Multi-Channel VHF Modulator Design
 

A little off the rails (well a bit anyway), but is there a modulator available from anyone for Band III Channel 8 use?

Cheers,


Andy

Re: VHF Modulator wanted
 

I'm in the process of building a modulator that covers all 13 System A channels. I have posted some pictures on my website http://homepage.ntlworld.com/david.robinson3 . Sorry the circuit diagrams are a bit scruffy - I haven't got round to Orcad - ing them yet. :-[ The sound modulator of course has the same peripheral components as the vision one, but I couldn't be bothered to draw them. This is very much 'work in progress', but it does work. Top of the to-do list is completing the stereo mixer for sound, and a 3.5MHz notch filter on the vision side to prevent buzz on sound.

If anyone wants to have a go at this, send me a PM and I will send you the software for the PIC. :)

The modulator chips can be obtained as free samples from Motorola Semiconductors (now inexplicably renamed Freescale). The chip supports System L, hence positive vision modulation. Understandably however it doesn't support the 3.5MHz sound spacing. To get round this I use a second chip with the vision modulator used for the sound. The actual sound modulators in both chips are switched off. The level pot on the sound modulator biasses the input to disable the video clamp on the chip. The chip does have one limitation in that the VHF carrier is derived by dividing the UHF carrier. This inevitably means a squarewave carrier with resultant odd harmonics. This isn't a problem in practice so long as you don't use more than one modulator feeding each TV.

Nothing to do with modulators, there are also some historic pictures of Practical Television mag on the website. ;D

Re: VHF Modulator wanted
 

That sounds very interesting David!

I bet you've spent ages developing that. Hats off to you sir!

I must say that channels 1 through to 13 inclusive is just what the doctor ordered.

By the way, I tried the link to your website and I get the dreaded, 'Cannot find server' message - perhaps it's just down at the moment. I'll try again later.

As for PICs, well I must confess to not ever having played with them, but you never know, this might just be the kick-start that I need.

I'll drop you a PM as you suggest.

Many thanks.

Regards,


Andy

Re: VHF Modulator wanted
 

Apologies to all about my homepage - ntl have some problems at the moment - major outages and all that. I can't access my email either. This does happen rather too often. >(( I'll post again when it's back up. >:(

Re: VHF Modulator wanted
 

Good morning David,
you make this thread very interesting now.
Please tell me the name of the IC. Maybe I can find a datasheet at
Googel for it. Or do you have a link?

You wrote:
Top of the to-do list is completing the stereo mixer for sound, and a 3.5MHz notch filter on the vision side to prevent buzz on sound.

Look how I solved it. To make stereo to mono you need two 4K7 Ohms
resistors.;)

Kind regards
Darius

Re: VHF Modulator wanted
 

Good morning Darius. Info on the MC44BS373CA can be found at http://www.freescale.com/webapp/sps/...01Bfq657442465

Unfortunately the ntl homepages are still down, and my jpg files are too big to attach on the forum. My email is now working so it looks like ntl are on the case. If you can't wait, and you can accept big attachments, send me a PM with your email address and I will send you the pictures directly. 8-)

Re: VHF Modulator wanted
 

The homepages appear to have been fixed. Quick, have a look before it falls over again! 8-\

Re: VHF Modulator wanted
 

Hi David,
I had a quick view on your schematic.
It is good that you are able to use the SDA/SCL!
This is my great problem for me!

I must look at the data sheet for the IC maybe there is a way to
use the audio section too.

You can use my video stage at the input pin 9 via a cap.
So the 3,5MHz filter problem is solved.

In the audio stage you don't need a tandem pot.
The other problem is the bias adjust via the emitter follower.
It must be temperature compensatet.
You can use a OPamp or a pnp npn emitter follower.

This is the way I would do this:

Re: VHF Modulator wanted
 

http://www.freescale.com/files/app_s...C44BS373CA.pdf

I think this is what Darryl is searching for.
Having a look at the pdf I noticed the IC makes not 100% video modulation.
So maybe there is a intercarier option for the sound.
But it seems so easy to use this IC so your idea David to use one
with the video input for audio is not a complicated way.
The audio carrier must be 7dBs under peak white.
I did not find anything about this in the pdf.
But it doesn't matter if you use a seperate IC for audio.
Hope the audio carrier can be switched off.

