On 3/16/2014 1:26 PM, Jeff wrote:

It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this, and you
will suddenly get nothing).


That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

No one said the NTSC had to be noiseless. But the 43dB is a bit high,
even for older sets. Input from the cable tv company to our equipment
was 10-20dB; we tried to push 10dB to all of the outputs but never had a
problem even down to 7dB (the lowest we would let it drop to).

That makes no sense; a 7dB CNR would be pretty much unwatchable on
analogue, it would be a very very noisy picture, if it even locked at all!

Jeff


I'm not talking CNR - I'm talking signal strength. 7dbm is plenty of
signal. Most later TV's would work even at 0dbm.

HDTV, not so much.

--
==================
Remove the "x" from my email address
Jerry Stuckle

==================

In message , Jerry Stuckle
writes
On 3/16/2014 1:26 PM, Jeff wrote:

It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this, and you
will suddenly get nothing).


That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

No one said the NTSC had to be noiseless. But the 43dB is a bit high,
even for older sets. Input from the cable tv company to our equipment
was 10-20dB; we tried to push 10dB to all of the outputs but never had a
problem even down to 7dB (the lowest we would let it drop to).

That makes no sense; a 7dB CNR would be pretty much unwatchable on
analogue, it would be a very very noisy picture, if it even locked at all!

Jeff


I'm not talking CNR - I'm talking signal strength. 7dbm is plenty of
signal. Most later TV's would work even at 0dbm.

HDTV, not so much.

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!
--
Ian

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On 3/16/2014 7:17 PM, Ian Jackson wrote:
In message , Jerry Stuckle
writes
On 3/16/2014 1:26 PM, Jeff wrote:

It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this,
and you
will suddenly get nothing).


That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

No one said the NTSC had to be noiseless. But the 43dB is a bit high,
even for older sets. Input from the cable tv company to our equipment
was 10-20dB; we tried to push 10dB to all of the outputs but never
had a
problem even down to 7dB (the lowest we would let it drop to).

That makes no sense; a 7dB CNR would be pretty much unwatchable on
analogue, it would be a very very noisy picture, if it even locked at
all!

Jeff


I'm not talking CNR - I'm talking signal strength. 7dbm is plenty of
signal. Most later TV's would work even at 0dbm.

HDTV, not so much.

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv companies
must deliver at least 10 dbm to the premises.

TV signals (at least in the U.S.) are not measured by CNR - they are
measured by dbm. CNR is not important because the bandwidth does not
change.

Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

In message , Jerry Stuckle
writes
On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv
companies must deliver at least 10 dbm to the premises.

You do realise that 20dBm (appx 68dBmV) is a massive 100mW? With a
modest 50 channel analogue cable TV system, that would be a total input
power of 5W - which would have a TV set or set-top box sagging at the
knees - if not even beginning top smoke!

TV signals (at least in the U.S.) are not measured by CNR

Well of course they aren't. CNR is a ratio - not a level.

- they are measured by dbm.

No. The US and UK cable TV industry definitely uses dBmV.

0dBmV is 1mV - a reasonable signal to feed to a TV set (especially
directly from an antenna).

0dBm is appx 48dBmV (250mV) - and that's one hell of a TV signal!

With a 75 ohm source impedance (antenna and coax) - and no significant
levels of outside noise-like interference, a 0dBmV (1mV) analogue NTSC
signal, direct from an antenna, will have a CNR of around 57dB. A TV set
with a decent tuner noise figure (5dB?) or a set-top box (8dB) will
produce essentially noise-free pictures.

However, with an analogue TV signal from a large cable TV system, the
signal CNR will be much worse than 57dB (regardless of its level). If I
recall correctly, the NCTA ( National Cable Television Association)
minimum spec is a CNR of 43dB (UK is 6B). At this ratio, it is judged
that picture noise is just beginning to become visible.

CNR is not important because the bandwidth does not change.

You're havin' a laff - surely?!

Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.

I'm not insisting on anything. However, an analogue with a poor CNR will
produce noisy pictures - regardless of the signal level. Similarly, a
digital signal with a too poor an SNR/MER will fail to decode -
regardless of the signal level. I think the UK cable TV spec for digital
signals is 25dB (although a good set-top box will decode down to the
mid-teens).
--
Ian

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On 3/17/2014 11:32 AM, Ian Jackson wrote:
In message , Jerry Stuckle
writes
On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv
companies must deliver at least 10 dbm to the premises.

You do realise that 20dBm (appx 68dBmV) is a massive 100mW? With a
modest 50 channel analogue cable TV system, that would be a total input
power of 5W - which would have a TV set or set-top box sagging at the
knees - if not even beginning top smoke!

TV signals (at least in the U.S.) are not measured by CNR

Well of course they aren't. CNR is a ratio - not a level.

- they are measured by dbm.

No. The US and UK cable TV industry definitely uses dBmV.


Which is generally shortened to dbm here. What you are talking about is
dBmW - which, unfortunately, is also often shortened to dBm. But most
people on this side of the pond who are in the business understand that.

0dBmV is 1mV - a reasonable signal to feed to a TV set (especially
directly from an antenna).

0dBm is appx 48dBmV (250mV) - and that's one hell of a TV signal!

With a 75 ohm source impedance (antenna and coax) - and no significant
levels of outside noise-like interference, a 0dBmV (1mV) analogue NTSC
signal, direct from an antenna, will have a CNR of around 57dB. A TV set
with a decent tuner noise figure (5dB?) or a set-top box (8dB) will
produce essentially noise-free pictures.

