Loopthrough amplifies signal?

I happened to have an analogue programme on TV when I disconnected the power
lead from my Sagem IDT602. The picture quality deteriorated somewhat, and
became slightly snowy. On reconnecting the power, the picture quality
returned to normal.

On opening the Sagem, there was a small screened box, about the size of a
modulator (although the Sagem doesn't have one) on the circuit side of the
aerial in/out coax plugs. It was a Samsung TDCC2305TV34A - a single
conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that - at
least in the case of the Sagem - a small amount of gain is introduced by the
Samsung tuner to perhaps compensate for the loss in signal through the
loopthrough.

Is that correct, or am I way off beam?

(Oh, by the way, I opened the Sagem because it has suddenly lost colour in
the signal from Scart 2. So It's fine if I am happy with B/W pictures from
several channels via the inbuilt timer. But if not, I'm stuck with a single
channel, and the box has to be left on when I need to make a recording).

--
Jeff
(cut "thetape" to reply)

Jeff Layman wrote:
I happened to have an analogue programme on TV when I disconnected the power
lead from my Sagem IDT602. The picture quality deteriorated somewhat, and
became slightly snowy. On reconnecting the power, the picture quality
returned to normal.

On opening the Sagem, there was a small screened box, about the size of a
modulator (although the Sagem doesn't have one) on the circuit side of the
aerial in/out coax plugs. It was a Samsung TDCC2305TV34A - a single
conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that - at
least in the case of the Sagem - a small amount of gain is introduced by the
Samsung tuner to perhaps compensate for the loss in signal through the
loopthrough.

Is that correct, or am I way off beam?

(Oh, by the way, I opened the Sagem because it has suddenly lost colour in
the signal from Scart 2. So It's fine if I am happy with B/W pictures from
several channels via the inbuilt timer. But if not, I'm stuck with a single
channel, and the box has to be left on when I need to make a recording).

--
Jeff
(cut "thetape" to reply)

Yes, I've noticed the same thing with the two freeview boxes I have
owned (Ferguson FDT2000-this has a modulator, and a Digifusion FVRT200)
and I would be interested to hear the reason for this.

"Jeff Layman" wrote in message
...
I happened to have an analogue programme on TV when I disconnected the
power lead from my Sagem IDT602. The picture quality deteriorated
somewhat, and became slightly snowy. On reconnecting the power, the
picture quality returned to normal.

On opening the Sagem, there was a small screened box, about the size of a
modulator (although the Sagem doesn't have one) on the circuit side of the
aerial in/out coax plugs. It was a Samsung TDCC2305TV34A - a single
conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that - at
least in the case of the Sagem - a small amount of gain is introduced by
the Samsung tuner to perhaps compensate for the loss in signal through the
loopthrough.

Is that correct, or am I way off beam?

(Oh, by the way, I opened the Sagem because it has suddenly lost colour in
the signal from Scart 2. So It's fine if I am happy with B/W pictures
from several channels via the inbuilt timer. But if not, I'm stuck with a
single channel, and the box has to be left on when I need to make a
recording).


The best passive splitter will attenuate the signal by about -4db,
so putting an amplifier of the same ballpark gain before the split
is wise.

Is there an option to switch SCART 2 to output S-Video?
The lumanance signal uses the same pin as composite video would
normally use, maybe that accounts for the monochrome output.
--

Graham.

%Profound_observation%

I thought that a simple loopthrough was just that. But it seems that - at
least in the case of the Sagem - a small amount of gain is introduced by the
Samsung tuner to perhaps compensate for the loss in signal through the
loopthrough.

Is that correct, or am I way off beam?

Yes thats correct. Its always good engineering practice to at least try to
ensure 0dB loss through a unit and a couple of dB's extra will always help.

Dave

In article .com,
writes

Jeff Layman wrote:
I happened to have an analogue programme on TV when I disconnected the power
lead from my Sagem IDT602. The picture quality deteriorated somewhat, and
became slightly snowy. On reconnecting the power, the picture quality
returned to normal.

On opening the Sagem, there was a small screened box, about the size of a
modulator (although the Sagem doesn't have one) on the circuit side of the
aerial in/out coax plugs. It was a Samsung TDCC2305TV34A - a single
conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that - at
least in the case of the Sagem - a small amount of gain is introduced by the
Samsung tuner to perhaps compensate for the loss in signal through the
loopthrough.

Yes, I've noticed the same thing with the two freeview boxes I have
owned (Ferguson FDT2000-this has a modulator, and a Digifusion FVRT200)
and I would be interested to hear the reason for this.

The internal tuner has a much less than infinite input impedance and
loads the antenna line, therefor a small amount of gain is introduced to
compensate for this. It is precisely because of the consequent
degradation in signal/noise that I used a distribution amp instead of
daisy-chaining all my items.