Kind regards
Darius

Re: VHF Modulator wanted
 

Trust me, there isn't! The sound-vision spacing can only have certain values and 3.5 isn't one of them! You could put an external audio subcarrier into the MC44BS373CA in the 20-pin version. Except the 20-pin version is unobtainable. :(

The problem with your circuit with the stereo mix to mono directly on the input, is if the source device (in my case a set top box) also feeds other places (my stereo amp). The mixer may reduce stereo separation on all destinations due to the output impedance of the source. I put the pot right on the input so that the system can't be overloaded.Good point! :)True, but think about what the diode detector in the average vintage TV will do to the signal if it reaches 100% mod!Not at the moment, but I can add that to the software. 8-)

Re: VHF Modulator wanted
 

Good evening David,


"The problem with your circuit with the stereo mix to mono directly on the input, is if the source device (in my case a set top box) also feeds other places (my stereo amp). The mixer may reduce stereo separation on all destinations due to the output impedance of the source. I put the pot right on the input so that the system can't be overloaded."

I don't hope you use this boxes with 5 scart sockets or more to split the
signal...
Normally in the set top box and DVD's it is made in the way I showed
it in the picture.
Do you loose signal at scart when you make a short a RCA?
Don't think so.

I ever try to use less components as possible in the sound stage.
Every bit damages the signal.

"True, but think about what the diode detector in the average vintage TV will do to the signal if it reaches 100% mod!"

100% is standard at 405. Michael toled me he had problems with a set
when he made less than 100%.
Maybe?!?
I tried less than 100% and I had no trouble.
I think the IC makes this to make intercarrier FM sound possible.

What kind of converter do you use?

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

http://www.freescale.com/files/app_s.../MC44BC373.pdf

Good morning David,

look what I have found.;) Maybe you can use this one with your soft!
It uses the MC44BS373.

What do you think about it? (Less than 50 Euros;D)

Kind regards

Darius

Re: Multi-Channel VHF Modulator Design
 

As the modulator is using the a PIC to program the chip(s), could the pic also be used to (optionally) output a very simple test card of some description too ? Just a basic cross hatch perhaps ?

TTFN,
Jon

Re: Multi-Channel VHF Modulator Design
 

Great idea. Any volunteers? ;D Interesting. You would need two of them to do vision and sound, and you would have to get the I2C data into the chips from the PIC.

Regarding your concerns about modulation depth, the Freescale data sheet quotes 93% typical which sounds pretty good. I decided to find out what this looked like in the real world. The attached photo shows the output from the vision detector on a typical TV (actually a Thorn 980 series). I have set the second trace to show where ground is. Of course the voltage drop in the detector diode (germanium in this case) increases the apparent modulation depth. I think this is known in technical terms as "near enough". :)

By the way the sound modulator in my design is programmed to negative vision modulation mode and will actually give 100% mod.

Re: Multi-Channel VHF Modulator Design
 

Hi,
the only thing you have to do is to change the soft in the red marked
IC.
Every thing else is " on board".
The chip includes a test pattern. It is all a question of the right software.
I am not able to change the soft.
But I think David is!

I don't think it is a problem with the modulation deep.
Main thing is you have a picture and maybe sound.

Kind regards
Darius.

Re: Multi-Channel VHF Modulator Design
 

Hi,
I am going to ask the people who made this thing on monday.
It is a german company.
Maybe it is possible to get a soft to change the modulation and
channels.

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

Good evening,
I made a picture of the I2C bus controller.
It was difficult to take a photo and I had to clean it to make
visible what is written on it.

Maybe It helps.
Do you know this David?
If not does anyone have a link to the datasheet?

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

It's a Zilog Z8 single chip microprocessor. There are loads of different types and I guess it's probably a "one time programmable" (OTP) part. This would mean replacing it competely in order to do a different program.

http://www.zilog.com/

Re: Multi-Channel VHF Modulator Design
 

Good evening Jeffrey,
I hoped the software is stored in the 24C82.
But I don't know enough about this things.

Maybe it is helpfull for David to have a PCB with the 44BC373.

