Reply by Jerry Avins February 21, 20082008-02-21
glen herrmannsfeldt wrote:
 > Jerry Avins wrote:
 > (snip)
 >
 >> Separating the sound from the video went out well before color came 
on the scene. By taking the sound after the video detector, the 4.5 MHz 
frequency is not affected by any fine tuning of the LO. It used to be 
that the sound would be distorted if the picture was, but not with what 
is called intercarrier sound.
 >
 >> The detector beats the sideband to baseband using the TV carrier as 
the beating signal. All rectifying (and some other) AM detectors do that.
 >
 > Which one uses fewer tubes?

Most of my cousin's tube detectors used only one tube. There was one in 
my TV set that needed touching up, so I got the schematic out of the 
library. It was kind of weird. The sound carrier was fed to the control 
grid of a pentode. There was a passive tank circuit tied between the 
suppressor grid and ground, a bias arrangement that kept the tube near 
cutoff when quiescent. Audio was pulled off the plate, which was 
bypassed by a capacitor. How?

I eventually figured it out. The tank was (supposed to be) tuned to the 
4.5 MHz. The positive parts of the swing made pulses, the negative 
swings simply drove the tube further into cut-off. It was a plate 
detector with a twist. The tank circuit was the twist. Its Q was high 
enough so that it rang at the average carrier frequency. As the 
frequency of the input varied, the pulses got wider or narrower, 
changing the average DC in the pulses.

It turned out that ten years of component drift had thrown the tank too 
far out of tune. I was able to restore it by ear. I dubbed it a flywheel 
detector, the tank being the flywheel. I took the schematic to my 
cousin, the TV expert in the family (check it out: the discriminator 
patent is Avins and Seely) and asked him if he had ever heard of the 
weird thing, and did it work the way I thought. I suspect he was 
offended; after all, it was an RCA set. He told me the correct name 
(which I forget) and that it was one of his.

As for the video detector, all that needs is a diode.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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Reply by Jerry Avins February 21, 20082008-02-21
dtsao wrote:
> This is why I was confused. I was not sure if a demodulator is basically a > mixer that puts the carrier to baseband, or if there was something else > involved in the demodulation process. > Because sometimes I see demodulators that take 2 inputs. One is from the > tuner output at IF, and one input to the demod is composite video > itself... So I guess when it takes in composite video, it is the tuner > that has already mixed it to baseband and the demod just passes it > through. But the 2nd input to the demod is attached to the tuner in case > it has to take an input at IF (as an option too)...?
There is a mixer. The inputs are RF and a local oscillator (LO). the output contains sum and difference frequencies; one of them is selected by a bandpass filter to be the intermediate frequency (IF). The process is called "heterodyning" and a receiver bilt on that model is called a "superheterodyne". There are advantages and drawbacks to the design (invented by Edwin Armstrong of FM fame), For a long time, the advantages far outweighed the disadvantages. Direct-conversion receivers are regaining popularity as modern methods change that balance. There may be IF and composite video inputs to a TV, but the composite goes where the output of the demodulator goes, not to the demodulator's input. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by Jerry Avins February 21, 20082008-02-21
glen herrmannsfeldt wrote:
> Jerry Avins wrote: > (snip) > >> Separating the sound from the video went out well before color came on >> the scene. By taking the sound after the video detector, the 4.5 MHz >> frequency is not affected by any fine tuning of the LO. It used to be >> that the sound would be distorted if the picture was, but not with >> what is called intercarrier sound. > >> The detector beats the sideband to baseband using the TV carrier as >> the beating signal. All rectifying (and some other) AM detectors do that. > > Which one uses fewer tubes?
Most of my cousin's tube detectors used only one tube. There was one in my TV set that needed touching up, so I got the schematic out of the library. It was kind of weird. The sound carrier was fed to the control grid of a pentode. There was a passive tank circuit tied between the suppressor grid and ground, a bias arrangement that kept the tube near cutoff when quiescent. Audio was pulled off the plate, which was bypassed by a capacitor. How? I eventually figured it out. The tank was (supposed to be) tuned to the IF frequency. The positive parts of the swing made pulses, the negative swings simply drove the tube further into cut-off. It was a plate detector with a twist. The tank circuit was the twist. Its Q was high enough so that it rang at the average carrier frequency. As the frequency of the input varied, the pulses got wider or narrower, changing the average DC in the pulses. It turned out that ten years of component drift had thrown the tank too far out of tune. I was able to restore it by ear. I dubbed it a flywheel detector, the tank being the flywheel. I took the schematic to my cousin, the TV expert in the family (check it out: the discriminator patent is Avins and Seely) and asked him if he had ever heard of the weird thing, and did it work the way I thought. I suspect he was offended; after all, it was an RCA set. He told me the correct name (which I forget) and that it was one of his. As for the video detector, all that needs is a diode. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by dtsao February 21, 20082008-02-21
This is why I was confused. I was not sure if a demodulator is basically a
mixer that puts the carrier to baseband, or if there was something else
involved in the demodulation process.
Because sometimes I see demodulators that take 2 inputs. One is from the
tuner output at IF, and one input to the demod is composite video
itself... So I guess when it takes in composite video, it is the tuner
that has already mixed it to baseband and the demod just passes it
through. But the 2nd input to the demod is attached to the tuner in case
it has to take an input at IF (as an option too)...?


