeli]

Is it really necessary to go to the trouble of converting CVBS
directly to digital with a high speed ADC? My approach would be to
find a circuit to decode from CVBS to digital YCbCr or RGB, inject the
overlay information, and then re-encode back to CVBS using a video
display circuit. Would the degradation in video quality due to
decoding and re-encoding really be noticeable?

Pure analog overlay injection could better preserve the video quality,
but is the complexity of preserving the color integrity worth it? The
Obsolete Motorola MC1378 "Color Television Composite Video Overlay
Synchronizer" would simplify this task if it were still produced
(http://www.directics.comshows some inventory of these devices).

-- David

I've implemented this circuit in both analog and digital. Perhaps you
could elaborate on the requirements ... if you're the OP.

You don't need an MC1378 do do this analog. If you could buy it, it's
not all you would need. You could generate your timing overlay in an
FPGA that was synchronized to the Hz scan rate, then use a multiplier
for the cookie cutter, as one of the other posters suggested.

More detail on the application is required for smarter discussion on
the options ;->

Frank

 

[Andreas]

The major requirement is that there will be overlayed some counters
(frame counter, current time and some measurement informatino) on each
frame, there can not be any glitches in this so that the frame counter
stays the same for two whole frames for example.
Other than that, the requirements are that the circuit must be able to
process a CVBS signal, either PAL or NTSC provided through a bnc-
connector - overlay the signal and then generate a CVBS signal(the
same standard as the input) on another bnc-connector.
Functional requirements are possibilities for graphical content
generation(i.e. graphical menus, sizable fonts etc.).

I guess the most important aspect of it is reliability in that it does
not "drop" any frames.

--

As another poster pointed out, if I were to take the FPGA route, I
would use both a digital encoder and decoder that performs all the
sampling and provide a 4:2:2 BT656-stream to my logic.

Best regards,
Andreas Eriksson

 

[[email protected]]

I've implemented this circuit in both analog and digital. Perhaps you
could elaborate on the requirements ... if you're the OP.

You don't need an MC1378 do do this analog. If you could buy it, it's
not all you would need. You could generate your timing overlay in an
FPGA that was synchronized to the Hz scan rate, then use a multiplier
for the cookie cutter, as one of the other posters suggested.

More detail on the application is required for smarter discussion on
the options ;->

Frank

I'm not the OP but I am working to a similar set of requirements. I
need to overlay text and graphic color symbology over composite
video. I have much experience with digital video processing and
understand how to genlock the overlay video to the incoming video.
I'm not sure how to ensure that the color burst or color subcarrier is
locked between the incoming video and the overlay video. This part is
where I though that the MC1378 would be useful.

My desire is to create the properly synchronized overlay video
digitally and then use a standard composite video encoder circuit to
generate the overlay video to be switched in using an analog
multiplier. However, I'm not sure how to force the overlay video
encoder subcarrier to match the incoming video. I saw your earlier
post regarding the need to insert the right chroma phase. I am aware
of the need but don't understand the implementation.

-- David

 

[Nobody]

I'm not the OP but I am working to a similar set of requirements. I
need to overlay text and graphic color symbology over composite
video. I have much experience with digital video processing and
understand how to genlock the overlay video to the incoming video.
I'm not sure how to ensure that the color burst or color subcarrier is
locked between the incoming video and the overlay video. This part is
where I though that the MC1378 would be useful.

My desire is to create the properly synchronized overlay video
digitally and then use a standard composite video encoder circuit to
generate the overlay video to be switched in using an analog
multiplier. However, I'm not sure how to force the overlay video
encoder subcarrier to match the incoming video. I saw your earlier
post regarding the need to insert the right chroma phase. I am aware
of the need but don't understand the implementation.

Regardless of whether you intend to decode the input signal to YCrCb
(or even to RGB) or keep it as CVBS, you'll need an oscillator for the
chroma subcarrier which is synchronised to the colour burst on the back
porch.

If you plan to decode to YCrCb, you need the oscillator to demodulate the
chroma signal into Cr and Cb. If you plan on keeping the source signal as
CVBS, you need the oscillator to generate the overlay's chroma signal from
its Cr and Cb.

The general principle is to first generate a "gate" pulse which identifies
when the colour burst is present, then while the gate is open, synchronise
the oscillator to the burst using a phased-locked loop. The rest of the
time, the oscillator runs freely. The frequency needs to be close enough
that the phase doesn't drift significantly over the period of a line.