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

David,

Yes the website is now up and running - just managed to have a nosey.

Interesting ....


Andy

Re: Multi-Channel VHF Modulator Design
 

Hi David,
I have a few ideas to use the AM sound section of the IC.

One way is to change the 4MHz XTAL.

But there is an other independent way I tested.

This IC uses an intercarrier sound mixing system.
You can see on the Analyzer two sound carriers one below
and one over the picture carrier.

This means the sound VCO works on 3,5 MHz not at the
high output frequency like the vision carrier does.

So you can use the 3,5 MHz sound osc without a PLL.
You have to put a DC voltage from a voltage divider
to the input SPLLFLT. I used a 10uF block and a 1K variable
resistor and 1K2 to VCCA. (ca.0,9V)
The frequency stability is much better than I expectet!

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

I have now drawn the schematic in OrCAD, saved as PDF, which is a lot easier to read than the previous photos. There is one more thing outstanding namely the 3.5MHz notch filter on the video input. I am just waiting for parts to arrive, then I'll update the drawing again.

Re: Multi-Channel VHF Modulator Design
 

Hi David,
thanks for posting your schematic. Looks very easy. The most work
is to programm the PIC. I am not able to do this.
Congratulationes:)



Please look at the basis of the PNP audio emitterfollower.
The input resistance is ca. 100K Ohms, so a 0,1uF cer. cap
is better than the 10uF electrolytic. The leak current of this cap
can change the bias! (picture thread 8)
It is better to place the electrolytic from the bias pot to ground
to block noise from the 5V.
The bias poti is not a level adjust.

I am sure your PIC will work in the axing modulator too.
Did you switch off the audio carrier or not?
Please note that the audio carrier level should be 7dB under peack
white of the vision carrier.

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

Hi all,

I agree with Darius about the Electrolytic as a coupler and the Bias de-coupling, I use Tantalums in my circuits.

Darius, the Bias pot will set the "resting" level of the sound carrier(which is really coming from a vision modulator with it's clamp overidden), with it you can set the ratio of the sound to vision carriers correctly.

On Domino I did not provide two L/R inputs for sound, I assumed only mono vintage material would be played, most Vintage DVD TV stuff is mono!.

I have never seen/heard the need for a 3.5MHz filter on the video feed, your video should be band limited to ~3.0MHz, is this somthing that you have found is a problem, maybe with 405 line VHS playback?.

Malcolm

Re: Multi-Channel VHF Modulator Design
 

Hi Malcolm,
not from VHS of course, but from the camera. My camera makes
vision components at 3,5MHz.
The converter too if you have no filter at the output of the DA converter.

The bias adj. must be set 0,5V higher than the sync tips would be.
(Hope you can understand my english)
Than you can use the full dynamic of the modulator and the carrier
is 6 dB under peak white. Of course the bias changes the audio level
too. But is it not the audio level adjustment!!!

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

If you want to build this design I can send you the programming file. You just need a PIC programmer (mine cost about £50).The cap I used is a ceramic chip (surface mount) not an electrolytic. The leakage is very low.Good point although I haven't noticed a problem.Well, keep this to yourself as it is very hush-hush, but I am developing a standards converter too, and this produces the input video bandwidth times 405/625 which is a bit more than 3MHz... :)

Re: Multi-Channel VHF Modulator Design
 

10uF????

Darius

Re: Multi-Channel VHF Modulator Design
 

Hi all in Telly land..

10µF in ceramic is get-able but at £1.20 each (1210 size, Farnell 578-289) thats a costly coupling cap! and you show three of them.

David_Robinson said..
"Well, keep this to yourself as it is very hush-hush, but I am developing a standards converter too, and this produces the input video bandwidth times 405/625 which is a bit more than 3MHz... "

Ooh er!, do tell, will this be a design you publish or something you will make to sell?.

I may have to add a "Go Faster Stripe" to Domino :-) or make a ++Version!

Malcolm

Re: Multi-Channel VHF Modulator Design
 

http://www.voti.nl/swp/

Good evening David,

thanks for your offer.

First I have to learn something about PIC's
I found some fundamental infos on this website.

