In article ,
says...

In message , Arny Krueger
writes

"Terry Casey" wrote in message
...


A couple of questions regarding that list:

Why is the HRC channel spacing offset[1] by 300Hz - 6.0003MHz instead of
6MHz?

I don't know.

I recall once specially tweaking a UK 8MHz HRC harmonic comb generator
(to which all of the TV channels were locked). It was a little above (or
was it below?) 8MHz. There was a reason for this, but at the moment, I
can't remember what it was.


One system where this was done was the old BT Westminster system -
probably very useful in an area where I would expect a lot of off-air
reception problems.

I don't know what offset they used but, as an example, if you alter the
comb to 7.990963855MHz, channel E45 is bang on (663.25MHz) so, if you
centred the five off-airs around this using E41, E43, E45, E47 and E49,
the worst case error will be +/-36kHz from the nominal frequency.

--

Terry

In message ,
Terry Casey writes
In article ,
says...

In message , Arny Krueger
writes

"Terry Casey" wrote in message
...


A couple of questions regarding that list:

Why is the HRC channel spacing offset[1] by 300Hz - 6.0003MHz instead of
6MHz?

I don't know.

I recall once specially tweaking a UK 8MHz HRC harmonic comb generator
(to which all of the TV channels were locked). It was a little above (or
was it below?) 8MHz. There was a reason for this, but at the moment, I
can't remember what it was.


One system where this was done was the old BT Westminster system -
probably very useful in an area where I would expect a lot of off-air
reception problems.

Same lot, in the land of the concrete cows and a thousand roundabouts.
;o))

I don't know what offset they used but, as an example, if you alter the
comb to 7.990963855MHz, channel E45 is bang on (663.25MHz) so, if you
centred the five off-airs around this using E41, E43, E45, E47 and E49,
the worst case error will be +/-36kHz from the nominal frequency.

Ah yes. That's certainly one of the reasons for using a weird reference
frequency. I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.
Almost certainly this is what I was involved in.

But wasn't it at the same place which ingeniously used a not-quite-8MHz
comb reference which was actually derived from one of the UHF off-air
channels? As the headend equipment was largely supplied by the 'other'
company, I doubt if I would have been involved with tinkering with it
(although I'm pretty sure that I did swap one or two of the modulator
SAW filters because of the problems which arose when NICAM started).
Maybe 'my' comb generator was a replacement.
--
Ian

In article ,
says...

In message ,
Terry Casey writes
In article ,
says...

In message , Arny Krueger
writes

"Terry Casey" wrote in message
...


A couple of questions regarding that list:

Why is the HRC channel spacing offset[1] by 300Hz - 6.0003MHz instead of
6MHz?

I don't know.

I recall once specially tweaking a UK 8MHz HRC harmonic comb generator
(to which all of the TV channels were locked). It was a little above (or
was it below?) 8MHz. There was a reason for this, but at the moment, I
can't remember what it was.


One system where this was done was the old BT Westminster system -
probably very useful in an area where I would expect a lot of off-air
reception problems.

Same lot, in the land of the concrete cows and a thousand roundabouts.
;o))

I don't know what offset they used but, as an example, if you alter the
comb to 7.990963855MHz, channel E45 is bang on (663.25MHz) so, if you
centred the five off-airs around this using E41, E43, E45, E47 and E49,
the worst case error will be +/-36kHz from the nominal frequency.

Ah yes. That's certainly one of the reasons for using a weird reference
frequency. I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!

It was more likely to be, in the case of Westminster, that, when CATV
systems rarely went above 600MHz, there was nowhere else to put them,
coupled with the fact that the off-air channels were left clear, so it
was convenient utilise to this for the n + 2 arrangement by straddling
the otherwise blank off-air allocation.

A comb of 7.988636364MHz would allow E25, E27, E29 and E31 to be used
with +/-34kHz error (off-airs being 23, 26, 30 & 33)

Of course, this was all long before the Channel 5 debacle - I can't see
a way of interleaving 5 channels around Crystal Palace without involving
the allegedly taboo n + 5 scenario - although I've never seen a problem
with any set I used directly connected to a CATV network


But wasn't it at the same place which ingeniously used a not-quite-8MHz
comb reference which was actually derived from one of the UHF off-air
channels?

Well, dividing E26 by 64 or E30 by 68 would do the trick. I based my
comb frequency on E28, being the centre channel but an off-air lock
would certainly produce a very stable result, and the offsets would
still be reasonable - +58/-11kHz or +11/-58kHz, depending on choice of
off air channel.

As the headend equipment was largely supplied by the 'other'
company, I doubt if I would have been involved with tinkering with it
(although I'm pretty sure that I did swap one or two of the modulator
SAW filters because of the problems which arose when NICAM started).
Maybe 'my' comb generator was a replacement.

My involvement with the Westminster system was at the time of the DTV
roll-out (or possibly Broadband Internet, I can't remember which) which
coincided with the transfer of the system from BT to ntl, so I never saw
the BT headend but I did see the documentation related to it, complete
with frequency details.

--

Terry

In message ,
Terry Casey writes
In article ,
says...




I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!

I don't know about 'unusual', but they were a problem. I think there
were only couple of budget brands which only tuned 'spot-on' to the UHF
channels (xxx.25MHz, in 8MHz steps). One might ask indeed "Why would you
need them to do otherwise?" Of course, even our cable set-top boxes
could normally only tune in 125kHz steps, but at least that got you to
within +/-63kHz of the correct frequency - and that was more than close
enough.

