Re: Valerian Vinyl

"Dave Plowman (News)" <dave@xxxxxxxxxxxxxxx> wrote in message
news:4f62147f00dave@xxxxxxxxxxxxxxxxxx
In article <eyaUi5EdvdjHFwO1@xxxxxxxxxxxx>,
tony sayer <tony@xxxxxxxxxxxx> wrote:
No you are not alone. I first piped TV sound through a Hi-Fi system in
the early 1970s. In those days TV sets required extensive modification
to provide a high-quality line-level audio feed, but it was worth the
effort.

David.

Indeed!, I was very surprised on just how good it was, AM and 405 line
TV let alone the later FM 625 which used to buzz well, intercarrier buzz

The potential sound quality of the AM sound channel that accompanied
405-line TV was surprisingly good. With a bandwidth of around 15kHz and in a
receiver that minimised vision buzz by good RF design and with a decent
audio amp and speaker the results were very acceptable. Most pre-war and
early post-war TVs had audio systems as good or better than the typical
table radio of the day. I have a 1936 HMV901 that has a KT41 driving an
8"x10" elliptical speaker mounted in a substantial wooden cabinet. With a
decent quality audio signal modulated onto a 41.5MHz carrier fed into it's
aerial socket the resulting sound is pretty good. After 1950 of course the
bean counters got to work.

TV set makers in those days didn't give much care to the audio circuit
design. Single ended valve output with a transformer the size of a golf
ball and the cheapest single cone speaker they could source. And that was
on the expensive sets. On my Philips we modified the black level clamp
which was causing a deal of the inter carrier buzz. Don't know the ins and
outs - a vision engineer pal worked out the mod.

Was that a Philips G6 (the hybrid one)?.

On normal TVs the intercarrier of just below 6MHz (in the UK) is created in
the vision detector by the non-linear mixing of the vision carrier IF
(nominally 39.5MHz) and the sound carrier IF (nominally 33.5MHz). If there
is any local oscillator drift both IFs move together, so the intercarrier
frequency remains the same.

On many earlier designs the intercarrier was amplified by the video amp
(luminance amp in colour sets) before being extracted by a 6MHz tuned
circuit and fed via a limiting amplifier to the FM discriminator. If the
video is clamped before the intercarrier is extracted it's likely that the
clamp will suppress the intercarrier signal whilst it is conducting. Either
modifying the clamp to not suppress 6MHz (which seems to be what your pal
did) or, better still, extracting the intercarrier before the clamp will
solve that one. But there are plenty of other ways of creating intercarrier
buzz, the hardest to solve being phase modulation of the vision carrier by
the video signal, which can occur in the transmitter or the receiver, but is
certain to be caused by the non-symmetrical sideband response of the IF
channel, necessary because of the use of vestigial-sideband transmission of
the video signal. The only total solution to that one is to use a separate
IF channel for the sound, which was almost unheard of in the pre-Nicam days.

Of course with the imminent switch off of analogue TV all this will soon be
of historical interest only.

David.


.

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