--
Ian G8ILZ

"Jeff Layman" wrote in message
...
I happened to have an analogue programme on TV when I disconnected the
power lead from my Sagem IDT602. The picture quality deteriorated
somewhat, and became slightly snowy. On reconnecting the power, the
picture quality returned to normal.

I think all loopthroughs (VCRs, STBs etc) amplify to compensate for the
losses due to the signal taken. In fact I'm surprised it worked at all with
the power to the STB off, as an amplifier with the power off usually hardly
allows anything through.

--
Max Demian

In article , Max Demian
writes
"Jeff Layman" wrote in message
.. .
I happened to have an analogue programme on TV when I disconnected the
power lead from my Sagem IDT602. The picture quality deteriorated
somewhat, and became slightly snowy. On reconnecting the power, the
picture quality returned to normal.

I think all loopthroughs (VCRs, STBs etc) amplify to compensate for the
losses due to the signal taken. In fact I'm surprised it worked at all with
the power to the STB off, as an amplifier with the power off usually hardly
allows anything through.

They probably have a small capacitor connected between input and output;
it would have to have a sufficiently high reactance so as to attenuate
the signal by more than the gain of the amplifier, otherwise it would
turn in to an oscillator.

--
Ian G8ILZ

Jeff Layman wrote:

I happened to have an analogue programme on TV when I disconnected
the power lead from my Sagem IDT602. The picture quality
deteriorated somewhat, and became slightly snowy. On reconnecting
the power, the picture quality returned to normal.

On opening the Sagem, there was a small screened box, about the size
of a modulator (although the Sagem doesn't have one) on the circuit
side of the aerial in/out coax plugs. It was a Samsung TDCC2305TV34A
- a single conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that
- at least in the case of the Sagem - a small amount of gain is
introduced by the Samsung tuner to perhaps compensate for the loss in
signal through the loopthrough.

Is that correct, or am I way off beam?

(Oh, by the way, I opened the Sagem because it has suddenly lost
colour in the signal from Scart 2. So It's fine if I am happy with
B/W pictures from several channels via the inbuilt timer. But if not,
I'm stuck with a single channel, and the box has to be left on when I
need to make a recording).

You can check to see if the loopthrough amplifies by removing power
from the STB. If the signal reduces then it must amplify.

--
Ashley
For Windsor weather see www.snglinks.com/wx/

"Ashley" wrote in message
...
Jeff Layman wrote:

I happened to have an analogue programme on TV when I disconnected
the power lead from my Sagem IDT602. The picture quality
deteriorated somewhat, and became slightly snowy. On reconnecting
the power, the picture quality returned to normal.

On opening the Sagem, there was a small screened box, about the size
of a modulator (although the Sagem doesn't have one) on the circuit
side of the aerial in/out coax plugs. It was a Samsung TDCC2305TV34A
- a single conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that
- at least in the case of the Sagem - a small amount of gain is
introduced by the Samsung tuner to perhaps compensate for the loss in
signal through the loopthrough.

Is that correct, or am I way off beam?

(Oh, by the way, I opened the Sagem because it has suddenly lost
colour in the signal from Scart 2. So It's fine if I am happy with
B/W pictures from several channels via the inbuilt timer. But if not,
I'm stuck with a single channel, and the box has to be left on when I
need to make a recording).

You can check to see if the loopthrough amplifies by removing power
from the STB. If the signal reduces then it must amplify.

No, you would have to make a direct through connection as an off amplifier
will attenuate the signal considerably.

--
Max Demian

In article , Max Demian
writes
"Ashley" wrote in message
...
Jeff Layman wrote:

I happened to have an analogue programme on TV when I disconnected
the power lead from my Sagem IDT602. The picture quality
deteriorated somewhat, and became slightly snowy. On reconnecting
the power, the picture quality returned to normal.

On opening the Sagem, there was a small screened box, about the size
of a modulator (although the Sagem doesn't have one) on the circuit
side of the aerial in/out coax plugs. It was a Samsung TDCC2305TV34A
- a single conversion 48 - 862MHz tuner.

I thought that a simple loopthrough was just that. But it seems that
- at least in the case of the Sagem - a small amount of gain is
introduced by the Samsung tuner to perhaps compensate for the loss in
signal through the loopthrough.

Is that correct, or am I way off beam?

(Oh, by the way, I opened the Sagem because it has suddenly lost
colour in the signal from Scart 2. So It's fine if I am happy with
B/W pictures from several channels via the inbuilt timer. But if not,
I'm stuck with a single channel, and the box has to be left on when I
need to make a recording).

You can check to see if the loopthrough amplifies by removing power
from the STB. If the signal reduces then it must amplify.

No, you would have to make a direct through connection as an off amplifier
will attenuate the signal considerably.

Pardon! How does a direct through connection tell you anything about the
STB?

--
Ian G8ILZ