However, with an analogue TV signal from a large cable TV system, the
signal CNR will be much worse than 57dB (regardless of its level). If I
recall correctly, the NCTA ( National Cable Television Association)
minimum spec is a CNR of 43dB (UK is 6B). At this ratio, it is judged
that picture noise is just beginning to become visible.

CNR is not important because the bandwidth does not change.

You're havin' a laff - surely?!

Nope.


Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.

I'm not insisting on anything. However, an analogue with a poor CNR will
produce noisy pictures - regardless of the signal level. Similarly, a
digital signal with a too poor an SNR/MER will fail to decode -
regardless of the signal level. I think the UK cable TV spec for digital
signals is 25dB (although a good set-top box will decode down to the
mid-teens).

External noise is somewhat consistent. Front ends are pretty much
comparable in their S/N ratio. The only problems with noise are
generally if you have something generating noise locally. But that is
not a problem with the signal nor the receiver.

That is why the real world uses signal strength to determine proper
signal levels. CNR in TV is not used nor is it required when the other
parameters are known.

--
==================
Remove the "x" from my email address
Jerry Stuckle

==================

On 17/03/2014 13:14, Jerry Stuckle wrote:
On 3/17/2014 3:45 AM, Jeff wrote:

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.


dBm is not a bandwidth dependant measurement such as CNR which is.
Putting +7dBm into a tv receiver is madness, it would cause severe
overload and inter mods. +7dBm is 50mW and that equates to about 61mV in
a 75 ohm system which is an enormous signal.

Jeff

Wrong. TV's are made to handle at least 20 dbm. And cable tv companies
must deliver at least 10 dbm to the premises.

TV signals (at least in the U.S.) are not measured by CNR - they are
measured by dbm. CNR is not important because the bandwidth does not
change.

Your insistence on using CNR shows you know nothing about how the
industry measures signal strength.


I have not insisted that CNR was used, it that was another poster who
mentioned CNR, what I was pointing out was the error that you made in
taking the 43db CNR value that was posted and then going on about
dbMmlevels.

I also dispute that televisions are made to handle +20dBm; that is 100mW
far in excess of what a tv tuner can handle without overload!!!!

+20dbmV may be but not +20dbm.

Jeff

On 17/03/2014 15:45, Jeff wrote:


I also dispute that televisions are made to handle +20dBm; that is 100mW
far in excess of what a tv tuner can handle without overload!!!!

+20dbmV may be but not +20dbm.

Jeff

I know little about TV broadcasting, but in my (audio) field, 0 dBm is
taken as a level of 1 milliwatt into 600 ohms (0.775 V).

Funny how all these "standards" arise!

Incidentally, during the early development of 10cm radar kit, the "dB"
was banished from the lab. the chaps preferring to express their results
in linear units.

Les.

In message , Jerry Stuckle
writes
On 3/16/2014 7:17 PM, Ian Jackson wrote:
In message , Jerry Stuckle
writes
On 3/16/2014 1:26 PM, Jeff wrote:

It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this,
and you
will suddenly get nothing).


That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

No one said the NTSC had to be noiseless. But the 43dB is a bit high,
even for older sets. Input from the cable tv company to our equipment
was 10-20dB; we tried to push 10dB to all of the outputs but never
had a
problem even down to 7dB (the lowest we would let it drop to).

That makes no sense; a 7dB CNR would be pretty much unwatchable on
analogue, it would be a very very noisy picture, if it even locked at
all!

Jeff


I'm not talking CNR - I'm talking signal strength. 7dbm is plenty of
signal. Most later TV's would work even at 0dbm.

HDTV, not so much.

7dBm is an absolutely colossal signal for a TV set. Even 0dBm is an
absolutely colossal signal!

Not in the United States. It was the minimum that the cable industry
provides to the TV set.

We are talking a signal 4.25Mhz wide signal, not SSB or CW.

The TV signal levels quoted for analogue cable TV don't really involve
bandwidth. The level is always the 'RMS during sync' (or 'RMS during
peak'), which is the RMS level of the vision RF envelope during the
horizontal (or vertical) sync period. This has the advantage of
remaining constant regardless of the video content (ie it's the same for
a completely black picture or a completely white picture).. The only
requirement is that the measuring instrument has sufficient bandwidth to
embrace enough of the low frequency sideband content of the video signal
to give a reading which IS independent of the video content. On a
spectrum analyser, 300kHz resolution will display the demodulated RF
waveform (thus enabling you to read the RF level), but IIRC many field
strength meters have an IF bandwidth of typically 30kHz. However,
regardless of the actual measuring bandwidth, noise levels are
normalised to a bandwidth of 4.2MHz (NTSC) and 5.2MHz (PAL). and
signal-to-noise measurements are adjusted accordingly.

Note that the cable TV industry generally uses units of dBmV (dB with
respect to 1mV - traditionally considered a 'good' level to feed to a TV
set). This is because most of the levels the cable TV guys work with are
generally in excess of 0dBmV (typically 0 to 60dBmV).

The off-air TV guys often use dBuV (dB wrt 1microvolt), as they are
usually dealing with weaker signals. As a result, cable TV guys are
always having to mentally deduct 60dB.

RF communications guys (and domestic satellite) tend to use dBm (which
is a slovenly version of 'dBmW' - dB wrt 1mW) - despite the fact that a
lot of their levels are large negative numbers. Also note that dBm tends
to imply a Zo of 50 ohms, and dBmV/dBuV 75 ohms - but it ain't always
necessarily so.

Anyone working in the RF industry would be well advised to ensure that
they always use the correct units - for example, don't say 'dB' or 'dBm'
when you really mean dBmV. Failure to do so can often result in people
needlessly arguing and talking at cross-purposes.
--
Ian

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