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

Blimey! You're right, that is expensive. I used Farnell 471-6711 at £0.12 each.To be honest I haven't really thought that far ahead and I'm some way from completion at the moment. :)

Re: Multi-Channel VHF Modulator Design
 

Hi all..

Yes Dave you’re right, upon further digging in the catalogue (Tiny print to go with Tiny components!) I see the range of SM Caps, it says there intended as an alternative to Electrolytic and Tantalum capacitors and cheaper too.

Very impressive, I didn’t know there was the capacity to make capacitors with that capacitance in cases with such a small capacity!!.

Apart from the ADC and Frame FIFO’s the only SMD in Domino is a BB149A varicap diode, it’s ‘the’ most fiddly part to fit!, I’ve often twanged one off into the land of the lost never to be seen again.

Dave said,
'To be honest I haven't really thought that far ahead and I'm some way from completion at the moment.'

I’ll hold off on the ‘go faster stripe and UV under lights’ then, please tell me your not using the dreaded ‘Xilinx’ or some such device in your converter..
(quiet Jeff B will hear! :-) ).

Malcolm

Re: Multi-Channel VHF Modulator Design
 

Hi Malcolm, have you tried the search facilities on the Farnell website? They are excellent - you can search for a 10µF capacitor and then sort the results by price. No tiny print here... As I'm sure you know, prices are typically 20% below RS.

Xilinx (spit)? No no, I'm using the market leader, ALTERA! (Sorry Jeffrey). 100 pin chips with 0.5mm pin spacing - steady hand required.

I'm all too aware though, that it's one thing to build a working prototype but it's an order of magnitude more impressive to produce a marketable product with a proper PCB and all that. I leave that kind of thing to others. :)

Re: Multi-Channel VHF Modulator Design
 

OK, attached is the latest version of the modulator schematic. This includes the 3.5MHz notch filter, if anyone wants it.Not at all - have a look at http://homepage.ntlworld.com/david.r...20Majestic.JPG . A 10" loudspeaker and 1 watt "undistorted" output. What more could you want? 8-)

Re: Multi-Channel VHF Modulator Design
 

Hi David,

why did you use an NPN in the video buffer?

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

Hi Darius

The video buffer is there so that the input cable sees a more accurate 75Ω termination. This avoids any reflections in the cable.Sorry I did not answer this question before. Yes, the sound carrier on both modulator chips is switched off. :)

Re: Multi-Channel VHF Modulator Design
 

Good morning David,

no question that you need a buffer.
But why an NPN and not a PNP Transistor?

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

I don't think it makes much difference either way. In theory an NPN will give better rejection of noise on the +5v rail, but I doubt you could measure the difference. The PIC doesn't run continuously, so there's really nothing to make any noise... 8-)

Re: Multi-Channel VHF Modulator Design
 

It really doesn't make much difference for an emitter follower (unlike a common emitter stage where the input and output are referenced to different power rails). A small amount of muck on the +ve rail can be injected via the bias chain for PNP or NPN. For NPN there is virtually no sensitivity to the +ve rail. For PNP it's a very slight sensitivity due to the finite output impedance.

Re: Multi-Channel VHF Modulator Design
 

Hi,

it is not the noise from the DC that makes an NPN a better choise.
With your biasing the output resistance is max 7 Ohms.

There is an other problem.
After this stage the DC restorer followes.
There goes a high current through the 10nF at the sync tips.
So you must make sure that there is a strong source to pull
down!
A NPN emitter follower is a strong source to pull up and
a PNP emitter follower is a strong source to pull down.
This is a very importand aspect.:)

In your case the 470 Ohms makes the pull down.
So if you have a DC restorer as a load and a negative sync
it is better to take a PNP !!!;)

The output resistance of the emitter follower should be as low
as possible to make sure that the sync will not be damaged
or gets a level reduce.

You placed a 180 Ohms at the output of the emitter follower
and this will be added to the output resistance.

In front of the DC restorer this is a bad mistake.:( It is a DC restorer
not a sync separator!!!

There are a lot of commercial modulators that makes trouble at the top of
the picture because of sync level changes and other mistakes
caused by a wrong driven DC restorer.

