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.
>>
>
>
Reply by dtsao●March 7, 20082008-03-07
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.
>
Reply by Vladimir Vassilevsky●March 6, 20082008-03-06
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
Reply by glen herrmannsfeldt●March 6, 20082008-03-06
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
Reply by Vladimir Vassilevsky●March 6, 20082008-03-06
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
Reply by Jerry Avins●March 6, 20082008-03-06
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.
�����������������������������������������������������������������������
Reply by dtsao●March 6, 20082008-03-06
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.