>If the output is at baseband, it is already demodulated. How not? > >Jerry >-- >Engineering is the art of making what you want from things you can get. >����������������������������������������������������������������������� >
Reply by glen herrmannsfeldt February 21, 20082008-02-21
Jerry Avins wrote:
(snip)

> Separating the sound from the video went out well before color came on > the scene. By taking the sound after the video detector, the 4.5 MHz > frequency is not affected by any fine tuning of the LO. It used to be > that the sound would be distorted if the picture was, but not with what > is called intercarrier sound.
> The detector beats the sideband to baseband using the TV carrier as the > beating signal. All rectifying (and some other) AM detectors do that.
Which one uses fewer tubes? -- glen
Reply by Jerry Avins February 20, 20082008-02-20
dtsao wrote:
> So if the Tuner output is an analog channel at baseband that has gone > through a low pass filter at 4Mhz (for CVBS) and a bandpass filter around > 4.5Mhz (for SIF), can we bypass the demodulator completely and connect > these directly to a video and audio processor? Or do we still need to > connect the tuner to a demod for some reason?
If the output is at baseband, it is already demodulated. How not? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by Jerry Avins February 20, 20082008-02-20
glen herrmannsfeldt wrote:
> dtsao wrote: > >> I am confused about the exact function of an analog TV demodulator. >> Does a >> demodulator for an analog channel take as its inputs already separated >> CVBS >> and Sound IF signals, or does it take as its Input the the full analog >> channel (including video and sound) at baseband (or IF?) and then >> filter/separate the CVBS and Sound IF as it's Outputs? > > I believe there are two ways, both having been used in production. > > One separates the audio and video at IF and separately demodulates > them, the other mixes down and separates the audio out later. > > In any case, at some point the video signal with the color subcarrier > in place is sent to the video processing section.
Separating the sound from the video went out well before color came on the scene. By taking the sound after the video detector, the 4.5 MHz frequency is not affected by any fine tuning of the LO. It used to be that the sound would be distorted if the picture was, but not with what is called intercarrier sound. The detector beats the sideband to baseband using the TV carrier as the beating signal. All rectifying (and some other) AM detectors do that. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by dtsao February 20, 20082008-02-20
So if the Tuner output is an analog channel at baseband that has gone
through a low pass filter at 4Mhz (for CVBS) and a bandpass filter around
4.5Mhz (for SIF), can we bypass the demodulator completely and connect
these directly to a video and audio processor? Or do we still need to
connect the tuner to a demod for some reason?
Reply by Mark February 20, 20082008-02-20
On Feb 20, 1:35&#2013266080;pm, "dtsao" <tsaod...@yahoo.ca> wrote:
> Hi, > > I am confused about the exact function of an analog TV demodulator. Does a > demodulator for an analog channel take as its inputs already separated CVBS > and Sound IF signals, or does it take as its Input the the full analog > channel (including video and sound) at baseband (or IF?) and then > filter/separate the CVBS and Sound IF as it's Outputs? > Take for example the following process: > 1) Take an analog TV channel at IF, then mix it down to baseband, so that > the video carrier falls at DC. The sound carrier is then at 4.5 Mhz. > 2) Use a low pass filter and band pass filter to separate the CVBS and > Sound IF. > I have seen the above as part of TV tuners which should then go on to > demodulators. But if the tuner is already separating the video and sound, > then what will the demodulator do with the CVBS and SIF outputted from the > tuner? > Demodulation is not exactly the same as mixing down to baseband, is it?? > (otherwise what's the difference between a demod and a mixer) > Thanks.
most demods will demodulate the main video carrier to create video and 4.5 MHz subcarrier and then FM demodulate the 4.5 MHz subcarrier to create audio. This is called the "intercarrier" archectecture. If you demodulate the RF sound carrier directly to audio (split carrier) you may have problems due to residual FM in the RF tuners and transmitters. The intercarrier arch eliminates this problem becasue any residual FM is applied to both the sound and video carrier together so that the 4.5 MHz delta is not impacted. Mark
Reply by glen herrmannsfeldt February 20, 20082008-02-20
dtsao wrote:

> I am confused about the exact function of an analog TV demodulator. Does a > demodulator for an analog channel take as its inputs already separated CVBS > and Sound IF signals, or does it take as its Input the the full analog > channel (including video and sound) at baseband (or IF?) and then > filter/separate the CVBS and Sound IF as it's Outputs?
I believe there are two ways, both having been used in production. One separates the audio and video at IF and separately demodulates them, the other mixes down and separates the audio out later. In any case, at some point the video signal with the color subcarrier in place is sent to the video processing section. -- glen