I hope I was able to show you that there is a big difference in
the use of a PNP or NPN emitter follower.
In this case the PNP is the better one.:D

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

I was looking at the audio emitter followers. For video you need to take account of the load as Darius correctly says. If the load is a DC restorer, this can take high-ish current pulses at the sync tips so for -ve syncs a PNP follower is better. If the input of the modulator chip is a keyed clamp the current pulses could be of either polarity. The series resistance definitely does not help a DC restorer but won't make much difference to a keyed clamp unless you are trying to make a very hard clamp. You do not want a hard clamp at the input to a modulator as it will distort the colour burst.

Re: Multi-Channel VHF Modulator Design
 

HI all,

David's KB Majestic looks well-kept, but can we see it on 405 lines! (and with real telly, not this set-top box muck!).

PNP, NPN, why not have a NPN on Tuesdays and Thursdays, PNP all other times!.

No but really, if you must have one (filter that is! and you don’t need it really) put the filter before the base with a stopper resistor like you have, maybe a series L/C type rather that a T filter, it (the MC44..) may be happier with a PNP as a source as the internals of the device (P31 of data sheet) shows a 'NPN emitter clamp' but have a look they show 500 ohm as a series feed!.

Jeff, remember the modulator is working on the whole of the video waveform, it 'clamps' the sync tips to a known level to suit its modulation so back porches and colour bursts are way up there from the sync tip level.

This goes back to what I said about the bandwidth of the video you feed to the modulator, it shouldn’t have anything near 3.5MHz in it!, make the output of the converter fit the 405 standard, then all is well.

The 'Gold Standard' Dinosaur converter has a 3.0MHz filter in its video output stage with a fancy feedback restorer in the modulator and that produces very good results.

Malcolm

Re: Multi-Channel VHF Modulator Design
 

Trust me Malcolm, that photo does show 405 lines! Thanks for the unwitting compliment to my standards converter / modulator. ;D

Darius, you make some good points. I do like a real technical discussion!

You could add another emitter follower between the filter and the modulator chip, but as Malcolm has already pointed out, there is 500Ω in series with the video inside the MC44 chip anyway so I don't think it would make much difference.

So how significant are these 'high-ish pulse currents'? The video input current is quoted as 1µA maximum. To supply this current on average, the current during syncs (just taking line syncs for a ball-park figure) will be

1 * 98.8 / 9 = 11µA approx.

The drop across (180 + 68 + 500) Ω will be 11µ * 748 = 8.2mV which is 2.7% of the sync pulse amplitude, worst case (0.6% typical). There is no cause for concern!

As for the ability of my emitter follower to supply this current, the drop across its emitter resistor caused by this pulse current will be

11µ * 470 = 5.2mV

This will not come anywhere near to cutting the transistor off so the difference from a PNP emitter follower will be negligible.

Malcolm's suggestion of NPN on Tuesdays and Thursdays is therefore vindicated.

Phew - I think I need a lie down! 8-)

Re: Multi-Channel VHF Modulator Design
 

So how significant are these 'high-ish pulse currents'? The video input current is quoted as 1µA maximum. To supply this current on average, the current during syncs (just taking line syncs for a ball-park figure) will be

Hi David,
congrats to your converter picture.

If you watch a test pattern your calculation is correct.:)

The problems take place when you have a jump from a black to
a white picture. When the resitance of the driver is too high,
the sync pulses get smaller for a moment.

When the current (the 1uA) is to low or the cap is to big,
the sync tips lift up from the clamp.
When the cap is to low the picture gets dark from left to right.

You must have a pattern generator where you can switch directly
from a black to a white and vise versa picture!;)

With the 500 ohms in the IC is a big problem.
You'll never be satisfied.

The best is to generate a clamp current only at the sync tips.
You can see this in my first converter at the input of the cxl's.

In my new converter I had that problem with the input resistor
too. >:(
In the M7403 ccd there is a resistor in front of the clamp too.
The only way to solve this problem is to make a seperate
DC restorer in front of it. And I made this:D.

Don't forget the cap is made for 625 at 405 you must
take 15nF at pin 9.
Look with the oscilloscope at 100% white
that the "line" does not fall from the beginning to the end of
the line. You first see this on the oscilloscope bevore you notice
it in the picture.


Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

Good afternoon Darius

Thanks for your suggestions. I have a Philips PM5570 generator that will do black to white jumps (0.25Hz), and many other functions. I will do a few experiments with this and then I will post again. :)

Re: Multi-Channel VHF Modulator Design
 

Hi David,
the problem is the changes of the sync takes place at the DC-restorer
and this is in the IC! So you must look for sync level changes at
the output of the vision demodulator in the TV set.
An eventually reduce of brightness from left to right can you measure
at pin 9 I think.

Kind regards
Darius

PS: The picture shows the input of the cxl's.
the clamp current comes from the LM1881 ony at the sync tips.
So the cap at the input will not be discharged during a line.

Re: Multi-Channel VHF Modulator Design
 

Me again!

David said,
'Trust me Malcolm, that photo does show 405 lines! Thanks for the unwitting compliment to my standards converter / modulator.'

Very good, but a 'real picture' would look better than that on screen message! and for a treat you could show us Test Card C.
(and what is on 6 music?, I cant get my DAB off 5 Live!)
I extended the blanking to chop of the converted line 23, gets rid of the WSS etc.

Not takers for the 'don’t have a filter' idea then?

And how about using a FET, maybe just on Sundays!...just teasing :-)

Malcolm

Already lying down!

Re: Multi-Channel VHF Modulator Design
 

Transistors? What are they? Don't people use opamps these days?

A client actually remarked to me that I'd used a transistor in one of my designs for him! You had to search for it between all the chips.

Use a cathode follower. It's more musical;D

If you've got a converter that really does have significant energy output above 3MHz then I suppose you ought to filter it to get rid of vision on sound. The acid test is listening to it on a good telly.

And I counted every single line and I claim a refund. I could only find about 380:o ;D :-)

Re: Multi-Channel VHF Modulator Design
 

Well I haven't got a full resolution copy of TCC so I hope TCF will do. I get big problems trying to take off screen photos - my camera keeps strobing out the interlace. >(( After many attempts this is the best I can do.

http://homepage.ntlworld.com/david.r...20Card%20F.JPG

The whole picture is really interlaced, honest guv.<off topic> Well it's all a matter of taste, but for me it's the station I've been waiting for. Of course you have to avoid the breakfast show (why do music channels feel the need for a show that's all talk?) but after 10am it's fab. None of that rap they have on Radio 1 (don't panic mods, I said RAP :-) ).</off topic>Yeah I know, I said it wasn't finished... :)Seems not!Of course it will have to be a set with proper gated AGC, to do the 0.25Hz test, none of that mean level AGC rubbish. Fortunately I have one such set. :) I have made the time constant on the base of the emitter follower nice and long, so the clamp is not presented with rapid changes in the sync tip voltage.

So how much white droop can we expect?

dV = I * dT / C

dV = 1µA * 80.55µs / 10nF = 8.1mV = 1.2% of (white - black) (worst case) (0.2% typical).

The result is attached with a 50Hz bar pattern from the Philips PM5570. As you can see any tilt on the white bar is negligible. I can't show you the 0.25Hz bounce test in a still photo, but I can tell you there was no problem :)

Re: Multi-Channel VHF Modulator Design
 

Good evening David,
Your test card looks really very good:)

dV = 1µA * 80.55µs / 10nF = 8.1mV = 1.2% of (white - black) (worst case) (0.2% typical).

I am not satisfied with 10nF.
You have more than 100 grey steps in a picture so I don't think
this is acceptable.
In your oscilloscope picture I can see that the
white level goes a little bit down. At the beginning is
the marking line of your oscilloscope in the middle and at the end at the top.
I think 15nF will make it better. I have a test pattern on DVD which makes this mistake visible.


so the clamp is not presented with rapid changes in the sync tip voltage.

I don't understand it. Does this mean the sync tips run away from the
clamp level with rapid changes? Where did you measure this?

Kind regards
Darius

Re: Multi-Channel VHF Modulator Design
 

I was referring to the time constant of C3 with R10 and R12. Following a sudden change in average picture level, this time constant causes a drift of the dc level which the clamp has to correct. For this to work, the clamp time constant has to be much shorter than the C3 one.

David