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video info below the carrier in analog channel?

Started by dtsao March 6, 2008
Hi,

in an analog channel, I know the video carrier is 1.25 Mhz above the
bottom of the channel. Also the color and audio info are 3.58Mhz and 4.5
Mhz above this video carrier. But I wonder, is there any info below the
carrier, between the bottom of the channel and the 1.25Mhz? My guess is
no, but I'm not sure.
dtsao wrote:
> Hi, > > in an analog channel, I know the video carrier is 1.25 Mhz above the > bottom of the channel. Also the color and audio info are 3.58Mhz and 4.5 > Mhz above this video carrier. But I wonder, is there any info below the > carrier, between the bottom of the channel and the 1.25Mhz? My guess is > no, but I'm not sure.
What signal are you referring to? It can't be NTSC composite video; that has the color subcarrier at 3.58 MHz and the sound carrier at 4.5 MHz. Since you refer to channel boundary, I presume you mean RF. An analog TV channel is 6 MHz wide, with the video carrier 1 MHz abofe the lower edge. Using vestigial sideband modulation, the luminance information ideally begins .75 MHz below the carrier and reaches full amplitude .75 MHz above it. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������

dtsao wrote:

> Hi, > > in an analog channel, I know the video carrier is 1.25 Mhz above the > bottom of the channel. Also the color and audio info are 3.58Mhz and 4.5 > Mhz above this video carrier. But I wonder, is there any info below the > carrier, between the bottom of the channel and the 1.25Mhz? My guess is > no, but I'm not sure.
Analog video uses AM hence the lower sideband is the mirror image of the upper sideband. They cut the big part of the lower sideband to reduce the bandwidth of the channel. However they couldn't cut the LSB completely, because the analog bandpass filter will introduce the phase distortion. If the cutoff of the filter will be at the low video frequencies, that distortion will be very noticeable, and it will affect the sync pulses, too. So they choose to leave the residual low sideband with the cut off at the compromise value of 1.25MHz. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Vladimir Vassilevsky wrote:

(snip)

> Analog video uses AM hence the lower sideband is the mirror image of the > upper sideband. They cut the big part of the lower sideband to reduce > the bandwidth of the channel. However they couldn't cut the LSB > completely, because the analog bandpass filter will introduce the phase > distortion. If the cutoff of the filter will be at the low video > frequencies, that distortion will be very noticeable, and it will affect > the sync pulses, too. So they choose to leave the residual low sideband > with the cut off at the compromise value of 1.25MHz.
And, if I remember it right, some filtering is done in the receiver to compensate for the above. But the OP asked if there was any information down there. I always did wonder how cheap home RF modulators did this, or even the ones built into VCRs. -- glen

glen herrmannsfeldt wrote:

> Vladimir Vassilevsky wrote: > >> Analog video uses AM hence the lower sideband is the mirror image of >> the upper sideband. They cut the big part of the lower sideband to >> reduce the bandwidth of the channel. However they couldn't cut the LSB >> completely, because the analog bandpass filter will introduce the >> phase distortion. If the cutoff of the filter will be at the low video >> frequencies, that distortion will be very noticeable, and it will >> affect the sync pulses, too. So they choose to leave the residual low >> sideband with the cut off at the compromise value of 1.25MHz. > > And, if I remember it right, some filtering is done in the receiver > to compensate for the above.
You are correct. There is a frequency boost above 1.25MHz after the detector.
> I always did wonder how cheap home RF modulators did this, > or even the ones built into VCRs.
They don't bother with filtering. If the modulator is on, then the adjacent channels don't matter for you. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Ok, I'm not sure if I am getting this 100%. I will try to repeat what you
are saying (please correct me if I'm wrong).
Basically, the data is symmetric around the video carrier, but on the
bottom only goes 1.25Mhz below the carrier. Anything beyond 1.25Mhz below
the carrier is not used to save on bandwidth, since that info is contained
above the carrier anyway...?
So why would you need to do some frequency boost? When you mention
"detector", does this mean tuner or demodulator?

>> Vladimir Vassilevsky wrote:
>You are correct. There is a frequency boost above 1.25MHz after the >detector. >
Before you get beat up by various members of group with weird senses of 
"humor", search out definition(s) of "detector"and "demodulator" ;)

Seriously, you'll learn more by searching for than by being given answer.

If the answers you get don't make sense, come back asking questions 
BASED ON YOUR RESULTS.


dtsao wrote:
> Ok, I'm not sure if I am getting this 100%. I will try to repeat what you > are saying (please correct me if I'm wrong). > Basically, the data is symmetric around the video carrier, but on the > bottom only goes 1.25Mhz below the carrier. Anything beyond 1.25Mhz below > the carrier is not used to save on bandwidth, since that info is contained > above the carrier anyway...? > So why would you need to do some frequency boost? When you mention > "detector", does this mean tuner or demodulator? > > >>>Vladimir Vassilevsky wrote: > > >>You are correct. There is a frequency boost above 1.25MHz after the >>detector. >> > >