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-   Television Standards Converters, Modulators etc (https://www.vintage-radio.net/forum/forumdisplay.php?f=95)
-   -   Multi-Channel VHF Modulator Design (https://www.vintage-radio.net/forum/showthread.php?t=1940)

af024 22nd Feb 2005 8:13 pm

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

David_Robinson 23rd Feb 2005 1:05 pm

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

af024 23rd Feb 2005 6:03 pm

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

David_Robinson 24th Feb 2005 8:36 am

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. >:(

oldeurope 24th Feb 2005 10:05 am

Re: VHF Modulator wanted
 
1 Attachment(s)
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

David_Robinson 24th Feb 2005 11:23 am

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-)

David_Robinson 24th Feb 2005 1:27 pm

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

oldeurope 24th Feb 2005 2:46 pm

Re: VHF Modulator wanted
 
1 Attachment(s)
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:

oldeurope 24th Feb 2005 3:40 pm

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

David_Robinson 24th Feb 2005 4:20 pm

Re: VHF Modulator wanted
 
Quote:

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

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. :(

Quote:

In the audio stage you don't need a tandem pot.
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.
Quote:

...the bias adjust via the emitter follower. It must be temperature compensatet.
Good point! :)
Quote:

...the IC makes not 100% video modulation.
True, but think about what the diode detector in the average vintage TV will do to the signal if it reaches 100% mod!
Quote:

Hope the audio carrier can be switched off.
Not at the moment, but I can add that to the software. 8-)

oldeurope 24th Feb 2005 5:04 pm

Re: VHF Modulator wanted
 
1 Attachment(s)
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

oldeurope 25th Feb 2005 9:39 am

Re: Multi-Channel VHF Modulator Design
 
5 Attachment(s)
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

Duke_Nukem 25th Feb 2005 12:14 pm

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

David_Robinson 25th Feb 2005 1:14 pm

Re: Multi-Channel VHF Modulator Design
 
1 Attachment(s)
Quote:

Originally Posted by Duke_Nukem
Just a basic cross hatch perhaps ?

Great idea. Any volunteers? ;D
Quote:

Originally Posted by oldeurope
look what I have found.

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.

oldeurope 25th Feb 2005 1:46 pm

Re: Multi-Channel VHF Modulator Design
 
1 Attachment(s)
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.

oldeurope 25th Feb 2005 2:27 pm

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

oldeurope 25th Feb 2005 5:03 pm

Re: Multi-Channel VHF Modulator Design
 
2 Attachment(s)
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

ppppenguin 25th Feb 2005 5:12 pm

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/

oldeurope 25th Feb 2005 5:26 pm

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

af024 25th Feb 2005 6:04 pm

Re: Multi-Channel VHF Modulator Design
 
David,

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

Interesting ....


Andy


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