It was more likely to be, in the case of Westminster, that, when CATV
systems rarely went above 600MHz, there was nowhere else to put them,
coupled with the fact that the off-air channels were left clear, so it
was convenient utilise to this for the n + 2 arrangement by straddling
the otherwise blank off-air allocation.

A comb of 7.988636364MHz would allow E25, E27, E29 and E31 to be used
with +/-34kHz error (off-airs being 23, 26, 30 & 33)

Of course, this was all long before the Channel 5 debacle - I can't see
a way of interleaving 5 channels around Crystal Palace without involving
the allegedly taboo n + 5 scenario - although I've never seen a problem
with any set I used directly connected to a CATV network

Sets generally seemed to improve a lot in later years. I think that the
change of IF from 39.5MHz to the European 38.9MHz made quite a
difference to N+/- problems. What surprises me is how well some sets
could tolerate having direct inputs of 48+ channels (without them going
through the converter UHF bypass filtering). Certainly, in the olden
days, when faced with more than half a dozen channels, some sets tended
to sag a bit at the knees.

But, as you have said, there used to be so many embargoed channels on a
cable TV system - no adjacent, no N+/-5, no N+/-9, no sums or
differences (with single-ended amplifiers) etc. It's a wonder anyone was
able to get more than two or three channels!


But wasn't it at the same place which ingeniously used a not-quite-8MHz
comb reference which was actually derived from one of the UHF off-air
channels?

Well, dividing E26 by 64 or E30 by 68 would do the trick. I based my
comb frequency on E28, being the centre channel but an off-air lock
would certainly produce a very stable result, and the offsets would
still be reasonable - +58/-11kHz or +11/-58kHz, depending on choice of
off air channel.

If this is what they did, they could have used either of those channels
from Crystal Palace. Next time I see him, I'll ask the man who will
almost certainly know (if I remember!).

As the headend equipment was largely supplied by the 'other'
company, I doubt if I would have been involved with tinkering with it
(although I'm pretty sure that I did swap one or two of the modulator
SAW filters because of the problems which arose when NICAM started).
Maybe 'my' comb generator was a replacement.

My involvement with the Westminster system was at the time of the DTV
roll-out (or possibly Broadband Internet, I can't remember which) which
coincided with the transfer of the system from BT to ntl, so I never saw
the BT headend but I did see the documentation related to it, complete
with frequency details.

I had little to do with the system in London (I think I only went there
once - underground, near Shepherds Bush IIRC). As I said, my involvement
was among the concrete cows and the roundabouts.
--
Ian

In article ,
says...

In message ,
Terry Casey writes
In article ,
says...



I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!

I don't know about 'unusual', but they were a problem. I think there
were only couple of budget brands which only tuned 'spot-on' to the UHF
channels (xxx.25MHz, in 8MHz steps). One might ask indeed "Why would you
need them to do otherwise?" Of course, even our cable set-top boxes
could normally only tune in 125kHz steps, but at least that got you to
within +/-63kHz of the correct frequency - and that was more than close
enough.


Apologies - I misread what you wrote!

I thought you wrote "the four set-top bypass channels had to be close to
the LOCAL off-air broadcast channels ..."



I had little to do with the system in London (I think I only went there
once - underground, near Shepherds Bush IIRC).

As I said, I didn't actually visit the headend. All of our equipment was
in a room in the basement of a large block not far from Marble Arch
IIRC. Obviously, the headend couldn't have been far away ...

--

Terry

Ian Jackson wrote:

I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!
They existed. We used to have problems with a system that had crappy
modulators that drifted. Some of the tellys just wouldn't tune the
signals unless they were exactly on the channel. Just skipped over them.


A comb of 7.988636364MHz would allow E25, E27, E29 and E31 to be used
with +/-34kHz error (off-airs being 23, 26, 30 & 33)
When I worked at CBC (near BH) about 20 years ago the cable input was on
channels 29 and 31.

On the subject of adjacent channel working we have several systems that
use almost every UHF channel and the tellys tolerate it perfectly well.
And they mostly cheap Vestels.

Bill

In article , says...

Ian Jackson wrote:

I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.


They would have to be very unusual TV sets!
They existed. We used to have problems with a system that had crappy
modulators that drifted. Some of the tellys just wouldn't tune the
signals unless they were exactly on the channel. Just skipped over them.


Yes, as I've said in another post, I misinterpreted Ian's comment.

I've also come across sets that jump in 8MHz increments but have usually
found a 'fine tune' option buried deep down in the menu somewhere ...

That's usually when you find out that the remote is not working or has
been lost ...

--

Terry

Terry Casey wrote:
In article , says...
Ian Jackson wrote:

I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.

They would have to be very unusual TV sets!
They existed. We used to have problems with a system that had crappy
modulators that drifted. Some of the tellys just wouldn't tune the
signals unless they were exactly on the channel. Just skipped over them.


Yes, as I've said in another post, I misinterpreted Ian's comment.

I've also come across sets that jump in 8MHz increments but have usually
found a 'fine tune' option buried deep down in the menu somewhere ...

That's usually when you find out that the remote is not working or has
been lost ...

I remember some sets that jumped in 8MHz increments but if the signal
was not exactly on channel the set would jump over it, so there was no
chance to fine tune.

Bill