Re: Valerian Vinyl

In article <5v6r3qF1l3863U1@xxxxxxxxxxxxxxxxxx>,
David Looser <david.looser@xxxxxxxxxxxxxx> wrote:
"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.

Yes. Pre-war TV was advertised as having better than radio sound quality.
Just as an aside, the landline feeding Brookman's Park which was the
original London area Radio 1 transmitter was an extra high quality one
capable of being equalised to 15 rather than 9 kHz - and was originally
installed for TV sound.

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)?.

In one. With the lethal EHT section. Colour difference drive.

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.

I *think* we added an emitter follower to the clamp. And for a different
reason a burst notch filter. For some reason Philips didn't seem to think
it necessary. Of course many makers didn't bother notching it out but just
rolled it off (and everything above).

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.

Thanks for the explanation which should have jogged my memory. My pal John
who designed the mods also designed and built several complete sets.
Despite being employed as a sound operator. ;-)

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

Yup. Other thing is just how good some PAL decoders are these days. Which
won't matter either.

--
*The man who fell into an upholstery machine is fully recovered*

Dave Plowman dave@xxxxxxxxxxxxxxx London SW
To e-mail, change noise into sound.
.

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